Scientific Forecaster Discussion
fxus66 kmtr 051622
Area forecast discussion
National Weather Service San Francisco Bay area
822 am PST Thu Dec 5 2019
Synopsis....after a brief break in wet weather conditions today,
another cold front will sweep through the area on Friday
resulting in widespread rainfall and gusty winds. Post-frontal
showers and potentially isolated thunderstorms will continue
through Saturday night before tapering off on Sunday.
Discussion...as of 8:22 am PST Thursday...there are still a few
areas of fog impacting the morning commute. The Livermore ASOS is
reporting 1/4 mile in fog, and Palo Altor is now indicating 1/2
mile visibility. Aside from these, visibilities are starting to
improve, and range from from 3 or greater across the San
Francisco Bay area. Shower activity has pretty much ended, with
no echoes being picked up by kmux at this time.
We should see a break in our rainy pattern today with the next
storm system due impact the area on Friday and Saturday with
rainfall and gusty winds. Light showers are expected to begin
Friday afternoon ahead of this system. The latest model guidance
has slowed down the timing of the cold front passage, with the
front moving into the North Bay late Friday afternoon then
gradually moving south and east overnight. The Friday evening
commute will likely be impacted across the North Bay and possibly
sf, but areas southward not as much. Given the dynamics with the
potent cold front and ample moisture, rainfall intensity will be
of concern across the North Bay, especially in the kincade burn
area. Hourly rainfall rates could exceed one half an inch in an
hour leading to debris flows or flash flooding. Therefore, a Flash
Flood Watch has been issued for the kincade burn area Friday
afternoon through Saturday morning. By Saturday morning the cold
front will have pushed through much of the Bay area. However, the
parent low and upper trough moving through will keep showers going
across the Bay area Saturday into Sunday. Enough instability will
be present with the frontal passage and with the upper trough
justifying a slight chance of thunderstorms for Friday and
Saturday. Rainfall totals from Friday into early Sunday range from
0.50"-1.25" in most urban areas, except 1.50-2.50" in the North
Bay valleys. In the hills/mountains, rain totals are forecast to
range from 1.50"-4.00" in the North Bay mountains, 1.25- 1.5" for
the East Bay hills, 1.50-3.00" in the Santa Cruz Mountains, and
1.00-3.00" across Monterey and San Benito co. Mountains. Lesser
amounts will be possible in the southern Salinas valley.
In addition to the rain, southerly winds will ramp up ahead of
and with the frontal passage. Winds will be strongest over the
coastal waters, near the coast, and in the higher terrain.
May need to consider possible Wind Advisory products later today.
A few lingering showers will be possible on Sunday as the upper
trough slowly exits. High pressure aloft building in over the
state will bring drier weather to the area Monday through at least
Wednesday. A weak system is forecast to move across northern
California on Wednesday which could bring a few showers to
northern zones late Wednesday.
Aviation...as of 3:45 am PST Thursday...for 12z tafs. Lingering
low level moisture and the remnants of the previous low pressure
system has resulting in a mixed bag of weather this morning with
patchy fog over select terminals and rain showers over the North
Bay. Have updated the tafs to reflect lower vsbys and cigs as well
as rain showers for the next several hours. Otherwise forecast
remains on track with light east to southeasterly winds across all
terminals. Southeast winds will begin to pick up slight this
afternoon with speeds approaching 8-10 kt at most terminals.
Vicinity of ksfo...borderline IFR/MVFR cigs around 800-1000 ft will
prevail this morning before lifting late morning to around
1500-2500 ft. Light east winds tonight become southeast 5-10 kt
by this afternoon.
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...mainly VFR conditions will prevail with
borderline MVFR/VFR cigs developing later this morning. Moderate
southeast winds will persist over ksns with speeds of 10-15 kt.
Light winds will persist over kmry.
Marine...as of 2:51 am PST Thursday...moderate southerly winds
ahead of an approaching low pressure will strengthen further late
tonight into Friday as the associated cold front approaches and
then moves through the coastal waters. Gale force winds are
expected late Thursday night and into Friday. Moderate northwest
swell will gradually build to around 10 to 15 feet Friday over the
northern waters then spread south across the coastal waters over
... Arena to pt Reyes 0-10 nm from 9 am
Small Craft Advisory...pt Arena to Pigeon pt 10-60 nm from 9 am
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm from 9 am
Public forecast: Sims
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