Scientific Forecaster Discussion
fxus66 kmtr 231035
Area forecast discussion
National Weather Service San Francisco Bay area
335 am PDT Sat Sep 23 2017
Synopsis...high pressure building near the West Coast will result
in dry weather, along with a warming trend, through at least the
middle of next week.
Discussion...as of 3:30 am PDT Saturday...clear skies across our
entire County Warning Area this morning along with a squashed marine layer and a
moderate offshore flow. Temperatures are generally running a few
degrees cooler than yesterday thanks in large part to drier air
and light winds. One exception is Napa where speeds are still
just enough to keep them currently at 50. Look for parts of the
North Bay to drop into the upper 30s (40s will be common) with the
remainder of our County Warning Area generally in the 40s. Low 50s will be mostly
confined to the coast.
Temperatures will quickly rebound as abundant sunshine will
combine with a synoptic pattern that will feature the longwave
trough overhead starting to progress to the east. In addition, 850
mb values will increase 1-2c by the afternoon. Highs will
generally be in the 60s at the coast with 70s to lower 80s
inland. Winds will mostly be from the north or northeast with
gusty conditions possible over parts of the North Bay. That has
lead to an issuance of a red flag warning starting this afternoon
for the North Bay hills. Please see the fire weather section
below for more information.
Temperatures will continue to warm each day for Sunday into
Wednesday as a ridge near the coast builds into our region and
850 mb values go up each day. The flow should remain offshore
while the marine layer will be non-existent. This will translate
into the warm weather making it down to the beaches. By Tuesday
highs will be in the 80s at most coastal spots with 80s and 90s
inland. Beyond that the range in solutions greatly increases
although the majority of the models keep very warm readings going
at least into Friday.
The big challenge for the forecast is just how warm will it get
both at the coast and inland. 850 mb temperatures and 500 mb
heights are not that impressive and anomaly values are generally 2
Standard deviations or less, so did not feel comfortable going
with extremely high numbers that some of the ensemble members are
suggestion (such as 90s to the coast). The current forecast keeps
the heat threshold in the low category at the worst, however if
the forecast trends upwards that could easily move to the moderate
level. Anyone with outdoor plans or at a location without air
conditioning should keep a close eye on the forecast.
Longer range guidance favors warmer than normal conditions
continuing all of the way into the middle of October. No
precipitation is expected.
Aviation...as of 10:12 PM PDT Friday for 06z Saturday
tafs...aside scattered or perhaps locally broken clouds at sunrise tomorrow
along and near the coast, VFR conditions are expected to prevail
throughout the San Francisco Bay area and central California coast
over the next 24 to 30 hours. Generally light winds in the
morning will increase on Saturday afternoon. High confidence.
Vicinity of ksfo...VFR conditions expected. West winds will
develop on Saturday afternoon at around 15 kt.
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...VFR conditions expected, though there is
a slight possibility of broken skies around sunrise Saturday.
Likelihood of occurrence at kmry or ksns is too low to put into
taf, but will monitor and amend if necessary. Breezy onshore winds
will decrease after sunset.
Fire weather...as of 3:30 am PDT Saturday...a warming and drying
trend will start this afternoon and continue at least into at
least Tuesday morning. In addition to the warm temperatures there
will be lowering humidity values and periods of gusty northeast
winds across the North Bay hills, especially across the Napa
County hills and the hills of northern Sonoma County. Fine fuels
will continue to dry given the lack of marine air and humidity.
Climatology also favors a greatly chance for critical fire
weather conditions for the weekend and next week.
Marine...as of 3:30 am PDT Saturday...high pressure will remain
off the northern California coast while low pressure over The
Rockies moves east. Winds will decrease Sunday except over the
... Bay from 2 PM
Small Craft Advisory...Pigeon pt to pt pinos 0-10 nm from 2 PM
Small Craft Advisory...Pigeon pt to pt Piedras Blancas 10-60 nm from 2 PM
Small Craft Advisory...pt Arena to Pigeon pt 10-60 nm
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm
Public forecast: Bell
marine: west pi
fire weather: Bell/rww
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