Thursday, October 31, 2013

Temperature Averaging

* Updated at bottom for clarity

Last week a discussion erupted regarding the calculation of average temperatures. Here is a little light reading on an example from down in Anchorage. The Anchorage forecast office is the official site where climate observations are made for the state's largest city. There is a thermometer on the site that publishes temperatures every 10 minutes. Some PAFC staff have indicated to me that it represents the official temperature but Rick has indicated that might not be the case. If we assume that it is, we can compare an average of the 144 daily readings with the official min plus max divided by two method for calculating average temps. For about 18 months I save the 10-minute readings for the PAFC site and put them into an Excel file. In June 2009 the web functionality changed any it only allowed the user to go back a few days before the data was purged.

The charts below show the daily difference between the 10-minute mean and the official daily mean. Over 90% of the time the two numbers were within 1 degree of each other. In the warm months, the 10-minute readings were typically larger than the official daily average. (*Update: the black line is a 10-day running mean). That is primarily due to a quick spike in temperatures at the official site late in the afternoon that pushed the official high upward for only a brief period. In the cold months, the pattern was much more chaotic and the magnitude was surprisingly smaller.

* Update: Here is an example of a single day's temperature profile. On 2/4/2008, the official high was +2 and the official low was -12. The strict arithmetic average of those numbers yields an average -5°F. What I am interested in knowing is exactly how representative this average temperature is as a proxy for the temperature at a much finer temporal resolution. Since there are 144 measurements throughout the day, we can compute an average in much greater detail. On this date, the average of the 144 measurements was -6.17°F. In my opinion, that is actually really good when compared to the simple high+low/2 method. 65% of the 10-minute observations were below the average but the short, intense peak nearly balanced the prolonged cold temperatures nearly perfectly. Over the course of the year, this value is pretty representative. In summer it is greater and in winter it is smaller. Hope that helps.

Temperature and Snow Cover

Temperatures in Fairbanks continue to run far above normal, as illustrated by the fact that this morning's 5 a.m. temperature of 19 °F is equivalent to the normal high temperature for this date.  An increasingly large contributor to this anomaly is the lack of snow cover, which profoundly influences the balance of infrared radiation and temperature at the earth's surface.  I thought it would be interesting to see how strongly snow depth is correlated with temperature in the autumn and early winter in Fairbanks - see the chart below.  Note that I've calculated each category's median temperature only for dates with 10 or more years available; and the anomalies are all with respect to the 1981-2010 normals.

When snow cover is absent or merely a trace in Fairbanks (blue line), the temperature tends to be increasingly above normal as October proceeds; this reflects the increasing departure from normal that the lack of snow represents.  Of course, this is a chicken and egg problem: do the warm temperatures cause lack of snow, or vice versa?  It's both, but as the season advances, a lack of snow becomes an increasingly unusual influence that prevents valley-level temperatures from dropping as they normally do.  Also, by late October even well above normal temperatures are not normally a hindrance to snow accumulation (this year being an extreme outlier with such abnormal warmth).

It's also interesting to see that prior to about 18 October, a thin snow cover (1-3") is associated with below-normal temperatures, but after that date it is more often observed with unusual warmth.  This is simply because thin snow cover is greater than is usually observed at the earlier dates, but becomes less than usual later.  The same is true for 4-6 inch snow depth; but snow more than 6 inches deep is associated with colder than normal temperatures throughout the autumn and early winter, and especially in October.

Tuesday, October 29, 2013

Feels Like Equivalency

Through October 28th, the daily average temperature in Fairbanks has been warmer than normal for 26 of the 28 days. One way to describe the actual temperature is to come up with an analog of which calendar day it most closely approximates. For example, the daily average temperature of 44.5 degrees yesterday would be expected to occur on September 17th. Therefore, a "feels like" equivalency of September 17th can be assigned to yesterday's climate. September 17th is 41 days earlier than October 28th. The following table shows the date, the actual temperature, and the feels like analog date. On average the analog date has been 15 days earlier that the actual date. So if it feels like the climate has been running about two weeks behind lately, you are correct.

Record Warmth Again

The warm, rainy conditions across interior Alaska have broken a number of longstanding records; here are a few, and I'm sure Rick and Brian can add more.

  • The 850 mb temperature above Fairbanks at 4 pm AKDT yesterday (9.4 °C) was the highest of record for so late in the season.  The previous record for this late or later was 8.6 °C on December 12, 1985.
  • The high temperature of 62 °F at Big Delta yesterday was by far the warmest recorded so late in the season; the previous record was 55 °F on (again) December 11, 1985.  Yesterday's high would have been the warmest so late in the season if it had occurred two weeks ago, because a temperature above 60 °F has never been recorded in Big Delta after October 13 (records back to 1941).
  • In Fairbanks, the high of 51 °F and low of 38 °F yesterday broke the Weather Bureau/NWS era daily records by 7 °F each.  Temperatures higher than this have been observed later in the year, but not since the 1930s.
  • Rick pointed out in an e-mail that this is the latest in the autumn that Fairbanks has ever observed measurable rain with no snow at all on the ground.  Rain has occurred later with snow already on the ground, but the rain with completely bare ground is unprecedented for the time of year.  The latest this has occurred before (since 1930) is October 24, in 1981.  This morning's soggy scene captured by the Alaska Climate Research Center webcam on UAF West Ridge (below) has never been seen before at this time of year.

The chart below shows the updated daily temperature anomalies for Fairbanks since August 1.  Remarkably, October so far is less than 1 °F cooler than the second half of September; and the October average high temperature is actually higher than in the second half of September.

Monday, October 28, 2013

Deep Warmth

A powerful southerly flow of air aloft is once again transporting extremely warm air for the time of year up to Alaska.  The morning balloon soundings from Anchorage and Fairbanks show a deep layer of above-freezing air, extending up to nearly 8000 feet above sea level at Fairbanks and an extraordinary 9500 feet at Anchorage (see below).  The 700 mb temperature of -0.1 °C at Anchorage is close to a record for the time of year.

The warmth won't last long in the interior this time, as the surge of warm air heads quickly east later today and the freezing level drops rapidly later today and tonight.  It will be interesting to see if valley locations cool enough for accumulating snow tomorrow; if not, then October may end as the least snowy on record in Fairbanks (0.1 inches so far).

Sunday, October 27, 2013

Most and Least Difficult Temperature Records to Break

Several of tomorrow's temperature records in Fairbanks are relatively easy to break. The low minimum record of -17 is only 1.97 standard deviations below the normal low of 6.3 degrees. Only two calendar days have record low temperature that are closer to their respective daily normal temperatures than October 28th. Therefore, the low minimum record for tomorrow is statistically, the third easiest low minimum record to break. Conversely, the record low minimum temperature for May 9th is pretty safe. It is 6.33 standard deviations below the 1981-2010 daily normal of 35.1 degrees.

The low maximum record for tomorrow is 4 degrees. That is only 1.61 standard deviations below the daily normal maximum of 21.4 degrees. No other low maximum record is closer to its respective daily normal temperature for any day of the year. Therefore, the October 28th low maximum record is the least difficult daily low maximum record to break during the course of the year. For reference, the coldest October 27 maximum is -1 and the coldest October 29 maximum is -6. Those records are more difficult to break.

The chart (graphic) below shows the most difficult daily records to break (left side) and least difficult daily records to break (right side) over the course of the year. There is a noticeable, and interesting, seasonality to the chart.

Four types of records are shown.

1) High maximum (a.k.a. record high)  --> shown in pink
2) Low maximum (a.k.a record coldest high temperature)  --> shown in yellow
3) Low minimum (a.k.a. record low)   --> shown in blue
4) High minimum (a.k.a. record warmest low temperature)   --> shown in green

Note 1: the standard deviations from normal are all based on the 1981-2010 climate normal period. The relative ease or difficulty to break a record should be compared to the current climate regime.

Note 2: The May 9 (1964) record was 5.32 SD below the mean for the 1931-1960 climate normal period.

Saturday, October 26, 2013

Minimum Diurnal Range

Fairbanks has just passed the date on the calendar when the climatological average diurnal range (the difference between the daily high and low temperatures) is smallest.  At the airport observing site, the minimum diurnal range is just under 15 °F and is smallest around October 21, according to the NCDC 1981-2010 climate normals.

The chart below shows the annual cycle in diurnal range for Fairbanks, Anchorage, and Barrow.  Of these three locations, the average diurnal range is highest at Fairbanks throughout the year, because cloudiness and relative humidity are usually much lower in the Alaskan interior than on the coasts; the relatively clear skies and low humidity in Fairbanks allow the temperature to show a greater response to the daily cycle in solar insolation (except in mid-winter).  Also, in Fairbanks in winter, changes in cloudiness within a 24-hour period can bring large changes in temperature, whereas in Anchorage and Barrow the generally more persistent cloudiness makes large daily changes of this kind less common.

But how do we explain the October minimum in diurnal range in Fairbanks?  I can think of three ingredients for a small average diurnal range:
  • a low sun angle and thus small solar insolation, minimizing daytime warming
  • high cloudiness, minimizing both daytime warming and nighttime cooling
  • lack of snow cover, minimizing nighttime cooling
Clearly there is a decreasing solar angle as autumn advances, and it's possible that cloudiness also increases from September to October in Fairbanks.  I don't have cloud cover data to back this up, but we've seen before that there is a peak in precipitation frequency in October.  In fact, the diurnal cycle is nearly inverse to the precipitation frequency in late summer and autumn, with a minimum diurnal range in August associated with damp weather, a maximum in September associated with drier conditions, and then a decline in October associated with more frequent precipitation again.

Finally, snow depth is typically not great in October in Fairbanks - and of course in some years may be non-existent.  If the ground remained snow-free for the rest of the year, then it's likely that the diurnal range would continue to decrease; but as snow depth builds in early winter in Fairbanks, then nighttime cooling increases and the diurnal range picks up again.  At least, that's my interpretation of what's going on.

The rapid increase in average diurnal range in late winter in Fairbanks is undoubtedly due to the return of the sun, combined with a typically deep snow cover and generally clear skies; all three factors favor large temperature swings between day and night.

It seems that some of the same basic physics comes into play in Barrow and Anchorage, as the diurnal range minimizes near the onset of winter and then increases in mid-winter (slightly in Anchorage) and again in spring.  However, I'm not sure what to make of the early summer variations in Barrow; any suggestions would be welcome.

To round out the analysis, below are maps showing the minimum diurnal range and the date of the minimum.  The annual minimum in diurnal range tends to progress southeastward across the state through the autumn and winter, with some Pacific coastal locations seeing their least variable daily temperatures after the turn of the year.

Thursday, October 24, 2013

Start of Freeze-Up

In a rapid response to the cooler - though still much warmer than normal - conditions, ice has begun to form on inland waterways across northern Alaska.  The webcam photo below, courtesy of Pro Music in downtown Fairbanks, shows some thin ice on the Chena River, and FAA webcams reveal a similar situation on the Yukon River at Beaver and the Koyuk River at Koyuk (also shown below).  It is interesting that in the case of Koyuk, the lowest the temperature has dropped is 27 °F, but with a stiff breeze today, that is all it took to begin the process of freeze-up.

Also of interest is the view from Kivalina, on the Chukchi Sea coast northwest of Kotzebue.  Despite temperatures only dropping below freezing yesterday evening, and a morning low temperature of only 24 °F, the lagoon behind Kivalina shows a substantial amount of ice today.  There's just no getting around the inevitable for this time of year.

Wednesday, October 23, 2013

First Sub-Freezing High Temperature

It appears that Fairbanks has recorded its first day this season with a high temperature at or below freezing. This is a full two weeks later than normal for the Airport station and only three days away from the station record (Note: the Fairbanks Airport station has been recording observations since 1948). The stations on this map all have at least 15 years worth of winter seasons. Seasons with more than 10 missing observations were not included. Today's date is at, or later, than the record latest first sub-freezing day for a few locations. It is possible that some stations broke this record. The analysis on this map does not include and data from this winter.

Gradual Cooling

Surface temperatures have cooled off quite a bit across the eastern interior in the past couple of days, with overnight minimum temperatures in the single digits and teens in many places, as befits the season.  The Chicken COOP observer reported 0 °F for the first time this morning, which is about 10 days later than normal; the climatological normal low temperature drops to zero in only three days from now at Chicken.  A number of higher elevation RAWS locations also reported 5 °F or lower, and Arctic Village in the northeast reached 0 °F.

Despite the surface cooling, above-freezing air was still observed in this morning's 4 a.m. sounding at Fairbanks.  The chart below shows that the 925 mb temperature (around 700 m above ground) has been above freezing at each 12-hourly measurement since early on October 11.  With gradual cooling continuing aloft today, and surface temperatures just below freezing, the column will soon drop below freezing if it hasn't already.  However, this won't last long, with warmer air returning soon.

Tuesday, October 22, 2013

October Warmth

Here are a few statistics that illustrate just how extremely unusual the recent warm spell has been across Alaska and northwestern Canada:

  • Fort McPherson, in the far northwest of Canada's Northwest Territories, experienced its warmest October week of record for the 7 days ending yesterday (Oct 15-21), with an average temperature of 44.1 °F.  The old record, in a fairly complete period of record back to 1893, was 43.9 °F for Oct 1-7, 2003.  It is quite astonishing that this record could be broken in the second half of the month, considering the rapid pace of seasonal cooling at this time of year.
  • In Anchorage, the week ending October 20 was the second warmest October week of record (weekly mean temperature of 49.0 °F, compared to 49.6 °F for Oct 1-7, 2003).
  • McGrath saw its third warmest (non-overlapping) October week of record.  More remarkably, the Oct 13-19 average low temperature of 40.9 °F was the highest on record for any week either wholly or partially in October; the previous record was 40.7 °F for Sep 25 - Oct 1, 2002.  The chart below shows the recent history of daily minimum temperatures in McGrath.

Monday, October 21, 2013

Snow is Late

The lack of snow in Interior is becoming quite notable. Fairbanks has had just 0.8" of snow for the season, and even higher elevations (least below 3500' MSL) that had significant snow in September and early October have lost it thanks to what we now know from Richard's analysis to be the third warmest late October warm spell of record.

Thus far, Fairbanks Airport has had any snow on the ground just one day this autumn, a trace on September 22nd. So how unusual is it to get to the last third of October without having had even one day with measurable snow on the ground? Below is a histogram of the first autumn date with a snow depth of 1" or more (this is NOT necessarily the date of establishment of the permanent winter snow cover). As you can see, we are moving into the late tail of the distribution. If, as seems likely, we get to Wednesday without an inch or more snow cover at the Airport, then this would move into a "top 10" latest first inch. The latest, and only occurrence in November (in the WB/NWS era since 1930) is November 6, 1938.

One caveat on the data here: snow depth is not, for climate purposed, monitored continuously. These dates are the snow depth at whatever the standard time for recording this element has been, and this as varied over the past 80 plus years (currently it's the snow depth at midnight). 

Will Warm Weather Continue?

In view of the exceptionally warm weather of the past week or so across interior Alaska, I thought it would be interesting to look at similar past events to see if the history reveals any consistent patterns in the subsequent weeks.  In other words, how does the weather typically evolve after unusual warmth at this time of year?  I looked up the warmest weeks ending between October 10th and 31st in Fairbanks, based on the mean daily temperature departure from the 1981-2010 daily climate normals.  Here's the list of weeks (only one per year allowed) along with the temperature anomaly in terms of standard deviations:

Oct 18-24, 1938      +2.86
Oct 7-13, 1969        +2.04
Oct 13-19, 2013      +2.01
Oct 18-24, 1980      +1.89
Oct 25-31, 2002      +1.75
Oct 21-27, 1981      +1.70
Oct 17-23, 1957      +1.68
Oct 22-28, 1936      +1.66
Oct 25-31, 1954      +1.61
Oct 23-29, 1942      +1.53
Oct 19-25, 1987      +1.49

The chart below shows the temperature anomaly over the 60 days following the end of each warm week, with each line representing a different year.  Admittedly it's a bit difficult to follow each individual year with so many lines, but focus on the thick black line which shows the median of the 10 years.  It's no surprise to see that temperatures tend to stay above normal for several weeks after a very warm October week in Fairbanks.  However, it's interesting that the median reaches its highest point at four weeks (28-29 days) after the end of the warm October week; indeed, several of the historical events (e.g. 2002, 1936, 1957, 1981, 1938) produced very warm conditions again (relative to normal of course) in the period from about 3 to 5 weeks later.  In fact, only 3 of the 10 years were not significantly above-normal in that period (1942, 1969, 1987).  This could be a statistical quirk, but it's not unreasonable to imagine that a natural periodicity in the global circulation might favor the reintensification of a pronounced feature such as the ridge over the northeast Pacific.

The other interesting aspect of the data is that half of the years experienced significantly below-normal temperatures around 8 weeks after the end of the warm October week; so there is no suggestion that unusual warmth is likely to linger into a second month.

In summary, the history of similar past events appears to suggest that relatively warm conditions are quite likely to continue in Fairbanks for another 4-5 weeks, but by mid-December it's entirely possible that below-normal temperatures will emerge.

Sunday, October 20, 2013

Temperature Deviations From Normal

The last four days in Fairbanks have all been more than 2 standard deviations from the mean. This brings the annual total to 42 days that the average daily temperature has been at least 2 standard deviations (more than +2 or less than -2) from normal. The total of 42 (through October 19th) far surpasses the record for an entire calendar year – the twice set value of 37. The values from 2011 onward were calculated based on published temperature and standard deviation normals published by NCDC. The values prior to 2011 were calculated based on normal temperatures calculated using the previous NCDC third-order spline method based on 30-year monthly average temperatures. The daily standard deviation calculation for dates prior to 2011 were determined according to Arguez (2011) which were also used by NCDC to calculate 2011 and onward daily standard deviations.

If temperature data were exactly normally distributed, we would expect that 16.6 days per year would be +/- 2.0 standard deviations from the mean. In actuality, the long term average for Fairbanks (and Anchorage) is 14.6 days per year. I believe this discrepancy is due to a few large outliers pushing the standard deviation values higher than what would be expected from pure normally distributed data.

The chart below shows the daily temperature standard deviations for different decadal climate periods fr Fairbanks and Anchorage.

Saturday, October 19, 2013

Climatological Inversion Season

As Rick alluded to in his post yesterday, this is the time of year when temperature inversions usually become more persistent and dominant in the Fairbanks area.  In fact, today is the day when the climatological daily mean temperature becomes lower at Fairbanks airport than on Keystone Ridge (the latter being over 1100 feet higher); this will remain true until mid-April.

The chart below shows the annual variation in normal daily maximum and minimum temperatures at the two locations, based on the 1981-2010 climate normals.  On average, temperature inversion occurs at the time of daily minimum temperature for nearly nine months of the year, while inversions tend to persist throughout the day for the four deep winter months of November through February.  In January the tendency for inversion is so strong that Fairbanks average high temperatures are close to Keystone Ridge average low temperatures.

Fun With Statistics... Cooling Ahead

This is rather frivolous, but I couldn't resist another opportunity to illustrate the extreme climate of interior Alaska.  The chart below shows the range of possibilities for the amount of cooling that will be observed in Fairbanks over the next month, starting from yesterday's remarkable high temperature of 57 °F.  The probabilities are derived simply by looking at the lowest temperature observed by November 18 in each year back to 1930.  We are virtually assured of seeing temperatures below 0 °F in the next month, and it's not impossible that the temperature could drop nearly 100 °F, i.e. to -40°.  Only two years ago the temperature reached -41 °F on November 17.

By the way, the largest temperature drop observed in Fairbanks in 30 days or less is 100 °F, observed in both 1934 (34 to -66 °F in only 8 days) and 1935 (58 to -42).  The largest temperature rise in 30 days or less is 103 °F.

Friday, October 18, 2013

Warm Week

The Fairbanks NWS releases a weekly summary of temperatures as a Public Information Statement (PNS). Here is a graphical representation of last weeks temperatures and liquid precipitation for Fairbanks. As you can see, the past week (10/12 to 10/18) in Fairbanks was the warmest since 1969 and one of the warmest on record. Each bar on the chart represents the 7-day average of the high and low temperature for this date range. The green bars at the bottom represent the total liquid precipitation (scale is on right side also in green).

(Lack of) Snow Cover

While browsing the UAF Puffin Feeder the other day, I noticed this image of the Arctic coastline. The image was taken with the Suomi NPP using the VIIRS sensor. Amazingly, there is no snow on the ground around the Mackenzie River Delta and no ice along the shoreline. The cyan (light blue) colors represent snow and ice and the milky white areas north of the coast are clouds. A similar image obtained today is much cloudier but the patterns visible through occasional breaks look nearly the same. This morning's sounding from Barrow showed a freezing level of 3,200' AGL and temperatures along the coast were at or slightly above freezing.

Inversion Season is Here…Sans Snow

Nature provided a perfect laboratory on Tuesday on the role that decreasing solar heating plays in Interior Alaska climate this time of year. A warm airmass covers the Interior: clouds thinned out a bit Thursday morning, allowing for about a 10ºF inversion to develop on the valley floor. However, by sunrise thick cirrus returned, and then there was even a bit of rain, just a hundredth or two in most places, in the early afternoon. Combined with a lack of wind, and the results are seen in the upper air sounding from Thursday afternoon. While the inversion is boring for winter, it's occurrence without any snow to be found below 3000" elevation is quite telling.

This inversion is reflected in the high temperatures Thursday:
Fairbanks Upper Air: 45ºF
Goldstream Valley Bottom: 45ºF
Aurora: 47ºF
Fairbanks Airport: 48ºF
Fort Wainwright: 48ºF
Eielson AFB: 51ºF

But in the hills...
Stuart Creek RAWS: 58ºF
Gilmore Creek CRN: 56ºF
Keystone Ridge: 54ºF
Wichersham Dome: 52ºF
Clearly Summit: 51ºF

Thursday, October 17, 2013

Records Tumbling

The unseasonable warmth across Alaska is breaking records for the time of year in many locations, particularly in terms of daily minimum temperatures.  Here are a few locations at which the daily low temperatures have been the highest on record for so late in the autumn:

  • Anchorage, both on the 15th (47 °F) and 16th (46 °F)
  • McGrath (44 °F on the 16th)
  • Kotzebue (38 °F on the 16th)
  • Nome (42 °F on the 16th, tied for warmest so late in the season in the Weather Bureau/NWS era)

In Fairbanks, yesterday's low of 38 °F at the airport was the warmest so late in the season since 1938.  Yesterday's high of 53 °F was the warmest so late in the season since 1969.

Perhaps most impressive of all is Eagle, where yesterday's low temperature came in at 44 °F at the airport.  The COOP observation appears to not be available yet, but the previous record for warmest this late in the season at either site is 38 °F.  [5 pm update: the COOP low temperature was 41 °F]

Arctic Canada is getting in on the action too: Fort McPherson reported a temperature of 53 °F yesterday.  The previous warmest for this late was 50 °F, in a period of record (with some gaps) back to 1893.

Needless to say, warmth and rain are eating up the snow cover even in the lower elevations of the Brooks Range.  Here's this morning's FAA webcam photo from Anaktuvuk Pass, elevation 2171' and a remarkable 43 °F overnight:

Tuesday, October 15, 2013

Updated Fairbanks Temperatures

Heading into some of the warmest late October weather in years, here's an update of normalized daily temperature departures for Fairbanks for 2013. The upper plot is the daily values, the lower plot the 5-day running mean (to help see the patterns). Recall that standardized anomalies are nothing more than the departure divided by the standard deviation, and makes for an equal comparison across the seasons. This is highly useful in Interior Alaska, where typical variability of cold seasons temperatures is several times greater than the warm season. Each passing day now without snow cover becomes more significant, with daily mean temperatures falling at nearly a degree a day, and it's quite likely that daily departures will exceed two standard deviations later this week,

Monday, October 14, 2013

Warm Forecast

The entire state of Alaska is experiencing unseasonable warmth this morning, and forecasts show very unusual warmth continuing throughout the interior and north this week.  For example, the National Weather Service forecast for Fairbanks airport indicates a high temperature of 50 °F on Wednesday and overnight minima of 37 °F on Wednesday and Thursday nights.  This is extremely unusual for the time of year.  A daily minimum temperature above 33 °F hasn't been observed after October 15 since 1981 in Fairbanks, and high temperatures of 50 °F or higher are very unusual for the second half of October.  The chart below shows the climatological (1930-2012) probability of reaching the temperature on the x-axis after October 15 in Fairbanks, showing that 50 °F is reached in slightly more than 10 percent of all years.
The chart above doesn't quite give the whole picture, however, because in fact 50+ temperatures have become rare in recent decades; the second chart below shows the annual values for the maximum temperature after October 15.  Interestingly, 50 °F was reached 9 times between 1930 and 1980, but only once in the years since; the more recent climate has produced these warm events less often.  This is consistent with the fact that October is the only month of the year that shows a long-term cooling trend in Fairbanks; the other 11 months show warming (and most at a much greater rate than October has cooled).

It will be interesting to compare the current warm event to some of the historical events - particularly that of 1938, when the temperature reached 61 °F on October 20 and overnight minima were 40 °F or higher for five consecutive days.

Sunday, October 13, 2013

Differetial Snow Cover, Low Angle Sun and Impacts on Maximum Temperatures

Temperatures the past month in Fairbanks have been on a bit of a roller coaster, as Richard has previously blogged about. And, as sometimes happens this time of year, snow cover has been quite variable, with very little snow at valley level but several rounds of accumulating snow above 1500' elevation. Combined with the rapidly decreasing solar heating, this makes for, at times, significantly greater differences in daily maximum temperature between the valley and hills than would be expected from that accounted for by differences in elevation.

Below is a plot (upper portion of the plot) of the daily highs since mid-September at Fairbanks Airport (434' MSL) and Keystone Ridge (1600' MSL). The high temperatures are the calendar day (AKST) max temperatures, so this is a direct apples-to-apples comparison. On the lower portion I've plotted the difference in max temperatures (blue) and the late evening snow depth at Keystone Ridge (Fairbanks Airport has had no days with measurable snow on that ground overnight). While not all days with big differences in max temperature are associated with measurable snow remaining, it is the case that the days with  differences less than 5 degrees have only occurred when no measurable snow was on the ground at Keystone Ridge. So clearly, snow cover isn't everything. Much of the difference is attributable to Keystone Ridge's northeast exposure, which greatly amplifies the lowering sun (solar noon solar elevation today, October 13th is 17º)

The Definition of The Seasons

The Seasons of Alaska
When does winter informally begin in Fairbanks, or Anchorage, or Juneau? Is it the same date for all cities or are there difference? I have always argued that the seasons, especially here in Alaska, do not neatly correspond to strict climatological or astronomical definitions. In addition, the seasons are not all the same length. Winter certainly lasts longer in Barrow than in Anchorage.

After thinking about the idea of seasonal definitions for a while (see First Snow Date, and First Sub Freezing High Temperature) I have finally come up with a first draft of seasonal definitions for Alaska. Coincidentally, Rick gave this some thought last January and came up with some suggested criteria. One of his ideas was to use a running mean of temperatures as a threshold value to transition from one season to the next.

Since every location, from Barrow, to Adak, to Ketchikan, receives freezing temperatures and snow, it is only natural to use those criteria to define the winter season. But what about summer? For this first iteration, only temperatures are used. The beginning of winter is arbitrarily defined as the first day that a 5-day average of the daily temperature is under 30°F. The last day of winter is the last day that a 5-day average of the daily temperature is under 30°F. The beginning of summer is arbitrarily defined as the first day that a 5-day average of the daily temperature is over 55°F. The last day of summer is the last day that a 5-day average of the daily temperature is over 55°F. Spring is defined as the time period between Winter and Summer and Fall is defined as the period between Summer and Winter.

There were 142 stations used in the analysis. Each of them contained (relatively) complete records between 1980 and 2012. Of that 32-year period, at least 15 were necessary for inclusion in the analysis. For some North Slope stations, there were years when so Summer occurred by these definitions.

I promise, this is the last one of these for a while!

Click on Image to See Animation of Seasons (~3.1Mb)

Click HERE to see the complete list of dates for all 142 stations

A list of the individual files:

Friday, October 11, 2013

Getting Warmer Relative to Normal

After the early taste of winter in September, Fairbanks temperatures have been generally warmer than normal, and this trend looks set to continue - possibly in a dramatic way - as deep southerly flow strengthens across the state.  The chart below shows the change from unusual chill to unusual warmth in less than three weeks even as the actual temperatures have been relatively stable.

The computer model forecasts from this morning are showing a strong ridge building over southeast Alaska this weekend and then perhaps intensifying further next week.  Some of the solutions look quite extreme, but it's too early to pin down the exact outcome.  But regardless, warm southerly flow should allow interior Alaska temperatures to defy the seasonal trend for a while longer.

For those interested in the technical forecast, the maps below show the ECMWF and GFS ensemble mean 500 mb height and 850 mb temperature forecasts for Wednesday afternoon.  The model agreement is pretty good at this point.

Thursday, October 10, 2013

Elevation Dependent Snow

As often happens in September and early October, snowfall Wednesday around Fairbanks was strongly elevation dependent. Valley elevations received almost two tenths of an inch of rain but hardly any snow, while elevations above 1500' MSL saw a significant whitening. Fairbanks Airport received just a trace of snow, UAF West Ridge 0.1" but Keystone Ridge (1600' MSL) had 3.2" with 0.43" total precipitation. The Wednesday evening photos below from UAF West Ridge and Keystone ridge, which are only about 1000' difference in elevation, illustrate the persisting difference.

Courtesy of ACRC

Tuesday, October 8, 2013

First Sub-Freezing High Temperature

There are a number of ways to define the informal beginning of Winter. Previous posts have looked at the date of the first freeze and the first measurable snowfall. In this post, I look at the first day where the high temperature is at or below freezing. For Fairbanks, the average date for the first sub-freezing high is October 9th. So far their coldest max temperature has been 34 on 9/22. For Anchorage the average date is October 27th. On the map, only stations with at least 15 years were used and the years were not required to be consecutive nor contemporary. The earliest average date for any station is for Barrow (September 2nd) and the latest average date belongs to Craig (December 24th).

As for coming up with a proxy measure for the beginning of Winter in Alaska, any thoughts on the different criteria are welcome. Here are some thoughts. 1) First snow, 2) First low temperature below 20, 3) Seven or more days with 1"+ snow on the ground, 4) First day with high temp below freezing, 5) Seven or more days with low temperature below freezing, 6) .....   I intend to write a paper on the relative dates of the seasons for Alaska. Very little research has been done on the subject.

Monday, October 7, 2013

Peak Cooling

Most of interior Alaska is now approaching the date at which the climatological average temperature drops most rapidly.  At most interior stations, the peak rate of climatological cooling is about 5-7 °F per week, or up to 1 °F per day.  The map below shows the maximum weekly rate of cooling at each station included in the 1981-2010 daily normals from NCDC.  Based on this data, the COOP station in Woodsmoke has the distinction of the most rapid cooling of any station, at 7.1 °F per week between October 15 and 27.  Woodsmoke is the only station with a weekly cooling of 7 °F or more.  The Aleutian town of Adak, with its extremely maritime climate, has the least rapid peak cooling.

The second map, below, shows the spatial distribution of the date of most rapid cooling; the colors indicate the starting date of the week with peak cooling.  It's interesting to note that in general, the date of most rapid cooling progresses eastward across the state, with most western stations seeing the most rapid seasonal change in early October, while the southeastern interior sees peak cooling in the second half of the month.  I expected to see the peak cooling occur later in the year along the west coast, because of the maritime influence, but in fact it's the other way around: in the more continental locations, there is nothing to keep the temperature from accelerating downward as the sun's strength diminishes.  At least, that's how I interpret the data.

As a point of comparison, it's fun to note that Oymyakon, the notorious pole of cold in Siberia, sees its average temperature drop by 9.4 °F in the week beginning October 18.  The average daily mean drops from 24.6 °F on October 1 to -15.3 °F on November 1... 40 degrees in one month.  Alaska's top competitor in the same period is, again, Woodsmoke, with a drop of 28.1 °F.

Saturday, October 5, 2013

Running Precipitation Total

Fairbanks has been on a precipitation roller coaster ride the last few months. It's been either feast or famine. It has seemed to me that Fairbanks is always running a deficit for precipitation whenever I looked at the daily or monthly climate summary. As it turns out, when looking at a 30-day running average of precipitation, Fairbanks has been in a deficit 59% of the time since January 1st, 2011, and in a surplus 41% of the time. Amazingly, Anchorage has been just the opposite. Since January 1st, 2011, Anchorage's 30-day precipitation total has been in a surplus 64% of the time and in a defect only 36% of the time. Quite a difference.

The chart below shows the 30-day precipitation surplus or deficit for both Anchorage and Fairbanks over the aforementioned time period.

Thursday, October 3, 2013

First Snow Date

Most areas of Northern and Interior Alaska have already received their first measurable snowfall this season. Even the Anchorage Bowl (except the official climate site) had a rare early season snowfall late last month. Looking back through the historical record, I decided to put together a map of average (not 'normal') first measurable snow dates.

For this analysis, a station had to have 15 complete years of snowfall data (July 1 to June 30). The 15 years were not required to be consecutive nor contemporary. It took several iterations of analysis to pin down a methodology that filtered out years with too much missing data. Even still, there are still many instances where data were not entered into the GHCN database properly. Still, the calculated data for most stations appears pretty reasonable. The map shows the results of the interpolation from the 189 stations used in the analysis. A 6-point inverse distance weighted surfacing technique was utilized to smooth out the data for the map. Below the map, is a table of all the dates (unsmoothed). To deal with leap years, I calculated the number of days after June 30th that the first measurable snowfall occurred. The table contains the average of those Julian dates and a conversion to calendar dates. For reference, September 1st is day #63, October 1st is day #93, and November 1st is day #124. (Hopefully the length of the table isn't too distracting. If it is, I will change it.

Elevation (m) Station Name Days after June 30th For First Measurable Snow Date Num of Years
5.2 ADAK 124.2 1-Nov 50
82.9 ALYESKA 119.1 27-Oct 32
39.9 ANCHORAGE FRCST OFC 113.1 21-Oct 15
36.6 ANCHORAGE INTL AP 108.8 16-Oct 60
42.1 ANCHORAGE MERRILL FLD 119.2 27-Oct 41
139.6 ANDERSON LAKE 110.0 17-Oct 27
8.5 ANGOON PWR 136.2 13-Nov 30
26.2 ANIAK AP 102.0 10-Oct 31
33.2 ANNETTE ISLAND AP 145.5 22-Nov 71
28 ANNEX CREEK 122.1 30-Oct 66
21.3 ATTU 132.7 9-Nov 15
13.4 AUKE BAY 127.9 4-Nov 47
6.1 BARANOF 130.8 7-Nov 22
9.4 BARROW POST ROGERS AP 42.5 11-Aug 85
11.9 BARTER ISLAND WSO AP 46.1 15-Aug 40
10.7 BEAVER FALLS 149.1 26-Nov 40
3 BELL ISLAND 135.2 12-Nov 19
38.7 BENS FARM 126.7 3-Nov 26
31.1 BETHEL AP 102.1 10-Oct 84
195.7 BETTLES AP 90.2 28-Sep 62
389.2 BIG DELTA AP 92.1 30-Sep 57
18.3 BIG RIVER LAKES 116.2 24-Oct 19
6.1 CALDER 142.7 19-Nov 19
25.9 CANNERY CREEK 123.5 31-Oct 26
649.8 CANTWELL 2 E 88.5 26-Sep 24
14.9 CAPE DECISION 182.9 29-Dec 15
56.4 CAPE HINCHINBROOK 128.4 5-Nov 20
13.7 CAPE LISBURNE 61.9 30-Aug 20
15.8 CAPE LISBURNE AFS 87.4 25-Sep 17
144.8 CAPE NEWENHAM 100.3 8-Oct 26
71.9 CAPE NEWENHAM AFS 103.1 11-Oct 16
132.3 CAPE ROMANZOF 93.3 1-Oct 24
53.9 CAPE SARICHEF LT STN 131.2 8-Nov 19
27.1 CAPE SPENCER 158.6 5-Dec 15
14.9 CAPE ST ELIAS 146.3 23-Nov 24
280.4 CENTRAL #2b 104.9 12-Oct 29
577.6 CHANDALAR LAKE 94.6 2-Oct 16
548.6 CHICKEN 95.2 3-Oct 15
196 CHITINA 109.6 17-Oct 15
412.7 CHULITNA RIVER 97.6 5-Oct 33
182.3 CIRCLE CITY 103.3 11-Oct 19
262.1 CIRCLE HOT SPRINGS 91.2 29-Sep 40
150.9 CLEAR 4 N 98.2 6-Oct 19
335.3 CLEARWATER 97.0 5-Oct 19
23.8 COLD BAY AP 110.6 18-Oct 62
298.1 COLLEGE 5 NW 94.7 2-Oct 34
182 COLLEGE OBSY 95.4 3-Oct 62
1.5 COLVILLE VILLAGE 72.7 10-Sep 16
153.9 COOPER LAKE PROJECT 122.7 30-Oct 40
114.3 COOPER LANDING 5 W 122.6 30-Oct 30
304.8 COPPER CTR 110.5 18-Oct 16
9.4 CORDOVA M K SMITH AP 125.7 2-Nov 85
7.6 CORDOVA NORTH 127.2 4-Nov 22
13.1 CRAIG 159.5 6-Dec 22
26.2 DILLINGHAM FAA AP 119.9 27-Oct 44
3 DUTCH HARBORb 136.9 13-Nov 44
259.1 EAGLE 100.2 8-Oct 39
151.8 EAGLE RIVER 5 SE 117.2 25-Oct 19
166.7 EIELSON FLD 92.9 30-Sep 47
173.1 EIELSON FLDb 93.4 1-Oct 21
11.6 EKLUTNA PROJECT 110.1 18-Oct 31
15.8 ELDRED ROCK 135.6 12-Nov 22
6.1 ELFIN COVE 131.7 8-Nov 32
58.5 ELMENDORF AFB 105.2 13-Oct 36
131.7 FAIRBANKS INTL AP 92.6 30-Sep 65
458.1 FAREWELL FAA AP 90.8 28-Sep 27
9.1 FIVE FINGER LT STN 165.3 12-Dec 23
99.1 FLAT 106.9 14-Oct 22
39.9 FORTMANN HATCHERY 132.2 9-Nov 18
143.3 FT RICHARDSON WTP 121.4 29-Oct 16
132 FT YUKON 95.9 3-Oct 43
46.6 GALENA AP 95.5 3-Oct 17
288 GILMORE CREEK 97.7 5-Oct 23
13.7 GLACIER BAY 136.4 13-Nov 29
671.2 GLEN ALPS 100.2 8-Oct 33
421.5 GLENNALLEN KCAM 105.2 13-Oct 28
6.1 GUARD ISLAND LIGHT STN 155.8 2-Dec 16
476.1 GULKANA AP 96.5 4-Oct 57
12.2 GUSTAVUS 131.0 8-Nov 44
249.9 HAINES 40 NW 109.3 17-Oct 23
4.6 HAINES AP 126.9 3-Nov 28
53.3 HAINES TERMINAL 150.9 27-Nov 22
304.8 HAYES RIVER 100.6 8-Oct 16
448.1 HEALY 2 NW 96.2 4-Oct 25
6.1 HOLY CROSS 107.4 15-Oct 54
345 HOMER 5 NW 107.4 15-Oct 21
329.2 HOMER 8 NW 107.0 15-Oct 32
156.1 HOMER 9 E 113.8 21-Oct 21
19.5 HOMER AP 119.6 27-Oct 61
39.6 HOONAH 140.4 17-Nov 20
54.9 HOPE 126.5 3-Nov 19
43.6 ILIAMNA AP 117.8 25-Oct 49
388.9 INDIAN MTN AFS 90.7 28-Sep 16
36.6 INTRICATE BAY 116.2 24-Oct 40
52.1 JUNEAU DWTN 151.5 28-Nov 30
15.2 JUNEAU DWTN(b) 142.3 19-Nov 28
4.9 JUNEAU INTL AP 127.7 4-Nov 64
21.3 KAKE 170.1 17-Dec 19
21.3 KASILOF 3 NW 118.4 26-Oct 48
38.4 KENAI 9N 115.2 23-Oct 16
27.7 KENAI MUNI AP 115.3 23-Oct 62
673.9 KENNECOTT 101.1 9-Oct 28
23.2 KETCHIKAN INTL AP 156.5 3-Dec 51
487.7 KEYSTONE RIDGE 86.8 24-Sep 16
19.2 KING SALMON 112.9 20-Oct 77
18.3 KITOI BAY 135.8 12-Nov 41
46 KODIAK 130.1 7-Nov 29
24.4 KODIAK AP 126.6 3-Nov 64
9.1 KOTZEBUE RALPH WEIN AP 97.3 5-Oct 72
19.5 KUPARUK 71.9 9-Sep 25
14.9 LATOUCHE 135.7 12-Nov 25
223.4 LAZY MTN 107.8 15-Oct 23
4.3 LITTLE PORT WALTER 144.4 21-Nov 60
17.4 MAIN BAY 119.9 27-Oct 23
82.9 MANLEY HOT SPRINGS 101.2 9-Oct 45
381 MCCARTHY 3 SW 96.0 4-Oct 19
101.5 MCGRATH AP 95.7 3-Oct 73
630.9 MCKINLEY PARK 91.8 29-Sep 76
14 MIDDLETON ISLAND AUTO 141.9 18-Nov 15
312.4 MINCHUMINA 96.1 4-Oct 39
150 MOOSE PASS 3 NW 124.5 1-Nov 22
6.1 MOSES POINT FAA AP 97.8 5-Oct 20
890 NABESNA 94.0 2-Oct 25
109.7 NENANA MUNI AP 102.9 10-Oct 54
4 NOME MUNI AP 101.4 9-Oct 86
144.8 NORTH POLE 94.9 2-Oct 41
9.1 NORTHEAST CAPE 94.9 2-Oct 15
522.1 NORTHWAY AP 89.5 27-Sep 57
34.1 NULATO 101.9 9-Oct 16
402.3 OLD EDGERTON 98.8 6-Oct 22
67.1 PALMER 1 N 109.7 17-Oct 16
65.8 PALMER JOB CORPS 117.8 25-Oct 45
823 PAXSON(b) 84.3 22-Sep 16
3.7 PELICAN 130.6 7-Nov 24
32.6 PETERSBURG 1 135.3 12-Nov 62
20.4 PLANT MATERIALS CTR 116.9 24-Oct 16
6.1 PLATINUM 120.4/td> 28-Oct 15
6.1 POINT RETREAT LT STN 143.5 20-Nov 22
3.7 PORT ALEXANDER 145.3 22-Nov 40
79.2 PORT ALSWORTH 118.3 26-Oct 43
28 PORT HEIDEN 128.8 5-Nov 21
12.2 PORT SAN JUAN 132.3 9-Nov 25
566.3 PUNTILLA 104.8 12-Oct 40
116.1 RAMPART 95.7 3-Oct 27
157.9 SEWARD 19N 117.9 25-Oct 22
129.2 SEWARD 8 NW 107.2 15-Oct 17
6.7 SEWARD AP 131.8 8-Nov 88
694.9 SHEEP MTN CAA AP 99.7 7-Oct 20
37.2 SHEMYA USAF BASE 124.2 1-Nov 39
3 SHISHMAREF 98.6 6-Oct 17
4.3 SITKA AIRPORT 144.6 21-Nov 48
20.4 SITKA MAGNETIC OBSY 144.4 21-Nov 71
10.7 SKAGWAY 136.4 13-Nov 39
45.7 SKWENTNA 108.0 15-Oct 46
668.1 SLANA 105.9 13-Oct 34
704.7 SNOWSHOE LAKE 92.9 30-Sep 35
597.4 SOURDOUGH 1 N 94.9 2-Oct 19
481.6 SPARREVOHN 87.8 25-Sep 21
484 SPARREVOHN MTN AFS 101.3 9-Oct 16
10.7 ST PAUL ISLAND AP 106.9 14-Oct 63
734.9 SUMMIT WSO AP 83.6 21-Sep 34
167.6 SUTTON 1 W 105.3 13-Oct 29
798.6 TAHNETA PASS 91.9 29-Sep 17
106.7 TALKEETNA AP 109.9 17-Oct 76
469.7 TANACROSS 91.8 29-Sep 18
69.2 TANANA CALHOUN MEM AP 100.9 8-Oct 82
293.8 TATALINA 99.2 7-Oct 27
82 TIN CITY 81.6 19-Sep 28
495.3 TOK 95.4 3-Oct 36
481.3 TONSINA 107.0 15-Oct 35
11 TREE POINT LT STN 159.7 6-Dec 29
99.1 TRI NAL ACRES 116.7 24-Oct 16
734.9 TRIMS CAMP 85.5 23-Sep 20
81.1 UMIAT 88.4 26-Sep 28
5.5 UNALAKLEET FLD 101.3 9-Oct 48
144.8 UNIVERSITY EXP STN 103.5 11-Oct 101
29 VALDEZ WSO 109.6 17-Oct 82
7.6 WALES 96.6 4-Oct 46
152.4 WASILLA 2 NE 104.2 12-Oct 15
15.2 WASILLA 3 S 120.5 28-Oct 38
82.3 WHITES CROSSING 118.6 26-Oct 31
307.8 WHITESTONE FARMS 91.8 29-Sep 16
18.3 WHITTIER 118.6 26-Oct 40
62.5 WILLOW WEST 119.9 27-Oct 17
349.6 WISEMAN 94.1 2-Oct 29
17.1 WRANGELL AIRPORT 149.8 26-Nov 70
8.2 YAKATAGA AP 129.1 6-Nov 24
10.1 YAKUTAT STATE AP 122.3 30-Oct 86