Second Summer

It's all change again in Alaska's weather, with summer returning this week for many places.  The temperature rose into the 80s F yesterday for much of the interior, and the next couple of days will be the hottest of the year for many.  The NWS expects Fairbanks to see 90°F, which has only happened 6 times this century so far (2009, 2010, two days in 2013, 2017, and last year).

Here is yesterday afternoon's 500mb analysis, courtesy of Environment Canada.  Instead of a strong Bering Sea trough dominating the circulation over Alaska, we now have a strong west-east ridge with a connection to western Canada and the western lower 48, where heat and wildfires have been running rampant lately.

This has the look of a classic early/mid summer heat wave; it's not as typical of late July (and would be very unusual in August).  It's certainly interesting to see this unfold after a very "westerly" first half of July.  Here's how this year compares to previous years in terms of the Seward-Deadhorse westerly wind speed index that I described a few weeks ago (when the previous pattern change occurred).

By this metric, July 1-15 this year fell in the top 10 for westerly winds aloft.  Not surprisingly, very strong westerly flow at 700mb (approximately 10,000' above sea level) corresponds to suppressed daytime temperatures in Fairbanks, owing to increased cloud and rainfall, as well as the predominance of cool North Pacific air masses.

Is it unusual for such a strong westerly regime to be followed by a heat wave later in summer?  No: the historical data shows no relationship between early July westerlies and the highest subsequent weekly temperature (looking at daily high temps):

And if we look at the highest daily temperature in the rest of summer, it's interesting to see that a strong westerly pattern in early July tends to have at least one subsequent episode with temperatures into the 80s in Fairbanks - unlike some years.

It's possible that the high-amplitude pattern that produces a strong westerly regime in early July is also conducive to subsequent hot weather, at least for a day or two.

In other words, don't write off summer in early July.

Finally, for completeness, here's the July 16 - August 31 total precipitation in Fairbanks, in relation to early July westerly flow.  Strong westerly patterns tend not to be very dry for the rest of summer, but neither are they typically very wet.