Scientific Forecaster Discussion
fxus66 kmtr 171724
Area forecast discussion
National Weather Service San Francisco Bay area
issued by National Weather Service Los Angeles/Oxnard California
910 am PST Wed Jan 17 2018
Synopsis...areas of dense fog this morning will give way to
mostly cloudy skies as high clouds increase ahead of the next
Pacific front. Light rain will start in the North Bay Thursday
morning and then spread southward through the Bay area and central
coast by Thursday evening. Rain will quickly turn to showers
Thursday night into Friday as much colder air settles over the
region. Saturday morning will start out chilly but skies should be
mostly clear and dry for the start of the weekend. The next front
approaches later Sunday into Monday with the next chance of rain.
Discussion...as of 09:10 am PST Wednesday...areas of fog and low
clouds continue this morning, but will likely lift and dissipate
for most areas later this morning. Otherwise the forecast in the
near term is on track. Mid to high level clouds will continue to
stream across the region, out ahead of an approaching cold front
that was about 500 miles northwest of the Bay area. Similar high
temperatures are expected for today as yesterday with highs
generally near 60 to 70 degrees. Please see the previous forecast
discussion below for additional details.
Previous discussion...as of 3:25 am PST Wednesday...short term concern is
dense fog being reported throughout the region. 3 am obs include
dense fog at Napa, Concord, Novato, Monterey, Salinas and Half
Moon Bay. Fog product is starting to become obscured as high
clouds pass overhead but dense fog appears to be over the coastal
waters and down the Salinas valley. Heads up for the early morning
commuters across the region.
Similar to the last few days once any morning fog clears skies
will be partly to mostly cloudy today as mid and high clouds
continue to come in off the Pacific ahead of the incoming trough
and surface cold front. Upshot is that today remains dry with
highs upper 50s to mid 60s most areas.
Long anticipated cold front looks to arrive on Thursday. In terms
of bringing significant rain, have to say the latest model trends
have been drier. The system will have plenty of cold air
associated with it and not much of a moisture tap (no talk of
atmospheric rivers lately) as the main upper trough and cold air
are dropping down from the Gulf of Alaska.
Following the latest model runs we expect light rain in the North
Bay Thursday morning then reaching the greater Bay area
midday/afternoon before washing out over the central coast by
Thursday night. The fairly fast moving nature of the surface front
will preclude significant rainfall with totals in the 0.25-0.75 on
Steady precip looks to shut off rapidly on the backside of the
front with Post-frontal showers more favorable over the Sierra,
our coastal hills and over the ocean due to the warmer SST. Anyway
the forecast will keep shower chances going on Friday but there
will be plenty of sunny breaks and a bite in the air as daytime
highs struggle to reach the mid 50s.
Friday night will feel cold as temps aloft will plummet leading to
widespread 30s and lower 40s by sunrise Saturday morning. The
start of the weekend looks dry but cool with highs in the mid 50s
for Saturday with an upper trough aloft.
Latest trends for the next system show rain moving into the North
Bay Sunday night and then dropping through the Bay area and
central coast for the Monday morning commute. Initial signals
suggest another light to low end moderate rain event as the front
will move through quickly.
General consensus suggests one final system around midweek then
high pressure looks to build heading into the last weekend of
Aviation...as of 4:30 am PST Wednesday...metar observations show
fog is limited to the coast and the inland valleys. Low cigs cover
much of the sfo Bay area but so far vsbys have remained 5 miles or
greater. Cloudcover should prevent dense fog from developing at
sfo or sjc but the lack of clouds at Oak may allow vsby there to
drop lower possibly down to 1-3 miles. Models indicate more drying
in the afternoon so cigs should burn off after 20-21z. Warm front
approaches late tonight with some pre-frontal rains beginning in
the North Bay after 06z spreading south during the night. Winds
will remain light.
Vicinity of ksfo...MVFR through 21z with vsbys down to 3-5 miles
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...some clear skies are mixed with fog
around mry as the drainage wind struggles to develop. Looks like
fog will be in and out of mry this morning so latest forecast
calls for tempo dense fog through 16z. Dense fog could possibly
linger on a little longer. For sns dense fog is expected through
18z then lifting to IFR cigs. Clouds becoming sct after 20z as
models forecast drier air to move in.
Marine...as of 09:10 am PST Wednesday...high pressure to the
west will shift south as a low pressure system approaches the
Pacific northwest. A frontal system associated with the low will
move through the area on Thursday. Gusty southerly will form
through tonight, then shift quickly to westerly on Thursday,
before becoming northwesterly by Friday. A large long-period swell
will bring dangerous breaking waves to the coastline tonight
through at least Friday. Dense fog with low visibility will impact
some areas today.
... Arena to Pigeon pt 10-60 nm
Small Craft Advisory...rough bar advisory for sf bar
Public forecast: rm/rww
aviation: west pi
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