A World of Difference

The last 3-4 weeks have seen a remarkable turnaround in weather fortunes for much of Alaska, with dramatically wetter - and considerably cooler - conditions bringing a very welcome end to the wildfire season.  Compare the two 30-day precipitation anomaly maps below: the top one showing the situation as of a month ago, and the latest analysis on the bottom.

 

On July 5, 18% of Alaska's land area was classified as having moderate or severe drought, according to the U.S. Drought Monitor, but now drought remains in only a small area (less than 1%) just to the south of Fort Yukon.

Reader Gary commented that "it seems like August in July for Interior Alaska", and indeed the change to notably wetter and cooler weather is very typical for August.  I've pondered whether it would be possible to identify a sudden regime shift that marks this transition in many years; it would be nice to measure the change in an objective way, and look at how the transition varies from year to year and from decade to decade.  For example, has it become earlier over time?  Any suggestions from readers would be welcomed.

In the absence of objective criteria to define an August-like "regime shift", I thought it would be interesting to look at the timing of the warmest week of the year in Fairbanks: see below.  Early July is the most common time for the warmest week to occur, like this year, but there is a lot of variability, with some years seeing the warmest week in early August or before the middle of June.  There's a hint of a trend towards earlier peak warmth, although it's certainly not statistically significant.

A much more significant trend is evident if we look at the wettest week of the year in Fairbanks, based on rain amounts only (I've excluded weeks with any measurable snowfall).  The timing of peak rains really has changed a lot: prior to about 1970, the wettest week was more often in August or September than in July, but in the past 20 years it has become rather unusual to see the peak in August, let alone in September.

From this perspective, then, it does look like the transition from dry spring weather to wet "late summer" weather has become earlier over time.  But we should also recognize that changes in July rainfall have been dominated by an increase of infrequent but heavy events, as discussed elsewhere on this blog (e.g. here), and this is not necessarily the same thing as the August wet regime arriving earlier (i.e. July rains are more convective, whereas August is typically more stratiform).

Another intriguing angle on the timing of the regime shift is found in the midpoint date for the wildfire season: see below for one of Rick Thoman's excellent Twitter graphics.  Only 30 years of data are available, but there's a hint of a trend towards earlier fire season midpoints.  Interestingly this season was a big one (over 3 million acres burned), and yet its midpoint was on June 30 (assuming acreage does not increase much from here).  No other year in the series had both a large acreage total and a relatively early midpoint.

That's enough speculation for one post.  But for the sake of completion, here's the timing of the coldest week in Fairbanks.  Given the profound warming throughout the Arctic in autumn and early winter, it's no surprise to see that it has become less common for Fairbanks to have the winter's coldest weather before the turn of the year.