Chinook Warmth

The middle Tanana River valley broke out of the chilly mid-autumn weather yesterday, with southerly flow bringing warmth across the Alaska Range and producing downslope warming in the lee of the mountains.  Fairbanks airport reached 58°F, which is the highest temperature in October in five years, and the warmest this late in the season since 2006.  Approaching 60°F is rare after the first few days of the month in Fairbanks.

Here are yesterday's maximum temperatures around the region (click to enlarge).  Note how much colder it was to the north of the Yukon-Tanana uplands; low-level cold air is already hard to displace at this date in the Yukon Flats.

The burst of warmth in Fairbanks marked a sharp contrast to an unusual chill in the closing days of September; last Sunday's high temperature of 35°F was the coldest September day in four years.

Despite the warmth yesterday, the SNOTEL instruments in the hills above Fairbanks report that there is still some snow on the ground; snow water equivalent values range from 0.2 inches to 2.0 inches, depending on elevation.  As usual, Munson Ridge leads the way and is above the seasonal normal.

In the 15 years with at least 5 sites reporting snow data in the Chena River basin (2011-present), only 4 other years had snow on the ground at all locations as of October 5, so it's a little bit unusual.  Those years were 2014, 2015, 2019, and 2021.

Much Bigger Chinook

Following on the heels of last weekend's Brooks Range chinook, a much more significant warm event has unfolded across much of the state in the past few days, with deep and strong flow from the south rather than low-level flow from the north.  Here's the 500mb setup at 4pm AKDT on Friday; the big western Canada ridge and Bering Sea trough are classic for a warm surge into mainland Alaska.

 

The result has been one of the most notable warm episodes on record for the time of year; Fairbanks has exceeded 60°F for the past 4 days, with 68°F on Thursday and 66°F on Friday.  This is only the 4th year since 1930 with 65°F or higher in Fairbanks in October.  It's also the first time since the 1930s that Fairbanks has seen a daily low temperature above 45°F after September (the low was 47°F on Friday).

In terms of departure from normal, Friday's mean temperature anomaly of +22°F was one of the largest daily departures from normal on record prior to mid-October.  The greatest, of course, occurred in the extreme chinook of September 1995, when a low temperature of 65°F occurred on the 20th of the month, followed by a high of 78°F the next day.

Readers may recall that I mentioned the 1995 event in this post back in July, when I noted that 1995, like this year, saw its highest temperature of the year in May.  It's interesting that 2020, like 1995, has produced highly unusual warmth in both May and in mid-autumn, while some of the major global climate drivers are similar (developing La Niña during summer, a very warm North Atlantic, and a very active Atlantic hurricane season).

Here's a look at high temperatures on Thursday the 1st (click to enlarge).  That's a lot of 60's, including at Chicken in the southeast interior; the co-op site (not shown here) had 61°F, which is only the second time with 60s in October (previously 64°F on Oct 2, 2003).

And the following map gives a sense of the scale of the departures from normal, based on gridded model data:

Finally, what if the similarity with 1995 persists?  Then we would expect a cold start to winter in the interior; the winter of 1995-96 was colder than normal through January, particularly in eastern and southeastern Alaska.  January was particularly cold in the southeast, with Northway seeing a monthly mean temperature of almost -30°F: about the same as last year, in fact.  Juneau also had their coldest January of recent decades in 1996.

Brooks Range Chinook

Yesterday afternoon the temperature reports across northern Alaska caught my eye, with a striking temperature contrast across the Brooks Range.  Mid-elevation stations on the north side of the divide, such as Anaktuvuk Pass and Howard Pass, saw temperatures around 20°F - nothing unusual for the time of year - but downsloping produced dramatic warming on the south side of the mountains.  Bettles reached 54°F, and the Norutak Lake RAWS hit 56°F, which is very warm for so late in September.

Here's a map of temperature reports at 5pm AKDT - click to enlarge.

A very strong Gulf of Alaska storm explains this situation, as there was a pronounced north-south pressure gradient across the state at 4pm yesterday - see below.  As a result, strong cross-barrier flow occurred over the Brooks Range, and impressive chinook warming was able to develop even though the elevation changes are more subtle than in southern Alaska (Anaktuvuk Pass and Norutak Lake differ by only 1300').  Such is the power of elevation for modifying weather and climate.

Chinook Winds

A wave of warmth has inundated Alaska in the last few days, and chinook winds have amplified the warming in much of the interior.  On Wednesday temperatures rose above freezing at locations spanning the state from west to east, including Nome, Kotzebue, Bettles, Fairbanks, and Eagle.  There have only been 10 previous days since 1952 (the common period of record) when all these stations made it above freezing on the same day in December through February; 6 of these 10 days have occurred since 2009.

The only portion of the interior that has stayed well below freezing is the far southeast, where the proximity of the upper ridge prevented winds from mixing down the warmth aloft; Northway has not exceeded 14°F.

Fairbanks airport reached 45°F on Wednesday, as the southerly chinook winds managed to break through the surface-based inversion for just a brief time in the afternoon; the temperature rose from 34°F at 1pm to 45°F at 2pm, and then dropped back to 30°F at 3pm and to 15°F by mid-evening as the stagnant colder air rolled back in.  A more extended period of gusty warm winds yesterday afternoon took the temperature back up to 40°F with a bit of mixed rain and snow.

Wednesday's fluctuations in temperature and wind speed in Fairbanks are evident in the half-hourly observations from UAF's Smith Lake site, see below.  The second temperature spike, with the larger wind speed spike, is the one that produced the high temperature at the airport, but at Smith Lake the temperature jump occurred between 12 and 12:30 (presumably AKST), i.e. at least 30 minutes earlier than at the airport.  The elevation difference between the two sites is only about 100 feet, but this might explain the delay in warming at the airport as the wind burst took time to mix down.

The relative humidity plot from Smith Lake shows the low humidity of the chinook air - see below.  The humidity fluctuations nicely highlight the contrast between the two air masses, one of them cold, humid, dense, and stagnant, and the other warm, dry, less dense, and moving quickly northward.  The density contrast between two such air masses is so great that they might be thought of as immiscible, like oil and water: they simply do not mix.  The warm air tends to slide over the cold air, and in the Fairbanks area the chinook flow doesn't often make it to the valley floor unless the pressure gradient is large or winds become unusually strong.

Similar charts from the Poker Flat Research Range, about 30 miles north of Fairbanks at ~700' elevation on the Chatanika River, show that temperatures were above freezing for two lengthy periods on Wednesday - see below.  The early afternoon spike in wind speed was also observed at this location (note that the time axis appears to be UTC), but the temperature rise was more gradual as there was less stagnant, stable cold air to displace at the higher elevation.