Weather Notes

Large parts of Alaska have been much wetter than normal recently; here's a map of estimated 14-day precipitation totals (NOAA data) and the "percent of normal" (click to enlarge):

Several longstanding climate observing sites saw near-record rainfall over the weekend, including:

Bethel 1.98" on Friday-Sunday: the highest 3-day rainfall total in the month of July since 1955.

Anchorage  2.28" on Saturday-Sunday: the second highest 2-day total in the month of July; only 2001 had more (July 4-5).

Sitka  2.35" on Saturday, the highest calendar day total in July since 1948.

Of course all these numbers would be less notable in August, when heavy rain storms are less rare.

Wet and unsettled weather is common across southern Alaska in summer when the PDO (Pacific Decadal Oscillation) is strongly negative, as it is now.  Here's a map showing the percent of years with above-normal sea-level pressure in summers with a bottom-10 PDO index since 1950:

And the PDO is certainly entrenched in the negative phase: here's the daily index for the last several years.

Sea surface temperatures are much lower than normal in the eastern Bering Sea, which is both cause and effect of the atmospheric trough (low pressure) that is tending to recur in that area.

The weekend rainstorm was produced by a strong trough over that very location; here's the 500mb chart from 4pm AKDT on Saturday afternoon (click to enlarge):

The deep, strong southerly flow into south-central Alaska is amply illustrated on the 4pm balloon sounding from Anchorage: observe all the wind barbs pointed out of the south on the right side of the profile.

Also notice the very high freezing level - over 11,500 feet above sea level - and the high water content (precipitable water) of well over an inch (PW=3.02cm).  No wonder it rained a lot.

The combination of cloud, rain, and a chilly ocean nearby also produced unusually low daytime temperatures: Anchorage and Bethel haven't seen back-to-back days with such low daily maximum temperatures in July since 1970 (54-55°F in Anchorage) and 1971 (48-49°F in Bethel).  Guess what: the PDO was significantly negative back then too.