Periodic Thermal Waves

*Updated to add 500mb anomaly reanalysis maps at the bottom *

A few days ago I thought it would be interesting to see what would happen when calculating what day of the week was the warmest in the U.S. It was intended to be a silly exercise to demonstrate that even if one day was the highest it would be statistically trivial. I looked at 'primary' stations only since my poor computer could only handle two years at once before crashing. So I queried 2012 and 2013.  Well, much to my surprise, there were very distinct patterns. Not only were the patterns geographical, but the were temporal and propagated from west to east. Figure 1 shows the warmest days of the week for all primary stations in the U.S. Across areas with minimal longitudinal variation, the daily percentages are staggeringly variable. In Alaska for example (see Figure 2), an amazing 41% of stations were warmest on Thursday and only 4% are warmest on Tuesday or Wednesday. To me, that is a signal indicating a possible climate connection.

Fig 1. All primary stations in the U.S. mapped according to which day of the week during 2012 and 2013 was the warmest (1=Sunday, 2 = Monday, and so on.) Only stations with 95% complete data were used.

Fig 2. All primary and RAWS stations in Alaska mapped according to which day of the week during 2012 and 2013 was the warmest (1=Sunday, 2 = Monday, and so on.). Only stations with 90% complete data were used.

I then set out to check time periods other than seven days. Of course a seven day period corresponds very nicely with a calendar week. I checked time periods between 2 and 30 days. For the 30-day period think of it as grouping Jan 1, Feb 1, March 1, etc. and calling those "Group 1". Then take Jan 2, Feb 2 March 2, etc. and call those "Group 2". 

After a little trial and error, it was pretty obvious that 4-days was a very prominent pattern for the Lower 48 and modestly prominent for Alaska. Figure 3 shows which of the repeating 4 days is the warmest for the Lower 48 and Figure 4 shows the same but for Alaska.

Fig 3. All primary stations in the U.S. mapped according to which day, of a repeating 4-day pattern, during 2012 and 2013 was the warmest. Only stations with 95% complete data were used.

Fig 4. All primary and RAWS stations in Alaska mapped according to which day, of a repeating 4-day pattern, during 2012 and 2013 was the warmest. Only stations with 90% complete data were used.

Looking at the ESRL Reanalysis data (see Figures 5, 6, 7, and 8 below), a very clearly defined 4-day thermal wave is evident. I compared each of the four repeating days with the other days in the 4-day set to track the atmospheric patterns for 2012 and 2013. I am quite amazed that given the number of days used in the analysis (728) that any pattern whatsoever holds up. If the 4-day pattern was 3.5 or 4.5 days it would break down in the reanalysis data after a while. When I ran the same analysis at the 7-day interval (not shown) a noticeable, but less prominent thermal wave was also evident. I can share those images with anyone who may be interested.

So what recurs at 4 and 7 days. One obvious answer is planetary (Rossby) waves. They tend to recur at 7-day intervals according to the literature but their period is pretty variable and is highly dependent on the number of waves. The fact that the 7-day pattern in Figure 1 is more prominent between 40°N and 50°N lends credence to the planetary wave origin for that length time period. But what about the 4-day pattern? It has a much stronger signal. Is it related to planetary waves? What drives the thermal push depicted in Figures 5-8? Also, might the 7-day pattern actually be a 1/2 strength version of the 4-day pattern since 7 is almost, but not exactly, a multiple of 4?

Surely this is not a new discovery. My brief search of the literature didn't lead to any obvious answers beyond the Rossby wave solution. Any ideas would be much appreciated.

Fig 5. Day 1 of the 4-day thermal wave as depicted by the ESRL daily composite reanalysis. A wave axis is superimposed as a red line. This is a compilation of all Day 1s minus the average of Days 2, 3, and 4. 728 days were used in the analysis.

Fig 6. Day 2 of the 4-day thermal wave as depicted by the ESRL daily composite reanalysis. A wave axis is superimposed as a red line. This is a compilation of all Day 2s minus the average of Days 1, 3, and 4. 728 days were used in the analysis.

Fig 7. Day 3 of the 4-day thermal wave as depicted by the ESRL daily composite reanalysis. A wave axis is superimposed as a red line. This is a compilation of all Day 3s minus the average of Days 1, 2, and 4. 728 days were used in the analysis.

Fig 8. Day 4 of the 4-day thermal wave as depicted by the ESRL daily composite reanalysis. A wave axis is superimposed as a red line. This is a compilation of all Day 4s minus the average of Days 1, 2, and 3. 728 days were used in the analysis.

* Update section

After reading a section from a paper from 1976 (Blackmon, M. L. 1976. A climatological spectral study of the 500 mb geopotential height of the Northern Hemisphere. J. Atmos. Sci. 33, 1607 -1623.) that describes the propagation period of Rossby waves as 20° longitude per day, I decided to make reanalysis plots of the 500 mb height anomalies. The 20° value corresponds to 1/4 of the width of the Lower 48 states. Therefore, the 4-day thermal wave may be entirely explained by that.The wave train of anomalies stands out very nicely and clearly propagate from west to east. 

Fig 9. 500mb height anomaly on Day 1 of the 4-day thermal wave as depicted by the ESRL daily composite reanalysis. This is a compilation of all Day 1s minus the average of Days 2, 3, and 4. 728 days were used in the analysis.

Fig 10. 500mb height anomaly on Day 2 of the 4-day thermal wave as depicted by the ESRL daily composite reanalysis. This is a compilation of all Day 2s minus the average of Days 1, 3, and 4. 728 days were used in the analysis.

Fig 11. 500mb height anomaly on Day 3 of the 4-day thermal wave as depicted by the ESRL daily composite reanalysis. This is a compilation of all Day 3s minus the average of Days 1, 2, and 4. 728 days were used in the analysis.

Fig 12. 500mb height anomaly on Day 4 of the 4-day thermal wave as depicted by the ESRL daily composite reanalysis. This is a compilation of all Day 4s minus the average of Days 1, 2, and 3. 728 days were used in the analysis.

Near-Record Inversion

The temperature inversion above Fairbanks reached near-record strength yesterday morning, with a temperature difference of more than 30 °C between the surface and the warmest level aloft.  This has been observed only three times before in February, on three consecutive days in 1989.  The February record for strongest inversion (from the surface to the warmest level aloft) is 32.9 °C.

While we're on the topic of upper-air temperatures, I created a graphic to visualize the evolution of temperatures aloft through the winter thus far - see below.  This is a time-height cross-section of the balloon sounding temperatures at Fairbanks; the freezing line is denoted with a black contour.  The remarkable warmth in October and January is readily apparent, as is the relative scarcity of deep cold air.  Of course this graphic might be more useful if it showed temperature anomaly rather than actual temperature, but it will take a bit more work to develop the daily climatology at each level.

Strong Morning Inversion

A building ridge aloft is a good set-up for strong valley based inversions in the winter in Interior Alaska, as temperatures this Saturday morning illustrate. Here's the temperature trace from the lowest 1000 meters from the early morning sounding at the Fairbanks Airport.

As usual, most of the vertical temperature gradient is in the lowest 100 meter (-29C to -15C). The pressure gradient is increasing across the area this morning, with 10 to 20 mph east winds blowing at elevation, and the wind is starting to reach the valley floor in places. Surface temperatures from Fairbanks-land from 7 to 8am AST Saturday show a similar spread:

Goldstream Creek: -31F
Fort Wainwright RAWS: -30F
Woodsmoke CWOP: -30F (near North Pole)
Eielson AFB: -28F

UAF West Ridge -14F
Fairbanks Airport -15F (winds NE 5 mph)
Fort Wainwright Airport: -19F

Wickersham Dome: +10F (2200' MSL)
Caribou Peak RAWS: +10F (2500' MSL)
Keystone Ridge: +12F (1600' MSL)
Fox Ridge (CWOP)" +14F (2100' MSL)

A 40 degree spread is pretty impressive even by Fairbanks standards. Being two months past winter solstice, clear skies, and some wind I'd expect temperatures will rise dramatically at valley elevations today.

8-14 Day Forecast Visualization for Alaska

A few weeks ago I decided to download the GIS shapefiles than NCEP makes available on their website for the 6-10 Day and 8-14 Day extended forecasts. I am particularly interested in these because the push the boundary between a weather forecast and a climate forecast. By downloading the shapefiles and then generating temperature anomaly maps from reanalysis data, we can visualize how effective the forecast actually was (note: this is not an actual verification). The YouTube video below is a compilation of 30 consecutive 8-14 Day forecasts beginning with the December 31 to January 6th time period and ending with the the February 11th to February 17th time period (no forecasts are issued on the weekends). The GIS maps (left) are placed next to the reanalysis data (right) for comparison. There are a few days where the GIS data may not have been processed correctly (repeating polygons on consecutive days). Nevertheless, it makes for an interesting visualization. With the exception of a few days in mid-January, they 8-14 Day forecasts were actually pretty good.

The Howard Pass Wind Chil and Meteorlogical Records

The discussion here on Deep Cold and elsewhere concerning the wind chill values reported from the Howard Pass RAWS platform over the weekend have been useful in helping to clarify the way the meteor/climatological community thinks about "records" and what constitutes a record.

From an operational viewpoint, "on the fly" data quality evaluation is a routine, every day, indeed every hour part of the job. Doing it correctly though requires a detailed understanding of not only the general meteorological situation but details through the meso and mircoscales, the dominant physical processes, the impact of orography on the variables in question and the details of instrumentation measuring and reporting the physical parameters of interest.

Alaska, with it's complex terrain and high latitude location is an exceptionally difficult arena for meteorological quality control. In Interior Alaska, it's boringly common in mid-winter for temperatures in two locations five miles apart to be 30ºF different, and for such differences to persist for days. How would that fly in Minnesota? Now in this case we happen to have a good physical understanding of how that happens. But temperatures are simple compared to winds.

Details of terrain and instrumentation are even more important for winds than temperatures. Consider the variability of winds but relative uniformity of temperatures in the heavily instrumented areas along the Delta River south of Fort Greely or in the Healy/Denali Park Entrance area. No one who has ever driven through Glitter Gulch in winter in  during a Chinook doubts the 70 mph winds reported at Antler Creek from the RWIS and at the same time the 15 mph or less winds at the AWOS at the Denali Park train station.

So how does all this play into the Howard Pass report? We know that outlets of constricted terrain can be remarkably windy in the appropriate meteorological conditions. We know the details of the instrumentation and know it is research grade equipment. We know it was installed and is maintained by experts in high latitude meteorological instrumentation (thanks Ken and Pam!). We know that the highest resolution numerical models routinely run over Alaska do capture something of the extreme winds and temperatures observed (thanks to Richard for this useful post illustrating this). I made an educated but incomplete scour of observations some years ago for this kind of information and Brian has now systematically checked hourly observations for most of the North Slope sites that might have ever have wind chills in the 90s below.

So, as I see it, to the best of our ability to determine, Howard Pass RAWS did observe the lowest wind chill reported in real-time in Alaska, and I think we can say so with confidence. That does not mean it is certain. And certainly we can argue whether the wind speeds should be adjusted due to the low height. But this is detail: the base observation has verisimilitude, and with no other observations anywhere close (KTZA2 is 67 miles away, and IMYA2, in a completely different physiographic setting, is 42 miles away), the best hope we have for verification would be higher, say 1km, horizontal resolution modeling. Even this will simply make the case more or less solid. There can be no proof.

In the past ten years we've seen a dramatic increase in the amount of quality surface data available in Alaska. but in winter. I expect we'll see more of these kind of "oddball records".

January Post-Mortem

I've been meaning to post a brief analysis of January climate conditions over Alaska, especially in view of what proved to be a very misleading analog forecast for the month.  Here's what I wrote in December:

http://ak-wx.blogspot.com/2013/12/analog-seasonal-outlook.html

"While the uncertainty is great, and we can't have high confidence in this kind of forecast, the analogs suggest a cold January may be in store for Fairbanks."

However, January proved to be very warm over all of Alaska, with a prolonged chinook event in the second half of the month.  Daily mean temperatures were below normal on only four days in Fairbanks.  All-time records for January high temperatures were set in some locations, the state record was tied, and Nome and Barrow set very significant records as discussed on this blog.  Precipitation was well above normal at most locations, with notable exceptions being Fairbanks where the chinook effect prevailed, and Annette Island, which was close to the apex of the anomalous ridge; the third chart below shows the average 500 mb height anomaly for the month.

So what went wrong with the long-range forecast?  Well, in retrospect it was unwise to focus on just a single predictor (2013 conditions in Barrow) rather than also examining potential predictability from other sources.  A major influence on Alaska climate for some months now has been the unusually warm ocean temperatures in the northeast Pacific.  The maps below show (top) the sea surface temperature anomaly for January and (bottom) the mean temperature anomaly between the surface and 100m depth.  These are remarkably large anomalies over a large area and represent a major perturbation in the climate system.  The unusual ocean warmth has developed in tandem with a persistent circulation pattern that keeps bringing warm air to the same areas over the northeast Pacific, thereby reinforcing the ocean anomaly; and of course it's a positive feedback, as the unusual ocean warmth also reinforces the strength of the atmospheric ridge.  The persistent ocean anomaly seems to be at least part of the reason why a similar weather pattern keeps developing near and over Alaska: compare the third chart below, showing the October 500 mb height anomaly, to the January map above.

We can go all the way back to May 2013 and find a very similar ocean temperature pattern, as shown in the next two charts; and of course the summer brought record-breaking warmth to interior Alaska.  The northeast Pacific sea surface anomaly did go away for a time in the early fall, as colder air swept down and erased the surface warmth (third chart below), but a subsurface chart from September shows that there was still anomalous warmth lurking beneath the surface (fourth chart below) and the overall pattern remained unbroken.

In summary, the forecast failure for January was a good lesson that we should not rely too heavily on one analog predictor - especially one without a strong physical basis.  This is not to say that the Barrow analog was without value; but it seems the ocean conditions south of Alaska were a more potent factor and prevented the January climate from following the pattern of the Barrow analog years.

Chena River Ice Thickness

**  Update section added at the bottom of the post on 2/18 **

In two of the last three years I have traveled to Fairbanks in February or March. Whenever I go I always stay at Pike's Waterfront Lodge. One year the Chena River was open and the other year the thickness was only 5". However, upstream at the Steese Highway the thickness was quite healthy. This is apparently a common occurrence. But why?

The simple answer is that a power plant between the two sites puts warm, clear water into the River thereby reducing the ice downstream at Pike's Landing. However, the ice thickness is pretty close early in the season at the two sites. Also, Rick noted that another powerplant is upstream of the Steese Highway that generates power for Fort Wainwright. So, there is probably a simple answer but I don't know what it is. Thoughts?

Fig 1. Ice thickness measurements (dots) 2001 to present for Chena River at Pikes Landing and at Fairbanks (Steese Highway).

Fig 2. Smoothed ice thickness 2001 to present for Chena River at Pikes Landing and at Fairbanks (Steese Highway) using monthly averages.

Fig 3. Google Earth screenshot of the two measurement locations. The GVEA Aurora power plant is also shown.

Fig 4. Picture I took of a van stuck on the Chena River Ice Bridge in March 2012.


* Update section  *

Google Earth has a 'historical photography' utility that I just decided to check out. Fortunately there is some photography from March 9, 2009, that we can look at. The following four screenshots are ordered from east (Steese Highway) to west (Pike's Landing). The effect of the Aurora Plant is quite apparent. What is still unresolved is why the early season ice at Pike's is 'normal' but it deteriorates quickly in February. Presumably the power plant is in full production mode in December but the ice hasn't started to melt out yet. Perhaps the thermal 'front' takes a long time to make it down that distance from when the pkant operations kick into high gear earlier in the winter.

Fig 5. Google Earth historical image from March 9, 2009 (1 of 4).

Fig 6. Google Earth historical image from March 9, 2009 (2 of 4).

Fig 7. Google Earth historical image from March 9, 2009 (3 of 4).

Fig 8. Google Earth historical image from March 9, 2009 (4 of 4).

Lower Howard Pass Wind Chill in 2013?

As I was searching for additional information about the installation and siting of the Howard Pass RAWS, I came across the following NPS report discussing the summer 2013 and winter 2012-2013 weather conditions in the Western Arctic Parklands.

http://www.nps.gov/akso/nature/documents/WEAR Summer 2013 Weather Summary.pdf

A note in the document provides the following interesting statement: "At Howard Pass (see cover photos), winter temperatures were as low as -48° F in February and wind gusts as high as 84 mph in January.  The minimum wind chill for February was -101° F on February 21.  The average wind chill for February was -39.5° F."

The Howard Pass statement is accompanied by a time series chart for winter 2012-2013, complete with wind chill measurements:

If these measurements are correct, then the all-time Alaska wind chill record may have been broken last year at Howard Pass, and the 2013 event may still stand as the existing record.  Curiously, the RAWS data archive at the Western Regional Climate Center shows corrupted or missing temperature data from Howard Pass prior to late July 2013, so I wasn't able to find the data to match the chart shown above.  The wind chill of -101F would correspond to the minimum temperature of -48F combined with a wind speed of 54 mph.

Brian points out that some of the temperature reports from other stations in northern Alaska on February 21, 2013 were low enough to make the Howard Pass wind chill at least plausible; Anaktuvuk Pass reported a high temperature of -38F and a low of -45F.

Rick will be able to answer the question of whether this data was thrown out for some reason, or if it might have been overlooked by NOAA at the time.

Model Depiction of Howard Pass Event

In view of Brian's suggestion of "further investigation" of the Howard Pass wind chill report, I thought it might be worth looking at some of yesterday's numerical model forecasts of conditions in the western Brooks Range.  The idea here is that if a numerical model forecast showed wind chill values anywhere near the reported values, this might boost the credibility of the RAWS data for this event.

There is a multitude of global weather forecasting models that could be examined for this event, but the coarse resolution of global models wouldn't tell us much about the terrain-modified flow conditions near Howard Pass.  Fortunately, the National Centers for Environmental Prediction produce a daily 5-km resolution run of the WRF forecast model in two different configurations for an Alaska domain; more information can be found here:

http://www.emc.ncep.noaa.gov/mmb/mmbpll/nestpage/

The model is initialized at 9 am AKST every day, which is conveniently just a few hours before the purported record wind chill measurement at Howard Pass.  Below are images showing the 6-hour forecasts of 2m temperature and 10m wind from the two alternative configurations of the model (NMM and ARW); these are forecasts for approximately the time of the minimum reported wind chill (-97F at 3:39 pm yesterday).  Note that the black contours show terrain elevation at 600m (thick line), 800m (medium), and 1000m (thin).  The location of Howard Pass is indicated with a white dot.

The forecasts are considerably different, with the NMM version showing much lower temperatures almost everywhere, and especially over and north of the north side of the Brooks Range.  A cursory comparison to observations suggests the NMM was closer to reality at high elevations (Killik Pass, -30F) and on the North Slope (Umiat, -34F), but the ARW was better in the lower terrain to the south and west (Noatak RAWS, -15F).

Let's look at the NMM a bit closer: the charts below show the forecast wind speed and wind chill at 3 pm yesterday.  Note that the wind arrows "begin" at the grid point that they represent, i.e. they point away from the grid point in question.  Also, not all the grid points are plotted as the grid is much finer than the arrows suggest.  The forecast wind speed at Howard Pass was only 25 mph, compared to 60-70 mph sustained according to the RAWS.  Nevertheless, with a forecast temperature of -36F, the forecast wind chill was -73F.  As one would expect, the lowest wind chills are along the ridge line where extremely cold air from the North Slope was being lifted and forced over the ridge and through gaps in the high terrain.

In the NMM forecast, the temperatures proceeded to drop overnight, and wind speeds increased slightly, with the result that the predicted wind chill dropped even lower by 9 am today: the forecast wind chill was below -80F over a large area near and west of Howard Pass.  The lowest forecast wind chill value at the Howard Pass location was -83F at 11 am.

I hope to look at some of the station comparisons in more detail next week, to see if we can gain any more insight as to how well the NMM run performed.  The biggest difference from the Howard Pass RAWS data is obviously the wind speeds, which purportedly reached an almost incredible 91 mph sustained at 11 am today.  It may be that an even higher resolution model run would be required to capture the localized flow effect that would have caused such extreme speeds - if indeed they actually occurred.

Alaska (and United States) Record Low Wind Chill at Howard Pass RAWS?

On February 14th, the National Weather Service in Alaska noted that the Howard Pass RAWS station achieved a wind chill value of -92°F at 5:39 a.m. That value was close the the presumed statewide record of -96°F set at Prudhoe Bay on 1/28/89 (temp -56°F and wind speed of 21 mph at Prudhoe Bay). Later in the afternoon of the 14th, the Howard Pass station measured a wind chill value of -97°F at 3:39 p.m. (temp -42°F wind speed of 71 mph). If verified, this would represent a state of Alaska and possibly a United States record.

The Howard Pass RAWS weather station is in the Brooks Range of northern Alaska. The station is located at an elevation of 2,062' along the edge of the Noatak River valley and can be found here via Google Maps. Figure 1 shows some pictures of the station that were posted on the NWS Facebook page on 2/15/14. Figure 2 shows a satellite image taken near the time of the record low wind chill value.

Figure 1. Howard Pass station photos posted the the NWS Alaska Facebook page.

Figure 2. Suomi NPP satellite image from 2/14/14 at 2:17 p.m. Alaska Standard Time.

At first glance the wind chill values seems quite improbable. The temperature is not too surprising but the wind speed is remarkably high. This station has data going back several years and unfortunately the temperature data quality is not always very good. However, looking at the wind frequency distribution tables on the RAWS site (not shown), it appears that wind speeds over 60 mph occur pretty regularly during the winter months and a month-long average winter wind speed of 20-25 mph is typical. Based on the station climatology, extreme winds are not uncommon. Additionally, the wind speed show in Figure 3 does not appear to resemble the chart of a malfunctioning station anemometer. Note: I was not able to find metadata about the station's equipment.

Figure 3. Four-day chart of temperature, wind speed, wind gust speed, and wind chill values (updated 9:00 p.m. on 2/15/14).

The station is located north of the Arctic Circle and by mid-February it receives only a small amount of solar radiation. Therefore, the diurnal temperature is primarily a function of advection, clouds, and vertical mixing. However, the little bit of solar energy they did receive on February 14th correlates with the spike in wind speeds which suggests some thermal mixing occurred; although the soundings indicated winds under 20 knots in the lowest 3,200'. Local topographic constriction likely influences the channeling of northerly winds through the passes along the northern slopes of the mountains. Figure 4 shows the surface map near the time of maximum wind chill. Temperatures measured in the lowest 5,000' feet as recorded by weather balloons were quite cold. The Kotzebue and Barrow RAOB soundings showed temperatures between -20°C and -25°C at 850 mb.

Figure 4. NWS surface map near time of maximum wind chill.

Figure 5. Maximum 24-hour wind speed beginning at 2 p.m. on February 14, 2014. Howard Pass had a reading of 103 mph. (retrieved from NOAA interface of Mesowest data server)

Regionally, wind speeds were high, but not necessarily extreme (see Figure 5). The direction of the maximum wind speed was north or northeast at nearly every station. Wind speeds of these magnitudes are uncommon, but not unheard of.

 Figure 6. Low temperatures on February 14, 2014, in western Brooks Range. Howard Pass is -43°F. (retrieved from NOAA interface of Mesowest data server)

The Howard Pass station had a high temperature of -38°F (not shown) and a low of -43°F (Figure 6). Given the high wind speeds and the 850 mb temperatures in the -20s°C, temperatures in the -40s°F seems a little cold in my opinion.

Is it a valid reading?

That is the question. Neither the high wind speed nor the low temperature are especially surprising by themselves but the combination of the two is a little curious. On the surface there doesn't appear to be a 'smoking gun' to disqualify the reading but further investigation is warranted. Richard has conducted an impressive assessment of the pre-event numerical guidance. I highly recommend reading it.

* 2/18/14 Update *

Preliminary communication with the NPS technician  (not by this blog entry's author) who installed and maintains the station indicated that the equipment is designed for this type of environment and that the wind readings are unlikely to be revised downward. The anemometer is replaced annually and the design makes readings that are too high improbable. In addition, data from the previous several seasons appears to confirm the fact that wind is funneled through the north-south oriented mountain gaps and that especially low temperatures are entirely plausible. As the data continues to accumulate, the confidence in the observations in increasing.

Barrow Record Warmth

* Updated with ESRL Reanalysis maps at end of post (2/13) *

Looking at the temperatures in Barrow since the beginning of October, it is evident that warm temperatures have been the rule. In fact, 77% of the days between October 1st and February 12th have been above normal.

The first chart shows the average daily temperature departure from normal (1981-2010) during that time period. Since October 1st, the average day has been 7.5°F above normal.

The second chart shows the same information as the first but instead of daily temperature departure measured in degrees Fahrenheit, it shows the daily departure in standard deviations. The median daily departure from normal as measured in standard deviations was 0.67 based on the 1981-2010 climate normal period. According to a standard z-score table, 25% of days should be more than 0.67 standard deviations above the mean – not 50%.

Finally, the last chart shows the average temperature between October 1st and February 11th for each winter since '20-'21 until the present. The current winter is far and away the warmest during those 90+ years – by 2.1°F! There is data that goes back before 1920 at Barrow but it has significant data quality issues. Still, this year is warmer than any complete year prior to 1920 as well. Certainly the lateness of the sea ice in recent decades has significantly contributed to early season warming in particular.

*Update Section*

The two maps below are from the ESRL Reanalysis data sets. The first map shows the temperature anomalies for Alaska during the months of October through December for the last 10 years. The second map shows the temperature anomalies for Alaska during the months of February through April for the last 10 years. The lack of sea ice in the Fall and early winter dramatically warms the northern portions of Alaska. Once the winter ice pack has set in, the temperature anomalies are greatly reduced. On the second map you can see the cold anomaly in the southeast Bering Sea due to high spring ice extents and cold water temperatures.