Rain Returns to a Dry Pacific Northwest

We have just experienced an extraordinarily dry second half of March, one in which weather systems and their accompanying precipitation was directed southward into California.

Well, all good things have to come to an end, and for us the spigot will be turned back on this week.

But first, let me impress you.  Here is the percent of normal precipitation for the past two weeks over Washington State.  The dark red is less than 25% of normal!    No wonder some some media outlets are throwing around the D word (drought!) or predicting a dry spring.

But percent of normal precipitation can be very deceptive, particularly in places or times that are relatively dry (like our summers or east of the Cascade crest).  Far better is to view the departure of precipitation from normal (in inches) as shown below.  Dry eastern Washington was generally less than an inch below normal.   The Puget Sound lowlands, 1-2 inches down.  More over the usually wet western slopes of the Cascades and Olympics.

But everything changes this week as the large-scale atmospheric configuration changes profoundly.  

For nearly two weeks,  there has a deep low over the eastern Pacific, with the jet stream and accompanying storms heading into California.  This is illustrated below by a weather map for 11 AM Wednesday for an upper tropospheric  level (300 hPa pressure level, about 30,000 ft).   Yellow colors indicate the strongest (jet stream) winds. Washington State gets little precipitation from such a pattern.  

But the pattern greatly changes this week, as low pressure retreats into the Gulf of Alaska (the normal position) and the jet stream moves northward right into us (see the forecast map for 8 AM next Sunday)  We get nailed and California finally dries out.

Let me show you the precipitation forecasts from the UW WRF modeling system.  Here in Seattle we have two more days of dry conditions, but Oregon gets hit earlier.   The total accumulated precipitation through 5 PM Tuesday (shown below) predicts that western Oregon will be very wet during the next two days.

One day later (5 PM Wednesday), it is clear that the hose has reached western WA, with the Olympics and north Cascades receiving several inches.

But why stop there?  Here is the accumulated precipitation through 5 PM Sunday.  Wow.  The Olympics and mountains of southwest BC get 5-10 inches, with the rest of western WA and Oregon enjoying 1-5 inches.

We had a cold/snowy February, a dry March,  and now a wet April?  We will see.

But one thing is sure....  you can not believe the long-term forecasts.  Here are the extended predictions of the NOAA/NWS CFSv2 seasonal forecast model run in late February for the precipitation anomalies (difference from normal) for the end of March.   The CFS was going for WETTER than normal conditions over our region for the period....when it was very, very dry.
No skill.   

from Cliff Mass Weather and Climate Blog https://cliffmass.blogspot.com/2019/03/rain-returns-to-dry-pacific-northwest.html