Weather Service Report


471 
FXUS66 KMTR 151130
AFDMTR

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).

&&

.AVIATION...as 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 
percent. 

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. 

&&

.MARINE...as 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.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .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
&&

$$

PUBLIC FORECAST: Dykema
AVIATION: AS
MARINE: AS
FIRE WEATHER: Dykema

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Western Regional Climate Center, wrcc@dri.edu