The Incoming System

 I wanted to provide a brief update on the incoming system.

A strong front is approaching, which will bring modest winds and substantial mountain precipitation...but nothing like we endured last week.

The latest forecast precipitation totals through 4 PM Monday is for "only" 3-5 inches in the Olympics and northern Cascades.

That will drive some of the local rivers up to moderate flow levels, but not nearly as high as last week.

Here is a sample for the Snoqualmie River,  where levels just reach major flood levels.  Also, a much lower peak this time.

Then there will be some winds associated with a strong cold front and the low pressure accompanying it. 

This forecast surface map for 7 AM Monday shows the  pressure pattern (solid lines), winds, and temperature (shading).  A strong front is offshore at this time, with a large north-south pressure difference over western Washington.  That means strong winds from the south.

By 4 PM, the front has moved in, and cooler air has swept into western WA.  A big pressure difference across the Cascades means strong winds descending into eastern WA.   Good for wind energy, bad for power outages around Ellensburg and Leavenworth.

Here in western WA, wind gusts should reach 40-50 mph, as illustrated by the Seattle WindWatch graphic for Seattle (below).

A few outages should be expected in western Washington--tens of thousands, but no more.

Finally, the big upcoming weather story will be SNOW.   Lots of it in the mountains, as illustrated by totals through next Sunday (below).

The Other Weather Disaster Last Week: A Downslope Windstorm

Although the big local weather story this week has been the heavy rain and serious flooding over the Pacific Northwest, there WAS another extreme weather feature:   powerful downslope winds on Wednesday that caused massive tree damage and power outages over the eastern slopes of the Cascades.

Some of the winds exceeded 100 mph from the west.

Leavenworth, on the eastern slopes of the Washington Cascades, was hit particularly hard, with the entire town losing power, forcing the cancellation of the Christmas Lights festival.  Here are a few samples of the destruction:

The maximum wind gusts on Wednesday (from the limited collection of locations with power) are shown below. 106 mph above Lake Wenachee, 112 mph at Mission Ridge, 77 mph on the slopes just above Leavenworth. Much lighter winds over the lower Columbia Basin.

The winds at Mission Ridge (6730 ft) are shown below (the dark blue line indicates the sustained wind, the top of the light blue area indicates the gusts, the bottom of the light blue indicates the low winds during the hour).

Very windy from early Wednesday through early Thursday, with many gusts over 100 mph.   And the winds were exceedingly gusty.

 Were these winds predicted ahead of time?   You bet they were.  Below is the forecast surface winds from the UW WRF modeling system for 1 PM Wednesday.  

Reds are over 70 knots (81 mph).  You can see bands of localized strong winds on the eastern side of the Cascades.  

The NOAA high-resolution model (HRRR, High Resolution Rapid Refresh) was doing the same thing, as shown by the forecast wind gusts at 7 PM Wednesday.

So why such localized winds?    

Answer: There was a downslope wind event in which strong winds approaching a mountain barrier are accelerated as they descend the lee slopes (see schematic below).

Favorable conditions include strong winds approaching the barrier with sufficient vertical stability, conditions that can occur with the passage of the weather system from off the Pacific or the approach of a strong atmospheric river, which occurred during that day.

The eastern slopes of the Cascades are well known for strong downslope winds. In my Northwest weather book, I note several incidents.

Finally, many of you would like to know whether another major atmospheric river is headed to our region.   The answer is yes.

Here is the atmospheric river diagnostic for Monday morning.  A juicy plume heading right into us.

Consider the predicted precipitation totals through next Friday.

Yikes!  Some places will get over 10 inches. On top of saturated soils and rivers that are running high. Some rivers will flood again.

Will discuss in future blogs.