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.
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 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.
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.
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.