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

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FXUS66 KMTR 212354
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
354 PM PST Tue Feb 21 2017

.SYNOPSIS...Scattered showers and isolated thunderstorms will
persist into this evening before dissipating overnight. Drier
weather and cold temperatures then take hold through the later
half of the week, with frost and freezing temperatures possible
tomorrow night through Friday night. A wet and unsettled pattern
then looks likely to return to our area for this weekend.

&&

.DISCUSSION...as of 2:35 PM PST Tuesday... Satellite imagery
shows that the San Francisco/Monterey Bay areas are now residing
in the unstable post frontal environment between an upper level
low to the northwest and a sluggish frontal boundary and the
remnants of the Presidents Day storm atmospheric river to the
south. Mosaic radar reflectivities depict scattered to widespread
rain showers and isolated thunderstorms across a southwest to
northeast corridor stretching roughly from Point Reyes to the
north to the Monterey Peninsula to the south, extending inland
across the width of the Golden State and into the Great Basin.
These convective cells are developing along a dry boundary on the
southeastern periphery of the core of the upper low and are
expected to decrease in intensity and coverage into this evening
as the upper low shifts deeper inland, displacing this convective
air mass boundary. Rain showers are forecast to dissipate by the
overnight hours, however, a few weak isolated showers may linger
into early tomorrow before this storm system completely exits the
region.

The core of the aforementioned upper low will advect in a colder
air mass aloft (down approx 7C at 850MB over the next 48 hours) as
it shifts inland, which will then filter into the lower levels and
result in much colder temperatures to surface. This atmospheric
cooling will coincide with clearing skies and lighter winds. These
combined ingredients make up the recipe for idealized radiational
cooling for maximum diurnal heat loss, and the coldest possible
temperatures. As such, between Wednesday night and Saturday night,
widespread low to mid 30s are expected for all inland valleys away
from any shorelines. Locally colder temperatures, specifically mid
to upper 20s, will be possible for our extreme inland locations.
The coldest temperatures are expected Friday morning around
sunrise. Areas of frost and freezing conditions will result in
impacts to vulnerable populations, outdoor livestock, and
plants/crops planted early in the season. Black ice over bridges
or elevated roadways may also be a concern.

Finally, forecast confidence is somewhat increasing model to
model and run to run regarding the evolution of a Pacific storm
system that is expected to bring wet, windy, and cool conditions
to the San Francisco/Monterey Bay areas through this weekend and
possibly into early next week. Storm total rainfall and winds are
NOT currently expected to as high as the President Days storm. One
other major difference from other previous storm will be the
possibility of lower snow levels (down to around 3000 ft) with
this storm. The details on this next potential storm system are
likely to evolve over the coming days, so stay tuned!

&&

.AVIATION...As of 3:45 PM PST Tuesday...Showers to continue
through this evening with borderline MVFR cigs. Abundant low
level moisture for tonight but believe there may enough wind to
keep clouds or fog from developing.

Vicinity of KSFO...Cigs near 3000 feet with scattered showers
through 03Z. VFR after 03Z.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR with sct-bkn clouds at or above 4000
feet. Southwest winds gusting to 20 kt decreasing after 02Z.

&&

.MARINE...as of 01:55 PM 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
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Pigeon Pt to Pt Pinos 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Rough Bar Advisory for SF Bar

&&

$$

PUBLIC FORECAST: DRP
AVIATION: W Pi
MARINE: MM


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