Strong Winds and an Atmospheric River Hit the Northwest

Strong winds, some exceeding 50 mph, are hitting some parts of the Northwest today.  And a moderate atmospheric river promises substantial rain in the mountains.

The maximum wind gusts over the past 24 h, shows a fascinating picture of very strong wind in some locations, while lighter winds are evident in others. 45-55 mph gusts in the San Juans and along the coast.  And 40 mph gusts east of North Bend as one ascends towards Snoqualmie Pass.   But light winds around Puget Sound.

Such huge differences result from our terrain and land-water contrasts.

A forecast from the UW WRF modeling system of sustained winds (not gusts) for 10 AM this morning shows the pattern (see below).  In particular, look at the strong winds from the eastern Strait of Juan de Fuca across the San Juans. 

Why are they there?  Blame the Olympic Mountains!   The sea level pressure forecast is shown by the light brown lines.  There is relatively high pressure on the windward (south side) of the Olympics and a leeside pressure trough on the northern side.   This pressure pressure pattern results in a strong pressure difference on the northeast side of the Olympics that accelerates the winds from the southeast.  And strong winds are found on the western side of Snoqualmie Pass because there is a large pressure difference across the Cascades and air is accelerating towards lower pressure to the west.

Smith Island, just west of Whidbey Island, has had recent gusts to 48 knots (SISW1) and  similar winds are being experienced along the coast, such as at Destruction Island (DESW1).   Destruction winds are particularly vicious because of the Olympics as well, since the barrier enhances the pressure gradients in the area.

But winds are only part of the story today.   A moist flow, known as an atmospheric river, is heading right into us, as shown by the infrared satellite image at 7:30 AM (below).  A really juicy frontal cloud band that stretches well into the Pacific.

We can see the plume of moisture clearly in a short-term forecast of total atmospheric water vapor in vertical column of air:

The resulting precipitation over our region during the next 24h  will be substantial, with parts of the Olympics and north Cascades getting over 5 inches of liquid water equivalent:

Note the light precipitation to the NE of the Olympics?   A world-class rainshadow is happening as the air sinks on the lee side of the barrier.  Here is the latest weather radar image--the rainshadow is spectacular:

And then mid-week the jet stream goes south and California gets hit hard by precipitation and mountain snows.  Fire season is over for central and northern CA.

from Cliff Mass Weather and Climate Blog https://ift.tt/2FHNELa