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

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FXUS66 KMTR 191615

Area Forecast Discussion
National Weather Service San Francisco Bay Area
815 AM PST Mon Nov 19 2018

.SYNOPSIS...Dry, stagnant, hazy, and lingering smoke will
continue today as high pressure gradually moves east.
A pattern change will develop by late Tuesday and widespread
rainfall is likely early Wednesday, with showers possibly
lingering through Wednesday night. A second system will likely
bring additional rainfall to the area from Thursday evening
through Friday. Forecast models are trending drier for next
weekend, but there is still a chance for shower activity then.

&& of 08:15 AM PST Monday...Upper level ridge axis
now situated just to our east and extends northwestward into
British Columbia. Upper low to our southwest is pushing high
clouds into the area this AM. Surface pattern remains stagnant
with light offshore flow overnight resulting in continued smoke
coverage over the Bay Area. Visibilities are between 1 1/2 and 3
miles in most locations around the Bay with air quality in the
unhealthy category. Little change in smoke extent is expected
today as winds will remain weakly offshore. Overnight dew points
were lower than the previous night preventing much fog/stratus
formation and allowing cool overnight lows once again. Conditions
today will be similar to the past couple of days with temperatures
in the 60s in lost locations to lower 70s in inland Monterey and
San Benito counties. Smoke/haze will continue through tonight and
into Tuesday.

Focus for the afternoon will be the upcoming pattern change.
Southerly to southeasterly flow will increase on Tuesday ahead of
an approaching trough, with a cold front and rain late Tuesday and
into Wednesday. The increasing southerly flow will help push some
of the smoke/haze out on Tuesday, with additional improvement on
Wednesday. Look for travel impacts on Wednesday with wet roads
areawide and mountain snow in the Sierra Nevada. No updates this


.PREV DISCUSSION...As of 3:15 AM PST Monday...Similar theme to the
forecast 24 hours ago - short term impacts will be smoke related
and then pattern change with rain by Wednesday.

The longwave pattern continues to show an upper level ridge over
NorCal into the PacNW as an upper low begins to undercut the ridge
to the south. Weak surface gradients from N-S and W-E combined
with weak offshore flow has kept smoke/haze around the Bay Area
overnight. Latest observations show visibilities hovering in the
2-4 miles with a mixed bag of smoke and/or haze. A few sites are
reporting some patchy fog, but haze and smoke are more likely.
Additionally, a quick look at air quality stations continue to
show moderate to unhealthy air quality. There is likely some low
stratus off the coast, but it`s hard to see underneath the higher
cirrus streaming in from the south. For today and Tuesday,
similar to Sunday with hazy smoky conditions lingering.
Temperatures will be somewhat mild (60s and 70s) with filtered
sunshine due to haze/smoke and high level clouds.

Southerly flow will increase Tuesday and Tuesday night as a cold
front approaches the Bay Area. This initially uptick in wind may
be enough to help improve some of the air quality around the Bay
Area pushing lingering smoke/haze northward.

As mentioned on the previous discussion the timing of the
approaching cold front has slowed a little bit, but still expect
rain to develop late Tuesday night. By Wednesday morning (4AM)
the cold front will be knocking on the doorstep of the Bay Area,
before pushing S and E during the day. Latest guidance continues
suggest a possible NCFR, which could bring periods of brief
intense rainfall during the Wednesday AM commute. The exact
timing could shift a little, but still expect impacts to the AM
commute. Additionally rain will continue through the day as the
front pushes through the Bay Area. Wednesday is also a big travel
day for Thanksgiving so travel could be dicey with wet roads and
mountain snow (Sierra). The front also looks to tap into to some
instability over the coastal waters. Therefore added a slight
chance for thunder over the waters on Wednesday. Confidence is low
over land, but may need it later. Post frontal showers will be
possible Wednesday night into Thursday, but the heaviest rain will
have exited the area. Initial rainfall estimated will be
0.25-0.75 inches and 1-1.5 inches for terrain favored locations
through Thursday. Do expect some rain shadowing in the Santa
Clara and Salinas Valleys given the robust SW flow ahead of the
front. It should be noted that WPC has much of the Bay Area in a
marginal risk for excessive rainfall on Wednesday with the fropa.

As for Thanksgiving holiday, a few lingering showers possible
early, but then break is possible before another system arrives
late. Medium range guidance brings another system to the West
Coast late on Thanksgiving. Locally, rain will be possible over
the North Bay late in the day. Rain will gradually push southward
Thanksgiving night into Friday. For what it`s worth, models have
been trending drier south of San Jose with the second system.

The drying trend continues into the weekend with only a few
showers possible over the North Bay.

Details beyond the weekend begin to wane as model consensus
diminishes. However, a somewhat active pattern remains as
additional system may bring precip to NorCal. Stay tuned.


.AVIATION...As of 03:29 AM PST Monday...For 12Z TAFs. High clouds
continue to stream overhead thanks to a low pressure system off
the coast. These clouds are making it difficult to see to the
surface on the satellite, but it appears there are some patches of
stratus along the coast with some stations reporting fog. Patchy
fog may be possible this morning in the North Bay and around the
Monterey Peninsula. Otherwise, the region will continue to see
reduced vis (~MVFR/IFR) due to wildfire smoke as well as slant
range vis issues. May be some improvement in smoke conditions
tomorrow as we undergo a pattern shift and southerly winds
increase. Light winds will persist through the period.

Vicinity of KSFO...Light winds, reduced vis to around 2SM-4SM and
SCT-BKN cigs around 1000 to 2000 ft due to smoke and haze.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...High clouds and light winds. May see
reduced vis to MVFR due to wildfire smoke. Patches of low cigs and
fog will be possible this morning, although confidence is low.

&& of 02:12 AM PST Monday...Generally light winds will
continue into Tuesday before southerly winds increase ahead of an
approaching front. Southerly winds will increase Tuesday night
into Wednesday with gale force gusts possible, especially along
the immediate coast from Pigeon Point to Point Piedras Blancas.
There will also be a slight chance of thunderstorms across the
waters Wednesday morning through the evening. Winds will diminish
and gradually transition out of the northwest on Thanksgiving.
Light mixed swell will continue through mid-week before a larger
northwest swell arrives late this week followed by another storm






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