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fxus66 kmtr 060607 
afdmtr

Area forecast discussion
National Weather Service San Francisco Bay area
1007 PM PST Thu Dec 5 2019

Synopsis....gearing up for another cold front that will sweep
through the area later Friday into Saturday resulting in
widespread rainfall and gusty winds. Post-frontal showers and
isolated thunderstorms will continue through Saturday night
before tapering off on Sunday.

&&

Discussion...as of 9:56 PM PST Thursday...no significant
forecast updates this evening as its the calm before the storm.
Satellite imagery shows well defined frontal boundary west of
Cape Mendocino with lots of cold core cumulus clouds farther back
near the low center. Latest model trends have continued to slow
the timing of main frontal rains. All short term guidance
including the NAM, rap and hrrr models bring initial rain bands
into the North Bay during the afternoon hours of Friday. Given the
main upper low is well west of the surface front its slowing the
arrival. This is some good news in terms of impacts to the Friday
afternoon commute as rain may not make it to the Golden Gate until
early evening before spreading southward during the overnight
hours with perhaps less impacts in terms of people on the
roadways.

Before the rain falls the southerly winds will ramp up ahead of
the front and be strongest along the coast and in the hills.
Southerly winds will gradually ramp up through the day and into
the evening ahead of the frontal passage. Currently the Wind
Advisory is in effect for the whole district with wind gusts from
30-50 mph expected, again strongest late afternoon/evening coast
and hills.

The main surface cold front will be through the district by
Saturday morning. However, expect a cool and dreary Saturday with
numerous rain showers as the core of the upper low passes
overhead. Isolated thunderstorms with small hail will certainly
be possible under this pattern with highs in the mid 50s to around
60. Snow levels will remain Above All Bay area peaks with this
system.

Flash Flood Watch remains in effect for the kincade burn scar from
4 PM Friday through 4 am Saturday. This covers the period when
intense cold frontal rains are expected to pass over the burn
area. Other trend of note with the 00z models is the NAM model has
trended wetter than the GFS model with the NAM showing a large
area of 5.5 inch storm totals across northern Sonoma by Sunday
afternoon while the GFS tops out around 3 inches. Its a little
unusual for the higher resolution NAM to be so much wetter at
this time scale. Will monitor trends but the NAM has a secondary
boundary Saturday afternoon as the core of the upper low passes
through.

Saturday looks wetter than Sunday but still plenty of shower
activity around for Sunday as well. Finally a drying trend for
Monday and Tuesday of next week.

&&

Previous discussion...as of 1:56 PM PST Thursday...under mostly cloudy
skies, area temperatures are mild, ranging from the mid 50s to
lower 60s, which are warmer than yesterday at this time by several
degrees. Tonight is expected to see low temps generally from the
mid 40s to lower 50s.

The next storm system is due impact the area on Friday and
Saturday with rainfall and gusty winds. Light showers are expected
to begin Friday ahead of this system. The cold front is expected
to move into the North Bay late Friday afternoon then gradually
moving south and east overnight. Given the dynamics with the
potent cold front and ample moisture, rainfall intensity will be
of concern across the North Bay, especially in the kincade burn
area. Hourly rainfall rates could exceed one half an inch in an
hour leading to debris flows or flash flooding. Therefore, a Flash
Flood Watch has been issued for the kincade burn area Friday
afternoon through Saturday morning. By Saturday morning the cold
front will have pushed through much of the Bay area. However, the
parent low and upper trough moving through will keep showers going
across the Bay area Saturday into Sunday. Enough instability will
be present with the frontal passage and with the upper trough
to justify a slight chance of thunderstorms for Friday and
Saturday. Rainfall totals from Friday into early Sunday range from
0.50"-1.25" in most urban areas, except 1.50-2.50" in the North
Bay valleys. In the hills/mountains, rain totals are forecast to
range from 1.50"-4.00" in the North Bay mountains, 1.25- 1.5" for
the East Bay hills, 1.50-3.00" in the Santa Cruz Mountains, and
1.00-3.00" across Monterey and San Benito co. Mountains. Lesser
amounts will be possible in the southern Salinas valley.

In addition to the rain, southerly winds will ramp up ahead of
and with the frontal passage. Winds will be strongest over the
coastal waters, near the coast, through south-east oriented
valleys, and in the higher terrain. A Wind Advisory has been
issued for the entire County Warning Area from 10 am Friday through 7 am Saturday.
Although the advisory is for the entire area, there will be wind-
sheltered areas that will see lesser wind speeds, including the
Monterey Bay coastline, and in the southern portion of the Santa
Clara Valley.

Lingering showers will be possible on Sunday as the upper trough
slowly exits. High pressure aloft building in over the state will
bring drier weather to the area Monday through at least Wednesday.
A weak system is forecast to move across northern California on
Wednesday which could bring a few showers to northern zones late
Wednesday, although latest model runs have weakened this system
further.

&&

Aviation...as of 10:05 PM PST Thursday...models indicate
abundant low level moisture over the area through the evening so
the latest forecasts call for MVFR cigs impacting sfo and the
approach. S-southeast winds begin to pick up above the surface tonight
causing possible low level wind shear. Winds at the surface increase Friday
morning under warm air advection ahead of the next frontal system.
Light rain should reach the Bay area by 20z. Moderate rain
expected to be confined to the North Bay Friday evening.

Vicinity of ksfo...cigs will be in and out of MVFR range through
the night. Bases predominately 3000-4000 ft. Low level wind shear possible
overnight tonight through Friday when winds at 2000 ft increase
more than the surface winds. Surface winds increase after 15z
with gusts to 15 kt. These winds will only increase as the day
GOES on Friday. Gusts to 30-35 kt after 20z.

Sfo bridge approach...similar to ksfo.

Monterey Bay terminals...cigs remaining in the VFR range. South
winds increasing in the Salinas valley with gusts to 20 kt after
14z. Light rain holding off until Friday night.

&&

Marine...as of 10:01 PM PST Thursday...a 981 mb low centered 800
miles west of Cape Blanco will wobble east towards the northern
California coast by Saturday night. This will bring another round
of gusty southerly winds to the coastal waters with gale force
winds expected by early Friday and persisting through Friday
night. Moderate northwest swell will gradually build to around 10
to 15 feet Friday over the northern waters then spread south
across the coastal waters over the weekend.

&&

Mtr watches/warnings/advisories...
... Pt to pt pinos 0-10 nm
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm
Small Craft Advisory...Pigeon pt to pt Piedras Blancas 10-60 nm
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm until 3 am
Small Craft Advisory...pt Arena to Pigeon pt 10-60 nm until 3 am
glw...pt Arena to pt Reyes 0-10 nm from 3 am
glw...pt Arena to Pigeon pt 10-60 nm from 3 am
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm from 12 am

&&

$$

Public forecast: rww
aviation: west pi
marine: west pi

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