Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA

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FXUS66 KMTR 201149

Area Forecast Discussion
National Weather Service San Francisco Bay Area
347 AM PST Mon Nov 20 2017

.SYNOPSIS...A weather system will bring periods of light rain to
the North Bay and possibly San Francisco today. High pressure will
bring dry and warmer weather to the area Tuesday through Thursday.
The next chance of rain will be late this weekend.

&& of 3:15 AM PST Monday...Cloudcover and southwest
flow over the area is keeping temperatures in the 50s early this
morning which is about 10 degrees warmer than the past couple of
nights. A warm front is bringing light rain to the North Bay this
morning. Rain amounts so far have generally been .10" or less.
Periods of light rain will continue through early afternoon in the
North Bay with the possibility of a sprinkle as far south as San
Mateo and Alameda counties. Total rainfall amounts will remain
under one-quarter of an inch.

Things will dry out tonight as the warm front lifts north. An
upper level trough deepening along 145W will cause an upper level
ridge to build over southern California and Nevada. This will
keep rain north of the district while bringing warmer temperatures
Tuesday through Thursday. The warmest temperatures will be
Wednesday and Thursday with highs in the 70s to lower 80s in
Monterey and San Benito counties. Temperatures around the SFO Bay
Area will warm into the upper 60s to lower 70s.

The ridge flattens on Friday and temperatures will cool off
slightly. Then the ridge moves a little to the east Saturday and
Sunday and the trough starts to move to the northeast. Medium
range models indicate a warm front will bring a chance of rain to
the North Bay Saturday night and most of the rest of the district
Sunday. Then another chance of rain Sunday night and Monday as the
trough passes.

Based on recent performances by the models...would not be surprised
if future model runs trend drier with the weekend system and track
it further north. In any case rainfall rates and amounts will be


.AVIATION...As of 3:47 AM PST Monday...It`s VFR at the terminals
except cigs are MVFR over the North Bay this morning. Cloud bases
have been steadily lowering since last evening as a very moist,
milder, and stable SW flow from the sub-tropics becomes displaced
to the Pacific Northwest and northernmost California this week.
This plume of moisture will brush the North Bay with light-moderate
rain today, while it remains mainly dry to dry elsewhere to the

20-30 knot SW winds at the 925 mb and 850 mb levels over the North
Bay counties will result in marginal low level wind shear (llws) at
KSTS this morning til early afternoon. Lower level winds are much
less robust over the remaining Bay Area and areas to the south today
thus llws is not expected to the south.

Vicinity of KSFO...Light wind, VFR. Cloud bases lowering today, and
forecast is dry runways. Low to moderate confidence.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Light E-SE winds this morning, except SE to
10 knots Salinas Valley. VFR and dry. Moderate to high confidence.

&& of 3:35 AM PST Monday...The eastern edge of a very
moist sub-tropical flow will overlap the northern coastal waters
through early this week. A warm frontal boundary connected to this
moist flow will approach the coastal waters today and become nearly
stationary while another warm front to the southwest advances toward
it and the Bay Area. By late week into early next week a cold front
will move southeastward over the coastal waters. Seas are expected
to remain light to moderate through the first part of next week.





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