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Area forecast discussion 
National Weather Service San Francisco Bay area
909 PM PDT Tuesday Sep 23 2014

Synopsis...a Pacific weather system will approach the northern
California coast on Wednesday...and rain could spread into the
North Bay by Wednesday afternoon. Rain chances will then develop
south through the remainder of the San Francisco Bay area on
Wednesday night and Thursday morning...and perhaps as far south as
Monterey on Thursday morning. This weather system will also
produce cooler than normal daytime temperatures during the second
half of the week. Dry weather is forecast to return by the
weekend.

&&

Discussion...as of 9:05 PM PDT Tuesday...weather system over
eastern Pacific looks rather impressive on evening infrared satellite
imagery...especially for late September. A vertically stacked and
relatively deep low pressure center is located off the Pacific
northwest coast near 47n/135w. An associated frontal rain band stretches
from Vancouver Island SW to 28n/150w. Precipitable water (pw)
satellite analysis shows a plume of very moist subtropical air
along the frontal boundary which stretches all the way back to
north of Hawaii. Precipitable water values in this plume are between 1.5 and 2.0
inches. So this system has a good deal of moisture to work with
and portions of northwest California will likely get soaking
rains over the next couple of days. The front is forecast to
weaken considerably by the time it presses south into our forecast
area late on Wednesday. But all models indicate that at least the
northern half of our area will see measurable rain and that the
North Bay could see isolated amounts up close to an inch.

As far as timing is concerned...the models disagree a bit on the
onset of precipitation with the European model (ecmwf) and Gem models being about 6-12
hours faster in moving rainfall into our area compared to both
the NAM and GFS. The general idea is that rain will spread across
much of the North Bay on Wednesday and then press south through
the remainder of the sf Bay area on Wednesday night. Light rain
may then perhaps reach as far south as the Monterey Bay area and
Big Sur coast by Thursday morning as the front rapidly dissipates.
There may even be some light warm advection precipitation across the
North Bay as early as overnight tonight. But the bulk of the
precipitation will fall with the main frontal rain band from midday
Wednesday through midday Thursday. As far as impacts...the primary
impact will be wet roadways and associated traffic issues. Most
areas should have a dry commute on Wednesday morning. Also...most
areas south of San Francisco should remain dry into the
Wednesday evening commute...although rain should begin to spread
south of San Francisco after about 6 or 7 PM. Rain is then
expected to begin tapering off by the time the Thursday morning
commute gets underway.

Rainfall amounts are forecast to range from 0.10 to 0.75 inches in
the North Bay...and generally a tenth of an inch or less for
points south.

Precipitation is forecast to taper off quickly after the front
moves through on Thursday. The upper trough will then gradually
move across northern California from late Thursday through Saturday. The
airmass will become somewhat unstable as the upper trough moves
across our area...especially on Friday. However...most models
indicate only isolated light showers from Thursday night through
Saturday. The outlier is the Gem which forecasts widespread precipitation
across the northern part of the area from Friday through Saturday.
May need to consider adding at least a slight chance of showers
across the north during that time frame depending on what the 00z
models show.

Daytime temperatures will drop several degrees by Thursday as a
much cooler airmass spreads across our area. Cool weather will then
continue into at least the first half of the weekend. Warmer and
drier weather will then develop late in the weekend and into the
early part of next week as the upper trough moves off to the east.

&&

Aviation...as of 5:30 PM PDT Tuesday...an approaching cold front
has strengthened the onshore gradient which has resulted in
stronger onshore flow as well as an anticipated earlier onset of
the stratus this evening.

Vicinity of ksfo...IFR ceilings expected to return early this evening
as the onshore gradient strengthens. Ceilings this evening may move in
with breaks in the status possible through 04z. Stratus expected
to stay in after 04z. Moderate to locally strong and gusty onshore
flow expected through this evening. Winds will ease and veer
increasingly to the south overnight.

Ksfo bridge approach...similar to ksfo.

Monterey Bay area terminals...IFR ceilings expected to return early
this evening as the onshore gradient strengthens. Moderate to
onshore flow expected through this evening. Winds will ease
becoming light and variable overnight.

&&

Marine...as of 9:00 PM PDT Tuesday...high pressure remains over
the eastern Pacific maintaining light to moderate northwest winds
this week. A cold front will go through the area Wednesday into
Thursday bringing a chance of rain and increased moderate period
northwest swell.

&&

Mtr watches/warnings/advisories...
...

$$

Public forecast: dykema
aviation/marine: CW

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