Scientific Forecaster Discussion
fxus66 kmtr 170541
Area forecast discussion
National Weather Service San Francisco Bay area
941 PM PST Sat Dec 16 2017
Synopsis...gusty northerly winds and dry conditions will persist
through Sunday morning. Lighter winds, with above average
temperatures and dry conditions through the early part of next
week. A slight chance of light precipitation is expected over the
northern portion of the region Tuesday night into Wednesday
morning, then dry through most of the remainder of the year.
Discussion...as of 08:45 PM PST Saturday...breezy to locally
gusty winds persist over the region this evening, especially in
the higher elevations. While winds will diminish some in the
valleys overnight, stronger winds will persist generally above
1,000 feet and combine with very dry conditions to produce
critical fire weather conditions. Thus, a Wind Advisory and red
flag warning remain in effect for the higher elevations across the
entire region through Sunday morning. Temperatures overnight are
forecast to cool into the 30s to lower 40s for most locations as a
colder air mass advects into the region. However, if winds do not
diminish as much as expected, could see temperatures stay a bit
warmer. With that said, the overall forecast remains on track and
no updates are needed at this time. Please see the previous
forecast discussion below for additional details.
Previous discussion...as of 12:45 PM PST Saturday...a dry but
energetic vorticity wave swept through the state early this
morning at a very quick pace. As of noon, this feature has already
pushed eastward beyond the Sierra Nevada and was rapidly
approaching the Utah state line. In the wake of this feature, the
northerly surface gradient has been trending upward (from 3mb
acv->sfo 24 hours ago, to nearly 10mb acv->sfo in the last hour),
resulting in gusty north to northwest winds throughout the region.
These strong winds initially arrived overnight with the passage
of the cold frontal boundary and were focused primarily over the
highest elevations, with lighter winds near the surface due to the
presence of a nocturnal inversion. As the nocturnal inversion was
eroded by the Rising Sun, the energy from these winds aloft was
able to Transfer their momentum to lower elevation locations,
including most of our major population centers. So far today, the
strongest wind gust reported was 65 mph at Mount Diablo, with 61
mph at los Vaqueros RAWS (situated southeast of Mount Diablo and
north of the Altamont pass), with several other locations from
around the area clocking in above 45 mph: Mt tamalpais, Bodega
Bay, Point Reyes, Oakland Hills, Half Moon Bay, inland San Benito
County, the Carquinez Strait/Delta, and others.
Due to these winds, a Wind Advisory remains in effect for
locations at or above 1000 feet throughout the area. In addition,
gale warnings are in effect for most of the coastal waters.
Finally, due to the dry antecedent conditions and these gusty
northerly winds, a red flag warning is also in effect for
locations at or above 1000 feet throughout the area for the
potential for extreme fire risk and potential for rapid fire
growth. Overall, humidity values at our driest locations are
ranging from the mid teens to mid 20s as of noon, however, these
values will continue to drop into the afternoon as temperatures
rise (to somewhat above average values for this time of the year).
The Wind Advisory and red flag warning will remain in effect
through 10 am Sunday morning as these dry conditions and gusty
north winds are expected to gradually taper off through this
window before dropping below criteria tomorrow morning. The most
common question associated with all these products is if the area
has ever had a red flag warning this late into the year before.
The answer is, yes, we have, but it is rather rare. Not
unprecedented, but rare, which would go along with the fact we on
track for one of the driest decembers on record (per downtown San
Francisco, only 1989 and 1876 were drier).
Forecast models then shows a moderately amplified ridge building
into the region as early as tomorrow in the wake of todays
disturbance, bringing continued dry conditions and above average
temperatures through at least Tuesday. Models then show a positively
tilted trough (a negatively tilted is generally preferred for a
stronger system) descending into northern California on Wednesday.
This system lacks sufficient moisture to bring widespread, or any,
rainfall to most locations, however, it could provide a glancing
blow to the North Bay, and bring a few hundredths there. Otherwise,
the 384-hour GFS shows dry conditions persisting throughout the
state through essentially the remainder of the year. The CPC 8-14
day analysis would tend to agree with this drier diagnosis, however,
the CPC 3-4 week forecast shows above average rainfall chances for
the region, so that is something to keep an eye on for 2018. For
what its Worth, both the GFS and Euro show a very high amplitude,
but narrow, ridge situated over the West Coast on December 25th.
Aviation...as of 9:30 PM PST Saturday...VFR. Surface winds gradually
decoupling from winds aloft this evening will cause low level wind
shear tonight into Sunday over the Bay area. A brief period of low
level wind shear vicinity kmry is possible late tonight and early
Sunday morning. Cold air drainage winds in the Salinas valley linking
up to lingering gusty NE-east winds aloft should offset potential for
low level wind shear ksns.
Vicinity of ksfo...VFR. Gusty northwest-north surface winds gradually decreasing
this evening, but a separation of the surface wind at the terminal from
gusty north-NE winds aloft will result in low level wind shear tonight and
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...VFR. Northwest winds becoming east-southeast and locally gusty
late tonight and Sunday morning in the Salinas valley. Low level wind
shear possible kmry early Sunday morning.
Fire weather...as of 9:40 PM PST Saturday...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 poor overnight recoveries expected as relative humidity values may
only rebound into the 25% to 35% range, especially in the higher
elevations. Along with the very dry air mass, breezy to gusty
winds continue this evening and will persist through the overnight
with gusts of 30 to 40 miles per hour. Stronger gusts are possible in the
highest peaks, such as Mt. Diablo where current winds are gusting
to 68 miles per hour. Wind speeds are then forecast to diminish late Sunday
morning, however dry conditions will continue through the weekend.
For what it's Worth, fire weather concerns are not that unusual
for this time of year. December 16, 2013, a wildfire broke out in
Big Sur called the Pfeiffer fire.
Marine...as of 9:25 PM PST Saturday...gale conditions will steadily
ease overnight. Steep seas will remain through Sunday before
subsiding on Monday. A cold front moves southeastward over the
coastal waters late Tuesday into Wednesday morning bringing some
building swell. Northerly winds will increase behind the front
causing building steep seas for mid to late next week.
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm
Small Craft Advisory...Pigeon pt to pt pinos 0-10 nm
glw...pt pinos to pt Piedras Blancas 0-10 nm
glw...pt Arena to Pigeon pt 10-60 nm
glw...Pigeon pt to pt Piedras Blancas 10-60 nm
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm
Small Craft Advisory...sf Bay until 3 am
Small Craft Advisory...mry Bay until 3 am
Public forecast: rgass/drp
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