Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA
FXUS66 KMTR 091719
Area Forecast Discussion
National Weather Service San Francisco Bay Area
919 AM PST Fri Dec 9 2016
.SYNOPSIS...On-again, off-again light rain will continue today
under mostly cloudy skies and moist onshore flow. The next
organized front arrives early Saturday morning and then spreads
rain southward across the Bay Area and Central Coast through
Saturday. Dry weather returns Sunday and Monday with another
chance of rain by the middle of next week.
.DISCUSSION...as of 09:00 AM PST Friday...A moist onshore flow
persists this morning with only light rainfall amounts being
reported across the region. Meanwhile, another weak disturbance
approaching the Central Coast will allow for on-again, off-again
rain through the day, especially along the coastal ranges. With a
lack of good mid/upper level support, many inland valley locations
will experience little to no rainfall through the day today.
With that said, rain chances increase later today across the North
Bay region as a frontal boundary approaches from the north. This
system will then spread southward into the San Francisco Bay Area
late tonight into Saturday morning and through the Central Coast
late Saturday morning into the early afternoon. Rainfall associated
with this system is forecast to be greater than the past few days
with the heaviest rain occuring from late tonight over the northern
portion of the region that will then spread southward along the
coast through early Saturday afternoon. Have made adjustments to the
forecast through the next 24 hours to reflect the latest forecast
guidance as they are in decent agreement with one another in timing
of this boundary pushes through. A full updated forecast package
will be out by early afternoon.
.PREV DISCUSSION...as of 3:39 AM PST Friday...Some weak returns
continue to show up on the radar with moist onshore flow still in
place. Rain gages show many buckets still tipping but rainfall
remains light and widely scattered this morning with mild
temperatures in the mid and upper 50s. The forecast for today will
continue to show shower chances but more nuisance than anything
for today. Nam model does indicate some more organized rain will
occur over the Big Sur hills by this afternoon as a jet streak
feature takes aim on the Central Coast.
Then by later this evening the next significant warm front starts
to enter the north bay with rain developing overnight into early
Saturday morning. The front will slowly sag southward on Saturday
spreading periods of moderate rainfall across the district,
somewhat similar to the event that moved through on Weds night
into Thursday. Once again with a warm advection type pattern
expect the heaviest rain to fall in the hills of the North Bay and
Santa Cruz mountains where 1-3 inches of rainfall should be common
by Saturday afternoon.
Things will dry out in the Bay Area by late Saturday afternoon
while rain will linger for the Central Coast into Saturday
High confidence for drying conditions now on Sunday and Monday.
Pattern looks to remain active next week but confidence on
details is low. The Gfs wants to bring the next AR/warm front as
early as Tuesday afternoon while the ecmwf holds off until later
Weds. In this fast moving zonal flow off the Pacific were seeing
pretty dramatic run to run changes in the timing of systems so for
now the extended forecast will show rain chances. Low confidence
on specific timing at this time but high confidence that we`ll see
one or two more rain events next week which in combination with
the rain expected over the next 24-36 hours will keep our year to
date rainfall totals well above 100% for so early in the rain
.AVIATION...As of 9:20 AM PST Friday...Unsettled weather with
predominately southerly winds, periods of rain, restricted
visibility, and various cloud decks expected through the TAF
period. Model data shows rather wet air mass confined to first
4000 feet AGL down to the surface. Ceilingsvisbys will range from
IFR to VFR through the TAF period depending on positioning of
frontal boundaries and showers. Winds will mostly be light
southerly. As for the rain, look for VCSH to isolated rain showers
through most of the day today with heavier rain expected on
Saturday. The next best shot of rain will move into the North Bay
by 06-09Z before spreading southward through the night and early
Vicinity of KSFO...MVFR ceilings expected through the taf period,
however, brief periods of IFR or VFR may also be possible. Lower
confidence on the timing of breaks/deterioration given poorly
resolved nature of these system. Winds generally southerly .
Heavier rain returns at and prior to sunrise Saturday morning.
SFO Bridge Approach...Similar to KSFO.
Monterey Bay Terminals... Bulk of energy/moisture north of the
area this morning, but expect worsening conditions through the day
as moisture shifts southward. Winds generally light S-SE. VCSH
possible through the day with increasing rain rates on Saturday.
.MARINE...as of 8:18 AM PST Friday...Light to occasionally
moderate southerly winds will prevail today then increase tonight
and into early tomorrow as a frontal boundary approaches the area.
Winds will weaken and veer towards the northwest behind this front
for the remainder of the upcoming weekend. A dominate westerly
swell train will move across the waters through the remainder of
the work week.
PUBLIC FORECAST: RGass
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