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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

                            
000
FXUS66 KMTR 160609
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1109 PM PDT Sun Sep 15 2019

.SYNOPSIS...A cold front will spread light rain across much of 
our area tonight and Monday, and result in much cooler weather 
for inland areas. Tuesday will be dry and mild. A second weather 
system may bring scattered showers to the North Bay on Wednesday,
and also keep temperatures on the cool side of normal through 
Thursday. Warmer weather is forecast from Friday on into next
weekend.

&&

.DISCUSSION...As of 8:55 PM PDT Sunday...The marine layer rapidly
deepened today as an early season storm system approached from 
the northwest. In addition, onshore flow ramped up this afternoon 
and locally strong and gusty winds are currently being observed, 
primarily near Altamont Pass in the East Bay where winds are 
have been gusting as high as 50 mph this evening. The combination
of a deep marine layer and robust onshore flow resulted in 
significant inland cooling today, with high temperatures in the 
inland valleys and hills cooling by as much as 20 degrees from 
yesterday. The only portion of our forecast area that remained 
very warm today was the southern interior of Monterey County and 
southern San Benito County where highs were in the lower to mid 
90s. 

Current satellite shows a well defined frontal rain band dropping
south along the northern California coast. Crescent City, on the 
far northern California coast, has picked up nearly three-quarters
of an inch of rain (0.72") from this system today. Latest 
satellite-derived precipitable water (PW) values remain 
relatively impressive, with a narrow band of PWs around 1.5 inches
along the frontal boundary. The models agree that precipitation 
potential with the front will decrease as the front pushes south 
into a drier airmass. However, given the history of rainfall with 
this system, and current satellite data, it seems reasonable to 
expect measurable rain across much of the SF Bay Area late tonight
and Monday morning and even some measurable rain as far south as 
Monterey and Big Sur by Monday afternoon. Drizzle is already being
reported in San Francisco as low level moisture convergence 
increases ahead of the approaching front. Will likely see areas of
drizzle continue near the coast and in the coastal hills prior to
the actual frontal rain band reaching our area later tonight and 
Monday morning. 

The 00Z NAM shows precipitation along the frontal boundary
dissipating just as the boundary reaches the North Bay late this
evening. But the NAM then forecasts enhancement along the front
as it pushes just to the south of San Francisco shortly after 
sunrise on Monday morning. The 18Z GFS shows this enhancement too,
but earlier and farther north. Forecast precipitation totals from
this system are hard to pin down as the models, particularly the 
mesoscale models, forecast very spotty QPF, presumably due to 
embedded convective elements developing along the frontal 
boundary. The latest HRRR forecast values ranging from nothing in 
the North Bay Valleys to isolated amounts up to a half inch along 
the San Mateo County coast and locally in the East Bay Hills. The 
NAM also forecasts up to a half inch in a band from southern San 
Mateo County across northern Santa Clara County. The upshot is 
that precipitation totals by late tomorrow will likely vary 
considerably, and potentially over short distances.

IR satellite is showing lots of cold air behind the incoming front
and models forecast a fair amount of instability along the 
frontal boundary late tonight and Monday. But the most unstable 
air is projected to be to our east and this is where the Storm 
Prediction Center shows a slight risk of thunderstorms on Monday.
If isolated thunderstorms do develop in our area, they would most
likely occur along the eastern fringes of the East Bay.

Dry conditions will return to all areas by late Monday and Tuesday
looks to be a dry and mild day. The next system is forecast to
drop in from the northwest late Tuesday night into Wednesday, and
perhaps generate scattered showers in the North Bay. The midweek 
system will have more of an inland trajectory, which could mean 
the development of gusty northerly winds on the back side of the 
trough Wednesday night into Thursday, mainly in the North and East
Bay Hills. 

The midweek system will bring a reinforcing shot of cool air into
our region and keep temperatures on the cool side of normal
through Thursday, especially inland. Warming is then anticipated
late in the work week and into next weekend when the models
generally agree that a ridge will build over California. 

&&

.AVIATION...As of 11:09 PM PDT Sunday...A fairly solid influx of 
low clouds, patchy drizzle, gusty winds aloft with increasing 
water vapor per higher dew point temperatures in the upper 50s to
lower 60s all since mid afternoon. 

The 06z tafs indicate increasing areal coverage of MVFR cloud 
ceilings with local IFR ceilings and visibilities lowering in 
additional patches of drizzle to rain developing early Monday 
morning. Strong mid-latitude influence arriving in our cwa with an
amplifying 500 mb trough and surface cold frontal passage Monday 
morning. With aforementioned moisture in place and vigorous 
dynamics rolling through Monday am abandoning recent model output 
to some extent and decided to go slightly more adverse weather 
conditions with morning frontal passage including bumping up post 
cold frontal winds a bit more above statistical guidance. The 500 
mb trough energy is splitting to some extent, but also think the 
models are exhibiting difficulty resolving the placement of key 
features. Thus would rather lean a bit more pessimistic than 
optimistic in this taf cycle. Low level wind shear added for KSTS
and KLVK til 11z and 12z respectively.

Vicinity of KSFO...MVFR tonight with light rain arriving by 10z 
then moderate to possibly briefly heavy rain 13z-18z Monday. 
Decided to bump up post frontal winds above guidance for Monday.
Improving ceiling conditions and drier Monday afternoon.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...MVFR-IFR tonight and Monday morning, patchy
drizzle possible tonight. Light rain Monday morning, but could be
briefly moderate until frontal passage. Improving ceiling conditions
and drier Monday afternoon. 

&&

.MARINE...as of 10:34 PM PDT Sunday...Expect locally breezy
evening winds along the inner coastal waters south of Pigeon Point
and over the bays. Winds will ease early this week as high 
pressure over the eastern Pacific weakens. A cold front will move 
through the waters on Monday bringing a chance of showers. Mixed 
seas will continue with moderate period northwest swell and a 
slightly longer period southerly swell.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm until 3 AM

&&

$$

PUBLIC FORECAST: Dykema
AVIATION/MARINE: Canepa

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