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.

The Atmospheric River Event is Ending--- And ANOTHER One is Predicted

The heavy rainfall is over, but some rivers are still rising.   The most impactful atmospheric river event of the past few decades.

Consider the 5-day rainfall totals below, and be prepared to be impressed.  

Many of the windward slopes of the Cascades and Olympics received more than 10 inches, with some experiencing 15-17 inches.

You will also notice extreme rain shadow areas in the lee (east) of major barriers, which received less than an inch.

Numerical weather prediction models were STUNNINGLY good, as shown by the predicted 5-day precipitation totals by the UW WRF model made last Sunday (see below).  

Close correspondence between predicted and observed rainfall.

The weather prediction world has changed dramatically during the past decades, with virtually every major extreme weather event predicted days before.

The problem is that governments, local agencies, and others are not sufficiently taking advantage of these skillful forecasts to save lives and property.  The media do not understand the profound impact of improved weather prediction technology.  

This problem is compounded by the National Weather Service, whose forecasts and warnings have declined substantially compared to the state of the art.  

 I can provide a half-dozen cases of the NWS failing to provide timely warnings even when model forecasts are excellent, such as this event and the Seattle ice storm of December 2022.

I hate to reveal this, but  ANOTHER significant atmospheric river/precipitation event is in the forecast.

Consider the predicted 72-h precipitation total ending 4 AM on Thursday.  Yikes...pretty wet, with some mountain areas getting 10 inches.  This is problematic, falling on saturated soils and rivers already well above normal.

To address the reliability of this forecast, one should consult ensembles of many forecasts.  Below are plots of forecast precipitation at Stampede Pass in the central Cascades from many forecasts.

All are going for a major precipitation event starting in the middle of next week.  You can take this to the bank.

All of this is a reminder that the most impactful severe weather of the Pacific Northwest is flooding.  Not heatwaves, not wildfires, not windstorms.