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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

FXUS66 KMTR 180547

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1047 PM PDT Wed Apr 17 2019

.SYNOPSIS...Warmer temperatures are forecast for tomorrow, especially
inland areas where widespread highs in the lower to mid 80s are 
expected. A cooling trend is then expected to begin Friday and 
continue into the weekend as onshore flow redevelops and 
strengthens. This will be followed by renewed warming for the 
first part of next week. No precipitation is expected through the 
forecast period.


.DISCUSSION...As of 09:05 PM PDT Wednesday...Mostly clear skies
prevail over the district, although latest fog product satellite
imagery shows areas of marine stratus already developing near the
waters from around Tomales Bay southward. Highs this afternoon
were up significantly from those on Thursday, with several inland
valley locations topping out at close to 80 degrees. Current
temperatures around the Bays are generally running in the mid 50s
to near 60.

Basic large-scale synoptic pattern, with an upper level ridge 
building along the west coast and 500 mb heights projected to 
exceed 582 dm over us by tomorrow, can and has resulted in record
temperatures for this time of year. But that's when it occurs in 
conjunction with at least moderately strong offshore flow at lower
tropospheric levels, clearing out any marine air influence and 
adding in subsidence warming. 

In the present situation, pressure differences between the Great 
Basin and the coast look to remain pretty minimal. Thus despite 
the several degrees of additional warming expected at say the 850 
mb level, it may only be fully realized at the surface well 
inland. Elsewhere it will be a mix of mid-April sunshine and a 
warm air mass aloft vs a residual pretty deep and cooler boundary 
layer (as evidenced by the 00Z KOAK sounding) and continuing 
marine air influence especially closer to the coast. 

Present forecasts remain consistent both with latest output from 
the NBM (National Blend of Models) and current forecast thinking, 
and so no updates presently anticipated. 


.PREV DISCUSSION...As of 02:01 PM PDT Wednesday...This morning's 
low clouds have dissipated leaving mainly clear skies across the 
region. Temperatures this afternoon are running around 5 to 10 
degrees above this time yesterday with most locations in the upper
60s to low 70s. Expect a few degrees of additional warming this 
afternoon before temperatures begin to cool off for the evening. 
Patchy low clouds may develop again overnight tonight along the 
coast, but conditions are forecast to clear by late tomorrow 

Warming will continue and peak tomorrow as the upper ridge 
building over the region strengthens and shifts inland over 
California. Light offshore flow will allow warming to affect
coastal areas as well. Temperatures tomorrow are expected to be as
much as 10 to 15 degrees above normal with interior locations
experiencing more widespread low to mid 80s in the afternoon. The
coast will see highs in the 70s with isolated spots reaching the 
upper 70s. For many places tomorrow will be the warmest day of 
2019 thus far, although record highs are not anticipated. San 
Jose, for example, has not seen 80 degree weather since early 
November 2018.

Cooling will then begin on Friday as onshore flow returns and the
upper ridge weakens and shifts to the east. The biggest change 
will be felt along the coast where marine layer clouds will likely
increase into Friday morning. Temperatures are forecast to fall 
back into the 60s for coastal areas. While some cooling will be 
felt inland, highs will remain in the upper 70s to low 80s across
the interior. More substantial cooling can be expected on 
Saturday as an upper trough moves into Northern California. Latest
runs of the GFS and ECMWF are starting to show a brief period of 
light precip Saturday morning with the ECMWF showing light precip 
as far south as Santa Cruz County as well as light drizzle over 
the Monterey Bay. The 12z NAM is also showing something similar. 
Kept POPS minimal at this point, but patchy light drizzle may be 
possible Saturday morning as the trough axis moves through. Will 
keep an eye on model trends as the weekend approaches.

Once the trough moves through Saturday night, high pressure will
begin to build in over the region once again bringing back a
warming trend. Temperatures will begin to rise on Sunday and
Monday with more a robust increase possible into mid-week as the upper
ridge builds over California.

&& of 10:40 PM PDT Tuesday...Upper level ridge will 
be moving over the area tonight and Thursday. Surface high 
pressure west of San Francisco with a weak ridge building into the
Oregon coast. This will keep the stratus patchy and limited 
mainly to the MRY Bay Area and around the Golden Gate which 
includes OAK. Currently there are patches of low clouds along the
coast and Monterey Bay.

Vicinity of KSFO...VFR through at least 12Z. Stratus around the
Golden gate could possibly spread briefly into SFO between 12-17Z
but given that it remained VFR this morning and that the marine 
layer will be shallower is less likely that stratus
gets to SFO. 

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Currently only a few clouds over the bay
but this is an indication of moisture in the area. Expecting 
IFR conditions at MRY 09Z-16Z and 12Z-16Z at SNS.

&& of 10:33 PM PDT Wednesday...High pressure off the
California coast will keep light to moderate northwest winds
through Thursday night. The high will shift north and build into
the Pacific Northwest on Friday resulting in  increasing winds
over the northern waters that will last through the weekend.





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