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Forecast

December 17, 2020/pm update

December 17, 2020

Summary: The shower activity has ended over the valley floor with only dismal amounts of rain.  Most locations reported just trace amounts to a few hundredths.  The trough of low pressure is now moving into Nevada and southern California.  There is currently a  brisk northwest flow wrapping around the backside of the storm.  Winds on the valley floor at just about every location were out of the northwest in the 10 to 20 MPH range with locally stronger gusts.  The sun has broken through towards the center and west side of the valley.  Upslope clouds are developing courtesy of that northwest flow along the valley facing slopes of the Sierra Nevada and the north facing slopes of the Tehachapi.  These clouds will more than likely continue to regenerate tonight, especially in Tulare and Kern Counties.

 

Drier air will begin to move into central California as upper level high pressure over the eastern Pacific builds into California and eventually the Great Basin.  As it does, a warm air inversion will begin to build above the valley floor, setting the stage for extensive fog and low clouds during the night and morning hours, possibly lasting through the day in some areas.  It seems in recent years these fog regimes have found it difficult to take hold, mostly because of drought conditions.  So, we’ll see how the fog monster behaves this time around.

 

By Tuesday, a vigorous winter storm will be developing in southern British Columbia and Montana. On the back side of this low, a sharp ridge of high pressure will build northward into Alaska and northwest Canada.  The low will quickly shift eastward with the first bone chilling arctic blast moving into the Rockies and Midwest by Christmas Eve.  A very strong surface high will then build over the Great Basin, generating a strong off shore flow.

 

Most models this afternoon don’t project as ominous situation as could be with a reduced chance of arctic air moving into central California.  Most of it is now projected to move into the Rockies and points eastward.

 

 

That off shore flow will no doubt spread cooler and drier air  into inland sections of central California.  Thankfully we have a 15,000 foot wall called the Sierra Nevada which, more often than not, keeps winter just to our east.  The blocking ridge pattern will continue for what appears to be all of next week, though a cutoff low shows up on some models ofof the northern Baja coastline around Christmas Eve, but for now it doesn’t appear to be something to be concerned about.

 

 

Forecast: Partly to mostly cloudy into Friday morning.  clearing later Friday morning and afternoon.  Mostly clear skies Friday night and on through Christmas Eve with areas of night and morning fog and low clouds, possibly continuing through each afternoon.

 

Short Term:                                                                        

Madera 35/55/29/55 Reedley 34/54/29/56 Dinuba 34/54/28/56
Porterville 36/55/29/57 Lindsay 34/55/28/57 Delano 37/55/29/58
Bakersfield 42/55/36/58 Taft 43/54/43/58 Arvin 40/55/31/58
Lamont 40/54/32/58 Pixley 36/55/29/58 Tulare 34/54/28/56
Woodlake 36/54/29/56 Hanford 36/55/30/57 Orosi 34/54/28/56

 

Winds: Winds this evening will continue out of the northwest at 01 to 20 MPH at times, decreasing later tonight to around 5 to 10 MPH.  Winds Friday through Sunday will be generally less than 10 MPH late mornings and afternoons and at or less than 5 MPH nights and early mornings.

 

Rain:  Expect dry conditions for the next week to ten days.

 

Frost:  All locations will be above 32 degrees tonight, but near to slightly above in the coldest locations where skies clear and winds die off.

 

Dew points are generally in the mid to upper 40s this afternoon and, with plenty of mixing and upslope clouds, most locations will remain in the 30s.

 

From Saturday morning on, any night with clear skies and no wind has the potential to drop down into the 27 to 28 degree range in unprotected river bottom, etc. and generally from 29 to 32 in flat terrain.  Above freezing conditions can be expected on hillsides.

 

With a warm air inversion increasing above the valley floor, fog is likely to form on a widespread basis.  In recent years, fog regimes have been few and far between mainly because of drought conditions and that may be the case here, as well, as rainfall from the current system is almost meaningless.  Still, there is that chance that the fog could lock in and lift into a low overcast with only partial afternoon clearing.  If that occurs, then above freezing conditions would prevail with colder afternoons.

 

The latest model projections beginning Tuesday of next week show a vigorous winter storm developing over Montana then moving quickly southward over Utah and Colorado.  Along the back side of this storm, a sharp ridge of high pressure will build into Alaska and western Canada.  The latest models this afternoon are looking more in our favor in keeping what will be a bitterly cold air mass far enough to our east so as to not infiltrate the valley.  So, continue to keep your fingers crossed.

 

I did see new model information for December 29 through January 2 which shows a deep low developing over southern California with another ridge into western Canada.  This pattern could also possibly cause challenges, however it’s much too far away to put much credence in.

Next update:  December 18/am