Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
343 PM PST Wednesday Nov 26 2014
..substantial rainfall event slated for Friday through at least
..Hydro concerns likely by next Tuesday...
Synopsis...high pressure will bring dry weather with seasonable
temperatures through tomorrow. Rain will return for Friday into Saturday
and continue into next work week. Another round of rain will then
move across our area around Tuesday and continue into Wednesday or Thursday.
Discussion...as of 3:00 PM PST Wednesday...mostly sunny
conditions across our area today with very comfortable
temperatures -- generally in the 60s. Tonight should be a near
repeat of last night with patchy fog over the North Bay along with
cool temperatures for almost all spots. Thanksgiving day should be a near
repeat of today.
On Friday conditions will change as a strong (983 mb surface
low/538 dm heights at 500 mb) Pacific storm system 1200 miles to
the northwest of sf 650 miles continues to approach our area while an
associated cold front 650 miles to the west nears our County Warning Area. This
will represent the switch to active weather and the return of
rain. Model solutions are in good agreement and bring the start of
the rain to the North Bay Friday evening...down to the sf Bay
metropolitan Friday night...and down to the Monterey Bay region on
Saturday. This initial push is associated with a moist plume of precipitable water
values up to 1.25" that will take a direct aim on our County Warning Area. On the
flip side, isentropic values are not impressive (10-20 kt) and positive vorticity advection
only looks marginal at best. Favorable instability for convection
will be limited to the northern waters plus the coastal North Bay.
In those spots some small hail is possible.
Immediately following the passage of the front, another slug of
moisture will move in from the SW and bring more rain to our area
late Saturday into Sunday. This one will have similar precipitable water values
with better positive vorticity advection plus isentropic flow, so amounts could be greater
with the second impulse compared to the first. Southerly winds
will also pick up with speeds of 15 to 25 miles per hour and local gusts to
40 miles per hour possible. Higher values can be anticipated in the hills and
coastal spots that have a southwestern exposure. Rain will taper
off by late Sunday.
By the time we get to Monday morning, rainfall totals are expected
to be up to 3" for higher elevations with generally 1/2"-1.5" for
most valley and urban spots. Worth noting that some of the latest
guidance that has just come in has backed off a bit on values.
Newest cnrfc numbers are also a bit of a departure from the past
few values. For now, will hold off dropping our precipitation numbers
until at least one more series of models comes across.
After a fairly dry Monday, the models bring the third and likely
most substantial amount of rain to our area either late Monday or
Tuesday. This will be due to both the surface and upper level low
moving overhead. Due to some disagreement with the track of the
low, the start and end times for the rain remains a difficult
call. Decision was made to go with a compromise between the faster
GFS and the slower European model (ecmwf). There is also disagreement with where
the best precipitable water values will be aimed. European model (ecmwf) which tracks a bit slower
and more to the south has the rain focused south of sf while the
GFS shows a widespread event with the highest rainfall totals over
the North Bay. In addition, the models disagree in regard to wind
concerns with the GFS solutions much windier. Depending on which
one verifies, rain will end either Wednesday or Thursday.
Additional rainfall amounts for the system next week should be as
great -- if not greater -- than what we will see this weekend.
Urban spots should see another 1/2"-1.5" with higher elevation
locations in the 3-6" range. If things line up just right, some of
the local spots in the hills could end up with 7 day rainfall
totals of nearly a foot!
High pressure is forecast to build back to our area for the second
half of next week allowing for a return to dry conditions.
Aviation...as of 3:40 PM PST Wednesday...VFR conditions with
light winds through the forecast period. High cloud continue to
stream in over the North Bay this evening. These insulating clouds
will work against fog development... however cant quite rule out
fog at ksts... but may do so at the 06z tafs.
Vicinity of ksfo...VFR.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...VFR.
Marine...as of 03:33 PM PST Wednesday...southerly winds will
prevail north of Pigeon Point and northerly winds will prevail
south of Pigeon Point through Friday. Southerly flow will develop
over all of the coastal waters this weekend as a storm system
begins to impact the region. Unsettled weather is expected Friday
into early next week with periods of rain and gusty winds. The sea
state will likely deteriorate rapidly early next week as the
strongest storm moves through.
Public forecast: Bell
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