Scientific Forecaster Discussion
fxus66 kmtr 070648
Area forecast discussion
National Weather Service San Francisco Bay area
1048 PM PST Fri Dec 6 2019
Synopsis....southerly winds will continue to ramp up ahead of an
incoming cold front. Rain will develop across the North Bay this
afternoon and then spread southward overnight. Showery and cool
on Saturday with isolated thunderstorms possible. Lingering
showers on Sunday but trending drier by afternoon. Dry weather
returns Monday and Tuesday with a slight chance of rain by
Discussion...as of 09:15 PM PST Friday...a potent cold front
continues to March through the Bay area this evening producing
light to moderate rain with pockets of heavy rain. If there is one
thing to be said about the evolution of the front is slower.
Several days ago models had the front through much earlier in the
day and with some speed too. The last day or two models really
slowed down the arrival and fropa speed. The slow speed is
definitely playing out tonight, which is a likely due to a deeper
surface low off the coast with a large occluded front. The latest
wpc surface analysis is rather interesting and shows the complex
set up currently off the West Coast. Rainfall amounts thus far
with the front have been highest over the North Bay coastal
mountains at over 2" and counting. North Bay valley locations are
less, but still a few tenths to nearly 1". South of the Golden
Gate has generally been a few hundredths to a few tenths, which
makes sense since the front is still moving S and east. The lower
amounts to the south are also likely due to the fact the front is
weakening slightly as it progress S and east.
Two other items of note this evening are lightning and wind. Off
the coast behind the front and east the main low is an area of
instability. Glm and other lightning detection networks have been
picking up a decent amount of lightning the last 6-12 hours.
Satellite derived instability puts that region in an area of
200-400 j/kg cape. Winds have been strong over the coastal waters,
coastline and higher terrain since this morning. Highest gusts
have been 50-70 mph this far.
For the rest of the evening and into Saturday - the front will
continue to move S and east with rain and gusty winds. As the 00z
models begin to roll in they show the front eventually weakening
and falling apart as it moves S and east by 09-12z Saturday. Quick on
its heels will be another area of instability and a developing
second cold front as the upstream low moves eastward. This
secondary cold front has more instability associated with and has
a higher likelihood for thunder over the Bay area on Saturday.
MOS guidance gives the North Bay 40-50% chc of thunder, which is
pretty decent. Storm Prediction Center also has much of the area in general mention of
thunder on Saturday. Therefore will leave the Flash Flood Watch
out for now as the next pulse of instability may arrive by
Saturday morning. If anything the next shift may need to extend the
watch if the offshore lighting holds together through Saturday
afternoon. Will also leave the Wind Advisory up for now as well.
As the front falls apart winds will weaken and the advisory could
be canceled early.
No updates to the forecast are being made this evening as wet and
windy weather continues.
Previous discussion...as of 1:30 PM PST Friday...the approaching storm
system continues to slowly near the coast. The low center is now
about 550 miles west off Cape Mendocino, with the associated
frontal band nearing the north coast. Latest computer models
bring the first frontal band into Sonoma County just after 00z (4
PM pst), holding off rain in San Francisco until after 7 PM. The
bulk of the rainfall with this first wave is forecast to fall in
the North Bay, and to a lesser extent, the Santa Cruz Mountains. A
second front will sweep through the entire district on Saturday
and will bring more rainfall to the area through Saturday evening,
along with a slight chance of thunderstorms with small hail
possible. The upper low will then finally move south along the
coast on Sunday keeping showers going in its vicinity as it exits
the area Sunday night. Latest storm-total rainfall estimates
still show some 3-4 inch maxes for the North Bay hills with 1.5-3
inches for the Santa Cruz/Santa Lucia mtns. Rain shadowed Santa
Salinas valley 0.25-0.50 on average with 0.50-1.00 inch elsewhere
through Sunday night.
The Flash Flood Watch for the kincade burn area still looks valid
with all models showing a bullseye of qpf in that area and the
front strongest as it moves through northern Sonoma County this
evening. Will need to monitor any convective lines and short term
rain rates that could lead to problems over the burn area.
A Wind Advisory remains in effect for all areas through Saturday
morning. Strongest winds are expected along the coast and in the
higher terrain. Gusty southeast winds will ramp up again during
the day Saturday as the second front moves through.
Dry weather returns Monday and Tuesday with seasonable
temperatures as high pressure rebuilds. A weak front may try and
knock into the ridge by Wednesday with a slight chance of rain,
but it looks insignificant at this time.
Aviation...as of 10:45 PM PST Friday...frontal system weakening
as it moves through the Bay area but is bringing light rain and
MVFR cigs. A break is expected later tonight into Saturday morning
behind the front. Lightning strikes associated with a second
front have been detected about 150 miles off the coast. This
system will arrive in the North Bay after 15z and the Bay area
after 18z bringing light to moderate rain that will last through
Saturday night. Latest models show a 35 percent chance of tstorms
in sfo Saturday afternoon so have added thunderstorms in the vicinity to Bay area tafs.
Winds gusting to 25 kt may decrease some overnight but should pick
up again with the next system. Gusts to 30-35 kt possible in
heavier rains. Things should wind down Saturday night with just
some scattered showers.
Vicinity of ksfo...MVFR cigs with bases 2500-3000 feet.
Light rain ending after 10z but resuming after 18z. Possibility of
a thunderstorm in the afternoon with occasional moderate rain.
Winds gusting to 25-28 kt through Saturday...locally higher in
heavier rains and thunderstorms. Stronger winds above the surface
could cause low level wind shear up to 2000 ft.
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...VFR cigs with bases around 4000 ft. Light
rain through 15z with a seconds front bringing another period of
light rain by Saturday evening. Gusty south to southeast winds to
20-25 kt at mry and 30-35 kt in the Salinas valley. Stronger
winds above the surface could cause low level wind shear at mry up to 2000 ft.
Marine...as of 09:36 PM PST Friday...a pair of frontal boundaries
will move through the waters from north to south over the next 24
to 36 hours bringing unsettled weather. Scattered thunderstorms
have been detected over the outer waters and could move into the
northern waters on Saturday. Gale force southerly winds can be
expected through early Saturday before slowly decreasing.
Flash Flood Watch...caz506-507
glw...sf Bay until 3 am
glw...pt Arena to pt Reyes 0-10 nm until 3 am
glw...pt Reyes to Pigeon pt 0-10 nm until 3 am
glw...Pigeon pt to pt pinos 0-10 nm until 3 am
glw...pt pinos to pt Piedras Blancas 0-10 nm until 3 am
glw...pt Arena to Pigeon pt 10-60 nm until 3 am
glw...Pigeon pt to pt Piedras Blancas 10-60 nm until 3 am
Small Craft Advisory...mry Bay
Public forecast: mm/Sims
aviation: west pi
marine: west pi
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