Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA
FXUS66 KMTR 250402
Area Forecast Discussion
National Weather Service San Francisco Bay Area
902 PM PDT Mon Oct 24 2016
.SYNOPSIS...Periods of light rain will persist over the northern
half of the region through Tuesday morning before conditions dry out
region-wide by Tuesday night. Our next system will arrive on
Thursday and bring widespread rainfall to the region through Friday
morning. Unsettled conditions will persist into the weekend with
another round of widespread rainfall likely Saturday night into
.DISCUSSION...as of 8:59 PM PDT Monday...A cold front associated
with a large upper level low spinning near 48N/132W and located
approximately 350 mile west of Cape Flattery, Washington has
stalled off the California coast this evening. This has resulted
in a very slow southward progression of the rain band stretched
across the greater Bay Area. The heaviest rainfall accumulations
have been over the mountains of the North Bay with a 6 hour total
of 2.88" in Venado, 1.46" at the Hawkeye RAWS and 1.3" over Mount
St. Helena. Along the coast 6 hour accumulations reached anywhere
from 0.50" to 1" while North Bay Valleys reported 0.10"- 0.50".
Lighter accumulations of a tenth or less were been reported from
the San Francisco Bay south with dry conditions persisting south
of the Monterey Bay.
Latest forecast solutions show very little southward progression
of the rain band as the low shifting northward overnight and
Tuesday. The frontal boundary over the area is forecast to weaken
and shift northward as it is stretched apart. Therefore
anticipate mostly dry conditions south of the Bay Area Tuesday
with periods of light rain over the North Bay through Tuesday
afternoon. Mainly dry conditions are forecast Wednesday as the
region transitions from one system to the next.
From previous discussion...By late Wednesday into Thursday, the
forecast models have been persistent in showing a mid/upper level
low developing west of the central/southern California coast. As
this feature slowly shifts toward the coast and then begins to
lift to the northeast, rain will return to a good portion of the
region. There continues to be differences in the forecast models
on the timing and exact location of this system off of our coast.
Regardless, it appears precipitation will increase in coverage
over the region from Thursday morning through the remainder of the
daytime hours. Rainfall amounts will be more widespread and less
focused on just the coastal ranges than this current system
Unsettled conditions will likely persist into Saturday yet a lull
in widespread rainfall is expected during the daytime hours. The
past few model runs have picked up on yet another chance of
widespread rainfall from late Saturday night into Sunday as yet
another mid/upper level low rotates toward northern California
through the broader upper level trough. Through this weekend,
rainfall amounts of 1.00" to 4.00" will be possible over the
Coastal Ranges with 0.50" to 1.50" in the urban areas (lower
amounts inland and to the south with greatest in the North Bay).
Given the unsettled conditions and upper level trough along the
West Coast, cooler than average daytime temperatures will likely
persist through the forecast period.
.AVIATION...as of 5:18 PM PDT Monday...Plume of moisture on SW
flow aloft with MVFR/VFR conditions over the forecast area.
Moderate rain continues over the North Bay counties, also pockets
of light rain or sprinkles continue to the south as a weakening
surface trough approaches the coast. A weakening ripple of low
level thermal troughing moves eastward over the forecast area this
evening likely squeezing out additional pockets of light rain or
sprinkles to the south of the Golden Gate; localized IFR cigs are
possible due to this cooling over the North Bay.
Areas of gusty south to southeast winds per area metars is resulting
in a rain shadow effect in the lee of terrain in areas south of
the Golden Gate. On the other hand, showers have been getting some
localized enhancement from a combination of low level cooling
(earlier today and last night) and orographics over the Santa Cruz
mtns and Big Sur coast/Santa Lucia mtns.
Vicinity of KSFO...VFR/MVFR cigs with SE winds gusting to the 20
knot range this evening. Tempo light rain this evening with a few
passing showers lingering tonight. Wind flow likely staying high
enough to preclude IFR in low cigs and/or fog tonight and Tuesday
SFO Bridge Approach...Similar to KSFO.
Monterey Bay Terminals...VFR/MVFR cigs with SE winds in the Salinas
Valley, but variable winds along the Monterey Peninsula to the Big
Sur Coast. Pockets of light rain or light drizzle possible later
.MARINE...as of 8:45 PM PDT Monday...A frontal boundary continues
to drift southward across the coastal waters. Short period
southerly fresh swell will continue to mix in with west to
northwest swell causing hazardous seas. The front is forecast to
stall and weaken over the San Francisco bay area late tonight.
Light to moderate showers will persist off and on through the
forecast period. Another low pressure system is expected to
impact the coastal waters Thursday through Friday.
.Tngt...SCA...Pt Arena to Pt Reyes 0-10 nm
SCA...Pt Reyes to Pigeon Pt 0-10 nm
SCA...Pigeon Pt to Pt Pinos 0-10 nm
SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
SCA...Pt Arena to Pigeon Pt 10-60 nm
PUBLIC FORECAST: CW
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