Scientific Forecaster Discussion
fxus66 kmtr 221702
Area forecast discussion
National Weather Service San Francisco Bay area
1002 am PDT Fri Sep 22 2017
Synopsis...a gradual warming trend will begin today as the upper
level trough shifts east and high pressure builds over the
eastern Pacific. As the region transitions to a warmer pattern
temperatures will approach seasonal norms still maintaining
moderately breezy conditions. Significantly warmer temperatures
are forecast for the weekend and into early next week.
Discussion...as of 10:00 am PDT Friday...mostly clear skies
across our County Warning Area today with most locations in the 50s. Worth noting
that a few spots did drop close to 40 degrees overnight. Main
story for the public forecast will be the change in the synoptic
pattern into next week as a ridge of high pressure rebuilds into
our region from the west. At the same time winds will become more
offshore which will allow the warmer weather to work down to the
coast. Highs on Tuesday will be particularly warm -- even San
Francisco could see mid 80s.
Current forecast appears to be on track. No major updates planned.
Previous discussion...little change has been observed over the
last 6 hours with respect to the cloud cover. A few high clouds
continue to stream in with the flow aloft while at the surface a
few patches persist over the hills of the Santa Lucia mountains,
Diablo Range, and Santa Cruz Mountains. The marine layer remains
mixed out as a cooler/drier air continues to advect in aloft
hampering any chance of an inversion. Mostly clear skies are
expected today with patch cumulus possible over the hills once
again this afternoon.
The broad trough currently over the western U.S. Has allowed a
significantly cooler and drier airmass to advect in over region
result in unseasonably cool temperatures. Comparing temperatures
from 24 hours ago one can see a drop of 2 to 5 degrees across most
locations this morning. Some locations such as San Jose and
Watsonville have seen as much as 8 to 13 degrees of cooling,
respectively. Temperatures are currently in the upper 40s to mid
50s with further cooling expected as the morning progresses.
Widespread 40s are expected by morning with several pockets of mid
to upper 30s possible over the hills.
This trough of low pressure will gradually shift east allowing
high pressure to build over the eastern Pacific. As the ridge of
high pressure nudges in toward the West Coast the region we will
see 850mb temps and 500mb heights steadily increase leading to a
steady increase in temperatures across the district. While the
region will likely only see 3 to 5 degrees of warming today,
significantly warmer temperatures are expected over the weekend
with temperatures returning to seasonal norms. Today also Marks
the autumnal equinox, which will occur at 1:02 PM PDT.
This weekend will bring widespread temperatures reaching into the
80s over inland areas as winds shift offshore. The warming trend
and an offshore winds pattern is forecast to continue into early
next week with forecasted highs flirting with 90s over the warmest
inland areas. Along with the warming temperatures the region will
see relative humidity values plummet leading to fire weather
concerns over the weekend and through the first half of next week.
Aviation...as of 04:35 am PDT Friday for 12z tafs...satellite
imagery showing low clouds beginning to develop. Confidence low as
to whether or not these clouds will push into the surrounding
airports. By late this morning expect VFR conditions to prevail
throughout the day. Breezy westerly winds expected this afternoon.
Vicinity of ksfo...clouds developing to the west of ksfo.
Confidence low as to whether or not these will move over the
terminal. Otherwise VFR conditions expected through the period.
Westerly winds this afternoon expected at around 15 kt.
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...MVFR cigs were around kmry earlier this
morning and have recently drifted back over the surrounding
hills. Expecting cigs to redevelop over the next couple of hours
but confidence is low.
Fire weather...as of 2:39 PM PDT Thursday...clear and cool
overnight with easing northwest winds. Continued dry and
seasonable on Friday. By late Friday night northerly winds will
develop across the North Bay hills into early Saturday morning but
humidity should stay fairly moist. Noted warming and drying trend
over the weekend with periods of gusty northeast winds across the
north and East Bay hills later Saturday night and then again
Sunday night. Afternoon highs well into the 80s and lower 90s
inland areas with humidity lowering into the teens. Long range
trends show a prolonged yet seasonable warm and dry pattern
through the weekend and into next week with high pressure aloft
and a thermal trough near the coast. Fine fuels will continue to
dry given the lack of marine air. Climatology suggests potential
for near critical fire weather conditions for much of the next 5-7
days starting this weekend. Fire weather watches and or warnings
may become necessary should the pattern develop as expected,
however no strong offshore wind events are forecast at this time.
Marine...as of 10:00 am PDT Friday...upper trough will exit the
area over the next 24 hours with high pressure building in from
well offshore behind the trough. As a result, northwest winds will
decrease through this morning before increasing once again,
primarily over the northern outer waters. Northwest swell will
continue to decrease over the weekend while steep wind waves will
remain through today.
... Arena to Pigeon pt 10-60 nm from 4 PM
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm from 5 PM
Small Craft Advisory...Pigeon pt to pt Piedras Blancas 10-60 nm from 4 PM
Public forecast: Bell/CW
fire weather: rww
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