Tuesday, January 30, 2018

Record Chukotka Warmth

Unseasonably cold conditions are hanging on in Fairbanks-land, with temperatures around 10-20°F below normal for the time of year, but some really exceptional warmth occurred yesterday in Russia's Chukotka district, on the other side of the Bering Strait.  Temperatures rose above freezing in a zone stretching from the northern Sea of Okhotsk and the Kamchatka Peninsula up to the Arctic Ocean, and the town of Ostrovnoe at 68°N saw a high of 42°F.

The location of Ostrovnoe (see the map below) means that it is usually a very cold location in winter; it's in northeastern Siberia, above the Arctic Circle, 100 miles south of the Arctic Ocean, and in a river valley.  The normal high temperature in January is well below -20°F and so it's considerably colder than any inhabited place in Alaska.  According to my calculations, yesterday's high temperature was 64°F above the 1981-2010 normal, and the low temperature of 33°F was a remarkable 69°F above normal.  The GHCN daily data from 1936-present (which is a bit patchy in some years) indicates that this is the first time Ostrovnoe has seen a daily minimum above freezing in January or February.

To put these daily anomalies in context, the largest positive daily temperature anomalies on record in Fairbanks are just over 50°F (for example, a high of +52°F on January 16, 2009, compared to normal of about 0°F).

Here's a chart of daily mean temperatures in Ostrovnoe since November 1; the daily mean temperature was 67°F above the 1981-2010 smoothed normal.  It should be noted that this is actually not unprecedented: December 21, 1955, was 68°F above the 1981-2010 normal (and presumably even farther above the then-colder normal).

The profound temperature gradient across the Bering Sea is being caused by an intense ridge of high pressure centered right over the date line - see this morning's 500mb height analysis below.  Powerful southerly flow to the west of the ridge is pulling warm air up into eastern Siberia and the high Arctic, but northerly flow is keeping Alaska rather cold.

The map below shows the average 500mb height anomaly in January and February, for 10 winters with strong La Niña conditions; a Bering Sea ridge is one of the main elements of the mid-high latitude atmospheric response to La Niña.  Although the current La Niña episode is not particularly strong, the current flow anomaly clearly fits the pattern rather well.

Thursday, January 25, 2018

Deep Cold and Persistent Snow

[Update 5am Jan 26:  Snow ended before 2pm yesterday in Fairbanks; one more hour would have tied the record for most hours of snow in a 7-day period.  As for consecutive hours with snow, this ended at 69 hours - nowhere near a record, although 45 hours of snow with temperatures below -20°F does tie a record from 1999 (thanks to Brian Brettschneider for that one).]

Very cold air associated with the upper-level low over the western interior has spread east in the past 36 hours, and as a result temperatures have dropped markedly in the Fairbanks area despite plentiful cloud cover and continuous light snow.  Normally the higher elevations in the hills provide some refuge from the valley-level cold, but this morning's sounding shows no low-level temperature inversion; however, there's a very deep inversion extending up to 700mb, and this is more characteristic of a true Arctic air mass.

Temperatures at the standard levels of 925mb (~2000' above ground) and 850mb (~4000') are very low this morning; the 850mb temperature of -30.3°C is the lowest since January 2013, and the 925 mb temperature of -32.1°C is the coldest since late January 2012.

Here are the minimum temperatures (°F) at the surface from the last 24 hours; colder conditions occurred just to the west of Fairbanks where the sky cleared out for a time yesterday.

Also of note is the persistence of the light snowfall in Fairbanks in recent days, caused by the lingering zone of ascent aloft to the east of the upper-level low.  As of the latest hour (6am AKST), snow has been falling continuously for 61 hours.  This is nowhere close to the record from late December 1984, when snow was reported every hour for 5 and a half days (133 hours).

However, if we look at the number of hours of snow in the past 7 days, we are getting close to a record: starting last Friday, there have only been 13 observations at the top of the hour that did not report snow falling - it has been snowing for more than 90% of the time in the past 6+ days.  Based on data since 1950, the lowest number of no-snow hours in a period of 7 calendar days (midnight to midnight) is 22 (in January 1996), so this record will be broken if Fairbanks gets another 9 hours of snow before midnight tonight.

For reference, here's this morning's 500mb analysis.  It's interesting to note the very cold air aloft to the east of the low; the temperature of -41°C at 500mb above Fairbanks is accompanied by wind from the south-southeast.

Tuesday, January 23, 2018

Wintry Chill

The coldest conditions of the winter so far have developed in western and northern Alaska over the past few days, and today there is quite an extensive area across the northwest with temperatures in the -30s and below.  So far it's a fairly run-of-the-mill cold snap for this time of year - after all the normal seasonal cycle of temperatures reached its low point just a few days ago in the interior - but it's very cold nonetheless.

Here are the lowest temperatures that I've seen so far:

-57°F  Clear Creek HADS site, 40 miles NE of Huslia
-54°F  Norutak Lake RAWS, near the upper Kobuk River at the southern edge of the Brooks Range
-52°F  Huslia airport
-51°F  Hozatka Lake SCAN site, between Galena and Huslia

This morning's FAA webcam photo from Huslia shows the calm and clear conditions that we expect at these temperatures.

Looking back at the winter thus far, there were 3 earlier spells when the temperature dropped below -40° somewhere in Alaska, but up until now the coldest conditions have been restricted mainly to the southeast interior.  The chart below shows the daily minimum temperature in Alaska since October 1, and the red markers denote days when the minimum occurred (or was tied) at the settlement of Chicken.  On another 8 days the minimum was nearby on the Fortymile River, and other southeastern sites show up for a number of other days - so from early November through mid-January the daily statewide minimum was nearly always in the southeast interior.  In the current cold spell, however, clouds have blanketed most of the eastern part of the state, keeping temperatures higher (including in Fairbanks).

The maps below show this morning's 500mb and surface analyses, courtesy of Environment Canada.  Note the strong cold low aloft with a strong contrast between dry northerly flow (thus clear and cold) to the west and moist southerly flow (thus cloudy and milder) to the east.

Saturday, January 20, 2018

Freezing Rain

Freezing rain created a fair bit of disruption in the Fairbanks area this week, with an initial round of light rain for a couple of hours on Monday morning followed by much more persistent and significant rainfall on Tuesday evening and overnight into Wednesday.  A very substantial snowfall also occurred, and the liquid-equivalent precipitation total was well over half an inch; this was the wettest January storm in 18 years.  There was also enough wind to produce Fairbanks' first ASOS report of "blowing snow" this winter - not a common occurrence in the sheltered Fairbanks bowl.

Here's the surface analysis from early Wednesday morning, courtesy of Environment Canada; a trough axis and low pressure center are indicated very close to Fairbanks.  The Fairbanks sounding from the same time is also shown below - note the strong southwest flow throughout the troposphere and the high precipitable water (total moisture content) of 1.22 cm.

In view of this rather significant freezing rain event, it's interesting to look back at the long-term history of similar conditions in Fairbanks winters.  The chart below shows the percentage of hourly observations in Fairbanks with either drizzle or rain reported at the top of the hour, for winters back to 1950-51  (thanks to Rick Thoman for the idea for this chart).

There has certainly been an uptick of winter rain and drizzle events in recent years, but the 1960s saw a couple of remarkable winters with a lot of freezing precipitation.  In particular, January 1963 stands out as highly anomalous; here's how it unfolded:

- freezing drizzle for 19 hours beginning at 9am on the 14th
- 6 hours of freezing rain on the evening of the 15th
- 4 more hours of freezing rain on the evening of the 16th
- freezing rain throughout the first half of the 19th, ending as plain rain in the afternoon when temperatures rose above freezing
- the afternoon sounding on the 19th recorded the highest precipitable water on record for the month of January in Fairbanks (1.66 cm)
- more plain rain on the morning of the 20th
- intermittent freezing drizzle and a bit of freezing rain on the 25th and 26th
- a few more hours of freezing drizzle late on the 29th and overnight Jan 30-31

Road conditions must have been somewhere between treacherous and impossible by the end of that month.  A quick look at the 500mb height map for January 1963 (below) shows an interesting dipole pattern, with much above-normal heights near and south of Alaska and also near Iceland.  The Arctic Oscillation was strongly negative, indicating a weak circumpolar flow that allowed cold air to spill south and, conversely, warm air to surge north.  Consequently it was a very cold month in the lower 48, and Europe was extremely cold; England had its coldest calendar month of the entire 20th century.  In some ways the topsy turvy pattern resembles recent conditions: both winters (1963 and 2018) produced headline-making cold at lower latitudes but exceptional warmth in Alaska and the Arctic.

Tuesday, January 16, 2018

Warmth on the Kuskokwim

Western Alaska's exceptionally warm weather is in the news again today, as open water on the Kuskokwim River has forced a change of plans for the Kuskokwim 300 sled dog race that begins this Friday:

I mentioned the warmth in Bethel a few days ago, but it really is remarkable: since October 1, the total number of freezing degree days is easily the lowest in the modern record (1930-present).  As a reminder, freezing degree days are a simple measure of accumulated "freeze units"; we just sum up all positive daily differences between 32°F and the daily mean temperature.  So far this winter the freezing thermal units have amounted to less than 40% of the 1981-2010 normal for the same period.

The previous lowest FDD total for October 1 - January 15 was in winter 2000-2001, which turned out to be Bethel's warmest winter on record overall (November - March).  It was an analogous winter in some respects, with a weak La Niña and without a negative PDO phase to match.  The absence of a negative PDO this winter is the most obvious reason for the lack of colder weather that some of us expected to accompany the La Niña episode.

As another illustration of just how warm it's been in Bethel, consider that more days so far this winter (since November 1) have had a high temperature above freezing (37 days) than below freezing (32 days).  This too is unprecedented for 1930-present.

Friday, January 12, 2018

Chena Basin Snowpack

The Fairbanks area has seen no significant precipitation in over 3 weeks now, but a look at monitoring data from SNOTEL sites in the nearby hills reveals that the snowpack is healthy; indeed the current water content of the snowpack ranges from 124% to 184% of normal, where "normal" is the 1981-2010 median at each location.  Somewhat remarkably, most of the sites already have about as much snow on the ground as they normally do at the end of the snow season; and the Munson Ridge SNOTEL, the highest site at 3100' elevation, is already reporting over 8" of water in the snowpack.  Here's a chart of normal snowpack trajectories that I showed last spring.

Down at valley level in Fairbanks the snowpack is close to normal for the time of year, but there's no question that precipitation has been above normal so far this winter.  Higher than normal precipitation has been observed widely across interior, northern, and western Alaska; here are the liquid-equivalent precipitation totals (percent of normal) from November 1 through January 10:

Fairbanks 134%
Bettles 185%
Nome 145%
Kotzebue 266% (a record 4.39" total, previous record 3.51" in 2010-2011)
McGrath 100%
Bethel 115%
Northway 107%

Looking more closely at the Chena Basin SNOTEL data from recent years, it's interesting to note that this is shaping up to be the 8th consecutive year with generally above-normal snowpack in the latter part of the snow season.  The chart below shows how each of these winters fared, and while some (like last winter) obviously had a slow start, each of the past 7 years was above the 1981-2010 normal by March.  This isn't quite what I expected to find, as several recent winters were exceptionally warm in Fairbanks, and historically most very warm winters are also dry.

The April 1 snowpack data from the past several years show surprisingly little variance compared to earlier decades (see below).  None of the years has been particularly unusual in isolation, but taken together it has been quite a string of relatively snowy winters in the Chena Basin hills; and this year seems to be continuing in the same vein.

Friday, January 5, 2018

Sea Ice Update

At the beginning of last month we noted the remarkable absence of early-winter sea ice in the Bering and Chukchi Seas, and in view of December's warmth it will surprise no one to learn that ice formation has continued to lag far behind normal for the time of year.  In the satellite era (1978-present), the Chukchi Sea ice extent has never before failed to reached 100% by the end of the year, but this year about 5% of the area remained open (less than 15% ice coverage) on December 31, according to NSIDC's regional sea ice index.  The ice extent finally exceeded 99% yesterday.

Here's a plot of the December average ice extent in the Chukchi and Bering Seas.  For the Chukchi, December 2007 ice extent (occurring in the wake of the then-record September melt-out) was nearly as low for the month as a whole, but the Bering Sea has seen nothing like recent conditions in the satellite era.

The warm ocean surface and slow ice growth to the west of Alaska was closely related to the exceptional warmth that prevailed throughout western and northern Alaska, as well as more widely across the Arctic, in December.  The map below shows an estimate of December's mean temperature anomaly from the CFS reanalysis.

The +6-8°C temperature anomaly estimated by the model for the Y-K delta region is supported by observations from Bethel, where the month was over 14°F warmer than normal.  The season-to-date accumulation of freezing thermal units (freezing degree days) is easily the lowest on record in Bethel, and consequently river ice conditions are very poor for the time of year across the region.