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

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FXUS66 KMTR 191821

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1121 AM PDT Thu Oct 19 2017

.SYNOPSIS...A cold front is still expected to move through the
area later today and tonight bringing widespread rainfall to the
region. The bulk of the rain will be in the North Bay Mountains
and coastal ranges. Along with precipitation, expect cooler
temperatures and breezy onshore winds through Friday. By late
Friday morning dry conditions will return lasting through the
forecast period. A warming trend is then on the way over the
weekend and into early next week. Temperatures are predicted to be
well above average during the beginning of next week.


.DISCUSSION...As of 8:30 AM PDT Thursday...Current satellite
imagery showing widespread stratus across the central coast and
inland valleys as the marine layer sits at around 1000 to 1500 ft
per the Fort Ord Profiler. Clouds this morning even seen as far
inland as Livermore. For those areas that were not covered in
stratus yesterday morning, temperatures are in some cases as much
as 10 degrees warmer than this time yesterday. Increased cloud
cover along with an approaching cold front will keep temperatures
cool today with inland areas in the North and East Bays seeing
highs in the mid to upper 60s.

Current surface analysis and satellite imagery show the
approaching cold front by the Washington and Oregon coasts.
Rainfall is expected to reach the Sonoma and Marin county coastal
areas by this afternoon before becoming more widespread late this
evening and eventually covering the remainder of the CWA by
around midnight and lasting through the early morning. As of this
morning, little change is expected in the rainfall totals. The 12Z
and 06Z NAM forecasts have accumulations trending downwards
slightly in the North Bay Mountains from last night, now showing a
maximum of 0.65" for favored locations. Otherwise, there has been
little change in rainfall totals with the Santa Cruz Mountains
still expecting up to 0.25", elsewhere in the hills and North Bay
Valleys totals from 0.10-0.25", and a tenth of an inch or less for
all other valley locations. Rainfall is forecast to end by Friday
morning from north to south, with rainfall out of the area
completely by late morning. Cool and breezy conditions are then
expected for the rest of the day Friday.


.PREV DISCUSSION...As of 03:43 AM PDT Thursday...A warming trend
is forecast during the weekend as an upper level ridge begins to
build over California. The ridge is then expected to strengthen
along the West Coast early next week, resulting in a marked
warming trend. light offshore flow is also forecast to develop
early next week resulting in warm and dry conditions across our
entire area on Monday and Tuesday, with widespread highs in the
80s to around 90. Slight cooling is indicated by midweek at the
coast with a return to onshore flow, but warm conditions will
persist inland.


.AVIATION...As of 11:15 AM PDT Thursday...For 18Z Tafs. Mixed
ceilings with some lingering overnight stratus now mixing out and
some mid level clouds above obscuring the view on satellite.
Ceilings are ranging from IFR to VFR conditions around the region
and are anticipated to gradually improve through the late morning
into early afternoon, but deteriorate later today ahead of the
arrival of a cold frontal system. Winds will shift and become
breezy to occasionally gusty, especially over the North Bay and
near KSFO/KOAK as the fropa occurs. Local WRF skew-t momentum
transfer indicates localized gusts up to around 30 knots will be
possible with the fropa as winds mix down from above. Some
nocturnal decoupling with these winds aloft could also bring
localized low level wind shear, especially to KSFO KSTS and KAPC
as frontal passage occurs. LLWS not yet added to TAFs but will be
looked at closer with next issuance. Rain will primarily being
between 01Z in the North Bay and closer to 04-06Z for the South
Bay/Monterey Bay. The heaviest amounts will be over the coastal
ranges and North Bay hills and mountains, with lighter amounts
farther southward and inland. Rains will be sufficient to wet
runways. Ceilings will generally be borderline MVFR to VFR by
tomorrow morning in the wake of the front.

Overall forecast confidence: moderate

Vicinity of KSFO...MVFR thru late morning becoming VFR until
around 03Z where borderline MVFR cigs return with FROPA.
Breezy west winds 15 to 20 mph with gusts 25 to 30 mph possible
with FROPA. Rain will wet runways but taper off but sunrise. LLWS
a possibility with FROPA (light at surface gusting to 25-30mph at
2000ft AGL). Cigs improving by sunrise FRI.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Mixed ceilings, bouncing between IFR and
VFR through the morning, low confidence. Ceilings will deteriorate
by mid to late afternoon and drop firmly into the IFR band ahead
of FROPA. Light onshore winds and light rain will accompany FROPA.
LLWS not a concern this far south from base of trough. CIGS
improving by sunrise FRI.


.FIRE WEATHER...As of 3:43 AM PDT Thursday...Relative humidity
recoveries have improved this morning with even most hilltop areas
seeing 80-100% values. Onshore winds will continue to gradually
increase today and moderate and locally gusty winds are expected.
Models continue to move an early season cold front through the
district late today and tonight. Rainfall will begin in the North
Bay this afternoon and spread across most of the remainder of the
district tonight. The models have just recently trended slightly
wetter with this system, although rainfall amounts are still
expected to be mostly light. Wetting rains are most likely in the
North Bay Mountains and perhaps locally in the Santa Cruz
Mountains. Rainfall should end in most areas by Friday morning.
Friday will be a cool day with brisk northwest winds.

A warming trend will begin over the upcoming weekend as a high
pressure ridge begins to build over California. This ridge will
strengthen into early next week and result in warm and dry
conditions districtwide from Monday through Wednesday. Will need
to be wary of possible gusty offshore winds in the hills by Sunday
night and especially Monday night.

&& of 2:47 AM PDT Thursday...A cold front is forecast
to pass through the region later this evening. Winds will increase
along and just behind the front, occasionally gusting around 25
knots at times. Showers will be possible ahead of and along the
cold front, as well. A very robust northwest, long period swell
will arrive late this afternoon and through the evening,
generating large waves and hazardous seas through at least
Saturday. Operators of small crafts are urged to use extreme
caution if out at sea over the next several days.


     .Tday...SCA...Pt Arena to Pigeon Pt 10-60 nm from 5 PM




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