'Critical' fire danger for parts of Northwest as unusual September wind storm arrives

SEATTLE -- A potentially historic late summer wind event is brewing for parts of the Pacific Northwest with hot and bone dry conditions already in place, making for critical fire weather conditions across much of the region and bringing on top of it the potential for power outages and wind damage.

The main Puget Sound area will escape most of the weather impacts with the storm's fury concentrated into the Cascades, northern Oregon and parts of Eastern Washington with wind gusts of 50-60 mph (or higher on the exposed ridgetops), but there could be some affects felt on this side of the mountains.

A rare-for-September Wind Advisory is in effect for the Cascade foothills from 8 p.m. Monday through 11 a.m. Tuesday for easterly winds gusting as high as 40-50 mph in places like Enumclaw, North Bend, Gold Bar and Black Diamond -- places that typically see strong east winds in the dead of winter. More on the "winter" connection in a moment.

Red Flag Warnings indicating critical fire conditions are in effect through Wednesday evening along the Cascade foothills -- even below 1,500 feet -- plus the Cascades themselves, the eastern slopes of the Olympic Mountains, the Black Hills and the lowland areas of the central coast and southwestern Washington.

NOAA's Storm Prediction Center even paints part of northwestern Oregon and the Portland/Willamette Valley area in their highest warning of "extreme fire danger" in their Fire Weather Forecast for Monday.

As has been obvious, the area has been in an extended dry streak with less than a half inch of total rain in Seattle since late June. But now the atmosphere is getting into an unusual summer pattern that actually is mimicking a pattern we normally only see in winter. A large ridge in the eastern Pacific has pushed an area of very high pressure into British Columbia and Alberta -- what forecasters at the National Weather Service office in Portland actually tongue-in-cheek called an "arctic high" though it's not that cold, but is acting in a similar manner.

That is in turn pushing up some of the intense heat that's been in Desert Southwest (111 in Los Angeles Sunday!) up the west coast in the form of a monster thermal trough -- which brings an area of lower pressure. Those forces are combining to make for an extreme pressure difference between the inland Northwest and the coast/offshore waters and will make for nearly unheard of east wind event for September - something historical records indicate would only happen 2-3 times a century.

What's worse, those east winds will undergo drying as the rise over and sink down the various mountain ranges (Cascades, Olympics, North Coast ranges in Oregon) and zap the air of much of its moisture, leading to expected relative humidity values in the 10-20% range.

So you have strong winds, extremely low humidity and an area that hasn't seen much rain in several weeks all combining for extreme fire danger. Any fire that starts would have potential for explosive growth in short order. Luckily there are no thunderstorms in the forecast but any man-made spark could become an instant problem.

Even worse in Oregon

While the Puget Sound area will see a warm and breezy day with high fire danger but minus fire issues not too many weather impacts, northern Oregon and areas east of the Cascades are looking at a much more active weather pattern.

High Wind Warnings are in effect across much of northern Oregon, including the north coast and Portland Area, for easterly gusts up to 60 mph -- though perhaps 65-75 mph on exposed ridgetops. In addition, the coastal areas from Newport up to Astoria will likely see high temperatures reaching into the 90s -- places that don't usually have air conditioning and are not typically used to an east wind. Those areas are where the fire danger is most critical.

Eastern Washington and Oregon will see strong winds too with areas of blowing dust an issue. In Mansfield, Washington, crews have issued some evacuation notices but say all roads out of town have extreme visibility issues and are telling residents to shelter in place or go to the high school shelter.

Wind calms Tuesday night

The wind will calm down Wednesday but in exchange the heat will build a little more around the Puget Sound region with highs Wednesday and Thursday reaching into the upper 80s to low 90s. Cooling weather looks to return Friday into the weekend.