Scientific Forecaster Discussion
fxus66 kmtr 221157
Area forecast discussion
National Weather Service San Francisco Bay area
457 am PDT sun Oct 22 2017
Synopsis...the warming trend that got underway across our region
yesterday will continue today. The warming trend will intensify
on Monday when offshore flow develops. Very warm and dry
conditions are forecast across the region on both Monday and
Tuesday. Dry weather conditions will persist through the week.
Gradual cooling is forecast during the second half of the week,
but temperatures will remain above seasonal averages.
Discussion...as of 3:20 am PDT Sunday...warming aloft under a
building ridge of high pressure has resulted in the formation of a
shallow marine layer near the coast overnight. However, onshore
flow is very weak and so morning low clouds and fog should remain
patchy and mostly confined to coastal areas. Otherwise, expect a
mostly sunny and mild Sunday with continued warming. Afternoon
highs are forecast to be mostly in the 70s and lower 80s, and
anywhere from 5 to 10 degrees warmer than yesterday.
Offshore flow is forecast to develop tonight as surface high
pressure builds over the Pacific northwest and into the Great
Basin. Moderate and gusty north to northeast winds will develop in
the hills late tonight and continue into Monday morning. The 00z
WRF backed off on the magnitude of wind speeds in the hills late
tonight and Monday morning, and winds will almost certainly
remain below advisory criteria. However, there will be enough wind
and drying by Monday to raise fire weather concerns to some
extent. See the fire weather discussion below for more details.
Offshore flow combined with robust airmass warming under a
strengthening upper ridge will result in much warmer and drier
conditions across our region on Monday and Tuesday. By Monday look
for widespread high temperatures in the 80s and lower 90s...even
in coastal areas. Tuesday is expected to be the warmest day of the
week with some mid to upper 90s possible across interior Monterey
County, and even a few lower 90s near the coast in places like
Santa Cruz. A few daily record high temperatures are possible.
Heat risks will increase into the moderate category by Tuesday,
especially at higher elevations where there will not be as much
cooling at night. Most lower elevation locations are not expected
to experience significant heat risk due to the long late October
nights which will allow for good overnight cooling.
The upper level ridge is forecast to weaken slightly and shift
offshore by midweek. This will result in gradual cooling during
the second half of the week. Even so, temperatures will remain
well above seasonal averages through at least Thursday.
Longer range models show no signs of precipitation through the end
of the month.
Aviation...as of 4:45 am PDT Sunday...warming airmass aloft has
allowed a shallow marine layer to form. Clouds have been forming
along the coast from Sonoma to San Mateo counties and spreads
locally into the North Bay valleys. If any clouds get into the sfo
Bay area it will be patchy and confined near the Golden Gate
which may briefly impact Oak this morning. Things will trend drier
by tonight as offshore flow develops. A strong high pressure
system builds over the Great Basin Sunday through Tuesday. The
acv-sfo pressure gradient nears 5 mb late Sunday into Monday and
the wmc-sfo gradient increases to 10-13 mb Sunday night into
Monday. There is potential for low level wind shear later Sunday
evening into Monday morning for the sfo Bay area terminals.
Vicinity of ksfo...VFR. Northwest wind increasing to 15 knots
Sunday afternoon and evening with gusts 20-22 kt. Low level wind
shear after 09z Monday.
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...VFR.
Fire weather...as of 3:20 am PDT Sunday...gradual warming and
drying will continue across the district today. Offshore flow is
then forecast to develop tonight, resulting in much warmer and
drier conditions by Monday and Tuesday. Locally moderate and gusty
north to northeast winds are forecast in the hills late tonight
and Monday morning. The latest WRF has scaled back on the
magnitude of wind speeds in the hills tonight and Monday morning.
Even so, local gusty winds are likely over the higher ridges and
peaks of the North Bay, particularly across eastern Napa County
where gusts up to 40 mph are possible. In addition, nighttime
humidity recoveries will be poor in the hills tonight. Were it not
for the recent wetting rains on Thursday night, these
developments may have resulted in critical fire weather
conditions. However, finer fuels are expected to retain enough
moisture in the short term to mitigate fire weather concerns to
some extent. Will continue to draw attention to local gusty winds
and drying conditions with headlines in the fire weather planning
Fuels are expected to dry significantly by late Monday.
Fortunately, winds in the hills on Monday night are not expected
to be as strong as tonight, with only some isolated patches of
gusty winds expected at the higher elevations of the North Bay.
Will continue to monitor fuel moisture over the next few days to
determine if critical fire weather conditions may occur. Winds are
expected to subside in all areas by Tuesday afternoon, but very
warm and dry conditions will continue through midweek. Slight
cooling is expected during the second half of the week as high
pressure over California weakens.
Marine...as of 4:55 am PDT Sunday...northwest swells will
continue to subside through day. A frontal system will move into
the Pacific northwest today with northwest winds increasing
tonight behind the front. Winds will decrease Monday through
Wednesday except over the northern outer waters as high pressure
builds into the Great Basin creasting offshore flow.
... Arena to pt Reyes 0-10 nm from 4 PM
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm from 4 PM
Small Craft Advisory...Pigeon pt to pt pinos 0-10 nm from 4 PM
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm
Small Craft Advisory...pt Arena to Pigeon pt 10-60 nm
Small Craft Advisory...Pigeon pt to pt Piedras Blancas 10-60 nm
Public forecast: dykema
aviation: west pi
marine: west pi
fire weather: dykema
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