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

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FXUS66 KMTR 211726
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
926 AM PST Tue Feb 21 2017

.SYNOPSIS...Lingering showers will continue across the region
through tonight before drying conditions return on Wednesday.
Along with the drying conditions, cooler temperatures are forecast
with the potential for late night, early morning frost late in the
week. Cool and unsettled conditions potentially return to the
region late in the week and into the upcoming weekend.

&&

.DISCUSSION...as of 08:40 AM PST Tuesday... Satellite imagery
shows the core of an upper level low shifting into the
PACNW/Northern California this morning. This upper low drove
yesterdays intensification of the atmospheric river that brought
heavy rain and gusty winds over the last couple of days. The
strongest winds (a few sites reported between 70-79 mph) and
heaviest rain (up to 8.50" storm total was reported in the coastal
North Bay) were associated with a cold front, which moved north to
south yesterday and has since exited into the southern half of
California. In the wake of this front, KMUX radar is detecting
scattered rain showers of varying strength in a southwest to
northeast band roughly stretching from the Santa Cruz mountains,
over San Jose, towards Livermore, and then on towards Lake Tahoe.
As of the last few minutes, lightning has now been detected east
in the northern San Joaquin valley.

As the core of the upper low shift inland, we are expecting to see
an uptick in convective rain shower coverage and intensity, with
embedded thunderstorms possible mainly. The best chance of these
thunderstorms will be over the North Bay, however, there exists a
small chance to see them throughout the forecast area today.
Impacts include copious amounts of small hail, brief heavy
downpours that can lead to localized flooding or mudslides given
the wet antecedent conditions, and erratic gusty winds that can
bring down trees or power lines. Shower coverage and intensity
will begin to wane by this evening, however lingering isolated
showers may continue into the night. See previous forecast
discussion for information about the upcoming weeks forecast.

.PREVIOUS DISCUSSION...as of 04:10 AM PST Tuesday...The strong
storm system that impacted the region during the past 24 to 36
hours continues to exit as the surface cold front pushes inland
early this morning. Wind speeds have also diminished as the deep
surface low pressure has ejected inland across the Pacific
Northwest. Meanwhile, lingering showers persist over the region
this morning in the post frontal environment. These showers have
produced brief heavy downpours with some locations picking up
between 0.10-0.25"/hr. Meanwhile, upsloping along the coastal,
especially across the Big Sur Coast, continues to result in
ongoing rain showers as well.

Isolated to scattered showers will remain possible through a good
portion of the day and into early Wednesday morning as a mid/upper
level trough remains over the region. However, widespread heavy
rainfall is not anticipated with this pattern. The parent
mid/upper level low will then shift inland across the Pacific
Northwest late Wednesday into Thursday and will result in mostly
dry conditions across the region. However, another weak
disturbance appears to drop southward down the West Coast and may
result in a few showers, mainly across the coastal waters and/or
along the coast Wednesday night into Thursday morning. Meanwhile,
a cooler air mass will advect southward across the region through
midweek and will result in much cooler overnight temperatures. By
tonight/Wednesday morning, temperatures will likely fall back into
the 30s and 40s under clearing skies. Even colder overnight
temperatures are forecast by Thursday and Friday mornings when
widespread 30s will be likely across inland areas. With this, late
night and early morning frost may be possible by late week,
especially in the wind sheltered inland valleys. Daytime
temperature will also be cooler than average, generally in the low
to mid 50s for the second half of the week with chilly
temperatures in the hills/higher elevations.

The latest forecast models are now hinting at a return to
unsettled conditions late in the week and into the upcoming
weekend. The GFS, ECMWF and Canadian models are all in decent
agreement in brining a cool, wet weather system into central
California. Current thinking is that a cold, mid/upper level low
will drop southward out of the Gulf of Alaska late in the week and
intersect a subtropical moisture tap. While we are several days
out and significant changes in the model solutions are possible,
this pattern would likely bring widespread precipitation to the
region this upcoming weekend and potentially into early next week
as the low lingers off of the West Coast. With this, lowering snow
levels will also be likely, potentially below 3,000 feet across
our region. Stay tuned!

&&

.AVIATION...As of 09:22 AM PST Tuesday...Low to moderate conf this
morning in dynamic weather regime. A mixed bag of cigs today as
scattered showers and possibly thunderstorms move across the Bay
Area today. Given the uncertainty decided to cover most TAFs with
VCSH through early tonight. Do expect some clearing late and could
even see some pockets of sunshine between now and then. Lots of
lingering moisture tonight, which could lead to stratus
development. Conf is rather low so decided to not include in TAFs
for Wed AM.

Vicinity of KSFO...Mixed MVFR to VFR with passing showers. Winds
will be be somewhat variable with passing showers.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...MVFR to VFR cigs with passing showers
through early tonight. Winds relatively light, but may increase at
KSNS later this afternoon.


&&

.MARINE...as of 09:20 AM PST Tuesday...Light to moderate winds
will prevail through the day as a storm system exits the region.
Hazardous conditions will prevail as squared sea impact the
coastal waters. Shower chances will continue through late
tonight...with a slight chance of thunderstorms through the
afternoon hours today. High pressure will bring moderate
northwest winds through the rest of the week with a moderate
westerly swell. Another storm system approaches by the weekend.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...SCA...Pt Arena to Pt Reyes 0-10 nm until 3 PM
             SCA...Pt Reyes to Pigeon Pt 0-10 nm until 3 PM
             SCA...Pigeon Pt to Pt Pinos 0-10 nm until 3 PM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm until 3 PM
             SCA...Pt Arena to Pigeon Pt 10-60 nm until 3 PM
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm until 3 PM
             SCA...Rough Bar Advisory for SF Bar

&&

$$

PUBLIC FORECAST: DRP
AVIATION: MM
MARINE: MM


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