Technical NOAA Weather Discussion

000
FXUS66 KMTR 171735
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
935 AM PST Sun Dec 17 2017

.SYNOPSIS...Gusty northerly winds and dry conditions will
continue this morning, especially over the North and East Bay
hills. Winds will then diminish, but with dry conditions and above
average daytime temperatures continuing through the first part of
the week. A Pacific weather system is projected to move rapidly
southeastward through Northern California Tuesday night and
Wednesday, with rain chances spreading southward into the greater
San Francisco Bay Region. High pressure then looks to rapidly
rebuild along the coast with dry weather and seasonal temperatures
for the latter part of the week and next weekend.

&&

.DISCUSSION...as of 9:30 AM PST Sunday...Visible satellite
imagery reveals mostly clear skies throughout the forecast area
this Sunday morning. Temperatures are generally in the 40s and 50s
at this time, though a few locales in the North Bay remain in the
30s. Fire weather and winds remain the short-term concerns as Red
Flag Warnings and Wind Advisories remain in effect for parts of
the forecast area until 10 am PST. Overnight observations have
mostly confined the strong winds to the higher elevations of the
North Bay Hills and East Bay Hills. Mt Diablo (as is usual)
reported the strongest speeds with max sustained winds over 50 mph
and gusts over 70 mph overnight. Our in-house NWS Monterey WRF
models indicates 925 mb winds will steadily decrease through the
day over the North and East Bay Hills. While the highest ridgetops
and peaks will probably still see sustained winds and/or gusts
above 35 mph after 10 am, not expecting these elevated winds to
remain widespread. Thus, at this time, plan to let the wind
advisory expire as scheduled at 10 am for the North Bay Hills and
East Bay Hills.

Relative humidity values generally recovered to the 25 to 40
percent range in the hills overnight. Though RHs will drop as the
day progresses, the decreasing wind speeds will help to alleviate
the widespread fire weather concerns that we`ve experienced over
the past day. Therefore, planning to let the Red Flag Warning
expire at 10 am PST as well. Despite the Red Flag expiration,
conditions will still remain exceptionally dry for December
throughout the San Francisco Bay Area and central California coast
this afternoon.

The short-term forecast remains on track and no major changes are
planned for this morning. For additional details beyond Day 1,
please refer to the previous discussion section.

&&

.PREV DISCUSSION...as of 3:30 AM PST Sunday...Aside from some
thin high cloudiness drifting through, mostly clear skies prevail
across our district. Winds continue to be locally strong and gusty
in the North and East Bay hills and am therefore continuing the
Wind Advisory for these zones into the morning. Elsewhere winds
have generally diminished and the Wind Advisories have been
cancelled. Will note that at a few locations the winds did
surface. Napa Airport, in particular, stood out as a low elevation
site with an interval of strong winds during the overnight hours,
recording a peak gust of 43 mph at 1:25 AM PST. The Red Flag
Warning remains in effect across all higher elevation portions of
our district until 10 AM PST.

The ridge of high pressure now building into the West Coast will
persist through the first part of the week, but then looks to
begin weakening and shifting inland as a strong upper level system
moves rapidly southeastward through the Pacific Northwest.
Although latest model output continues to show the primary
associated precip band moving inland to our north, more recent
solutions have trended westward and now do bring its tail end
through our district in a late Tuesday night to Wednesday time
frame. Have therefore begun ramping up associated PoPs, and noted
just received 06Z runs of the NAM and GFS are a bit wetter still.
That said, rainfall totals are likely to be on the small side,
given the limited moisture supply and rapid speed of movement.

As we then head into the latter part of the week, high pressure at
the surface and aloft again looks to rapidly rebuild along and
into the West Coast, with dry conditions and offshore flow
returning for the latter of the week and into next weekend.

&&

.AVIATION...As of 9:35 AM PST Sunday...Dry northerly flow will
result in continued VFR conditions through Monday. Stronger winds
aloft will result in low level wind shear, especially over the
northern portion of the region through this morning with
diminishing winds through 21Z.

Vicinity of KSFO...VFR.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR.

&&

.FIRE WEATHER...as of 3:35 AM PST Sunday...A Red Flag Warning
remains in effect through 10 AM PST Sunday for elevations at or
above 1,000 feet across the entire region. Dry conditions will
persist with early morning maximum humidity values likely confined
to the 20 to 30% range, especially in the higher elevations.
Along with the very dry air mass, breezy to gusty winds will
continue during the morning hours, especially over higher
elevations in the North and East Bay hills. Wind speeds are then
forecast to diminish, however dry conditions will continue into
the first part of the week.

&&

.MARINE...as of 09:35 AM PST Sunday...Winds will diminish through
the day as an upper level trough over the interior moves east.
High pressure will build off of the northern California coast
bringing light to moderate north winds tonight through Tuesday.
Northwest winds will increase Tuesday night and Wednesday after a
frontal system moves across the area.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...Wind Advisory...CAZ507-511
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm until 2 PM
             SCA...Pt Reyes to Pigeon Pt 0-10 nm until 2 PM
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm until 2 PM

&&

$$

PUBLIC FORECAST: Rowe/Blier
AVIATION: RGass
MARINE: RGass
FIRE WEATHER: W Blier

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Prepared by Boulder-Creek.com Weather at: Sun Dec 17 12:30:02 PST 2017
From the National Weather Service