Scientific Forecaster Discussion
fxus66 kmtr 210534
Area forecast discussion
National Weather Service San Francisco Bay area
1034 PM PDT Fri Oct 20 2017
Synopsis...a warming trend will begin over the weekend and
continue into early next week. Much warmer and drier conditions
are expected to develop by Monday and Tuesday. Slight cooling is
likely during the second half of the week, but temperatures will
remain well above seasonal averages.
Discussion...as of 8:40 PM PDT Friday...last night's rain-
producing cold front ushered a cool airmass into our area today.
High temperatures today were uniformly in the 60s and were as
much as ten degrees cooler than normal. Expect a cool night
tonight, especially in the interior valleys where overnight lows
will drop into the mid and upper 30s. There may be enough low
level moisture remaining for patchy fog to develop late tonight
and early Saturday, particularly in the North Bay valleys.
A warming trend is anticipated over the weekend, along with
mostly clear skies and dry conditions, as an upper ridge builds
over California and onshore flow gradually weakens. High
temperatures on Saturday are forecast to be about 5 degrees warmer
than today, with another 5-7 degrees of warming then expected for
Sunday. By Sunday, look for widespread highs in the 70s and lower
Winds are forecast to turn offshore by Sunday night. Based on the
latest WRF model moderate to locally strong north to northeast
winds will develop in the hills by late Sunday night, especially
in the North Bay hills, but also locally in the East Bay hills and
Santa Cruz Mountains. Fortunately, last night's rain has eased
fire weather concerns, at least temporarily, but dry offshore
winds may raise those concerns once again by late Sunday night
(see fire weather discussion below for more details).
The warming trend will ramp up by Monday as the upper ridge
strengthens over California and offshore winds dry out our
airmass. 850 mb temperatures are forecast to climb from about 5
deg c currently to more than 15 deg c by Sunday and then over 20 c
on Monday and Tuesday. Monday and Tuesday look to be the warmest
days next week with widespread highs in the 80s and lower 90s.
The upper ridge is forecast to weaken and retreat offshore
starting around midweek, and this will likely result in modest
cooling during the second half of next week. In any case,
temperatures are expected to remain well above average through
the week ahead.
Aviation...as of 10:34 PM PDT Friday for 06z Saturday tafs. VFR
conditions are expected to prevail at most taf sites through the
period. Could see some fog develop in the North Bay before
sunrise, largely due to increased ground moisture from Thursday
night and Friday morning's rain. Light winds overnight will
increase by Saturday afternoon to around 10 to 15 kt at most
Vicinity of ksfo.... VFR conditions expected. Breezy winds will
diminish over the next several hours.
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...VFR conditions expected through the
period. Could see some few/scattered low clouds overnight at both kmry
and ksns, but at this point don't expect any broken/overcast low clouds.
Fire weather...as of 8:30 PM PDT Friday...Thursday night's rain
was beneficial and 10 hour fuel sticks on area RAWS sites remain
very wet. Even the erc readings took a pretty good dip to
temporarily mitigate fire weather concerns. General trend for
Saturday and Sunday will be modest warming and drying trend. By
Sunday night there is the potential for another offshore wind
event in the hills, primarily the North Bay hills, but also
locally in the East Bay hills and Santa Cruz Mountains. If we
hadn't received rain on Thursday night, this Sunday night pattern
would be very concerning. However this upcoming moderate offshore
wind event will occur with somewhat higher humidity and with
fuels still on the moist side. We will continue to coordinate with
staff on-site at the Atlas and Tubbs fires to coordinate if any
fire weather watches or red flag warnings are needed.
A very strong ridge builds Monday through much of next week. This
will bring increased fire weather concerns with above normal
temperatures, low humidity and drying fuels. The upshot is that
fire season will continue despite recent rainfall.
Marine...as of 10:29 PM PDT Friday...a long period northwest
swell will continue to taper down through the overnight and into
Saturday. Northerly winds will remain fairly light overnight, but
gradually increase late weekend and into early next week.
Small Craft Advisory...Pigeon pt to pt pinos 0-10 nm
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
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm
Small Craft Advisory...rough bar advisory for sf bar
Public forecast: dykema
fire weather: rww/dykema
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