Scientific Forecaster Discussion
fxus66 kmtr 100626
Area forecast discussion
National Weather Service San Francisco Bay area
1026 PM PST Fri Dec 9 2016
Synopsis...moist onshore flow will persist over the region through
tonight ahead of our next approaching system. Rain will increase in
coverage along the coastal ranges overnight ahead of a frontal
boundary that will spread from north to south across the region late
tonight through Saturday afternoon/evening. More widespread rainfall
will accompany this boundary as it moves through the region Saturday
during the day. Clearing conditions are forecast from late Saturday
through Sunday with the potential for unsettled conditions to return
for a good portion of next week.
Discussion...as of 8:55 PM PST Friday...latest Doppler radar
imagery has picked up several returns over the coverage area in
the span of the last few hours. Showers will continue across the
region through tonight ahead of an advancing cold front that is
moving in over northern California. Temperatures in this warm
frontal environment are running anywhere from 2 to 9 degrees
warmer than yesterday at this time with observations in the upper
50s to lower 60s at this hour. With dew points in the mid 50s and a
nearly saturated boundary layer the showers moving over the region
will not have any trouble reaching the ground. Only a few
locations over the North Bay have reported any rain thus far. The
Hawkeye RAWS at 2024 feet, Mount Saint Helena at 4140 feet and
Atlas Peak at 1680 feet have all reported 0.04" over the last 3
Models indicate showers will yield greater accumulations along
the coastal ranges and over elevated terrain with rain shadowing
likely over the inland valleys. Rain will increase significantly
over the North Bay region in the next few hours as the cold front
sags southward into our area. This trend will continue through the
night with rain spreading across the San Francisco Bay area
overnight. The main band of rain will spread south toward the
Monterey Bay area Saturday morning and continue through the
afternoon. 1" to 3" of rain are forecast along the coastal ranges
and over higher elevation location with 1/4" to 1" expected
across the low lying and possibly less over rain shadowed areas.
With precipitable water values around 1.5" off the coast rain rate
will need to be closely monitored as this system move into areas
with significant burn scars from fires earlier this year.
Showers are forecast to come to an end from north to south
overnight Saturday with dry conditions expected Sunday. Cooler
temperatures are also anticipated as the cold pool settles in over
After a brief break in the rain Sunday into Monday rain chances
are forecast to return to the region as early as Monday evening.
Models have not yet come to a consensus with the timing of the
next rain event but it appears the storm door has been left open and
unsettled wet weather is likely to persist through the bulk of
Aviation...as of 10:30 PM PST Friday...radar indicates showers
moving into the sfo Bay area. This is still moisture out ahead of
the front which will not arrive until Saturday evening. Shower
activity will decrease late this evening and Saturday morning then
rain associated with the front moves through late in the morning
and in the afternoon. Mostly MVFR cigs through the night. Cigs may
lower into IFR category Saturday afternoon.
Vicinity of ksfo...MVFR cigs through the night with cigs lowering
to around 1500 feet by 09z and below 1000 feet Saturday afternoon.
Light rain becoming more widespread after 16z. South winds
increasing after 09z with gusts to 20 kt.
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...patchy infrared conditions for fog/cigs around
sns through 09z. A few showers moving into the area after 09z
should help clear out the fog. Conditions may improve briefly to
MVFR late Saturday morning before rain with the front moves in and
conditions deteriorate again.
Marine...as of 8:16 PM PST Friday...a frontal system will move
across the area Saturday night. Southerly winds will increase
ahead of the front Saturday then switch to west to northwest
Saturday night as high pressure builds in behind the front.
... Arena to Pigeon pt 10-60 nm
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm from 3 am
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm
Small Craft Advisory...sf Bay from 3 am
Public forecast: CW
aviation: west pi
marine: west pi
Visit US at www.Weather.Gov/sanfrancisco