Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
957 am PST sun Dec 28 2014
..a strong cold but dry storm system expected to arrive on Tuesday
will bring gusty winds and cold temperatures to the area from late
Tuesday through early Wednesday...
Synopsis...high pressure positioned over the forecast area will
provide dry and seasonable weather through Monday. On Tuesday a
strong but mainly dry cold storm system will move through the
state. Isolated showers with snow levels to around 2500 feet
across the Bay area. However the main story and impact will be
blustery winds later Tuesday into Wednesday...especially in the
hills. High pressure rebuilds Thursday through next weekend with
dry and seasonable weather.
Discussion...as of 9:40 am PST Sunday...although chilly
on average temperatures have started off between 2 to 5 degrees
warmer this morning than those observed yesterday morning. In fact
Napa Airport is reporting a temperature 36 which is 5 degrees
warmer than yesterday at this hour. There are a few cold spots
this morning near Santa Cruz where temperatures fell below
freezing such as Scotts Valley which is reporting a low of 30.
These spots should warm up quickly today. Most areas will
experience sunny skies and afternoon temperatures into the 50s
today as high pressure remains positioned over the area.
With benign weather expected today and Monday our focus shifts to a
cold dry storm system expected to arrive on Tuesday. The 1200z
runs of the gfs40...nam12 and the 0000z European model (ecmwf) have all initialized
well with the current synoptic pattern and are in good agreement
through the next 84 hours. Yesterday the models really started to
lock in on the timing and position of this storm system which will
slide in from the Great Basin on Tuesday. The track of this storm
will force cold dry air into our area with the greatest impact
being strong winds over the North Bay and East Bay hills in
addition to cold overnight temperatures. Snow levels are
anticipated to fall to 2500 to 3000 feet...so any precipitation
that does fall will snow in the mountains.
In terms of the wind the 1200z nam12 is showing 45 to 50 knots winds
at 925mb over the North Bay mountains and East Bay hills. The
1200z gfs40 is forecasting 50 to 60 kts over the same location.
Cross sectional analysis performed with the gfs40 and nam12 show
plenty of wind with strong downward motion up to 20 ubars of
Omega. This analysis provides ME with a high level of confidence
that the North Bay mountains and East Bay hills will see sustained wind
speed in the advisory level of 35 to 40 miles per hour with warning level
gusts up to 70 miles per hour. In addition...earlier in the season our
partners made US aware of the poor condition that the drought has
left trees in in this area and their susceptibility to wind. This
information suggests that a large number of trees may be blown
down by these strong winds. Therefore...we will be issuing a wind
message with the afternoon forecast package.
Aviation...as of 4:30 am PST Sunday...VFR with winds generally
10kt or less. A cool dry air mass over the area will keep mostly
clear skies through the forecast period. Favorable aviation
weather will continue ahead of a vigorous wind storm scheduled to
arrive later Tuesday.
Vicinity of ksfo...VFR. Winds 10kt or less.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...VFR.
Marine...as of 09:49 am PST Sunday...light winds under high
pressure through midday today. Increasing north to northwest winds
this evening and overnight. Winds will continue to strengthen
through midweek before subsiding late in the week. A period of
steep fresh north swell enters the northern waters Tuesday and
Public forecast: Larry
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