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.

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.

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).

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.

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.

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.

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.

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.

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.

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.

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.

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

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:

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,

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.

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

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:

• August 1st
• August 15th
  
• September 1st
• September 15th
• October 1st
• October 15th
• November 1st
• November 15th
• December 1st
• December 15th
• January 1st
• January 15th
• February 1st
• February 15th
• March 1st
• March 15th
• April 1st
• April 15th
• May 1st
• May 15th
• June 1st
• June 15th
• July 1st
• July 15th

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.

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

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.

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.

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.

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
52.4	MATANUSKA EXPERIMENT FARM	116.7	24-Oct	86
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