Scientific Forecaster Discussion
fxus66 kmtr 201502
Area forecast discussion
National Weather Service San Francisco Bay area
802 am PDT sun Oct 20 2019
Synopsis...a warming and drying trend will begin today and
continue through Thursday as high pressure strengthens over
California. In addition, offshore flow will develop by Monday, and
increase by midweek, which will result in warm temperatures
reaching all the way to the coast. Locally gusty offshore winds
may develop at times in the hills, especially in the North Bay
hills on Wednesday night and Thursday morning. In the longer
range, cooling is forecast next weekend.
Discussion...as of 08:00 am PDT Sunday... low level moisture
developed into low clouds and patchy fog overnight across portions
of the South Bay, Monterey coast, and Salinas valley early this
morning. These clouds and patchy fog have already begun to mix out
and should dissipate by mid morning.
High pressure will begin to build into the state today, marking
the beginning of a warming/drying trend that will extend through
much of the upcoming week. Temperatures this afternoon will run
around 2 to 5 degrees warmer than yesterday, with highs in the mid
60s to low 70s along most of the coast, and mid 70s to around 80
inland. By tomorrow, afternoon temperatures will be tangibly
warmer and the first day of this warming trend to see above
seasonal normal temperatures. Thursday looks to be the warmest
day of the upcoming week with highs along the coast in the 70s to
around 80 while inland areas will range in the 80s to around 90. One
forecast challenge today will be to determine if temperatures
need to be ramped up slightly on Thursday as models tend to
struggle to capture the extent of warming coastal areas see during
our fall offshore flow warming trends.
Speaking of offshore, north to northeast winds are forecast to
gradually increase as the sfo-wmc gradient increases through the
upcoming week, peaking later Wednesday into Thursday.
In addition to these winds, the unseasonably warm/dry air mass
associated with the building high pressure will lead to increase
fire weather risk, particularly across the higher elevations of
the North Bay. Longer term ensemble guidance is also picking up on
the possibility of another offshore wind event by next weekend,
however, that could change if the modeled trajectory and
positioning of the "insider slider" system shifts over the coming
days. See previous discussion for more information on the current
Previous discussion...as of 3:10 am PDT Sunday...satellite fog
product shows residual low clouds along the coast from about Half
Moon Bay south through the Monterey Bay, and extending into the
Salinas valley. Patchy fog is also around with sts reporting 3
miles visibility, and Salinas is at 5 miles. Dry air moving in
from the north by morning should serve to clear out these low
clouds and patchy fog.
An upper level ridge over the eastern Pacific will build and shift
eastward over the next several days. This will result in a warming
trend, especially by Monday and Tuesday. High temps today are
expected to only be a few degrees warmer than Saturday, with a
5-10 degree jump by Monday. High temperatures to level out by
midweek, but warmest inland areas could approach 90 degrees over
this time frame.
In addition to the warming temperatures, offshore flow is forecast
to develop. The warming and drying conditions coupled with
offshore flow will bring the potential for critical fire weather
conditions, especially after midweek when gusty offshore winds
could be the strongest. However 00z model runs have weakened the
forecast gradients somewhat, with the GFS indicating only about 6
mb from wmc-sfo Tuesday and Wednesday nights. The European model (ecmwf) is
substantially less than this. But, as previously noted, even if
winds aren't extremely high, the prolonged warming and drying of
the atmosphere will still make for potentially critical fire
weather conditions. Cooler weather is indicated by next weekend as
an upper level trough drops southward from western Canada.
Aviation...as of 4:39 am PDT Sunday...moist air is trapped
beneath lower level temperature inversions resulting in patchy
IFR-LIFR over the East Bay and South Bay to areas IFR-LIFR over
north the central coast; drier air arriving from the north will
lift ceilings and visibilities to VFR from mid to late morning.
VFR tonight and Monday.
Vicinity of ksfo...VFR. West wind increasing and becoming gusty to
20 to 25 knots from 21z today to 05z this evening. West wind near
10 knots mid to late evening shifting to light NE overnight.
Sfo bridge approach...same as ksfo.
Monterey Bay terminals...IFR-LIFR lifting to VFR from mid to late
morning. VFR late morning through the afternoon. Lowering confidence
VFR forecast continuing into the evening.
Fire weather...as of 3:55 PM PDT Saturday...heightened fire
weather concerns are expected next week as periods of offshore
flow combine with gusty winds at times to create critical to near
critical fire weather conditions. Locally breezy northwest to
north winds are expected in the hills Sunday, but relative
humidity values will remain somewhat elevated. A warming and
drying trend will get underway on Sunday, and especially on
Monday, as high pressure builds over California and offshore flow
develops. Offshore flow will mostly be light through Tuesday
night, except locally moderate and gusty at times in the hills.
Relative humidity values will drop into the teens by Monday
afternoon and very poor overnight humidity recoveries are expected
in the hills starting Monday night. Warm and very dry conditions
will then persist for the remainder of next week. The period of
greatest concern is Wednesday night and Thursday morning when some
models indicate the potential for strong and gusty north to
northeast winds in the hills, mainly across the higher elevations
of the North Bay. Fire weather watches and/or red flag warnings
may be issued over the next few days if critical fire weather
conditions are deemed likely to occur.
Marine...as of 07:49 am PDT Sunday...high pressure over the
eastern Pacific will also extend over far northern California and
into the Great Basin through early to mid week.
... Pt to pt pinos 0-10 nm from 11 am
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm
Small Craft Advisory...Pigeon pt to pt Piedras Blancas 10-60 nm
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm from 1 PM
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm from 1 PM
Public forecast: drp
fire weather: dykema
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