Wednesday, October 5, 2016

Heavy Rain to Strike the Southeast U.S. and the Pacific Northwest/N. California

During the next two weeks, both the southeast U.S. and the the Pacific Northwest will get large amounts of precipitation, with some locations forecast receiving more than 15 inches of rain.

Here is the latest 13-day forecast total from the National Weather Service GFS model.    Along the southeast U.S. coast, precipitation will mainly be the result of Hurricane Matthew this Friday to Sunday.   For the the Northwest, a series of strong storms will hit Washington and Oregon, followed by a major atmospheric river that will ram into northern CA next week.

For folks here in the Northwest, the first step will be Friday's storm, with a modestly deep low center crossing Vancouver Island on Friday AM (see sea level pressure map).  Strong N-S pressure gradients will result in substantial winds along the coast (30-50 mph) and over NW Washington.  This is the first major blow of the fall and one that is sure to produce some power failures from untested branches.

Precipitation with this system will be substantial (2-5 inches) over the mountains of southern BC, the Olympics, and northwest Oregon (see below).

Outdoor watering season will be over.  Some showers may linger into Saturday as another weak system moves through in the morning, followed by dry conditions on Sunday and Monday.  More rain will follow Tuesday through Thursday for Oregon to BC.    

But the big story is what is forecast to happen later next week--a major atmospheric river event in which a plume of large moisture values plows into northern CA and southern Oregon, producing huge amounts of rain.  To illustrate, here is a forecast of precipitable water for 11 PM for next Thursday. (Precipitable water is the amount of water produced by condensing all the water in a vertical column of air.)  Large values (2-3 inches) are found in a "river" stretching SW to NE into northern CA.

Imagine courtesy of Weatherbell

As this moist air is forced to rise by West Coast terrain,  heavy precipitation will result   For the 24 h, ending 11 PM on Thursday we are talking about more than 5 inches.

 For the next 24 h, more heavy rain:

This situation will have to be watched carefully, since it has the potential to cause slope failures and flooding over NW California and SW Oregin if it verifies.


andrew said...

Will Seattle be affected by the river hitting northern California next week?

Michael Snyder said...

Its fun watching the model runs show potential for a few wind events coming up in the next two weeks.
Fall is here finally.

nutso fasst said...

NWS forecasts as much as 5.5 feet of snow on Baker by Columbus Day.

iron said...

hopefully this forecast will be more accurate than your last one (2 posts ago). it seems to me that most forecasts these days always call for more rain/clouds/other bad things than we ever get. is this the CYA method, or based on historical weather that no longer exists?

John Marshall said...

A wild evening in Sequim at 10pm... winds from SE gusting above 40mph with horizontal rain. Feels like a good November storm. Not boring.

My three water-loving Labs went to the door, stepped out into the inky darkness, wind and blasting rain, and spun right around and darted back inside to dive into a pile of blankets. Decided they could hold it a little longer. Guess they weren't ready for Fall storms.

You called this one nicely Cliff.