Scientific Forecaster Discussion
fxus66 kmtr 210532
Area forecast discussion
National Weather Service San Francisco Bay area
1032 PM PDT Fri Sep 20 2019
Synopsis...a warming trend will continue through Saturday. A low
pressure system moving southward to our east early next week may
create the potential for continued warming temperatures, dry
offshore flow and increased fire weather concerns. A trough of low
pressure will move through the region late next week resulting in
significantly cooler temperatures.
Discussion...as of 9:24 PM PDT Friday...temperatures rose on
average 5 to 8 degrees today as high pressure continues to build
in over the region. Temperatures along the coast warmed in the 70s
to low 80s while inland temperatures reached into the upper 70s
to mid 80s. The warming trend will continue into Saturday as light
offshore flow persists across the district and Ushers warmer
inland air toward the coast. Temperatures are forecast to inch up
another 3 to 5 degrees on Saturday with highs expected to warm
into the 70s to low 80s along the coast, 80s across most inland
locations and mid 80s to low 90s over the warmest inland areas.
Cooler temperatures are forecast for Sunday as an upper level
trough slides south across the state bringing with it a cooler and
more humid airmass. The cold front associated with this system is
expected to fall apart before reaching northern California,
therefore the region will not receive any precipitation from this
system. The trough is progged to move inland Monday allowing high
pressure to build in from the west. The high will ridge across the
Pacific northwest and into the Great Basin while a thermal trough
develops along the California coast. This pattern will result,
not only in warmer temperatures for Monday and Tuesday, but also
offshore winds across northern California. Strongest winds are
currently focusing over the north and East Bay hills Monday night
into Tuesday morning. However, current models only show and
offshore gradients of around 7 mb and current fuel moisture
observations remains well below normal. In other words, fuels are
wetter than normal. We will continue to watch this situations
closely as it develops.
Temperatures will be significantly warmer on Monday and Tuesday,
with Tuesday expected to be the warmest day. Highs will reach into
the 70s to near 80 along the coast with areas such as Santa Cruz
that is typically affected by downsloping offshore winds may see
temperatures in the mid to upper 80s. Widespread 80s are
anticipated around the Bay with inland temperatures soring into
the 90s and possibly approaching 100 degrees.
From previous discussion...another significant pattern shift is
then forecast to begin midweek bringing with it a substantial drop
in temperatures. 5-10 degree temperature drop is forecast from
Wednesday to Thursday as the ridge weakens and an upper level
trough approaches from the pacnw. Additional cooling is then
expected Friday and into next weekend along with the possibility
Aviation...as of 10:30 PM PDT Friday...clear skies this evening
as dry airmass continues over the area. Models not showing any
indication of a marine layer tonight so VFR is forecast through
Vicinity of ksfo...VFR.
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...VFR.
Fire weather...as of 2:55 am PDT Friday...a quick look at energy
release component (erc) and 100 hour fuel values are both well
below normal for the region. The below normal readings are likely
in response to the recent rain and cool weather. A warming and
drying trend is on track to develop into the weekend. There is
even a brief period of offshore flow Friday night across the north
and East Bay hills. Winds will be breezy with moderate to
possibly poor humidity recoveries Friday night. Not strong enough
or long enough to warrant any red flag warnings. What it may do is
help chip away at the low ercs and high 100 hour fuels. The
inside slider mentioned above will bring a more pronounced
offshore flow event Monday into Tuesday with poor overnight
humidity recovery and gusty winds in the hills. One potential
limiting factor is the actual offshore flow gradient. Models put
the wmc-sfo gradient at 7-8 mb, decent but not strong. If this
pattern holds true a Fire Weather Watch may be in order over the
weekend. Climatologically this is the favored time of year for
offshore flow events and red flag warnings for wind/relative humidity in the Bay
area. Taking a look at the bigger picture - far norcal has had a
few wetting rain events so fire season may be near the end to the
north. Socal - not so much and fire season will keep on going.
What about the Bay area? Fuels have been below normal for much of
the Summer and recent cool/wet weather has kept them that way.
That being said, the recent wet weather wasn't wet enough to end
fire season. Therefore, we're not out of the Woods yet and we
should remain weather/fire aware through the fall season.
Marine...as of 10:29 PM PDT Friday...winds will generally
decrease Saturday and Sunday as low pressure over the
intermountain region moves east reducing the pressure gradient.
Winds will increase again by Monday as another low pressure trough
drops into the Great Basin. A 10-second northwest swell will be
mixed with a long-period southerly swell.
... Arena to Pigeon pt 10-60 nm
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm until 3 am
Public forecast: CW
aviation: west pi
marine: west pi
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