Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA
FXUS66 KMTR 081755
Area Forecast Discussion
National Weather Service San Francisco Bay Area
955 AM PST Thu Dec 8 2016
.SYNOPSIS...Periods of rain will continue this morning and taper
to showers this afternoon and evening. Shower threat continues on
Friday but not as widespread. Next front arrives Saturday with
another chance of rain. Mainly dry Sunday and Monday then more
rain possible by middle of next week.
.DISCUSSION...as of 8:30 AM PST Thursday...Based off of current
radar trends plus observations from various spots across our
region, the bulk of the rainfall has shifted into the southern
part of our CWA. Latest 1 hour amounts indicate the highest values
over Monterey and Santa Cruz counties although no spots are above
a quarter of an inch. Please see SFOPNSMTR for the latest
unofficial 24 hour rainfall totals. Also worth noting that thanks
to more humidity and winds a few parts of the CWA are running well
ahead of yesterday. Monterey was 23 degrees above values from
Rain chances will remain in the picture at least through Saturday
as the moist flow remains overhead while weak disturbances go
across. Latest NAM brings widespread rainfall back Friday evening
to Friday night.
.PREVIOUS DISCUSSION...Radar still picking up plenty of returns
this morning but rain rates and totals have been light to
moderate. Some of the wettest locations around Marin county and
the Santa Cruz mtns are approaching 1 inch storm totals so far. In
addition to the beneficial rains overnight the warm front has
allowed for much milder overnight lows compared to last night with
readings in the 40s to around 50...feeling much warmer.
The overall pattern remains pretty complex and models are still
showing fairly significant run to run changes. In the short term
expect steady light...to locally moderate rain to continue through
the morning rush hour. Then the activity should start to become
less widespread in nature and more focused over the coastal hills
as synoptic forcing ends.
Have scaled back pops for tonight to just show lingering showers
overnight with the scattered shower chances continuing into
Friday. Models now bring another warm frontal type feature in by
early Saturday morning spreading from north to south...not to
different from our current event. It finally appears the nam/gfs
and euro are all on board with rain chances but exact timing and
location of rainfall remains frustratingly difficult to pinpoint
given the lack of a surface cold front and typical cold core upper
low. These warm advection rains lack the upper dynamics and
surface cold frontal passage to really crank up the rain rates.
Rain will end from north to south by Saturday afternoon/evening
and latest guidance now suggests things dry out for Sunday and
Monday. Even thats a low confidence forecast as with this fast
moving zonal flow the timing of individual shortwaves can change
Mid range forecast remains pretty generic with some type of precip
chances in the Tue/Wed time frame...best chance in the north bay.
Rather than try and time individual model runs thinks it prudent
to just keep some low pops in the forecast for now. Models do seem
to be honing in on a potentially deep and cold trough by about
next Thu. The good news is continued early season beneficial rain
and mountain snow looks to continue for the foreseeable future.
.AVIATION...as of 09:41 AM PST Thursday...While rain showers have
diminished in coverage this morning, expecting them to persist
over portions of the region through late morning as a warm front
continues to move across the San Francisco Bay Area. Thus, a wide
range of ceilings and visibilities will continue over regional
terminals and result in IFR to MVFR CIGS through much of the day.
In addition, moist quasizonal flow will persist across the region
and result in isolated to scattered showers off and on throughout
a good portion of the day. Similar conditions will continue into
the overnight with low ceilings and visibilities along with
generally light winds.
Vicinity of KSFO...IFR conditions to prevail through midmorning
with intermittent IFR to MVFR CIGS/VSBYS through the remainder of
the day as -SHRA move through the region. Northerly winds are
forecast to become southerly later this morning as a warm front
pushes through the San Francisco Bay Area. Showers should tapper
off some through the afternoon before returning overnight into
SFO Bridge Approach...Similar to KSFO.
Monterey Bay Terminals...MVFR to VFR conditions are forecast
through the morning with more prevailing MVFR to IFR conditions
this afternoon and evening. On and off again showers will also be
possible through the day and again during the overnight hours with
continued low CIGS/VSBYS.
.MARINE...as of 09:41 AM PST Thursday...A warm front will continue
to move through the region this morning. Strong and gusty south
to southeast winds will gradually weaken today and veer west. The
strongest winds are expected along the coast today. A dominate
westerly swell train will move across the waters today through the
remainder of the work week.
.Tday...SCA...Mry Bay until 3 PM
SCA...Pt Arena to Pt Reyes 0-10 nm until 3 PM
SCA...Pt Reyes to Pigeon Pt 0-10 nm until 3 PM
SCA...Pigeon Pt to Pt Pinos 0-10 nm until 3 PM
SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm until 3 PM
SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm until 3 PM
PUBLIC FORECAST: Bell/RWW
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