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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

FXUS66 KMTR 240001

Area Forecast Discussion
National Weather Service San Francisco Bay Area
501 PM PDT Tue Apr 23 2019

.SYNOPSIS...Well above normal temperatures today with high 
pressure and offshore winds in place. A few degrees of cooling 
possible by Wednesday for the North Bay, but no big changes in the
weather pattern. Onshore winds return by late in the week and 
into the weekend with temperatures gradually returning to seasonal
normal values.

&& of 2:23 PM PDT Tuesday...Warm-to-hot day across 
the San Francisco Bay Area and Central California Coast as an 
upper level ridge axis sits over the northern half of California. 
The 12Z Oakland sounding earlier this morning resulted in 925 mb, 
850 mb, 700 mb, and 500 mb heights and temperatures at/above the 
90th percentile for this calendar day (according to the Storm 
Prediction Center's Sounding Climatology Page). The SFO-SAC 
surface pressure gradient as of 19Z was -0.3 mb. All of these 
parameters are good ingredients for producing an anomalously warm 
middle to late April afternoon. Early afternoon surface 
observations are in agreement with this thinking as temperatures 
are generally running a solid 3-8 degrees warmer than mid-day 
yesterday. By the time the afternoon wraps up, we'll see 
widespread 70s for coastal communities, and 80s to low 90s for 
interior valleys and the Santa Cruz Mountains. This equates to 
around 10-20 degrees warmer than climatological normal values.

The positively-tilted ridge axis responsible for the warm-to-hot 
temperatures is progged to slide southeastward over the next 24 
hours as the center of the upper level ridge offshore weakens 
slightly. This should result in some cooling for Wednesday 
afternoon for the North Bay while interior locations in Monterey 
and San Benito Counties may actually see some day-to-day warming 
as the ridge axis shifts. The marine layer appears that it will 
struggle to redevelop tonight, though there are hints of it 
returning sometime on Wednesday night or Thursday for the Central 

Widespread day-to-day cooling should commence beginning Thursday 
as the ridge continues to weaken and upper level heights drop over
our region. By 00Z Friday, models place a weak upper level low 
around 31 deg N, 139 deg W (or about 1300 miles west of San 
Diego), and have it push eastward toward southern California 
through over the course of the weekend. For our area of the state,
most locations should remain dry, though parts of rural Monterey 
County have a slight chance of experiencing light rain showers. 

Model solutions begin to diverge by the beginning of next week.
The Climate Prediction Center's 6-10 and 8-14 day outlooks provide
equal chances for above and below normal temps for our part of 
the state (i.e., no clear signal one way or the other).


.AVIATION...As of 4:55 PM PDT Tuesday...for 00Z TAFs. VFR 
conditions and clear skies prevail across the district, with just
a few thin high clouds passing through. The KACV to KSFO 
pressure gradient remains pretty strong, presently up at 7.2 mb. 
Consistent with this, latest model output and statistical guidance
indicates minimal coastal stratus development overnight, with 
even much of the coast itself likely to stay mostly clear. 
Therefore expect both San Francisco Bay Area and Monterey Bay 
terminals to remain VFR through the night, with the possible 
exception of KSTS where visibilities could diminish into the IFR 
range for a few hours towards sunrise. Light to moderate onshore 
winds will gradually diminish later this evening.

Vicinity of KSFO...VFR conditions are forecast to prevail through
the night and on Wednesday. Moderate onshore winds 15 to 20 kt 
will prevail through around sunset, then progressively decrease. 

SFO Bridge Approach...Same as KSFO.

Monterey Bay Terminals...VFR. Only an outside chance of ceilings
developing overnight. Light onshore winds will become light and 
variable by mid-evening.

&& of 2:40 PM PDT Tuesday...High pressure off of the 
California coast will slowly shift northward today. Winds over 
the northern waters will remain moderate and gusty resulting in 
steep, short-period swell. Elsewhere, winds will be weaker overall
but will become locally gusty at times in the afternoon and 
evening hours near the coast.


     .Tngt...SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm




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