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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

FXUS66 KMTR 220551

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1051 PM PDT Sun Apr 21 2019

.SYNOPSIS...High pressure building over the region will result in 
a warming and drying trend through midweek. Temperatures will warm 
well above seasonal averages by Tuesday with weak offshore flow. 
Temperatures will then moderate and cool slightly later in the 
week as the ridge of high pressure weakens.


.DISCUSSION...As of 9:00 PM PDT Sunday...Thanks to plenty of sun 
and a drier airmass moving in from the north, temperatures today 
were warmer in most locations compared to Saturday, and were close
to seasonal averages. 

Skies remain mostly clear this evening, except for areas of low
clouds near the ocean, mainly along the San Mateo County Coast and
over the Monterey Peninsula. Expect a gradual increase in low
clouds along the coast and locally inland overnight. Most inland
development of low clouds will likely be in the valleys across 
the southern portion of our area. Farther north, a 6.5 mb surface 
pressure gradient from ACV to SFO may be enough to keep low clouds
confined to the coast. Expect low clouds to clear in most areas 
by late Monday morning as dry northerly flow increases just above 
the boundary layer.

An upper level ridge, currently centered offshore along 135W, is
forecast to shift eastward and build over California over the next
48 hours, resulting in an increase in 850 mb temperatures from 
about 10 deg C at present to about 18 deg C by Tuesday. Also, 
onshore flow is forecast to decrease through Monday and then low 
level flow will likely turn light offshore by Tuesday. These 
factors will result in a robust warming trend through Tuesday. On
Monday, temperatures are forecast to warm by about 5 degrees in 
coastal areas and about 10 degrees inland. Up to 10 more degrees
of warming is expected regionwide on Tuesday. By Tuesday we 
should see highs mostly in the 70s at the coast, with 80s and 
lower 90s expected inland. Nights will remain seasonably cool,
keeping heat impacts low in most areas. 

The upper ridge axis is forecast to shift inland by Wednesday
which will likely mean a return to light onshore flow. Thus,
expect temperatures to begin to cool at the coast and in the
coastal valleys on Wednesday, while the inland valleys and hills
remain very warm. In general, the warmest day of the week in
coastal areas will be Tuesday, while the warmest day inland will
probably be Wednesday.

More widespread cooling is forecast for Thursday as onshore flow
increases. However, most model guidance keeps temperatures at
least slightly warmer than normal through the week.

In the longer range, both the GFS and ECMWF bring an upper low to
the coast near Point Conception next weekend. However, there is a
disagreement among the various model ensemble members concerning
the likelihood of shower activity in our area with this low. For
now it appears that chances will be low and confined primarily to
the southern portion of our area. 


.AVIATION...As of 10:51 PM PDT Sunday...Evening satellite shows a
strong intrusion of low clouds along the San Mateo coast and the 
Monterey Peninsula. As high pressure builds over the area tonight,
the inward extent of the low clouds should remain with limited 
inland penetration. Therefore, expect most terminals to remain 
generally VFR. If clouds form, they will form early and dissipate 
early morning or shortly after sunrise. That said, where there are
terrain gaps, low clouds will have access to fill in to that 
valley and possibly spill into other portions of the Bay Area. 
Regardless, by the time the sun comes up the main air terminals 
should be VFR. Coastal low clouds will develop again Monday night.

Vicinity of KSFO...VFR. Low clouds are trying to squeak into SFO,
but the building high pressure and developing offshore flow are
expected to keep these limited. If clouds do thicken up, they will
dissipate late in the night with conditions expected to go VFR
right around sunrise. Monday night will likely be very similar to

SFO Bridge Approach...VFR is expected through the night as the low
level clouds should not be able to make it around the corner far
enough to impact SMB.

Monterey Bay Terminals...IFR. There is an outside shot that with
the building ridge the marine layer becomes compressed and fog
becomes an issue. This is not expected at this time as a departing
upper level low over Southern Cal should keep the low level
atmosphere mixed enough to lift the marine layer off the surface.
Thus look for low clouds tonight, VFR Monday, and some fog Monday

&& of 10:36 PM PDT Sunday...Strong to near gale
conditions out of the northwest will remain through the remainder
of the night. Some gale force gusts are  possible through tonight,
especially North of Point Reyes. These  gusty winds will generate
steep fresh waves that are hazardous  for small craft vessels.
Northwest winds dissipate into midweek.


     .Tngt...SCA...Pigeon Pt to Pt Pinos 0-10 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Mry Bay until 3 AM




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