Atmospheric River Blasts PNW, Northern California to be Brushed by Front

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While northern California has been seasonable for early fall, the Pacific Northwest has been blasted with a very moist atmospheric river including leftover/remnant moisture from what was a typhoon in the western Pacific.

September 28th Graphics

The first strong wave slammed into Oregon and Washington Saturday, bringing very heavy rainfall and strong winds, causing large power outages in some cities and towns.

The tail end of this wave will barely scrape across the northern edge of the state through Sunday morning, not making it south of Redding for the most part.

Behind that, the deep “parent’ low off the British Columbia coast will aid in the development of a new shortwave along the strong 140kt+ jet streak. This will slam into the Pacific Northwest Sunday afternoon, sending the associated cold front down south into northern California late in the evening. This front will eventually make it south of Redding during the early morning hours of Monday, however the upslope/orographic flow will assist in the development of some rain in the sierra by the early to mid evening (especially along southwest facing slopes).

(9/29) 00z GFS's 300mb winds and heights valid for Sunday evening, indicating a strong jet plowing into the western U.S. in association with a deep low in the Gulf of Alaska.

(9/29) 00z GFS’s 300mb winds and heights valid for Sunday evening, indicating a strong jet plowing into the western U.S. in association with a deep low in the Gulf of Alaska.

The cold front, in combination with the very moist sub-tropical atmospheric river (precipitable water values in the 1.3 to 1.6″ range), 700mb vertical velocities around 30 – 40 ub/s up the western slope of the sierra, and the generally strong south/southwest flow will create a pretty decent orographic enhanced precipitation event.

In the valley north of around Yuba City, the chance of rainfall will be best, as lift will be weakening the further south it slides. In the mountains, precipitation will be heaviest – especially in Butte, Plumas, Lassen, and Shasta counties. Along the northwest coast near the Oregon border, rainfall will be heaviest than basically everywhere else, though.

Even though lift is expected to weaken significantly south of the northern and central valley, there remains a chance of some showers as far south as Sacramento, perhaps. Valley rainfall totals will be very limited due to rain shadowing from the coastal mountains, while QPFs in the northern mountains/foothills/northwest coast will be decent… perhaps between .75 – 1.5″ in the northern foothills and mountains, with 2 – 5″ from around Crescent City north. In the valley, a trace to perhaps a quarter inch of an inch is possible, though, a quarter inch is quite high for this system.

Things dry out Tuesday in northern California as the jet lifts further north, and things will warm back into the low to mid 80s in most valley locations.

Not seeing any real action in the next 1 – 1.5 weeks on any long range models so far.

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