September 16, 2022
In the short term, the very pleasant weather of the past few days will continue through Sunday. A westerly flow prevails in the upper atmosphere and is resulting in much cooler air aloft. At the surface, marine air moves through the Delta from time to time, adding to those temperatures that are finally below average. Models this afternoon have nudged back the timing of our next likelihood of precipitation for the valley. The northern half of the valley could see rain as early as Sunday evening. However, Monday through Tuesday is my current best estimate. A low is already taking shape west of British Columbia and will slide southward right along the coast, centering just west of San Francisco Monday morning. This will generate a strong southwesterly component in the upper atmosphere which will pick up Pacific moisture, spreading rain initially over northern California with precipitation reaching into Merced County late Sunday night or early Monday. one component of this event will be the slow movement of the low. Most models show the center of circulation moving through the northern San Joaquin Valley Tuesday afternoon then finally into the Great Basin Wednesday. Rather impressive amounts of precipitation are projected for much of northern California. As much as one to two inches is likely from a Bay Area/Sacramento line north. A rather robust rain shadow will develop along the west side of the valley which, of course, always reduces precipitation amounts. My thinking is trace amounts to a few hundredths in Kern County and much of Tulare County, a tenth to a quarter of an inch over Fresno County, and near one-half inch as far north as Merced County. We are still more than three days out on this event and it’s possible models may be overdoing things a bit simply because this is so unusual so early in the season. We should be out of this by Wednesday as the center of circulation moves into the Great Basin with showers continuing over the Sierra Nevada. Isolated thunderstorms are also possible Monday and Tuesday afternoons, mainly from Fresno County northward. If and where any thunderstorms occur, locally heavy amounts could occur. Upper level high pressure will return later next week, resulting in dry weather and a warming trend.
Forecast: Mostly clear skies through Sunday morning. Increasing cloudiness from Fresno north Sunday afternoon. Mostly cloudy Sunday night with a chance of showers from Merced County north. Showers spreading southward Monday with a slight chance of isolated thunderstorms in the afternoon from Fresno County north. Periods of showers Tuesday and Tuesday night with a slight chance of thunderstorms from Fresno County north. A few lingering showers will be possible Wednesday morning near the Sierra Nevada foothills, otherwise expect partly cloudy skies. Mostly clear Wednesday night through Friday.
Temperatures: Lows tonight will cool into the low to mid 50s, but low to mid 60s in Kern County. Highs Saturday will warm into the mid to upper 80s. lows Saturday night will cool into the low to mid 50s north of Kern County and the lower 60s in Kern County. Highs Sunday will warm into the lower 80s.
Next report: Saturday, September 17