The first atmospheric river event of the season is taking aim for the Pacific Northwest, bringing widespread rain to the region beginning on the coast Tuesday night and spreading inland to moisten the ground in the Tri-Cities Wednesday night. It will be backed by a cold front spawned by a low pressure system that will be centered in the Gulf of Alaska. The strength of the system is impressive. I like how fellow WeatherTogether blogger Charlie Phillips put it yesterday in his post on the subject: “This storm would be a strong one for November, let alone September!”
This stands to be the first good soaking rain for the Tri-Cities in some time, with models indicating perhaps a quarter inch of rain falling Wednesday night into Thursday. The last time the airport in Pasco recorded that much rain in a single day was June 13, but precipitation values from summer thunderstorms were highly variable around the area. There have been a couple of instances where the Richland Airport received more than that since June, for example, but for everyone in the Tri-Cities this will certainly be the wettest non-thunderstorm event in months.
The image at the top of this post shows total precipitable water (TPW) from the 12z Monday GFS for Wednesday night from the WeatherTogether models page. TPW is useful in telling meteorologists how much water is in the atmosphere above a single point. This model shows TPW around 1.25 inches in the Columbia Basin overnight Wednesday into Thursday which means that if every bit of water rained out at once, it would accumulate to 1.25 inches on the ground. Much of the water shown here will remain in the atmosphere, but these high values coupled with lowered temperatures and dew points around 50° will let a reasonable amount of rain make it to the surface.
Rain isn’t the only thing headed for the Columbia Basin. Temperatures, especially high temperatures, will be impacted as well. Highs Monday through Wednesday are expected to be in the upper-70s to low-80s, but following the passage of the cold front that figure drops to the upper-60s to low-70s. Lows will remain rather steady in the low-50s. Higher dew points will help to keep lows from dropping too far. Warm temperatures will keep snow levels in regional mountain ranges fairly high with rain even expected in the highest elevations of the North Cascades Highway.
Breezy conditions are expected behind the front, with gusts reaching into the 30-40 mph range. Winds will be highest outside of town and on exposed hilltops where trees and buildings don’t get in the way quite as much.
Following the bulk of the rain event Wednesday night, a broad upper-level trough sets up over the Pacific Northwest. In this, some smaller disturbances (termed shortwaves) will keep rain chances in the picture through Friday, though this precipitation won’t reach the same extent as that overnight Wednesday into Thursday. During this period, 12-hour rain chances are around 30% to 40% in the Tri-Cities, with higher chances closer to the mountains.
Conditions are currently expected to begin clearing out Friday night into Saturday (though some lingering showers are still possible). High temperatures jump from the lower-70s Saturday to around 80° Sunday as the broad trough gives way to a building ridge of high pressure. This ridge may last for a while, though any forecast beyond about seven days out has a significant amount of uncertainty. If the pattern verifies, though, the Columbia Basin can look for above average temperatures to start October with precipitation being shoved well to the north.
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