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

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FXUS66 KMTR 220619

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1019 PM PST Tue Feb 21 2017

.SYNOPSIS...Scattered showers and isolated thunderstorms will
persist into this evening before dissipating overnight. Drier
weather and much cooler temperatures are then forecast through
the latter half of the week, with late night and early morning
frost possible. Unsettled weather will likely return by the
upcoming weekend, with a potential for widespread rain and gusty


.DISCUSSION...As of 9:00 PM PST Tuesday...KMUX radar this evening
continues to show scattered showers moving through the region,
mainly to the south and east of San Francisco. Showers and
isolated thunderstorms this afternoon and early evening produced
brief heavy downpours and small hail. Since sunset, shower
intensity has decreased and based on latest model output would
expect showers to continue to taper off overnight.

Water levels along the Coyote Creek in Santa Clara county remain
at record high levels this evening, and flooding continues. Water
levels have recently peaked along the creek and should gradually
recede from this point forward. However, it will take nearly 24
hours for the creek to fall below flood stage and a flood warning
remains in effect for the Coyote Creek until further notice.

00Z NAM indicates a few light showers may linger into Wednesday
morning, but for the most part Wednesday is expected to be a
dry and cool day under partly cloudy skies. A shortwave trough is
forecast to drop into northern California from the northwest by
late Wednesday, but precipitation with that disturbance is
forecast to remain to our north. That system will, however, bring
a reinforcing shot of cold air into our area Wednesday night into
Thursday, with 850 mb temperatures forecast to drop as low as -4
deg C by Thursday morning. What this will mean is possible late
night and early morning frost in our inland valleys later this
week. The coldest nights are expected to be Wednesday night and
Thursday night when valley frost is likely and freezing
temperatures are possible in the coldest locations. A hazardous
weather outlook has been issued to draw attention to the upcoming
cold temps.

Our area will see at least three consecutive dry days during the
second half of this week. But our weather pattern is expected to
turn unsettled again over the upcoming weekend when a cold system
drops in from the north. But just how much rain and wind that
system may produce is unclear at this point. The GFS has been
consistent for the past few days in forecasting a merger offshore
between the cold system dropping in from the north and a wet
system approaching from the west. The GFS then tracks this merged
system across California late in the weekend, with the result
being widespread moderate to heavy rain and locally strong and
gusty winds from late Saturday night through Sunday. The 12Z
ECMWF, however, does not expect these two systems to fully phase
with the result being significantly less wind and rain over the
weekend compared to the GFS. There are still four or five days for
the models to sort all this out, but in the meantime preparations
should be made for another potentially wet and windy system this


.AVIATION...As of 10:15 PM PST Tuesday...A line of showers moved
through the area around 03Z and is now exiting the East Bay
interior valley. Conditions drying out the rest of the night but
low-level moisture will keep some MVFR cigs near the hills and fog
near STS until early morning.

Vicinity of KSFO...VFR.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR with sct-bkn clouds above 3000 feet
through 12Z.

&& of 10:15 PM PST Tuesday...Winds will decrease
tonight as a storm system departs from the area. High pressure
building over the eastern Pacific with bring increasing northwest
winds to the southern waters Wednesday and the remainder of the
waters Wednesday night.






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