Saturday, December 28, 2019

More on Cold

A couple of items of quick follow-up are warranted regarding the holiday-season cold snap of 2019.  First, here's a summary of low temperatures from the NWS:

438 PM AKST SAT DEC 28 2019




LOCATION                     TEMP       LAT/LON            TIME/DATE 

MANLEY HOT SPRINGS 15 MI NE  -65 F      65.14N/150.22W     0800 AM 12/28 (OFFICIAL)      
ALLAKAKET (FAA-ALASKA)       -60 F      66.55N/152.63W     1220 PM 12/28
BETTLES                      -60 F      66.92N/151.52W     0153 AM 12/28       
CHALKYITSIK RAWS             -57 F      66.60N/144.35W     0120 AM 12/28  
NOWITNA RIVER                -57 F      64.50N/154.13W     1205 AM 12/28  
ARCTIC VILLAGE               -57 F      68.12N/145.57W     0956 AM 12/27  
FORT YUKON                   -56 F      66.57N/145.27W     0803 PM 12/27    
TELIDA RAWS                  -56 F      63.43N/153.35W     0953 AM 12/27      
NORUTAK LAKE RAWS            -55 F      66.83N/154.32W     0721 AM 12/28  
TANANA                       -55 F      65.17N/152.10W     1152 AM 12/27 (OFFICIAL) 
MANLEY HOT SPRINGS DOT CAMP  -54 F      65.01N/150.61W     1000 PM 12/27       
COLDFOOT (FAA-ALASKA)        -52 F      67.26N/150.19W     1040 AM 12/27  
HUSLIA                       -52 F      65.70N/156.35W     0656 AM 12/27  
WISEMAN NWS COOP             -52 F      67.42N/150.11W     0700 AM 12/27 (OFFICIAL)      
NIKOLAI                      -51 F      63.02N/154.35W     0655 AM 12/27  
KALTAG                       -49 F      64.33N/158.75W     0553 AM 12/28 (OFFICIAL)
MCKINLEY RIVER RAWS          -50 F      63.65N/151.64W     1050 AM 12/27  
GALENA                       -49 F      64.73N/156.93W     0756 AM 12/27
SEVEN MILE RAWS              -49 F      65.94N/149.85W     0219 AM 12/28     
CLEAR SKY LODGE NWS COOP     -46 F      64.25N/149.18W     0806 PM 12/27 (OFFICIAL)
UMIAT AIRFIELD RAWS          -46 F      69.37N/152.14W     1023 AM 12/28  
SHUNGNAK                     -45 F      66.88N/157.17W     1117 AM 12/28  
SALCHA RAWS                  -45 F      64.59N/146.15W     0852 PM 12/27  
UAF TOOLIK FIELD STATION     -45 F      68.63N/149.60W     1100 AM 12/28  
AMBLER                       -44 F      67.10N/157.85W     0456 AM 12/28
EIELSON AFB                  -44 F      64.67N/147.10W     0857 PM 12/27 (OFFICIAL)  
FORT WAINWRIGHT              -44 F      64.83N/147.62W     1013 PM 12/27 (OFFICIAL)
NENANA                       -44 F      64.55N/149.08W     0922 PM 12/27 (OFFICIAL)      
NUIQSUT                      -44 F      70.22N/151.00W     0353 PM 12/28 (OFFICIAL)             
MCGRATH                      -43 F      62.97N/155.62W     1213 PM 12/26 (OFFICIAL)      
BUCKLAND                     -42 F      65.98N/161.15W     0811 PM 12/27  
MINCHUMINA                   -42 F      63.90N/152.32W     0256 AM 12/27  
DEADHORSE                    -41 F      70.20N/148.47W     0253 AM 12/28 (OFFICIAL)     
GOLDSTREAM CREEK NWS COOP    -41 F      64.89N/147.88W     0954 PM 12/27 (OFFICIAL)          
FAIRBANKS AIRPORT            -40 F      64.80N/147.85W     0553 PM 12/27 (OFFICIAL)  

The "preliminary" label attached to the observations has some significance in this case, because there's serious doubt concerning the -60°F at Bettles.  The problem is that the ASOS thermometer reported a minimum of -57°F, but the climate observation was manually adjusted down to -60°F.  Here's the proof... the original midnight (actually 23:53 AKST) METAR was available by 23:55 as follows (I've highlighted the 24-hour minimum, -49.4°C or -57°F):

METAR PABT 280853Z 00000KT 10SM CLR M48/ A2983 RMK AO2 SLP148 T1483 414831494 58022

Then two minutes later another 23:53 METAR became available with a manual correction:

METAR PABT 280853Z COR 00000KT 10SM CLR M48/ A2983 RMK AO2 SLP148 T1483 414831511 58022

The ASOS instruments did not measure a temperature below -57°F; this was a manual correction by the FAA observer based on some other information.  NOAA/NCEI may decide to throw out the correction; it's obviously a small difference, but it has some significance in view of the potential for a new December cold record at the site.

On another note, with changing pressure patterns across the state, extreme cold in certain areas has given way to extreme wind chills in others.  For instance, Deadhorse this morning reported a temperature of -38°F with a 21 mph sustained wind, equating to a wind chill of -72°F.  Perhaps surprisingly, more than half of winters in Deadhorse do not see a wind chill this low; and only once in recent decades has the wind chill dropped below -80°F (January 23, 2012).  The record low wind chill prior to the New Year in Deadhorse is -75°F.

Conditions have been nasty in western and southwestern Alaska, too.  Bethel reported -18°F with 28 mph winds yesterday afternoon, for a wind chill of -49°F.  Again, this is very unusual for this early in the winter; in the ASOS era (1998-present), the record low wind chill for December is -56°F.

Here's the 3am surface/MSLP analysis from Environment Canada (click to enlarge).

Finally, it was interesting to see temperatures jump up in the coldest parts of the interior last night as winds broke through the inversion and mixed down warmer air.  Here's an example from the Chalkyitsik RAWS near Fort Yukon.

Friday, December 27, 2019

Cold Returns to Alaska

A fresh wave of Arctic cold has sunk south across Alaska in the past couple of days, and surface temperatures have plummeted in areas where skies have cleared.  Sub-minus 50°F was observed today in a wide range of locations from the Kuskokwim valley upstream of McGrath, northward to the Koyukuk River drainage, and east to the usual cold spots in the northeastern interior.  Fairbanks held off the cold for a while but has dropped to -40° this evening.  Click to enlarge the map below.

The coldest report was from a cooperative observer near Eureka, just northeast of Manley Hot Springs, where the thermometer apparently registered -65°F.  This is the lowest observed temperature in Alaska in nearly 8 years.

Another notable data point came in from the high-quality CRN site in the Nowitna NWR (Ruby 44 ESE); the temperature dropped to -57°F, which is the lowest temperature observed so far by any CRN site in the nation (in any year).  Most of the CRN sites in Alaska were installed in the last 10 years; the Nowitna site didn't start reporting reliably in winter until two years ago, but nevertheless it already held the CRN record with -54°F in January 2018.

Finally, it's worth noting that Bettles has not risen above -55°F since midnight, and if this holds through midnight tonight it will be the coldest December day in Bettles' history (data since 1951).  The last time the daily high temperature was -55°F or below was in January 1993, and it's never happened outside of January (again, since 1951).

Satellite data indicate that the heart of the cold was over the Kanuti Flats south of Bettles; this is certainly one of the coldest parts of the interior (try searching for "Kanuti" on this blog).  Below are a couple of images showing the lowest temperature estimated by the radiometer on the Suomi polar orbiting satellite today; I've marked several of the sites of interest, and the second image is a zoomed-in view with a different color scale.  Note that white indicates missing data.

The lowest estimated temperature in the satellite grid is -64°F on the Kanuti Flats, which seems very plausible in light of the nearby observations.  Allakaket, which is usually colder than Bettles, reported as low as -60°F and hovered around -58°F all day.

On the other hand, the satellite data don't seem particularly supportive of the -65°F near Eureka (MLYA2), although there's no reason to think it's physically implausible.

Here's a simple animation of the webcam view from the Allakaket airport today. It's interesting to see the slight and variable air movements in the half-light of a near-solstice day right on the Arctic Circle.

And for the record, here's yesterday morning's 500mb analysis showing the main culprit for the cold: the intense Arctic vortex north of the Alaska-Canada border.  Based on sounding data from Utqiagvik (850mb temperatures below -32°C), the air mass is the coldest prior to the New Year since 1983.  From a scientific perspective, it's nice to see something different after the pervasive warmth of recent months and years.

Monday, December 23, 2019

Cold and Dry

The past few days have brought a fairly notable cold spell to the eastern and northeastern interior, with the first sub-minus 50°F temperatures of the season emerging at the usual cold spots.  Today is the fifth consecutive day with a daily minimum of -50°F or lower at the Chalkyitsik RAWS site near Fort Yukon, and this appears to be the most since January 2010 for that particular site (although there's a lot of missing data from 2012-2015).

It's been pretty chilly in Fairbanks too, with daily low temperatures below -30°F for three days, and a high temperature of -30°F on Saturday.  Of course solar heating is essentially zero at the winter solstice.  As an aside, here's a link to a very attractive solstice video from the other day.

It has been a little unusual to see cold like this at valley level without colder conditions aloft.  According to the Fairbanks balloon soundings, the 850mb temperature did not drop below zero Fahrenheit; this temperature is colder than normal at 850mb, but not dramatically so - a colder air mass prevails about a quarter of the time in deep winter (per data since 1981).  In contrast, only about 5% of days see a high temperature of -30°F or lower in Fairbanks at this time of year.

This means, of course, that the surface-based temperature inversion has been strong; see Saturday morning's sounding below.  The surface-850mb temperature difference has been about 20°C in recent days, and this is exceeded on only about 7% of days in December and January - and more often than not it happens when conditions are warmer than normal aloft.  Big inversions tend to occur under high pressure, when clear skies and calm winds allow for strong radiational cooling, and at the same time subsidence aloft produces relatively warm, dry air above the inversion.  In contrast, cold air aloft tends to occur in association with a trough, which also typically brings cloudier and less calm conditions that don't promote a strong inversion.

So the unusual aspect of recent days has been the juxtaposition of relatively clear and calm conditions with a cool air mass, allowing surface cold to "outperform" over the northeastern interior.  One important reason for the absence of cloud is that the air aloft has been very dry indeed; the total column water vapor ("precipitable water") dropped to about 1mm late last week, which is down in the extreme lower tail of the historical distribution.  Less than 2% of soundings are this dry in Fairbanks in December and January.  The dryness of the air probably has to do with its origin in the high Arctic, as it seems to have been transported southward on the west side of a persistent and intense "polar vortex" near Canada's Banks Island.  This is illustrated below by the 500mb charts from last Friday morning and this morning respectively, courtesy of Environment Canada:

If we look at the history of precipitable water and surface-850mb inversion strength in Fairbanks, there is an inverse relationship, and this stands out particularly clearly when cold air is in place - see the figure below.  The blue markers show precipitable water versus inversion strength for the lowest 10% of 850mb temperatures in December and January; notice the uniformly low water content when the air is cold both aloft and at the surface (i.e. blue markers and strong inversion).

In conclusion, the combination of very cold and very dry air aloft is a recipe for extreme cold at the surface in Alaska's interior, and the last few days fulfilled the dry aspect of this.  There's a chance the "very cold" part may show up too in the near future; and this would be quite a shock to the system after all the warmth of recent years.   At the very least, the coming weekend looks to bring the coldest weather of the season so far.

Tuesday, December 17, 2019

Bering Ice Loss Modeling

Last week Rick Thoman, the founder of this blog and still occasional contributor, announced the publication of a paper of which he is lead author, entitled, "The Record Low Bering Sea Ice Extent in 2018: Context, Impacts, and an Assessment of the Role of Anthropogenic Climate Change".

The brief paper is well worth a read for those interested in Alaska climate; Rick and co-authors document various aspects of the unusual winter of 2017-2018 in the Bering Sea, with a focus on the extreme lack of sea ice.  As we discussed on this blog at the time (for example, here), late winter Bering Sea ice extent was far below previous records in the modern satellite era, with both pre-existing ocean warmth and extremely persistent southerly flow contributing to the anomaly.

The most striking conclusion of the paper arises from an analysis of climate model output, which indicates that the very low ice level of two winters ago "will likely by typical by the 2040s", i.e. barely more than 20 years from now.  This result is obtained by applying a bias correction to the climate model's ice predictions over the satellite era, and then finding the frequency with which the 2018 ice shortfall is surpassed in the model forecasts for future decades.  It should be noted that the model simulations use the RCP8.5 climate forcing, which is a high-emissions scenario that some suggest is unlikely to occur, although of course this is just one aspect of the uncertainty (see the following article for discussion of RCP8.5:

One point that is dealt with briefly in the paper, but may be worthy of further consideration, is the apparent discrepancy in Bering Sea ice trends between the climate model and the real world.  Despite the drastic drop-off in ice extent in 2018 and 2019, the 41-year downward trend is not yet statistically significant, and in fact prior to 2018 the trend was upward; it was even statistically significant in the upward direction for a while (e.g. 1979-2013).  See the figure below for the observed January-April ice extent.  According to the paper, most of the model's simulations (35 members of a 40-member ensemble) have 1980-2018 declining trends that are greater than what has really happened.

Does this mean that the model is not realistic?  Not at all.  As Thoman and co-authors explain, the variability of Bering Sea ice on time scales of a few years ("subdecadal-scale") is essentially random, and this means that the observed 41-year trend is only one of a wide range of trends that could have occurred.  In other words, the real world is only one of an infinite number of possible realizations based on the same physics, and the climate model suggests that the actual Bering Sea trend - with its lack of decline until recently - happens to have been an unlikely one.  Obviously it's very important to verify that the model is in fact simulating sea ice properly, and the paper cites other studies showing that this is the case.  And of course it goes without saying that sea ice loss in the Arctic as a whole has been extremely statistically significant, unlike the relatively small Bering Sea basin.

All in all - congratulations to Rick on the publication - this is good work.

Tuesday, December 10, 2019

Chukchi Beach

It's been a dramatic turnaround for temperatures in Alaska over the past few days, as the brief seasonal chill was quickly replaced by highly unusual warmth once again over much of the state.  Deep southerly flow has pushed warm air all the way up into the Arctic, and temperatures have been widely above freezing in western and southern parts of Alaska since yesterday.

As an illustration of the unusual conditions, the village of Kivalina on the Chukchi Sea coast has been above freezing (up to 39°F) with a strong offshore breeze since yesterday, and the FAA webcam shows open water along the coast.  It's striking to see the afternoon sun poke just slightly above a horizon of open water; tomorrow's sunset will be the last of the year in Kivalina.  For reference, the 1981-2010 climate normal points to a typical daily mean temperature of +5°F on this date.

Here's a view of sunrise today in Kivalina at about 1pm local time:

And here's a simple animation of the beach scene.

Thursday, December 5, 2019

Cold Spell

It won't last long, but interior Alaska is seeing conditions worthy of early winter today, with temperatures dropping below -30°F in the usual cold spots and below -40° in some locations.  Chicken dropped to -44°F this morning, and the Chalkyitsik RAWS reached -45°F.  Fort Yukon hovered near -40° all day, and Tanana and Eagle both reached -36°F.  Here's a map of minimum temperatures from midnight through mid-afternoon (click to enlarge):

Here's the view from UAF at close to solar noon, with a mere 3° of solar elevation above the horizon.

Up in Fort Yukon, polar night is now only a little more than 100 miles to the north, and this was today's view during the sun's brief traverse of the southern sky:

Clear skies and calm winds are the main ingredients for this kind of "normal" cold spell; the 3am surface analysis shows a modest anticyclone over northern Alaska.

The air mass aloft is chilly, but not especially cold; for example, the 850mb temperature over Fairbanks was just below -20°C this morning.  Based on 1981-2010 data, temperatures this low or lower are typically observed over Fairbanks about 15% of the time in December and 20% of the time in January; but in the past 10 years it's been more like 10% and 15% respectively.  Real cold tends to show up when 850mb temperatures get closer to -30°C, and that's a rather rare occurrence these days.

Here's the 6-10 day forecast from CPC; enjoy the cold, if you can, while you can.