Unusual Heavy Rain Event to Hit the Northwest

 Update 8:30 AM Thursday:  A line of thunderstorms is now moving westward across Puget Sound (see radar).  Should be through by 9-9:30 AM and then dry after that for several hours until the big action comes in later today.

 Later on Thursday and into Friday morning, a very unusual heavy rain event will strike the region.  Undoubtedly, some locations will break daily records, scattered urban street  flooding is expected, and some areas in the Cascades may well get 2-4 inches.

Right now (see graphic), the National Weather Service has much of the northwest U.S. in either a flood warning (dark green color) or a red flag warning (red color), the latter due to worries about lightning-caused fires.

NWS Warnings

The heavy action over western Washington really doesn't start until late on Thursday afternoon  (I am going to bike to the UW!).   Below I will show you the UW WRF model forecasts, which have been fairly consistent both between runs and with other modeling systems.

 Tomorrow afternoon the air is going to get very, very unstable over the region.  To illustrate, here is an important measure of the instability called CAPE: Convective Available Potential Energy.  Values are uber-high.   Normally we don't get above a few hundred.  Tomorrow, it gets to near 2000 in places.  This indicates the potential for strong thunderstorms if lifting occurs...and it will.

The 24-h precipitation ending 5 PM Thursday (shown below) indicates convective showers over much of the Northwest, with some areas getting .5-1 inch.   This folks is the "warm up".

The next 24h, bring rain Armageddon (see next map).  Just amazing amounts, broad areas receive 1 to 2.5 inches, with some areas, particularly near the southern Cascade crest into southwest Washington being hit by 2.5-5 inches of rain.  Such a broad area of heavy rain is very unusual for September.  And it is not associated with an atmospheric river from off the Pacific.

Here is a blow-up of the same map over western Washington.  Amazing amounts...even some 5-10 inch over Mt. Rainier.  A strong east-west and north-south precipitation gradient.  Tatoosh Is. would be an excellent place to go to stay dry.  The Cascades will be brutal, with the possibility of slope failures from such heavy precipitation.

The 24-h precipitation for the period ending Saturday at 5 PM shows drying west of the Cascades, but intense showers over the Cascade mountains and northeastern Washington.

 Forecasting thunderstorms is a tricky matter, but considering the consistency of the model predictions, something pretty major is going to happen--so be forewarned.

The origin of all this fun is an unusual cold-core low that slowly moves over our area (see the 500 hPa map on Thursday evening at 8 PM to illustrate)

Annoucement:  My Public Lecture Series on NW Weather

I am giving a five-lecture evening short course: "Reading the Northwest Sky: Understanding Our Weather and Climate"  

October 1, October 22, November 5, November 26, December 3  
Kane Hall: University of Washington  
Co-Presented by University of Washington Alumni Association   and Seattle Public Lectures.

If anyone is interested, more information here.