Hail in California, November 4, 2017

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1051 PM PDT Fri Nov 3 2017

.SYNOPSIS...A weak cold front approaching from the northwest will
generate periods of rain tonight through at least midday Saturday.
Rainfall amounts will mostly be light, but brief heavy downpours
are possible. Another cold front may bring additional light rain
late Sunday and Sunday night, mainly to the northern portion of
our region. Dry weather is forecast from Monday through most of
Wednesday. A potentially stronger and wetter system may impact the
region from late next Wednesday into Friday.

&&

.DISCUSSION...As of 9:00 PM PDT Friday...KMUX radar currently
shows bands of precipitation across the North Bay associated with
a weak cold frontal boundary that is slowly sagging southward.
Radar also shows isolated showers elsewhere across the SF Bay
Area. Rain rates are mostly light, although there have been
reports of brief heavy downpours near Santa Rosa this evening.
Rainfall totals over the past 24 hours have generally been less
than a tenth of an inch. However, locally up to a quarter of an
inch has fallen in the North Bay, with up to a half inch in
isolated portions of the Santa Cruz and Santa Lucia Mountains.

Periods of rain will continue overnight, mainly in the North Bay,
as the front continues its slow southward progression. According
to the latest NAM and HRRR models, rainfall along the frontal
boundary will shift south of the Golden Gate around sunrise
Saturday morning. By Saturday afternoon, scattered light
precipitation is mostly expected to be confined to areas from
Santa Clara County southward. The frontal boundary is forecast to
dissipate by late in the afternoon and precipitation will likely
end in most areas by Saturday evening. Additional rainfall through
Saturday evening is expected to be on the order of a tenth to a
quarter of an inch.

A shortwave trough currently located along the British Columbia
coast is forecast to dig south through Sunday. A cold frontal
boundary associated with this trough may produce additional
rainfall in our area from late Sunday afternoon through Sunday
night. The latest GFS and ECMWF both indicate this frontal system
will wash out before reaching Sonoma County, and thereby produce
little or no rainfall in our forecast area. The NAM and the
Canadian, by contrast, bring a band of light precipitation into
the North Bay around sunset on Sunday, with light rain reaching
as far south as San Francisco and Concord by late Sunday night.
Even if the NAM and Canadian verify, rainfall amounts will be very
light.

Tonight's system and Sunday night's system will bring cool
air into our region. So expect a couple of chilly nights once
skies clear out early next week. The coolest night will likely be
Monday night when overnight lows in the inland valleys could drop
into the mid 30s. Otherwise expect dry weather from Monday through
most of Wednesday, along with seasonably cool daytime
temperatures.

The longer range models agree that a stronger and wetter system
will impact our region by late next Wednesday, with the potential
for periods of rain and gusty winds from Wednesday night on into
next Friday.

Forecast updates this evening included adjustments to timing of
rain chances, and increases in overnight lows during periods of
cloudy wet weather.

&&

.AVIATION...As of 10:51 PM PDT Friday...The cold front moved by
Point Arena buoy at 00z (5 pm PDT), but has yet to pass by Bodega
Bay buoy. The front is gently sloped and a slow mover at about 10
mph, forward progression being primarily building surface pressures
behind the front while there's little contribution to forward motion
by nearly parallel winds aloft above the front. 06z tafs indicate
VFR and MVFR cigs, except thought is that IFR cigs and vsbys are
more probable over the North Bay as the surface to lower level cold
front presses into consolidating higher dewpoint air and 100-150 j/kg
MUCAPE just ahead of the front. Caveat, patchy IFR cigs may spread
southeast across the remaining forecast area overnight if warm air
advection light rain interacts with near surface cooler air advection
causing frontal fog to develop.

Vicinity of KSFO...S-SE wind 5-10 knots tonight into Saturday morning.
The cold front becomes nearly stalled along the San Mateo Coast Saturday
morning with best guess for wind shift to W-NW being 20z Saturday. Cigs
VFR/MVFR though as mentioned if conditions become more favorable for
frontal fog then visibilities may become lower than advertised in present
06z taf.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Light/variable winds ahead of a slow moving
cold front that'll reach the Monterey Bay Saturday afternoon. The 0
deg Celsius 850 mb isotherm reaches the Monterey Peninsula Sunday
morning. A chilly early November air mass will steadily modify through
Sunday, however expect a lowering freezing level and NAM/WRF forecasts
high boundary layer RH (at least 80%), and 925 mb level RH 90-100%
most likely resulting in low clouds Sunday.

&&

.MARINE...as of 10:38 PM PDT Friday...A cold front will move southeast
over the coastal waters and bays late tonight and Saturday with periods
of light rain, and post frontal winds becoming northwesterly. Seas will
gradually increase through the weekend. A second cold front will arrive
later Sunday into Monday.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...None.

PUBLIC FORECAST: Dykema
AVIATION/MARINE: Canepa

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