Scientific Forecaster Discussion
fxus66 kmtr 232059
Area forecast discussion
National Weather Service San Francisco Bay area
159 PM PDT sun Apr 23 2017
Synopsis...mostly clear this evening and overnight then
increasing clouds towards dawn as the next front approaches.
Expect some light rain in the North Bay Monday morning with a few
light rain showers reaching from the Golden Gate to Monterey Bay
Monday afternoon and early evening. Another weak front will keep a
slight chance of North Bay rain showers in the forecast Tuesday
into Wednesday but dry and seasonably cool from San Francisco
southward. Dry on Thursday then breezy winds develop into Friday
with a warming trend next weekend as high pressure builds.
Discussion...as of 1:59 PM PDT Sunday...its a sunny and mild
Sunday afternoon with highs in the 60s to lower 70s. Expect a
clear but cool evening with west winds off the ocean ushering in
some cool marine air overnight with sea surface temps back into
the mid 50s.
Next weather system will arrive by around sunrise Monday as mostly
cloudy skies greet the day. Some light rain should reach northern
Sonoma County by 15z Monday with light rain impacting the North
Bay Monday morning commute. Rain totals will be very light only a
few hundredths or perhaps a tenth of an inch but enough to wet the
roads and turn on the wipers. Periodic light rain or showers will
reach the Golden Gate and extend southward towards Monterey Bay
later Monday afternoon. Expect nothing more than some nuisance
light rain and most cells shouldn't penetrate too far inland. The
combination of cloud cover and onshore winds will keep daytime
highs seasonably cool in the mid 60s for most locations.
Yet another weak front will move into the North Bay on Tuesday and
keep at least a slight chance of showers in the forecast. Best
chances will be from Mendocino north with little or no chance
south of the Golden Gate. Forecast was fine tuned to try and keep
slight chance pops confined to the North Bay Tuesday and then
again weds as a final front rides over the ridge and tries to
bring some light rain to the North Bay region. All this talk of
North Bay rain should only add up to very light amounts of rain
through weds (trace to a couple tenths), while the rest of the Bay
area looks to remain dry and with seasonable or temps a few
degrees below normal.
The longwave pattern will start to shift by Thursday with a trough
digging over the Great Basin. This will set up some strong surface
pressure gradients across the state and will need to watch wind
potential, especially over our hills on Friday as gusty north
winds develop. Latest ECMWF then builds a warm ridge with some
lighter offshore winds next weekend suggesting dry and mild
weather to wind out the month of April. Long range models keep the
forecast dry as we head into may.
Aviation...as of 4:44 am PDT Sunday...for 12z tafs. Generally
VFR across the district as of writing this discussion. Could be
brief MVFR cigs early this morning, especially for Monterey Bay
terminals. VFR this afternoon, with breezy and gusty onshore winds
by late morning. Another tough cig forecast for tonight. Local
WRF model suggests a relatively moist boundary layer through the
day today, enough to keep few-scattered low clouds across the region.
It increases relative humidity values later this evening. Likewise, latest lamp
guidance has MVFR cigs at most terminals by 08-09z Mon., Earlier
for Monterey Bay terminals. Needless to say, confidence is low on
cig coverage and timing tonight.
Vicinity of ksfo...generally VFR through this afternoon. Cannot
rule out a couple of hours of MVFR cigs before sunrise, but not
enough confidence to include in tafs. West-northwest winds 15-20 knots
Sunday with afternoon and evening gusts to 25-30 knots possible.
Could be MVFR cigs late tonight, into the overnight hours, though
confidence is low.
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals..patchy MVFR cigs possible through about
15z or so, then VFR. Onshore winds 10-15 knots by late morning,
with gusts in excess of 20 kts at times. Potential for MVFR cigs
by 05z Monday.
Marine...as of 01:43 PM PDT Sunday...look for moderate
northwesterly winds to continue through the forecast period with
the highest speeds near the coast. Winds and seas will
dramatically increase by Thursday as the surface pressure
gradient tightens along the California coast.
... Bay until 3 am
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm until 3 am
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm until 3 am
Small Craft Advisory...Pigeon pt to pt pinos 0-10 nm until 3 am
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm
Small Craft Advisory...sf Bay until 3 am
Public forecast: rww
Visit US at www.Weather.Gov/sanfrancisco