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fxus66 kmtr 080039 
afdmtr

Area forecast discussion
National Weather Service San Francisco Bay area
436 PM PST Sat Dec 7 2019

Synopsis....showery conditions through the morning, then another
frontal band will move across the district bringing heavier
showers and a chance of thunderstorms. Lingering showers on
Sunday but trending drier by afternoon. Dry weather returns early
next week.

&&

Discussion...as of 1:09 PM PST...the next round of rain has
arrived to the North Bay. Latest kmux radar imagery shows light to
moderate rain falling across much of Sonoma and Napa counties as
of 1 PM PST. The rain is also starting to impact San Francisco
this early afternoon. Expect the rain to continue spreading
southeastward through the rest of the day. While we have yet to
observe any lightning strikes in the San Francisco Bay area, the
chance of thunderstorms will continue through the evening hours
for much of the region. The day 1 Storm Prediction Center has the
vast majority of the region in at least the general thunder
category. Additionally, most of the North Bay is under a marginal
risk for severe weather. The 18z hrrr does indicate some locally
impressive 0-1 km bulk shear and 0-1 km storm relative helicity
values, but think the best potential for any sort of rotating
storms will be to the east of the area in the Central Valley
where the skies have thus far remained generally clear today. In
addition to the thunderstorm risk, a Flash Flood Watch is in
effect for the kincade fire burn area through 10 PM PST this
evening.

Southerly winds will remain locally strong through the rest of
the afternoon. Winds have already gusted above 40 mph at sfo, 31
mph at Oak, and at least 25 mph at mry today. These winds will
diminish later this afternoon or evening near sea level with
breezy conditions likely persisting in the higher elevations into
the overnight hours.

Scattered rain showers are expected to continue through about
mid-day on Sunday as the upper level trough axis passes through
the Bay area. Behind this trough an upper level ridge will quickly
build over the West Coast, bringing dry conditions to the Bay
area and central California coast for Monday and Tuesday. Models
advertise a weak system that will impact parts of northern
California around Wednesday. At this point it would appear the
best chances for rain would be north of the Golden Gate with
perhaps a few tenths of an inch. Another system will then arrive
late Thursday or Friday to northern California, but again the best
rain chances will be to the north of the Golden Gate. Overall --
all locations are expected to be significantly drier this upcoming
week compared to the last few weeks. In fact, the latest 6-10 day
precipitation outlook (valid December 13-17) from the climate
prediction center places much of central and Southern California
with favored odds of experiencing below normal precipitation.

&&

Aviation...as of 4:36 PM PST Saturday...generally MVFR except
IFR ceilings and visibilities in a band of heavy showers with a
low level cold front coupled to an amplifying 700 mb trough
pressing eastward over the Bay area; echo tops 20 thousand feet
with isolated tops to 25 thousand feet, lightning strikes so far
observed east of our cwa. Additional amplification in the 700 mb
trough including the trough at 500 mb tonight and Sunday generates
more showers. Drier weather returning by the end of the 00z taf
cycle.

With convergent winds focusing low level isotherms together, I.E.
Frontogenesis expect gusty prefrontal south winds to continue into
the early evening before diminishing. Surface winds forecast to
shift to southwest and west direction, but timing is low confidence
due to initially mature and closed 500 mb low offshore transforms
to an open wave before passing eastward over the cwa Sunday morning.
The mid level trough axis will move east of the cwa Sunday afternoon
with dry weather returning.

Vicinity of ksfo...MVFR ceiling with showers, possibly heavy, through
03z this evening, surface wind direction close to 210 degrees with
winds subsiding by 03z. Best guess is that the surface wind will
wait until late evening or early Sunday morning before shifting to
240 degree wind direction. West wind late Sunday morning at latest.

Sfo bridge approach...similar to ksfo.

Monterey Bay terminals...VFR-MVFR ceilings with showers, possibly
heavy at times, surface wind from the south. MVFR tonight with
showers. Winds shifting to west to northwest by Sunday afternoon.

&&

Marine...as of 12:59 PM PST Saturday...a secondary frontal
boundary will move through the coastal waters today. Scattered
thunderstorms will move into the coastal waters which could
produce lightning, gusty winds, and possible waterspouts. Breezy
to gusty south to southwest winds will continue through the day
today before diminishing this evening. A moderate west swell will
continue through Sunday. Steep wind waves will continue through
the day today creating hazardous seas.

&&

Mtr watches/warnings/advisories...
... Flood Watch...caz506-507
Small Craft Advisory...sf Bay until 9 PM
Small Craft Advisory...mry Bay until 9 PM
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm until 9 PM
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm until 9 PM
Small Craft Advisory...Pigeon pt to pt pinos 0-10 nm until 9 PM
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm until 9 PM
Small Craft Advisory...pt Arena to Pigeon pt 10-60 nm until 9 PM
Small Craft Advisory...Pigeon pt to pt Piedras Blancas 10-60 nm until 9 PM

&&

$$

Public forecast: Rowe
aviation: canepa
marine: bfg

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