Diurnal Cycles

A few days ago reader Gary inquired about the time of daily minimum temperatures in Fairbanks throughout the winter.  The simplest way of addressing this is to examine the climatological mean temperature by hour of the day; the chart below shows the results for the modern ASOS era, 1998-2014.  Each curve shows the mean diurnal cycle for a different month, and the small diamond markers show the maximum and minimum points.

The seasonal changes in the diurnal cycle are very much as expected, with a tiny diurnal cycle in December giving way to a substantial daily oscillation by February.  According to the hourly means, the coldest hour of the day in December is 7 am AKST, which is well before the sun rises; but there is less than 0.5 °F difference from midnight to 11 am.

We could also look at the frequency with which each hour of the day records the lowest or highest temperature, and examine some histograms based on those results, but I'll leave that for another time.

As we're on the topic of short-term temperature fluctuations, it's worth noting that these have been very large in Fairbanks lately.  The plot below shows the 2m temperature trace from UAF's North Campus, near Smith Lake, over the past 4 days (with a period of missing data in the middle).  The 2m temperature rose from -39.4 °F on Sunday to +47.2 °F yesterday, a rise of 86.6 °F in three days.  Looking at the official Fairbanks history since 1930, only two occasions saw a larger 3-day temperature rise, and both of those were in January.

At Fairbanks airport, the recent 3-day rise was 84 °F (-37 to +47), the third largest on record and the largest ever outside of December and January.

The diurnal temperature range has also been very large in recent days: 46 °F yesterday, which is equivalent to the largest that is typically observed in any given year.  March is by far the most common month for observing the year's largest diurnal temperature variation in Fairbanks.

Coldest Airmass of the Winter

The 850 mb temperature observed by balloon sounding in Fairbanks dropped to -29.9 C on Saturday morning, which is the coldest of the entire winter.  Prior to the arrival of this cold airmass, the coldest observed at 850 mb was -27.7 C on January 25.  If the previous low point had stood, the winter's coldest measurement would have been the second warmest on record for an entire winter, but this new cold is closer to (but still above) the long-term normal for coldest of the season.

Is it unusual to see the coldest air (aloft) of the season in March?  Not especially; the chart below shows the month in which the coldest air was observed at 850 mb for each winter since 1948-1949.  It's nearly as common to see the coldest air in March as in December.  The last time the coldest air was observed in March was in 2003, which followed the very warm winter of 2002-2003; obviously the odds of this happening are heightened after persistent warmth earlier in the season.  Over the history since 1948, the earliest that the coldest air was observed was November 16, 1956, and the latest was April 18, 1959.

At the surface, Fairbanks airport observed 6 consecutive days with a daily minimum of -30 F or lower, culminating in -37 yesterday, March 15.  This is fairly impressive for so late in the season; it's the most consecutive days reaching -30 this late in the winter since 1966.  However, it can't compare to the 13 straight days reaching -30 F from March 9-21, 1959.

The -37 F yesterday morning is the coldest this late in the winter since 1995 (-37 F on March 24).  The record for coldest this late or later is -46 F on March 15, 1964.

Frequency of Extremes

I've spent some time recently working with the NCEP/NCAR reanalysis data to assess patterns of upper-level temperature and pressure extremes over the Northern Hemisphere, and the changing distribution of these extremes over time.  I won't dive into the details of this investigation now, but I did want to put up one figure that I thought was interesting - see below.

The map shows the long-term average of the frequency of +/- 2 standard deviation anomalies in the daily mean 850mb temperature.  The anomalies are calculated with respect to a baseline climatology that includes a long-term trend, so that patterns of extremes do not simply reflect overall warming.

The interesting result is that southern Alaska, western Canada, and the northern Gulf of Alaska see the highest frequency of 2 SD extremes of anywhere in the northern extratropics.  If the distribution of daily anomalies were perfectly Gaussian, then the frequency would be 0.0455, so the 850mb climate over southern Alaska has more frequent extreme temperature events than would be expected.  Most of the northern extratropics have slightly less frequent extremes than expected.

Looking at the distribution by season, it is mainly the winter and spring that contribute to the excess of extremes in the Gulf of Alaska.  The map below shows the mean frequency for December through February.

In contrast to the peak frequency near Kodiak Island, the lowest annual frequency of 2 SD extremes is over the North Atlantic, and the two distributions can be compared as follows:

Category (SD)	57.5°N 155°W	50°N 35°W	Expected Gaussian
-3SD or lower	0.0038	0.0001	0.0013
-3SD to -2SD	0.0226	0.0135	0.0214
-2SD to -1SD	0.1141	0.1608	0.1359
-1SD to +1SD	0.7233	0.6414	0.6827
+1SD to +2SD	0.1069	0.1701	0.1359
+2SD to +3SD	0.0256	0.0142	0.0214
3SD or higher	0.0037	0.0000	0.0013

The chart below shows how the frequencies in three categories have changed over time near the peak in the Gulf of Alaska.  Relatively high frequencies - higher than expected based on a Gaussian distribution - were observed prior to the PDO shift in 1976, but the frequency of extremes has been generally lower since then.

Inversion Season Dates

A late-season cold spell has descended upon Alaska, bringing -30 °F temperatures to Fairbanks airport on the past two mornings.  Temperatures this low are a bit unusual for this late in the season, occurring in fewer than 3 out of 10 years after March 10.  Rapidly strengthening sunshine is making a big difference, however, as daytime temperatures would be much lower with an air mass this cold in the depths of winter.

With solar insolation rising quickly, Fairbanks will soon reach the date at which mean daily surface temperatures rise above mean daily temperatures at 850 mb.  According to the 1981-2010 normals, that date is March 19.  Based on the history of radiosonde data from several stations around the state, we can look at how the surface-850 mb temperature difference varies throughout the year; the charts below show the results.  Note that I would prefer to look at the 925 mb level as a measure of low-level inversion characteristics, but the 925 mb was not regularly reported in radiosonde data until 1992.

It's not surprising to observe that more northerly locations in Alaska generally spend a greater fraction of the year with surface temperatures lower than 850 mb temperatures, i.e. with an inversion in place on average; Anchorage sees this condition only during a mid-winter period of about 6 weeks in length.  An interesting feature of the charts is that the surface-850 mb temperature difference is nearly constant during most of the warm season at each of the locations outside the Arctic, so the low-level vertical stability of the atmosphere is little changed for a period of several months during summer.

It's also interesting to note that the Fairbanks and McGrath curves follow each other very closely indeed, despite a fairly large distance separating the two locations; this illustrates the relatively uniform nature of some aspects of the climate in interior Alaska, as opposed to the strong spatial variation near and along the coasts.

Arctic Ice Update

Arctic sea ice is now close to its seasonal maximum extent, so it's a good time for an update.  The total area of ice cover has seen little growth since early February, and the extent is now running more than 2 standard deviations below the 1981-2010 normal according to the NSIDC.  The warm winter and lack of ice cover in the Bering Sea is contributing to the ice area shortfall, along with a large ice deficit in the Sea of Okhotsk and a smaller deficit in the Barents Sea.  The latest graphics from the NSIDC are shown below.

NSIDC commentary notes that if the ice extent does not advance to a new peak in the next few weeks, then this year will see the lowest maximum extent on record for the satellite era.  At first glance this would seem to bode ill for the upcoming melt season.  However, the University of Washington's ice volume analysis - last updated for the month of January - showed that ice thickness and ice volume were higher than in recent years.  First, the thickness: the PIOMAS model's estimate of average thickness at the end of January was the highest since 2006 - see below.

The estimated Arctic sea ice volume was close to that in 2009 and just over 1 standard deviation below the 1979-2014 mean (see below).  The volume was notably higher than last year in January, which reflects a persistence of the year-over-year gain that was seen during last year's melt season.  It will be most interesting to see if the volume recovery continues this year and if the ice extent and ice volume anomalies continue to show contrasting trends.

According to NOAA's CFS reanalysis, the mean air temperature has been well above normal across most of the Arctic Ocean this winter, with the highest anomalies located near Wrangel Island and the Chukchi Sea.  The extent and magnitude of the warm anomalies can be compared to the previous 10 years in the series of images below.  The previous 10 winters showed a persistent tendency for warmth over the Barents and Kara Seas and with less unusual warmth on Alaska's side of the ocean.  Last winter (2013-2014) saw warm conditions across the entire ocean, but this winter has been notably cooler between 0 and 90 °E, with the warmest conditions now prevailing on our side of the basin.

Kaktovik Wind

The severe blizzard that affected the eastern North Slope on Saturday night was notable for its ferocity in the Barter Island community of Kaktovik, as documented in a news article here.  The highest sustained wind measured at the Barter Island ASOS was 49 knots (56 mph), and the highest gust was 58 knots (67 mph), but the anemometer did not report for several hours during the height of the storm.  It seems very likely that winds were significantly stronger for a time.

How does this storm compare to past high wind events on Barter Island?  The history of hourly observations contains quite a number of bogus wind reports in recent years, but after removing these I found that there have been 18 separate dates (in 13 separate winters) since 1950 on which sustained wind speeds reached 60 knots (69 mph).  All but one of these occurred in November through March.  Without question, Barter Island is a very windy place in winter, and near-hurricane force winds are not all that uncommon.  The top three wind events since 1950 are as follows:

January 6, 1974    70 kt sustained
December 27, 1951    69 kt sustained
September 20, 1957    68 kt sustained

It's interesting to note that most of the 60+ knot events were in the 1950's and 60's, and there do not appear to be any reliable reports of 60 kt or higher since 1995.  This could be due to different wind speed measurement parameters with the introduction of ASOS - but it is surprising nonetheless.

The highest winds at Barter Island are nearly always from a westerly direction, as in this weekend's storm.  The climatological distribution of wind speed also shows a pronounced maximum for east-southeasterly flow (see below), but easterly flow does not bring the same extremes in wind speed.  Winds from the north and south are typically light and are also much less frequent.

Heavy Snow and Wind

Weather warnings have been hoisted across interior and northern Alaska today, as a powerful cold front makes its way eastward across the state.  Heavy snow was reported at Fairbanks airport between 3 and 4 pm, and this is a rarity in the Fairbanks climate.  The last time heavy snow was reported in Fairbanks was in the epic snowstorm of February 2011.  The current weather situation bears resemblance to the active weather pattern in the last 10 days of that month - read more about it here:

http://ak-wx.blogspot.com/2011_02_01_archive.html

Looking farther back in the history of hourly observations at Fairbanks, heavy snow has been reported on only 17 previous occasions since 1950 (I threw out a couple of bogus reports).  Only 12 of these were during the winter months of November through March, so that's an average rate of about once every 5 years during winter.  Interestingly February has seen more occurrences of heavy snow than any other month, with 4 previous events that produced snow totals of 16.8" (1977), 5.9" (1981), 14.5" (1996), and 18.6" (2011).

Here's the scene at the Golden Heart Plaza at 4pm, with Iron Dog festivities hurriedly wrapping up under the heavy snowfall.

The following maps illustrate the meteorological situation at 3pm AKST, with a pronounced 500mb shortwave and vorticity maximum just upstream of Fairbanks and a strong 925mb temperature gradient across northwestern Alaska.

The preliminary 3pm surface analysis from Environment Canada shows (in the far upper left, click to enlarge) the strong surface low just to the northeast of Barrow.

As the low moves east and colder air rushes in, a severe blizzard is expected to develop over the eastern North Slope.  The NWS forecast discussion notes that "wind gusts are expected to increase to at least 80 mph this evening".  The intense temperature gradient and wind associated with the front are evident in the graphics below, which show a time-height cross-section of forecast conditions at Barter Island.  As of this hour, winds have gusted to 51 mph at Deadhorse and at least 58 mph at Nuiqsut (although the anemometer has now stopped reporting).  Interestingly the Umiat RAWS is reporting wind gusts to 54 mph, so the high winds are extending well inland from the ocean.

Finally, for reference, here is a recent plot of METAR observations showing the heavy snow in Fairbanks and the building blizzard on the eastern North Slope.

Winter Rain Temperatures

As a follow-up to Monday's post on freezing rain trends, I thought it would be interesting to divide the historical winter rain events according to temperature so that we can see the relative changes in frequency of "plain rain" versus "freezing rain".  I define "plain rain" as rain at temperatures of 32 °F or higher, and freezing rain as rain at sub-freezing temperatures.  Of course, plain rain in winter generally still freezes on road surfaces in interior and northern Alaska, even if the air temperature is well above freezing, so the distinction may not be practically all that useful - but the results are interesting nonetheless.

The charts below show the number of days each winter on which rain was reported, with red columns indicating reports at which the temperature was simultaneously at or above 32 °F, and blue columns indicating "freezing rain" reports.  The 10-year trailing mean lines are colored correspondingly to highlight the long-term variations.

In Fairbanks, McGrath, and Kotzebue, there is a fairly clear trend towards plain rain taking up a larger proportion of all winter rain events in recent years.  The changes in Nome are not quite as clear, as freezing rain also appears to have become more common in the past 20 years.  Rain of any kind is rare in Barrow in winter, of course, although it does happen, and even at temperatures above freezing on three occasions since 1950.

As in the previous post, the 6-station mean consolidates the overall trend quite nicely (see below).  There has been quite a pronounced increase in the frequency of plain rain, beginning (it would seem) with the anomalous winter of 2002-3.  It is interesting to bear in mind that the PDO was predominantly negative from 2007-2012, so the warmer rain is presumably not attributable to warmer nearby ocean temperatures.  Based on a Mann-Whitney U-test for non-normally distributed data, the increase in plain rain frequency is very statistically significant (p<0.02) and is not sensitive to the choice of years for the two sub-samples.

To confirm the changes at Fairbanks, I examined the temperature observations that accompanied all rain observations from 1950 to 2014, and compared the distributions from 1950-1982 and 1983-2014 (see below).  The shift to warmer temperatures is notable, as temperatures above 34 °F have become much more common during winter rain events, while significantly sub-freezing temperatures have become much less common.  This matches what we saw last week: the rain occurred at temperatures of 32, 33, and 34 °F.  In other words, Fairbanks now tends to see plain rain relatively often in winter, as opposed to true freezing rain which was the favored outcome (when it rained) in the past.

Climatology of 2015 Iditarod Route

Due to poor snow conditions in the southern mainland, the Iditarod race start was moved from Willow to Fairbanks. This is the second time in history (1973-2014) that the race start was moved due to low snow in the southern portion of the route. The other year that the race start was moved to Fairbanks was in 2003. The official restart will occur on Monday, March 9th. Approximately half way through the race, the 2015 route will join with the traditional northern route at the Koyukon village of Ruby. A map of the 2015 route is shown in Figure 1.

Figure 1. Iditarod route in 2015. Populated places along the route are shown as yellow circles.

Analysis Period

What does climatology tell us about the expected conditions during this year's Iditarod? To answer that question, we need to select an analysis period. In the last 20 years, the winning time for the Iditarod is 9 days, 7 hours, and 28 minutes. The top tier of mushers usually finish in about 10 days. Therefore, I chose a time period of 10 days for my analysis. Since the start date this year is on March 9th, the time period for climatological analysis is March 9th to March 18th. This brings up an important point. The analysis in this post is not a climate analysis of past Iditarods. Previous races usually began on a Sunday and the calendar date differs from year to year. This analysis is looking at the same time period over a number of years to assess what the range of temperature and snow conditions have been like along the route used in 2015.

Stations

There are eleven stations along the 2015 route in the Global Historical Climatology Network (GHCN) database. In this analysis I have restricted the date range to the 1973-2014 time period. This corresponds exactly to the years in which the Iditarod race has occurred. That gives us 42 possible years of data. Some stations, like Fairbanks and Nome, have outstanding data for all 42 years. Other stations have incomplete or interrupted data. A map of the climate stations used in this analysis is shown in Figure 2.

Figure 2. Climate stations along the 2015 Iditarod route.

Temperatures

When assessing the temperatures on a yearly basis, I used an average of all stations for all 10 days of the analysis period. This yields a single temperature value for each year. Since the stations are fairly well distributed geographically, arithmetic averaging is sufficient.

As you can see in Figure 3, a wide range of temperatures have been observed since 1973. Keep in mind that these are average daily temperatures (high plus low divided by two). Four years have seen an average temperature of 20°F or warmer and seven years have seen an average temperature below 0°F. Interestingly, the years since 2000 have been, on average, a little cooler than the years in the 1970's and 1980's. Based on climatology, it is reasonable to expect temperatures to average between +10°F and -10°F.

A note of caution, do not over interpret the trend in Figure 3. Remember that this year's Yukon Quest happened to take place during a brutally cold week during one of the warmest winters on record.

Figure 3. Average temperature for the March 9 to March 18 time period for the 11 stations along the 2015 route shown in Figure 2.

Now that we have seen the annual temperature chart, let's look and see how the warmest and coldest years varied on a day-to-day basis during the March 9 to March 18 time period. Figure 4 shows the three warmest and three coldest years.

Figure 3. Average temperature for the March 9 to March 18 time period for the three warmest and the three coldest years along the 2015 route shown in Figure 2.

Even in the very warmest years, only the last day or two was above freezing – even then only by a couple of degrees. The coldest years were quite cold. While not even close to this year's Yukon Quest, temperature have average as low as -25°F (1995) with average low temperatures near -40°C/F. Keep in mind that temperatures on the river ice can be 10°F colder than at the climate stations.

On a broader scale, we can look at temperatures on a statewide basis using the ESRL Reanalysis tool. I put together a video of temperatures from March 9 to March 18 for each year between 1948 and 2014. Video 1 shows the time lapse of the temperature departures from normal from year to year.

Video 1. Temperature departure from normal from 1948 to 2014 during the March 9 to March 18 time period. The 1981-2010 climate normals are used as the temperature baseline.

Snow

What does a musher really care about? Snow, of course. How has the snow been during the last 42 years along the 2015 route? The short answer is that it is consistently good. Figure 3 shows the average snow depth and average new snow for all stations along the 2015 route during the 1973-2014 time period.

Figure 4. Average new snow and snow depth for the March 9 to March 18 time period between 1973 and 2014 along the 2015 route shown in Figure 2.

The average snow depth was 20.8" and the lowest was still 9" (1986). Six years have seen over 30" of snow on average with a peak of 42" in 2009. As for new snow, most years have recorded 2"-5" of new snow during the 10-day period – average is 2.4".

Conclusion

While there are no guarantees in life, history tells us that the route chosen for the 2015 Iditarod is consistently cold and consistently snowy. Mush on!

Thunder Snow in Nome

This morning the National Weather Service noted the occurrence two lightning strikes concurrent with snow in Nome – thunder snow. This is quite the event. Figure 1 shows the text of the Special Weather Statement that was issued by the NWS office in Nome. Since the event was observed by NWS personnel, we assume that the report is valid. Unfortunately the two closest lightning detectors to Nome are both non-operational at the moment so we cannot tell how widespread the event was.

Figure 1. Special Weather Statement issued this morning by the Nome, Alaska, NWS office.

The METARs (see Figure 2) indicate the beginning and ending of the event. Several "special" METARs were issued that note the presence moderate snow and blowing snow concurrent with the lightning. The temperature was noted to be -2°C (28°F) during the thunderstorm.

Figure 2. Screenshot of METARs for Nome, Alaska, between 17:15 Z and 17:31 Z.

Thunder snow requires an unusual set of atmospheric occurrence to come together at just the right time. I am not a forecaster and am not qualified to provide context on the specific atmospheric dynamics at the time. However, for those out there more qualified than I am, here is the 12 Z (3 a.m.) upper air sounding from Nome this morning.

Figure 3. Upper air Skew T/Log P plot from Nome, Alaska, for February 24, 2015 (12 Z).

Just how rare of an occurrence is this? Surprisingly, it is not unprecedented. In fact there are 11 instances of thunderstorms in Nome with a temperature below freezing prior to today's occurrence. Figure 4 shows all of the hourly observations (1960-present) with snow and/or sub-freezing temperatures and a thunderstorm. Only 69 days in Nome have recorded a thunderstorm since 1960; the fact that 11 of those days were winter occurrences is remarkable. Even more amazing is that three instances occurred with a temperature of 2°F or colder!

Figure 4. Hourly observations from Nome, Alaska, that contain a thunderstorm and sub-freezing temperatures.

Freezing Rain Trends

Fairbanks and many other interior communities were afflicted with freezing rain over the weekend in what seems to have become an annual winter ritual of late.  Last winter I looked at the history of significant (0.1" or more) winter rain and freezing rain events in Fairbanks; we can now add another event to the list, which means that four of the last five winters have seen a significant event.  This is quite a remarkable sequence, but as we noted before there have also been periods in the past when winter rain occurred with uncommon frequency.

Another way of looking at the historical trends is to extract reports of rain or freezing rain from the hourly observations.  This method includes all winter rain events, not just the most significant ones, but it does not account for the quantity of rain that fell; some of the events will have been just a trace.  The series of charts below shows the results when we do this for six locations with a reliable history of hourly observations since 1950.  Note that I looked at just the deep winter (Dec-Feb) period, and I've used the same vertical scale for all the charts to highlight the differences in the typical frequency of winter rain.

It's interesting to observe that in Fairbanks, none of the recent winters comes close to 1962-63 in terms of the number of days with rain.  I confirmed this by looking at the Local Climatological Data publication; remarkably, Fairbanks reported "glaze" in January 1963 on 10 separate days (some of these were freezing drizzle events).  However, the persistent recurrence of rain in recent winters appears to be unprecedented since 1950, with this winter (2014-15) being the 7th in a row with at least one report of rain.  The results also show that Nome and (especially) Kotzebue have seen a higher frequency of winter rain in the past decade than in earlier years.

Another interesting observation is that last winter (2013-14) was the first one with at least 2 rain reports (days) at all 6 locations.  Only 4 previous winters had at least one report at each location.  We can also note that only 5 winters had zero reports of rain at all of the locations - most recently in 2003-4.  In other words, nearly every year brings rain in deep winter to at least one of these locations.

The chart below shows an attempt to combine the 6 station series into an overall mean.  For each winter, I took the percent of normal at each station and then calculated the 6-station mean of these numbers.  This is better than just averaging the 6 series together, because that result would be dominated by the higher values at Nome and McGrath.

The mean chart illustrates the highly anomalous nature of last winter and also shows that the 10-year trailing mean is slightly higher than at any earlier point since 1950, although the winter of 1962-63 produced a similar trailing mean.  The 10-year trailing median provides an alternative measure of the long-term variations, but this is no higher than the value in about 1980.