Thursday, January 29, 2015

North Slope Wind Chill Climatology

A few days ago the hardy residents of eastern North Slope communities experienced a combination of windy and cold conditions, leading to some sustained low wind chill values.  The chart below shows recent observations from Deadhorse airport; the wind chill was sub-minus 50 °F for quite a lengthy period over the weekend, and the lowest value was -57 °F.


How unusual is this?  The answer is "not particularly" for this time of year.  The box-and-whisker plot below shows the monthly distribution of wind chill values from the hourly observations since 1982 in Deadhorse.  The central box in each column indicates the inter-quartile range, i.e. the top and bottom of each box show the upper and lower quartile respectively for the month.  The thick horizontal line in the middle of each box shows the median, and the "whiskers" above and below the boxes show the extreme values.

We see that the lower quartile of wind chill values in January is -49 °F, so the wind chill is below that value 25% of the time.  It is between -24 °F and -49 °F half of the time.  Interestingly there is little relief from low wind chill values until April, as temperatures don't improve until then (and average wind speeds change rather little from month to month at Deadhorse).

I also looked at data from Barrow and Barter Island; the chart below shows the median wind chill from all three locations by month.  Deadhorse is the chilliest of the three locations in winter, but Barrow is coolest in summer owing to the stronger marine influence.


Monday, January 26, 2015

Really Cold Snow

The other day Rick noted that snow was accumulating on Keystone Ridge with a temperature of -25°F. That got me wondering how low the temperature can go and still receive accumulating snow around Fairbanks. With the help of daily climate summaries and hourly observations, we can identify the very coldest days with measurable snow. A key part of this analysis is that we eliminated all observations with fog in the wx code. The reason for this is because I am using visibility as a proxy measure for accumulating snow. I sometimes wonder if dense ice fog creates a rime layer that is counted as new snow or if ice crystals can pile up into a minor accumulation. I expect the Fairbanksans reading this to chime in right about now.

In conversations with Rick and Richard, they both agree that a snow observation with a visibility of 3 miles or less will start to accumulate (actually they suggested 4 or 5 miles). Since ice fog nearly always forms when the temperature is colder than -35°F, for practical purposes, this means true snow events with temperatures under this value are not captured in this analysis.

Hourly observations that are coded to identify snow go back to 1960. I paired all of the hourly observations with the daily climate summary to see which days had accumulating snow. The reason we do this is because a day might have a high of -10°F and a low of -35°F with 0.3" of snow. But did the snow fall when the temperature was -10°F, -20°F, or -35°F? That is why pairing the daily and hourly observations makes a huge difference.

Table 1 shows the dates since 1960 (54 years) that observed accumulating snow (snow wx observation, visibility of 3 miles or less, and no fog or drizzle) with a temperature of -25°F or colder. In the table, the column labelled "Warm Snow Obs" is the warmest hourly temperature that met the selection criteria on that date and the column labelled "Cold Snow Obs" is the coldest hourly temperature that met the selection criteria on that date.


Table 1. Top 25 coldest "pure" snow observations (no fog) since 1960 at the Fairbanks International Airport.


Remember that the idea here is to set practical bounds on accumulating snow and not to identify extremes. In Table 2, you will see how fog plays havoc with the analysis. On December 28, 1964, one observation met the aforementioned selection criteria (temperature was -35°F) but other observations with fog probably has a snow intensity sufficient to accumulate.


Table 2. All snow observations on December 28, 1964 at the Fairbanks International Airport.

It seems that snow can occur at, and just under, -30°F every once in a while in Fairbanks. At that temperature, there isn't very much moisture in the air (mixing ratio about 0.2 g/kg). Unfortunately the hourly observations and the daily summaries leave much of the story untold. This is a great example of where local knowledge can supplement technical documentation.

Cold - Surface and Aloft

As advertised by computer forecasts from the middle of last week, clear skies and calm winds have allowed temperatures to drop off sharply over the central and eastern interior.  At the surface there is a 1030 mb high across north-central Alaska, but at upper levels the dominant feature is not high pressure but a cold low over the western Beaufort Sea.

The lowest temperature observation I've seen so far is -55F at the Granite Creek SNOTEL just east of Delta Junction, but there were many other worthy contenders including:

-53F  Huslia
-53F  Coldfoot SNOTEL
-53F  Arctic Village
-52F  Bettles airport

Here's a delightful webcam photo from Arctic Village this afternoon: -49F and some light ice fog.



In the Fairbanks area, -43F was reached at the airport, the coldest in nearly two years.  Fort Wainwright reached -48 and Eielson -47.

Yesterday afternoon's balloon sounding revealed deep cold with only a slight inversion (prior to the overnight surface cooling).  The column maximum temperature was -26.5 C, which is again the coldest in nearly two years.  It's interesting to compare this sounding with the coldest of this winter prior to last Wednesday (as measured by 1000-500 mb thickness): see below.


Saturday, January 24, 2015

Decreasing Extremes

With potentially much below normal temperatures possible in the next few days in the central and eastern interior (latest MOS number -65 °F for Fort Yukon on Tuesday), I started to wonder about historical changes in the frequency of large temperature anomalies.  We know that cold extremes are less common in winter now, even relative to the warmer modern "normal", but what about warm extremes - have these become more or less common?

The chart below is a histogram of the daily temperature anomalies in November through March in Fairbanks, for two 30-year periods: 1931-1960 and 1981-2010.  In each case I compared the daily mean temperatures to the daily normal values for the 30-year period in question, so this is a straightforward comparison of daily temperatures to the contemporary climate normal.  The results show that large anomalies were noticeably less common in the 1981-2010 period than in 1931-1960, and that the decrease in extremes occurred on both the warm and cold sides.


In the extreme tails of the distribution, there was a large proportional decrease in frequency of daily anomalies greater than +/- 40 °F.  On the cold side, anomalies more than 40 °F below normal decreased from 0.95% of days to 0.22 % of days, and on the warm side the frequency of +40 °F anomalies decreased from 0.48% to 0.13% of days.  The chart below shows the numbers of days by decade that saw +/- 40 °F anomalies on either side.  As noted in earlier posts, the 1930s was an extreme time for Fairbanks climate.


The decrease in temperature variance over time in Fairbanks was previously noted in Brian's analysis here.

As an aside, a corresponding analysis for the warm months of the year does not show the same decrease in extremes (see below); in fact the 1990's was the decade with the highest frequency of daily anomalies greater than +/- 15 °F.


Friday, January 23, 2015

Getting Wintry

The coldest temperatures of the winter so far in Alaska were observed this morning, with the -50 °F level broken for the first time to my knowledge: -51 °F was reported at Huslia in the lower Koyukuk River valley.  The infrared satellite image below, taken at 3:53 am AKST this morning, shows the areas that cooled off under clear skies in the western interior: dark shades are colder, and Huslia is located close to the prominent dark patch in the upper center of the image.


In contrast, heavy cloud cover has so far kept temperatures relatively milder in the eastern interior, although Arctic air has been working its way in at low levels despite the blanket of clouds.  The frontal zone aloft over Fairbanks created a band of snow today that became quite narrow and fairly intense as it persisted just east of the city (see below); a storm total of 7.7" was reported from North Pole as of 4:42pm.   This kind of "mesoscale" feature is nearly impossible to predict more than a few hours ahead of time but makes a great difference for total snowfall amounts.


As clouds and moisture migrate eastward in the next couple of days, and high pressure builds across the north, the cold conditions already observed in the west will envelop the central and eastern interior.  This morning's GFS MOS forecast numbers were remarkably cold, including -49 °F at Fairbanks airport and -50's at various locations along the Tanana River valley on Monday or Tuesday.  Readers can track the latest MOS (statistical) forecast temperatures at the following page:

http://www.worldagweather.com/alaska/mos.php


Wednesday, January 21, 2015

Turning Colder

This afternoon's balloon sounding from Fairbanks measured a column maximum temperature of +9 °F, which is - remarkably - the coldest such measurement of the season so far.  Much colder conditions are on the way; western Alaska is feeling a chill already, with Kotzebue reporting a stiff northwesterly breeze and temperatures hovering around -6 °F all day.

In what will be a nice change of scene both for residents and this blogger, we should have some material to discuss on the cold side in the next week or so.  Latest GFS MOS numbers for next Tuesday include -36 °F in Fairbanks, -48 °F in Eagle, and -59 °F in Chicken.  This is a long-range forecast, but gives a sense of what is easily possible given the cold airmass and the date on the calendar.

Here's this afternoon's sounding: at last, just a smidge cooler than normal at 850 mb.


Tuesday, January 20, 2015

More on Warmth Aloft

Recently there's been much attention given to the fact that - according to the National Climatic Data Center - 2014 was the warmest year on record in Alaska.  This verdict is based on surface station measurements, but is also consistent with record warmth aloft as observed by balloon soundings.  For instance, we can look at the 1000-500 mb thickness, which is an excellent measure of the average temperature of the lower half of the atmosphere; we find that the 2014 mean 1000-500 mb thickness was higher than in any other year (1948-present) in Fairbanks, Barrow, and Kotzebue.  The thickness was second only to 1957 in Nome, McGrath, Bethel, and Anchorage.

One question that arose in my mind when pondering these new records was whether there were any 365-day periods in the past that were warmer than the calendar year 2014; after all, calendar year boundaries are artificial, like week or month boundaries throughout the year.  So I calculated the running 365-day mean 1000-500 mb thickness in Fairbanks and found that - remarkably - the highest values on record occurred in the past week.  Here's a list of the top several 365-day means with data complete through today (January 20); the top 61 overlapping periods ended in 2014 or 2015, and next in line was the year ending February 7, 1994.

Jan 17, 2014 - Jan 16, 2015   5344.20 m
Jan 18, 2014 - Jan 17, 2015   5344.15 m
Jan 16, 2014 - Jan 15, 2015   5343.83 m
Jan 19, 2014 - Jan 18, 2015   5343.63 m
Jan 20, 2014 - Jan 19, 2015   5343.62 m
Jan 21, 2014 - Jan 20, 2015   5343.44 m
Jan 15, 2014 - Jan 14, 2015   5343.23 m
May 21, 2013 - May 20, 2014   5343.03 m
...
Feb 8, 1993 - Feb 7, 1994   5338.53 m

Below is a chart of the running means since 1949.  The new record obviously reflects the remarkable combination of persistent unusual warmth and an absence of unusual cold; the last really notable cold spell (compared to normal) in Fairbanks was the exceptionally cold spring of 2013.


Sunday, January 18, 2015

Warmest Winter To Date

The mean temperature in Fairbanks since November 1 has now moved into first place in the historical rankings since 1930; in other words, if we define winter as November-March, then this is now the warmest winter on record through January 17.  The winter-to-date temperature now exceeds that of 2002-2003 by the slimmest of margins.

The chart below shows the evolution of the winter-to-date mean temperature this year in red, compared to 2002-3 in blue and the historical range in gray.  This winter has been steadily climbing through the rankings as warmth at the surface has become increasingly anomalous.  We can see that mean daily temperatures close to +10 °F would be required to maintain first position for the remainder of winter; this would be more than 15 °F above normal for late January, so it will be a challenge to hold onto the record in the near-term.  In fact, we're very likely to fall back out of first place in the coming days, as January 2003 had a very warm spell around the 20th.


[Update: Brian coincidentally made a similar graphic, and his analysis shows that the winter of 1928-29 was warmer through January 17.  This is helpful context for the current warmth.  It's a matter of personal preference as to whether to include pre-1930 data in this kind of analysis - I usually don't do so, because the Weather Bureau/NWS era began in December 1929 and I have greater confidence in the quality of the Fairbanks data since then.  Rick Thoman went so far as to say that pre-1930 data from Fairbanks are "plagued by data quality issues", although this may apply more to pre-1920 data.  My own (relatively uninformed) opinion is that there's very useful information to be gleaned from the early years, but the data should be used with caution.]