Big Pattern Change

What a change a few days can make.  A dramatic onset of westerly flow has brought much cooler weather to western, interior, and northern Alaska, and substantial rains have provided major relief for the wildfire situation in many areas.  All burn suspensions have been lifted statewide.

Yesterday was really quite chilly in the interior: with cloud and rain, the high temperature was only 60°F in Fairbanks and 55°F in Bettles.  These are distinctly unusual conditions for this early in July, but would be less so in the second half of July.  Rain totals in the past couple of days exceeded 3" in some spots in the hills around Fairbanks:

Earlier in the week, even higher rain amounts fell in northwestern Alaska: as much as 4-5" was measured in the hills to the north of the Kobuk River delta:

Well over 3" fell in 12 hours at the ASIK RAWS site, elevation 1329': it was an onslaught of heavy, wind-driven rain on Tuesday afternoon.

The immediate explanation for this pattern change is the development of a strong trough on Alaska's side of the Arctic, bringing strong westerly flow across central and northern Alaska.  Furthermore, a powerful ridge has developed over the Aleutians, boosting the north-south pressure gradient that's driving the westerly flow.  Here's the 500mb chart from 3am today, courtesy of Environment Canada:

The estimated 500mb height of 597dm just south of the Aleutians is extraordinarily high and may be an all-time record for the location; I'll need to look into this.

It's interesting that a westerly episode of this magnitude has developed so early in July, when we typically think of late July and August as the time for onset of rainy, cool weather across interior Alaska.  The chart below shows estimates of the normal seasonal cycle of 700mb westerly flow across the 150°W meridian between 60 and 70°N, i.e. a north-south line from near Seward in the south to roughly Deadhorse in the north.  Two 30-year periods are shown: 1961-1990 and 1991-2020.

This confirms that westerly flow tends to see a seasonal minimum around late May and ramps up quickly to its seasonal maximum in August.  There seems to be a hint that recent decades have seen weaker August westerlies, but this difference may well be within the random error from a 30-year sample size.

In a subsequent post I'll take a look at the historical variability of "westerly onset" timing.