Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
459 PM PDT Friday Oct 24 2014
Synopsis...a cold front will approach the coast later tonight
increasing south winds and rain chances. The front will pass
through quickly in the morning...but cooler unstable air will bring
showers to much of the area Saturday afternoon and evening. A few
thunderstorms may develop especially Santa Cruz Mountains
northward. Drier and cooler weather is expected Sunday with a
warming trend beginning Monday as high pressure rebuilds along the
Discussion...as of 01:29 PM PDT Friday...offshore winds have
turned to southerly as a forerunner to the approaching cold front
and marine layer deepening and starting to mix out. Cold core
trough located around 135w with vorticity center around 35n and has
bottomed out. Feature will progress eastward and shear to the
northeast as it moves onshore Saturday. Given extent of cold front
to the south...believe this system will produce rainfall across
most of the area with the heaviest precipitation in the North Bay
and Santa Cruz Mountains northward. Amounts will vary considerably
with less than a tenth of an inch most of Monterey County to as
much as three quarters inch North Bay mountains.
Main meteorological concerns will be significant pre frontal south
winds along the coast and higher elevations of coastal mountains.
Planning on issuing Wind Advisory for these areas late tonight and
Saturday morning. Higher resolution NAM and WRF models indicate
quick frontal passage into the Bay area early in the morning and
through the rest of the district by early afternoon. Cool unstable
airmass in the wake of the cold front will trigger convection with
possible thunderstorms Santa Cruz Mountains northward. Should see
a brief lull in precipitation after the cold front...then Saturday
afternoon and evening should be active with convection.
Believe the NAM is a little overdone with high cape and helicity
values...but GFS is likely underdone with lower model resolution
issues. Interesting to note that unusually warm sea surface
temperatures could assist convective development Saturday
afternoon as the cooler airmass moves overhead. Confidence in
convective strength is low...but confidence is moderate for a
thunderstorm or two at least North Bay.
Dynamics with this system are not particularly impressive
with an apparent stable layer above 700 mb that should cap any
significant convection. However...if dew points stay high
enough...would be able to break through this stable layer aloft.
The coolest day should be Sunday with northwest flow and the
airmass will begin to dry out. Model trends are leaning toward
stronger high pressure next week keeping the jet stream well to
our north. Look for drier and warmer conditions through midweek.
Could be a few showers extreme North Bay as weak systems push through
the Pacific northwest. Lots of differences in the medium range guidance
late next week with a trend toward slower approach of next trough
and delaying next significant threat for rain until at least next
Aviation...as of 4:59 PM PDT Friday...for 00z tafs. A well
defined low pressure system spinning approximately 500 nm off the
northern California coast will approach the region tonight into
Saturday bringing stronger southeasterly winds and wet runways.
T-storms remain a concert for Saturday afternoon but keep out of
the taf at this time.
Vicinity of ksfo...VFR conditions expected through this evening. Light
southwest will turn southeasterly and strengthen overnight. -Ra
expected around 11z tonight. By Saturday morning moderate and
gusty southeast winds and wet runways will impact the morning
traffic. Winds expected to veer southwesterly Saturday afternoon.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...VFR conditions expected through
this evening. Light southwest will turn southeasterly overnight.
-Shra expected to develop around 12z-15z Saturday. By Saturday
morning southeast winds and wet runways will impact area
terminals. Winds expected to veer southwesterly Saturday
Marine...as of 3:00 PM PDT Friday...a 992 mb low will deepen
west of Cape Mendocino this evening with increasing southerly
winds. Gale force winds will develop overnight across the northern
waters with strong small craft winds developing elsewhere. A line
of rain and showers will move through the waters early Saturday
morning with thunderstorms possible north of the Golden Gate
Saturday afternoon. Winds will ease and showers will end by
Saturday night with lighter winds and moderate seas Sunday into
Wind Advisory...caz505-507-509-512-529 from 3 am until
Small Craft Advisory...sf Bay north of the Bay Bridge from 3 am
Small Craft Advisory...sf Bay south of the Bay Bridge from 3 am
Small Craft Advisory...Pigeon pt to pt pinos 0-10 nm from 3 am
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm from 3 am
Small Craft Advisory...pt pinos to pt Piedras Blancas 10-60 nm from 3 am
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm from 9 PM
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm from 9 PM
Small Craft Advisory...pt Arena to pt Reyes 10-60 nm from 9 PM
Small Craft Advisory...pt Reyes to Pigeon pt 10-60 nm from 9 PM
Small Craft Advisory...Pigeon pt to pt pinos 10-60 nm from 9 PM
Small Craft Advisory...rough bar advisory for sf bar from 5 am
Public forecast: kbb
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