Mid-Winter Precipitation Review

We are now solidly into mid-winter--a good time to check on the status of precipitation over the region.

Let's start with the total cool-season precipitation since October 1, the beginning of the "water year" (see below).

The majority of the western U.S. has received above-normal precipitation, with the North Cascades and the Los Angeles area being substantially wetter than normal.

There is one major dry area:  western Oregon.

There has been enough precipitation west of the Cascade crest so that the Yakima Reservoir levels are not only WAY above normal, but GREATER THAN THE HIGHEST AMOUNTS of last year (see below).  Wow.

Temperature is a contrasting story (below).  

Much warmer than normal over the Rockies,  and near normal over western Oregon and the interior valley of California.  Modestly warmer than normal over much of Washington.

The key driver of these patterns of precipitation and temperature has been persistent high pressure aloft over the western U.S, and the persistent trough of low pressure aloft over the eastern U.S.    This is illustrated below, which shows the difference from normal at 500 hPa (roughly 18,000 ft).  Higher than normal pressure is indicated by the yellow;   blue and purple indicate the oppoosite

High pressure is associated with sinking and warming aloft.  It also contributed to dry conditions.  

The complex distributions of precipitation and temperature have created an even more complex distribution of snowpack over the western U.S.

The southern Sierra Nevada is fine, and eastern Washington and the Rockies are just a little below normal.

But the real problem area is found over Oregon and northern CA, where the combination of a lack of precipitation AND warm temperatures has resulted in snowpack areas below 40%.

Looking forward, the latest European Center forecasts are predicting substantial precipitation along the Washington Coast, over southwest BC, and the north Cascades.

But far less to the south, where the real deficiencies exist.

Oregon folks should not panic yet.  

I can remember a number of years that were dry until early February, but made up most of the deficiency in late winter and spring.

An Extraordinary Inversion, Twinkling Lights, and the Return of Rain

This morning, an extraordinary low-level inversion (temperature increasing with height) developed over the western Washington lowlands.

For example,  at 4 AM, the temperature around SeaTac Airport rose from 41F to 56F (15F increase!!) between the surface and 1000 ft.

You could see the large temperature variations at the surface, such as in this plot of the minimum temperatures this morning, with temperatures ranging from 29 to 53F.   Just amazing.

The extreme temperature change in the inversion produced an atmospheric lens that caused lights to flicker when viewed from above.

To illustrate, there is a video taken from above the inversion by Peter Benda, who lives at high elevation in the hills above Bellevue.

The inversion was associated with a transient area of high pressure, which now moved to the east.

In its wake, precipitation has FINALLY moved in, as noted by the latest radar image:

Weather forecast models are predicting one rain event after another during the next few days, with the heaviest amounts in southwest British Columbia and northwest Washington (see totals through Friday afternoon below).

You will need to find your umbrella😊