Wednesday, February 7, 2018

Warmth to Return

The southern and eastern interior of Alaska has seen a decent spell of cold weather in the past couple of weeks, but it's on the way out now, and unseasonable warmth looks likely to return in the near future.  The chart below puts the recent cold in perspective relative to the anomalous warmth earlier in the winter; notice how few days have been more than 1 standard deviation below normal, compared to the many days of 1SD or more above normal.

As chilly as the recent spell may have seemed, the departure from normal for the past 2 weeks is "only" 14°F, which is quite modest for interior Alaska; the 2 weeks ending December 19 were over 23°F above normal.


The impending shift back to warmth is partly related to a very dramatic weather event that is unfolding in the stratosphere: a "sudden stratospheric warming" (SSW).  In these events, which occur a few times a decade, there is a weakening and disruption of the winter-time vortex of westerly winds that usually prevails in the stratosphere above the Arctic.

In some SSW events the vortex is merely weakened and displaced away from the pole, but in the more dramatic cases the vortex splits or breaks down completely and the flow reverses to easterly around the pole for a time.  The upcoming SSW will be of the latter variety; the two maps below, courtesy of tropicaltidbits.com, show the change over the course of a week beginning last Sunday.  The second map shows that the vortex will split into two daughter vortices by this Sunday, with high pressure and anticyclonic flow in the middle.



These events are followed very closely in the long-range forecast community, because the disruption of the vortex usually works its way down to the troposphere and leads to a weakening of the westerly flow closer to the surface in the subsequent weeks.  This in turn often allows blocking high pressure to set up and cold air to spill south into the mid-latitudes, and in particular Europe and western Asia have a strong tendency to be cold in the weeks following a SSW.  Conversely, the blocking pattern tends to favor unusual warmth in southern and interior Alaska as low pressure sets up over the Bering Sea.

Of course the SSW isn't the only driver of the current circulation pattern, but the latest long-range forecasts are certainly quite consistent with the expected SSW impacts.  Here are the latest MSLP and temperature forecasts for the North Pacific sector over the next 6 weeks from NOAA's CFSv2 model; the maps show the ensemble mean forecast anomaly of MSLP (left) and temperature (right) relative to a 2000-2016 normal.  Based on this forecast, it wouldn't be a surprise to see some record breaking warmth in Alaska in the next several weeks.

Week 1


Week 2


Week 3


Week 4


Week 5


Week 6




13 comments:

  1. This is good news Richard. Thanks for the outlook. It's not been extremely cold in the valley near Fairbanks, and even warmer in the surrounding hills above the temp inversion. The higher terrain unlike the valley is often subject to a slight breeze.

    What has been problematic is the unhealthy air pollution trapped in Fairbanks and especially North Pole to the east. It stinks like vehicle and stove oil fumes, and during mid-day a visible blue haze covers the town and nearby airport. The haze can often be seen in the image of town in the link above.

    Gary

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  2. Forgot link of air quality and visible haze: http://fnsb.us/transportation/Pages/Air-Quality.aspx

    Gary

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    1. Within the link above is this Fairbanks pollution data. If interested note the increase and peak of PM2.5, PM10, and CO February 7th. Wind data and temps are included:

      http://dec.alaska.gov/Applications/Air/airtoolsweb/Aq/Station/17

      The warming and windy trend Richard suggests might help reduce the inversion and decrease local air contamination. It's not just wood burners that are the recent source in my opinion.

      Gary

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    2. Thanks for the link Gary. That's some distinctly unhealthy air. It's obvious that despite the sunshine and warming by day, the inversion is keeping a tight lid on the pollutants. Presumably the situation is even worse in some of the larger high-latitude cities of the world.

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    3. During these colder times (now -28F) with clear skies and little wind, dwelling heat source exhaust rarely rises above tree heights (~50-75') and flow is typically North to South. Even the major coal burning power plant exhausts don't get much above 300' until afternoon when the inversion weakens, presumably due to solar heating and resulting air movement until sundown.

      We've not had ice fog nor extreme cold during recent winters yet the inversion remains and constrains the contaminants near the valley floor.

      Gary

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    4. Final comment on the current Fairbanks/North Pole air pollution. Here's a link to EPA's air quality standards: https://www.epa.gov/criteria-air-pollutants/naaqs-table

      Compare with this data offered with qualifications for 2/8/18 and note any air pollution problem: http://dec.alaska.gov/Applications/Air/airtoolsweb/Aq/

      Gary

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  3. This is really unsettling for Iditarod. If the forecast holds true, I wouldn't be surprised if a series of Chinook events destroys what little snowpack there is on the lee of the Alaska Range, forcing the restart to move north to Fairbanks once again.

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    1. It does seem like a possibility. Snow depth of 16" at McGrath is well below normal (but similar to recent years); presumably locations closer to the mountains are not in good shape.

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    2. So it seems the southern route is confirmed for this year.

      https://twitter.com/FDNMoutdoors/status/962076966668009477

      Let's hope it proves to be a good decision.

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  4. When would you expect the models to reflect this SSW event in the 500-250mb range? I don't see anything too dramatic yet...certainly lots of transitory ridging over mid Alaska but similar to what we've been seeing a lot this winter, it seems. If a blocking pattern does set up it could mean more cold outflow winds for SE AK depending on exactly where the ridge sets in. I'd vote for some more snow for everyone before any of this happens. Excellent post, thanks.

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    1. Thanks Jim. There are some indications of a strong ridge returning to 150-160°W around Feb 17-19, but predictability is rather low for the North Pacific at that lead time. It seems the models are not rushing to develop much Arctic blocking for at least 7-10 days, so we'll have to wait and see if and when it shows up.

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  5. The Alaskan climate's changing. Adapt, improvise, overcome. Our mutual experience is minimal in time.

    Some have noted how mild this last cold snap was compared to previous events, especially those that live elsewhere or in the warm hills nearby Fairbanks. I say good to the event...I recall mid-1960's on and that Deep Cold doesn't require a repeat.

    For the rest of Alaska I hope they can learn to adapt. For some their predecessors surely did as climate evolved during the end of the last Ice Age 11,700+- yrs ago when the NA glaciers melted and the oceans rose.

    Gary

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