Friday, June 19, 2015

Stagnant Flow Aloft

The blocking high pressure system that we noted on Monday has weakened somewhat and migrated a little to the west and north, but continues to have a strong influence over Alaska's weather.  It is residing near the Gulf of Anadyr, as seen in this morning's 500 mb analysis (see below).

The location of the persistent blocking high is unusual, as the Bering Sea is a climatologically favored zone for low pressure; blocking highs are much more common over eastern Alaska and northwestern Canada at this time of year.

With stagnant flow aloft and clear skies, the seasonally intense solar radiation is generating very warm temperatures throughout the interior and north of Alaska.  Many locations across the western interior have reached at least 86 °F today, including McGrath, Tanana, Kaltag, and Huslia, and even the North Slope is hot, with at least 63 °F in Barrow and 70s in other locations.  The 63 °F at Barrow is warmer than any temperature observed throughout last summer; the highest temperature for 2014 was only 58 °F.

The chart below shows how persistent the unusual warmth has been in Fairbanks since late winter.  Unfortunately - from the point of view of fire weather - it looks like relief will not arrive soon.  The map below shows the forecast 500mb height anomaly from 3 different computer models for 7-10 days from now, and all three show a very similar pattern of continued above-normal heights (and thus above-normal temperature) over central Alaska.

Update June 20: here's a map showing the climatological frequency of cut-off high pressure centers at 500 mb in June.  Northeastern Alaska and northwestern Canada lead the Northern Hemisphere in the frequency of these events, with an annualized rate of over 9 per year, which translates into nearly one per June on average.  This is also the highest frequency of any location in any month of the year.  However, the frequency is a lot lower over the Bering Sea.


  1. Would it be too difficult to look for similar Bering highs in the reanalysis? It would be interesting to see how often this pattern pops up and what the rest of the summer might look like.

    1. Eric, great idea! I did the heavy lifting last year in preparing this analysis:

      I've added the blocking frequency map for June at the end of the post.

      To address your question, I took the area-average of the number of June blocking events for 55-70N, 170E-160W, and the top 10 years were 1968, 1993, 1973, 1975, 2001, 2005, 1966, 1989, 1953, and 1988.

      Looking at the rest of the summer in Fairbanks for these years, we find that 8 of 10 were warmer than normal (based on earlier historical normals) for July, and 7 of 10 were warmer than normal in both August and September. For precipitation, 8 of 10 were drier than normal (median) for the July-September totals and in fact 3 of the 10 years were in the top 4 driest on record for Jul-Sep (1966 - 2nd driest, 1968 - 3rd driest, 1989 - 4th driest).

      Put another way, 3 of the analog years produced a top-10 dry July, 3 produced a top-10 dry August, and remarkably 5 produced a top-10 dry September.

      This is a very robust signal as far as analogs go and obviously it's bad news for the fire situation if it plays out the same way in 2015.

      A quick look at the pressure patterns shows no evidence that blocking tends to persist in the Bering Sea region; there is more of a tendency for high pressure over SE Alaska in July and then in the Chukchi and Beaufort Seas in August and September.

  2. A Summer of Fires? Not much else to say. I wondered a couple of months ago if it'd materialize into another hot fire season with the forecast outlook for warmth, and unfortunately it's developing.

    Now unstable air and any influx of moisture will increase the lightning activity.

    Sometimes winter isn't all that bad in comparison.