NWS Forecast Area Discussion - San Francisco Area

FXUS66 KMTR 060044

Area Forecast Discussion
National Weather Service San Francisco Bay Area
544 PM PDT Sun Jul 5 2020

.SYNOPSIS...Temperatures are warm and conditions are dry with breezy
onshore winds. Monday afternoon high temperatures will be slightly
cooler as onshore flow increases with a likely return of the
marine layer for the immediate coastline. Temperatures are
expected to be similar day- to- day midweek before warming likely
late in the week and into next weekend.

&& of 01:42 PM PDT Sunday...Temperatures Sunday are
running similar to yesterday given the high pressure still located
over the Bay Area region. A few observations in the North Bay are
slightly warmer, but generally speaking the pattern and conditions
are consistent to yesterday. Winds remain on onshore and have
started to increase this afternoon. Despite the onshore direction,
coupled with the dry conditions over inland areas, the winds could
make fire containment challenging. However, as an update to the
previous discussion, it appears that the Park Fire is now improved
to 50% contained and crews and stopped its forward progress. Sky
conditions remain clear as the nearest bit of cloud cover is over
100 miles off the California coast.

The pattern changes slightly beginning overnight tonight, being
driven by an upper level trough currently northwest of the state of
Washington. As it moves down the Pacific coast tonight, expect a
deeper marine layer and a return of low clouds around the Monterey
Peninsula. Winds will remain onshore but increase overnight as well,
reaching farther inland. These cooler northwesterly winds will drive
5-7 degrees cooler temperatures on Monday, with isolated locations
seeing a 10 degree cool down.

As the trough moves through, these cooler temperatures in the 70s
and 80s will remain through the midweek. Winds will remain onshore
and be breezy. In a return to the weather pattern from last week,
high pressure to the southeast will once again build by the end of
the week. The return of higher pressure puts a warming trend in the
forecast for the weekend. Drier conditions prevail with no
precipitation forecast.

&& of 5:44 PM PDT Sunday...for 00Z TAFS. It`s VFR, a
few patches of coastal stratus and fog may redevelop tonight and
Monday morning. At this time similar to yesterday a dry mid-latitude
flow continues to prevent fog and stratus development. 24 hr trends
in temperatures and dew point temperatures support high confidence
VFR forecast for the evening and VFR continuing for the Bay Area
tonight and Monday morning due to sufficient northerly pressure
gradient and winds. Lower level cooling and weakening of the flat
500 mb height ridge is forecast tonight, Monday and Monday night
which will likely weaken the marine layer temperature inversion
at least a little bit and increase the marine layer depth.

The marine layer depth varies from 1,200 to 1,600 feet. A surface
high pressure ridge over northern California is producing a 7.4
mb ACV-SFO pressure gradient and 2.2 mb UKI-STS gradient. The 18z
NAM is under-forecasting the ACV-SFO pressure gradient by almost 2
mb which is quite likely resulting in stronger northwest winds
over the coast than modeled. Also, visible imagery shows clear
conditions over the coastal waters thus turbulent mixing and
diffusion of much drier air at or above the marine temperature
inversion continues to enter the boundary layer preventing marine
fog and stratus from developing. On the flip side both UKI-STS and
SFO-SMX gradients are over-forecast by the 18z NAM, although as
mentioned the UKI-STS gradient is 2.2 mb, the gradient now
strengthening closer to 18z NAM forecasts; the northerly gradient
and winds is causing downsloping warming and drying over the North
Bay, e.g. STS has sustained NW wind near 15 knots.

Not much let up in the strength of the aforementioned pressure
gradients is forecast tonight and Monday morning.

Vicinity of KSFO...VFR. 5 minute obs showing a stronger west wind
with gusts up over 30 knots likely into the evening close to airport
weather warning criteria for winds, criteria is 35 knots or higher
sustained or gusts. Looks good for VFR through the taf period given
overall conditions mainly due to a strengthening northerly ACV-SFO
pressure gradient, currently 7.4 mb. Similarly, a gusty west wind is
forecast Monday and Tuesday afternoons and evenings per statistical

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR. Not a clear cut VFR forecast for
tonight with strengthening northerly gradients, though at least at
the onset for the evening with plenty of mixing of drier air into
the boundary layer should keep VFR going. Later tonight and Monday
morning there`s the possibility of patchy IFR in fog and stratus
briefly forming until diurnal mixing mixes out any fog and/or stratus
by late Monday morning.

&& of 01:42 PM PDT Sunday...A 1030 mb high centered 1000
miles west of Cape Mendocino will move closer to the coast through
Monday. As a result, gusty  northwest winds will continue over the
coastal waters into  midweek. Gale force gusts are possible along
the Big Sur coast  Monday afternoon and evening. Strong northwest
winds will generate steep fresh swell resulting in hazardous seas
conditions. A long- period southerly swell will continue through
Monday before swell  periods begin to subside.


     .Tngt...SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Pigeon Pt to Pt Pinos 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Mry Bay until 9 PM




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NWS MTR Office Area Forecast Discussion