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Displaying AFOS PIL: AFDMTR Received: 2017-10-10 04:46 UTC


010 
FXUS66 KMTR 100446
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
946 PM PDT Mon Oct 9 2017

.SYNOPSIS...Seasonable weather on Tuesday, then a dry cool front
passes through Tuesday night into Wednesday. This will bring
another spike of North winds by Wednesday morning but not nearly
as strong as the Sunday night event. After the midweek trough
moves through the trend should be for gradual warming and drying
as high pressure builds with a thermal trough near the coast
producing periods of light offshore winds.

&&

.DISCUSSION...as of 9:46 PM PDT Monday...For the last 25 years or
so the Oakland hills fire has been the seminal fire event that 
was seared into Bay Area residents psyche. For the current 
generation of North Bay residents todays firestorm will leave an 
indelible scar and for years to come we will all recall the 
Columbus Day firestorm. The wind event that roared through the 
North Bay last night is long behind us as ignitions occurred on 
the tinder dry fuels, fire literally exploded and raced along the 
landscape. Fuel analysis just ahead of the winds showed they were 
at all time record dry levels preceding the front. Recall we had 
nearly 5 years of extreme drought followed by record rains last 
winter that produced a bumper crop of grasses and fine fuels on 
top of drought and disease stressed heavier fuels. The Hawkeye 
raws station in northern Sonoma peaked to 79 mph during the event 
with the Santa Rosa raws at only 599 feet reporting an incredible 
gust to 68 mph that may have been fire induced as the temperature 
spiked to 91 degrees at 4:30 am. The NWS has meteorologists 
enroute to support the incoming incident management teams.

Looking forward. We have cancelled the Red Flags for the East Bay
and Santa Cruz mtns. The Red Flags for the North Bay will stay in
place overnight simply for the protection of fire fighting lives
and property. There is simply so much active fire on the ground
that even the normal onshore winds will be of concern for both
fire fighters on the ground and the general public through the
overnight hours.

For Tuesday the wind flow will turn onshore with a mainly south to
southwest direction. Again, its not really a significant weather
factor but with miles of open fireline it will represent a change
in wind direction for assets on the ground. The transport winds
look to be southerly as well so luckily for the greater bay area
much of the smoke should be pushed to the north of us and impact
other parts of CA.

In the fire business, the common refrain is fire goes where wind
blows and unfortunately there will be another dry cold frontal
passage later Tuesday night into Wednesday morning. To be crystal
clear the magnitude of this front will be MUCH weaker than the
Sunday night system. However, if there is still a lot of open
fireline at this time tomorrow night it will be of concern once
again. Following the 00z nam solution expect winds to be southwest
late tomorrow afternoon, then shifting to northwest during the
evening (as the front approaches) but then quickly be northeast
by 12z Weds. The 925 mb winds on the nam are about 35 kt over 
Napa county on Weds morning (they were 55-60 kt with the Sunday 
night event). Humidity will be higher and temperatures cooler by 
Weds morning so that will help but all bets are off when you 
already have active fire on the ground. Tuesday will be a critical
day in the fire fight.

By Weds afternoon dry and breezy northerly winds will persist,
especially over Napa and interior Sonoma. Looking a little closer
at the 00z nam its now keeping those dry northeast winds in the
hills Weds night into Thursday morning with humidity values in 
the 30-40% range.

Looking at the Euro for the long range would suggest warming and
drying for late in the week and into the weekend with troughs
staying well to our east and the Bay Area getting periods of warm
offshore wind flow with the thermal trough along the coast. Good
weather if you like going to the beaches but not so much if your
trying to suppress wildfires.

&&

.AVIATION...as of 5:18 PM PDT Monday...Smoke due to fires in the 
North Bay continues to reduce visibilities to IFR over the North
Bay, smoky and hazy conditions have swept as far southwest as the
Farallon Islands with a recent report of 7 miles visibility. Elsewhere
weak onshore winds and clear skies have helped keep VFR going at the
terminals. Not indicated in 00z tafs, but caveat here is that little
change in air mass stability is forecast tonight may trap more smoke
and haze overnight degrading slant-wise and horizontal visibilities
over more of the Bay Area into Tuesday morning.

Visible imagery shows an area of stratus and fog moving rapidly north
along the coast reminiscent of a southerly surge, however a nearly
flat SMX-SFO gradient does not support a typical wind reversal. As
best as can tell from obs and model forecasts this is part of a 
surface to low level eddy circulation spun up from the recently 
strong to very strong N-NE Bay Area winds; not a lot of confidence
in wind direction forecasts in the models for tonight, but for 
what it's worth the NAM is not forecasting a southerly pressure 
gradient. Best guess is that this circulation will continue to 
spin down with general onshore flow prevailing at the terminals 
overnight. Marine inversions are quite compressed, down to a few 
hundred feet, thus any stratus and/or fog should stay confined to 
the immediate coastline tonight and Tuesday.

Vicinity of KSFO...VFR. Recent wind shift to SW per observations will
probably stick for the evening, low confidence forecast. Otherwise
in general expecting W wind flow near 10 knots tonight and Tuesday.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Low to moderate confidence VFR. Light onshore
winds becoming E-SE overnight should help keep VFR going tonight. Onshore
winds redevelop Tuesday afternoon and evening.

&&

.FIRE WEATHER...As of 9:46 PM PDT Monday...Red Flag Warning will
continue through 5 am Tuesday for the North Bay. The warnings
for the East Bay and Santa Cruz mtns were cancelled. Please see
regular afd above for more details. Next period of concern will be
early Weds morning with another round of northeast winds for the
Napa hills. Not nearly as strong as the Sunday night event but
concerning none the less.

&&

.MARINE...as of 09:17 PM PDT Monday...Light winds tonight and
Tuesday morning then northwesterly winds will increase from north
to south later in the day Tuesday as a  cold front sweeps in from
the Gulf of Alaska. The cold front is  forecast to reach the
southern California coastal waters Wednesday morning. High
pressure strengthening over the eastern Pacific and over
northernmost California will cause an increase in northwesterly
winds across the coastal waters through mid to late week. Gusty
afternoon and evening westerly winds are forecast across the bays
mid to late week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...NONE.

&&

$$

PUBLIC FORECAST: RWW
AVIATION: RGass
MARINE: RGass
FIRE WEATHER: RWW

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