Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
403 am PST Sat Nov 22 2014
Synopsis...an incoming weather system will spread rain across
our area today. Dry weather will return tonight. High pressure will then
maintain dry and warmer weather through at least Thanksgiving day.
Discussion...as of 3:00 am PST Saturday...high level clouds in
advance of a shortwave trough approaching the pacnw coast continue
to be on the increase across our area. Been closely watching the
various coastal radars early this morning as rain has slowly been
working its way southward down the coast. Most of the returns
continue to be north of our County Warning Area (generally from Fort Bragg to the
north), however starting to get a few reports of light rain in the
North Bay at this hour. Rain is forecast to progress southward
through our County Warning Area today with the highest rainfall totals north of
San Francisco. Worth noting that the models are not initializing
that great -- they depict widespread rainfall already across Marin
County. Therefore, there remains the potential that the rain will
not be a plentiful as expected and may not advance as far to the
south. For now will continue with rfc guidance which has locally
more than 1" for the North Bay mountains, generally 1/4"-1/2" for
sf Bay plus North Bay valleys and the SC mountains, with lesser
amounts for the rest of our area. The system will depart to the
east later today with rain chances ending.
Ridge of high pressure (up to an impressive 593 dm at 500 mb on
the GFS/ECMWF, while navgem has it closer to 586 dm) out in the
Pacific will quickly rebuild back to our area forcing the jet
stream back well to our north. At the same time according to all
of the operational models 850 mb temperatures will jump as much as 10c
by the middle of next week to around 17c along with an offshore
flow. This will lead to much warmer than normal temperatures
including coastal spots. European model (ecmwf) MOS guidance brings highs into the
middle to upper 70s even down to Monterey while values from the
extended GFS guidance are nearly the same. Decided to bump up the
previous grid package close to what the guidance suggests. If it
continues to show those high values, next shift may want to bump
them up higher. By the way, looking back at the records shows
there have been some very warm episodes toward the end of November,
including one in 1956 where Monterey went into the middle 80s!
The ridge is forecast to break down by the end of next week --
probably on Friday or Saturday. Looking even farther down the
Road, the models seem to be converging on the idea of another
substantial amount of rain close to November 30th or December
Aviation...as of 3:59 PM PST Saturday...for 12z tafs. Light rain
will persist over the North Bay this morning as a frontal system
slides south across the area. Rain is anticipated to spread across
Bay area terminals in the couple of hours and continue through the
morning. Moderate south winds and wet runways are expected to
persist through the morning rush. West winds will return to the
region and showers will come to an end this afternoon once the
front moves across the district. Moderate confidence.
Vicinity of ksfo...MVFR ceilings will persist through the morning.
Rain is expected to move into the area around 13z-14z today.
Southerly winds around 12 knots will accompany the arrival of the
rain. Winds will veer to the west after 21z Saturday.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...VFR ceilings expected through much of
the forecast period with possible lowering of ceilings to MVFR
later this morning as the rain moves through. Rain is expected to
move into the area after 16z. Light to moderate east winds
expected through the morning. Winds will veer back to the west
Marine...as of 03:39 am PST Saturday...northwest winds will
return to the coastal waters today as a frontal system situated
across northern California slides south across the region.
Northwest winds and seas will increase this evening with strongest
winds confined mainly south of Point Reyes.
Public forecast: Bell
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