September 04, 2013

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.


  1. Thanks. I was just wondering what type of winds we might encounter. Battening down the hatches.

  2. yikes! get out the maybe the boat!

    Is the Forest Service worried about lightning strikes starting fires near here?

    Thanks for the heads up!

  3. this a taste of what our fall/winter may be like? I hope so! I love stormy weather!

  4. Any potential for Tornados? Seems like we're looking at major thunderstorm here and tornados are definitely associated with major storms.

  5. Here in the Salem area a couple of bright lightning flashes around midnight with loud thunder then nothing more.

    This weather system is like an 80 year old man trying to get up in the morning being very slow and puttering around the house. (No offense)

    I bet faster then you can say *Ka boom!" this storm system will fall apart and Cliff will have to add an explanation of what went wrong or right for the people who don't like extreme weather.

  6. I DO hope we get a snowstorm version of this kind of weather with howling east winds.

    BTW: Why is this year and last year so dang hard to get those easterly gradients forming?

    Recently just a little bit and PDX often runs as cold as rural stations canceling out UHI effects.


  7. Normandy Park, WA

    Fairly continous thunder from 530am-830am


  8. Lots and lots of heavy thunder and lightening here, though not a lot of rain. South Whidbey

  9. i've been watching the TPW and it sure looks like an atmospheric river to me!!! it also feels very tropical and has done all summer, with the high humidity indexes. i am curious how you concluded there is no atmospheric river activity. i have seen a lot of AR systems hitting the states this season, as well as a lot of flooding in those regions, and i know it ain't glacier melt. so what gives? -portland

  10. I really like this kind of weather.


Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

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