December 17, 2021pm
There is pretty much wall to wall low overcast in the valley this afternoon. Generally, from Stanislaus County southward through Kern County. Temperatures are only in the upper 40s at most locations, very little changed from this morning’s lows. This will be the main challenge through at least Monday night as upper level high pressure centered off shore ridges north/northeast into the Pacific Northwest and northern California.
In the meantime, at the surface, weak high pressure is over Utah and Nevada. This is generating an off shore flow. Even though the flow is off shore, the Sierra Nevada blocks these winds from affecting the valley. On occasion, they can sneak over the Kern County mountains and onto the valley floor in Kern County, but such is not the case this time around. Models are now showing the high collapsing as early as Monday as a cold low drops southward from the Gulf of Alaska and parks off the Oregon/northern California coast. If the elements stack up the way they’re being portrayed this afternoon, we could see a prolonged period of occasional wet weather.
The jet stream will move in from the west/southwest, slamming central California with periodic storms. This is one of the wetter patterns we deal with during the winter months, so let’s hope all of this comes to pass and drastically improves our water situation. Also, it’s possible we could have some very cold weather after the first of the year, however a great deal can change between now and then.
Forecast: Fog and low overcast through Monday with a chance of some hazy sunshine in the afternoons. Increasing cloudiness Tuesday leading to a chance of rain Tuesday night. There will be periodic chances of rain from Wednesday through Friday.
Short Term:
Highs where the cold, gray overcast remains will rise into the upper 40s to near 50. Locations with hazy sunshine will see highs in the low to mid 50s. Overnight lows where fog and low clouds do not persist will lower into the upper 20s to mid 30s. overnight lows where overcast remains will be in the low to mid 40s through Monday
Wind: Winds will be generally at or less than 6 MPH tonight through Saturday and generally out of the southeast. Winds Saturday night through Monday will be at or less than 6 MPH with periods of near calm conditions.
Rain: Expect dry weather through at least Monday night. If what I’m seeing on paper actually unfolds, we could see abundant precipitation next week. The pattern I see on paper beginning Tuesday is one of the wettest we deal with. A big low will sit off the northern California/Oregon coast with a trail of moisture moving in from the Pacific with embedded low pressure systems slamming into the coast from time to time. Not only do some of the models forecast abundant moisture in the short time, they also indicate precipitation in the medium and even longer range forecast. If we get a week’s worth of this, it would go a long way towards alleviating the water situation. All I can say is, so far-so good!
Frost: The good ol’ fog and low clouds have anchored themselves into the valley, ending any frost threat through tonight, at least. With a warm air inversion building above the valley floor and dewpoints well into the 40s, below freezing temperatures do not seem likely now through Monday. If clearing does happen to occur at any given location, upper 20s to loer 30s would be the result but for now I consider that unlikely. If models are correct, we will be entering a prolonged period of wet weather beginning Tuesday and lasting all the way through the Christmas weekend. For the first time this season, some models are picking up on a possible cold weather episode beginning in January. The GFS model in particular are showing a mammoth high building over the eastern Pacific and reaching into southern Alaska while a trough of low pressure resides over the interior US. This creates a polar express, moving from north to south into the west coast. If this were happening tonight, we’d be looking at a freeze. But there a lot of time between now and then and weather changes. The reason I even bring it up, since the chance is low this far out, is so you can keep it in your back pocket for future planning.
Next report: December 18