Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA

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FXUS66 KMTR 151130

Area Forecast Discussion
National Weather Service San Francisco Bay Area
430 AM PDT Mon Oct 15 2018

.SYNOPSIS...Offshore flow will result in warmer and drier
conditions region-wide today, along with critical fire weather
conditions in the hills of the North and East Bay through
midday. Gusty winds in the hills are forecast to diminish by this
afternoon. Dry weather is forecast to persist through the end of
the week, along with mild to warm daytime temperatures.


.DISCUSSION...As of 3:20 AM PDT Monday...North to northeast winds
have been locally strong and gusty in the North Bay Mountains
since last evening. Wind sensors near the summit of Mount Saint
Helena in the far North Bay (4300` elevation) have been
consistently reporting sustained winds of 40 to 60 mph, along
with gusts of 50 to 70 mph, since Sunday evening. Elsewhere in the
North and East Bay Hills, winds have generally been gusting
between 25 and 45 mph. Mesoscale models have indicated that winds
would peak between late last evening and about now, and then
gradually subside. Already seeing some evidence that wind speeds
are starting to come down, but the rate of decrease is expected to
be slow. A Wind Advisory remains in effect for the North Bay
Mountains through 10 am this morning.

Gusty offshore winds in the hills have brought a very dry airmass
into the North and East Bay, where relative humidity values did
not recover at all overnight, and mostly remained in the range of
15 to 25 percent. Widespread Red Flag conditions have been
occurring across the higher elevations of the North and East Bay
since yesterday afternoon. The Red Flag Warning is set to end at
Noon today when winds finally subside sufficiently. For more
details on critical fire weather conditions, see the Fire Weather
Discussion below.

The dry airmass that has arrived across most of our area, and
continued light offshore flow through the day today, will mean
warmer temperatures today. Highs are expected to be in the 70s in
most coastal areas, and 80s across inland areas. A shallow marine
layer has managed to persist in coastal areas from San Mateo
County southward, where offshore flow has not been strong enough
to mix into the boundary layer. Thus, our southern coastal areas
may not warm as much today as anticipated today, with
temperatures perhaps remaining in the 60s.

The current synoptic scale pattern consists of an upper ridge
centered to our north along the coast of the Pacific Northwest and
a trough to our south-southeast over the Desert Southwest and far
southern California. This Rex Block pattern is forecast to undergo
only minor changes as the week progresses. What this means for our
area is continued light offshore flow, mild to warm daytime
temperatures, and persistently dry air across most of our area.
The low confidence part of the forecast for the rest of the week
concerns the marine layer. The magnitude of offshore flow may be
too weak to prevent a shallow marine layer from persisting in
coastal areas (as is currently the case along our southern coast).
So, coastal temperatures will be difficult to predict this week.
Also, dry air and relatively long mid-October nights will mean
rather chilly nights for the inland valleys starting tonight, with
some of the coldest valleys expected to see overnight lows in the
upper 30s and lower 40s.

Longer range models indicate the blocking pattern will finally
break down by early next week, and we could see rain in the
northern portion of our area by next Tuesday (the 23rd).

&& of 04:30 AM PDT Monday...For 12z tafs. Offshore
flow has kept much of the area under clear skies overnight.
Stratus has developed along the San Mateo coast and southward to
Monterey and down the Salinas Valley. VFR conditions will prevail
through the period for all sites with the exception of KMRY and
KSNS. LLWS is forecast to continue through the morning when winds
aloft subside.

Vicinity of KSFO...VFR. N to NE winds around 10 kt will continue
with LLWS through around 18Z with NE winds 25-30 kt above 2000 ft.
Winds aloft will subside throughout the morning.

SFO Bridge Approach...similar to KSFO.

Monterey Bay Terminals...IFR cigs occasionally scattering out
through around 15Z-17Z. VFR conditions will then prevail.
Generally light winds through the period.


.FIRE WEATHER...As of 3:15 AM PDT Monday...Locally strong and
gusty north to northeast winds continue in the hills of the North
and East Bay early this morning. Most locations have seen gusts of
between 25 and 45 mph overnight, but Mount Saint Helena in the
North Bay (elev. 4300 feet) has consistently been reporting
sustained winds of 40 to 60 mph, along with gusts between 50 and
70 mph. Mesoscale models have indicated that peak winds would
occur from late last evening through about the present time, with
winds then forecast to gradually subside from about 3 am through
midday. Some higher elevation locations have recently begun to
show a downward trend in wind speeds, so the models appear to be
correct. Relative humidity (RH) recoveries have been almost non-
existent overnight, with RH values holding in a range of between
15 and 25 percent across the North and East Bay. The Red Flag
Warning for the North Bay Mountains and East Bay Hills is due to
expire at Noon today, and this seems reasonable as winds will
have decreased significantly by then. However, very dry air will
persist and afternoon RH values are expected to drop as low as 5

Latest local WRF model indicates there will be a modest increase
in north to northeast winds in the higher hills of Napa and
northeast Sonoma County tonight into Tuesday morning, but winds
tonight are not expected to be anywhere near as strong as they
are currently...perhaps local gusts to 30 mph across some of the
highest peaks.

High pressure is forecast to maintain dry conditions and
relatively warm temperatures across the district through the
remainder of the week. However, after today winds are expected to
remain light (except for the brief uptick in winds locally in the
North Bay Mountains tonight as mentioned previously). In
addition, relative humidity values are forecast to gradually
increase as the week progresses.

&& of 02:13 AM PDT Monday...Gusty offshore winds can be
expected through late this morning in the Suisun Bay and the West
Delta as well as over the waters near coastal gaps. Winds will
diminish this afternoon and then remain generally light to
moderate through the forecast period. Mixed swell will continue
with a long period southerly swell through mid week and a more
moderate period northwest swell. A longer period northwest swell
will arrive during the second half of the week.


     .Tday...Wind Advisory...North Bay Mountains until 10 am
             Red Flag Warning...North Bay Mountains and East Bay
              Hills until Noon
             SCA...Pt Reyes to Pigeon Pt 0-10 nm until 2 PM
             SCA...SF Bay until 11 AM



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