Friday, 11:30 A.M.
July and much of August were very dry in the Northwest. Since late August, though, there have been two healthy systems roll through the region, making the last few weeks as a whole wetter than average. Despite the added rain, it has remained warmer than normal. That's all about to change.
One front is rapidly closing in on the coast. A check of the Friday morning IR imagery:
There will be some rain with and behind the front later this afternoon into tonight, followed by a cool, unstable kind of day tomorrow from Washington to northern California as the upper-level trough swings inland. Here's the 12z Sept. 20 NAM 500mb forecast:
As this trough swings farther inland tomorrow night and Sunday, it will spread showers and a few thunderstorms across the Intermountain West, reaching the Continental Divide Sunday. The attendant cold front will bring some cooling across the Rockies Sunday and Sunday night into Monday, but this upper-level trough will be lifting northeastward Monday to reduce the amount of cooling that ever reaches the Plains.
Meanwhile, back in the northeast Pacific, another storm will roll into the Gulf of Alaska, and it won't be a small one. Here's the 12z NAM surface forecast for Saturday evening, near the point where the storm is apt to be at its strongest:
It appears likely there will be a nice jet stream on the south side of this storm, and it will take dead aim on Vancouver and the Olympic Peninsula. This should crash inland with a burst of heavy rain and strong winds Sunday, lowering temperatures even more. The main storm will stay in the northeastern Gulf, southwest of Yakutat, Alaska, through Sunday before starting to weaken Sunday night and Monday.
Following that onslaught of rain and wind, another upper-level disturbance will roll into the Gulf of Alaska from the west, eventually rolling a deepening upper level trough into the Northwest Tuesday. Here's the 6z September GFS 500mb forecast for Tuesday evening:
As cool as it will be following the first couple of disturbances, it will be that much cooler over a much broader area Tuesday and Wednesday as the trough dives into the West and plows inland. Yes, this means some snow for the Cascades and northern Sierra in time, and some of the higher ground all the way to the Divide will eventually see some snow out of the trough as it passes. Here's a projected look at the temperature anomalies for Wednesday:
As heights rise from the west starting Thursday, the chilly air will move more over the Rockies, while the showers dry up along the West Coast and it begins to warm. That warming trend will continue the weekend, but not after the week as whole will be destined to feel more like mid- to late October!
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