Scientific Forecaster Discussion
fxus66 kmtr 200627
Area forecast discussion
National Weather Service San Francisco Bay area
1027 PM PST sun Nov 19 2017
Synopsis...look for a weather system to bring rain chances late
tonight and Monday, mainly across the North Bay. There is a slight
chance of additional rainfall at times in the North Bay for much
of this week, but most of our area is forecast to be warm and dry
Discussion...as of 9:00 PM PST Sunday...very comfortable day
across our entire region with many locations into the lower to mid
60s. Considering that mid-November can often be rainy, breezy, and
cool, conditions were quite mild for the time of year. Satellite
and radar both show the leading edge of moisture approaching the
North Bay. This is associated with a system focused farther to
the north. Along our region, the southern extent of the moisture
will move into Napa/Sonoma tonight and potentially drop into part
of the San Francisco Bay area overnight into Monday morning. Rain
will then retreat farther to the north by the afternoon as a ridge
of high pressure builds in from the south. Rainfall amounts will
be minor -- less than a quarter of an inch for most of the North
Bay. Around San Francisco Bay less than 0.05" is likely.
The building ridge will lead to warmer temperatures with 80s
expected to return to parts of Monterey and San Benito counties
on Tuesday. With an offshore gradient in place, the warmer weather
will even make it to the coast with mid 70s to lower 80s likely.
Farther to the north, temperatures will warm, but not nearly as
much due to the closer proximity to moisture. Highs will generally
be in the upper 60s to the lower 70s.
Continued warm conditions are forecast through Thanksgiving day
along with mostly dry conditions. Temperatures will lower by the
end of the week as the ridge shifts off to the east. No sign of
any weather hazards with the rain expected to stay mostly to our
north and only a slight chance of precipitation north of San
Francisco. Latest model runs are also running drier than the ones
the day crew had access to.
Previous discussion...after active weather earlier this week the
weekend will end quietly. Filtered sunshine blanket the Bay area
this afternoon with mild temperatures.
Latest surface analysis shows a large area of high pressure over
the region and just off the West Coast. Much farther to the northwest a
surface low is located near 31.30n 149.90w with a front extending
to the NE and to the SW. Just south of the SW extending frontal
boundary is a very impressive moisture plume with pwats around two
inches. Over the next 24 hours or so a battle between the ridge
and frontal boundary will unfold. As of now, the models have the
ridge of high pressure winning out. That being said, enough
moisture and warm air advection will move over the North Bay to
warrant a few showers in the forecast late tonight and Monday.
Rainfall amounts will generally be light with a tenth to a
quarter possible over the North Bay. There could be a few spots
near the Sonoma/Mendocino line that could be over one quarter of
an inch. Did cut back on rain chances south of the of the Golden
Gate, but there could be a stray shower early tonight before the
ridge builds in tomorrow. South of sf Bay is expected to remain
Monday night into Tuesday ridging builds across the Bay area
bringing dry conditions but more notably well above normal
temperatures Tuesday and Wednesday. Current forecast will have
Monterey/San Benito counties Max temps eclipsing 80 degrees across
the interior and 70s at the coast. Elsewhere across the County Warning Area highs
will be in the upper 60s and 70s. That will put Max temps 10-15
degrees above normal for late November.
Ridge of high pressure weakens on Thanksgiving with a slight drop
in temperatures. The weakening ridge will also allow for moisture
from the north to sag southward. Medium range models have varying
solutions on Thanksgiving with the Canadian showing precip to the
Golden Gate, GFS is north of Golden Gate and European model (ecmwf) is north of
Santa Rosa. In all, rainfall amounts are light as well-tenth to a
quarter. Given the uncertainty, will at least keep a slight
chance for showers over the North Bay. The rest of the Bay area
will likely have a dry and mild Thanksgiving, especially south of
The longer range models indicate the ridge will shift far enough
to the east late in the week to allow precipitation to gradually
spread into our forecast area between Thursday night and next
weekend. However, there are considerable model differences
regarding timing and location of rainfall late in the extended
Aviation...as of 10:27 PM PST Sunday...VFR persists at the
terminals this evening, however cloud bases will steadily lower
tonight and Monday. A very moist, milder, and stable SW flow from
the sub-tropics primarily displacing to the Pacific northwest and
northernmost California this week will brush the North Bay with
light-moderate rain beginning tonight, while it remains mainly dry
to dry elsewhere to the south.
The WRF model indicates 20-30 knot SW winds at the 925 mb and 850
mb levels over the North Bay counties starting early Monday morning
and lasting until early Monday afternoon. With a stable lower level
air mass in place and lighter surface winds though marginal low level
wind shear (llws) is forecast at ksts Monday. Lower level winds are
much less robust over the remaining Bay area and areas to the south
Monday thus llws is not expected to the south.
Vicinity of ksfo...light wind, VFR. Cloud bases lowering tonight and
Monday, dry runway. Low to moderate confidence.
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...light east-southeast winds this evening, except southeast to
10 knots Salinas valley Monday morning. VFR and dry. Moderate to high
Marine...as of 10:20 PM PST Sunday...the eastern edge of a very
moist sub-tropical flow will overlap the northern coastal waters
through early this week. A warm frontal boundary connected to this
moist flow will approach the coastal waters Monday and become
nearly stationary while another warm front to the southwest
advances toward it and the Bay area. By late week into early next
week a cold front will move southeastward over the coastal waters.
Seas are expected to remain light to moderate through the first
part of next week.
Public forecast: Bell/mm
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