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

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FXUS66 KMTR 180031

Area Forecast Discussion
National Weather Service San Francisco Bay Area
531 PM PDT Tue Oct 17 2017

.SYNOPSIS...After two very warm days, a gradual cooling trend is
forecast for today and tomorrow. The pattern will then change as
a system drops down from the north and brings generally light
rainfall to parts of the area late Thursday into Friday.

&& of 1:39 PM PDT Tuesday...Mid and high level
clouds continue to stream over the San Francisco Bay Area and
central California coast this afternoon as a vort max approaches
Point Conception. This has allowed for some showers and even a
some thunderstorms to develop offshore earlier this morning. While
the best chances for any shower and thunder activity will remain
in San Luis Obispo and Santa Barbara Counties, can`t rule out a
stray shower or even a few rumbles of thunder to make it into the
southern third of Monterey County as well as over the southern
coastal waters south of Big Sur. This vort max will quickly push
east into Kern County overnight and continue east on Wednesday.

Across the region this afternoon, temperatures are generally
running about 3 to 8 degrees cooler than what they were at this
time yesterday. This is largely due to a weak shortwave that has
rippled through the area that has allowed for 500 mb heights to
fall from 590 decameters in yesterday`s 12Z Oakland sounding to
585 decameters at 12Z today. Additionally dewpoints are generally
up by about 3 to 9 degrees compared to yesterday. When all is set
and done today, expect high temperatures to reach the middle 60s
to middle 70s along the coast and middle 70s to middle 80s for
interior areas.

Temperatures are expected to continue a downward trend through
the end of the week as an upper level low currently centered south
of the Aleutian Islands is forecast to slide southeastward toward
coastal British Columbia by early Thursday. A front will extend
southward along the Pacific Northwest and into parts of northern
California, and will bring with it increasing chances of rain by
late Thursday and overnight into Friday morning. At this point,
models are becoming more consistent run-to-run and are advertising
up to around 0.25" in the North Bay, up to 0.10" around San
Francisco, and maybe up to a few one-hundredths around Monterey.
Motorists should be prepared for locally wet and/or slick roadways
during the Friday morning commute, particularly those commuting
in/around the North Bay.

Skies will eventually clear by the afternoon hours for many as the
decaying frontal system passes through. Forecast models continue
to advertise a building ridge over California heading into the
weekend and into early next week. This will result in a warming
and drying trend as temperatures climb back to above normal

&& of 4:50 AM PDT Tuesday...For 00z tafs. A
disturbance moving to the south of our forecast area is bringing
mid and upper level clouds across area terminals this afternoon.
Aside from high clouds smoke from wildfires burning in our area is
reducing vsbys across area terminals. Light onshore flow will
prevail through this evening becoming light and variable
overnight. Patchy coastal fog and low clouds may return to the
immediate coast overnight under a shallow marine inversion with
minimal impact to area terminals. KMRY and KSNS are anticipated to
see cigs around 14z Wednesday morning.

Vicinity of KSFO...VFR conditions will prevail through the
forecast period with scattered to broken high clouds moving
across area terminals through tonight. Moderate onshore winds will
become light easterly to variable overnight.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR conditions will prevail through early
Wednesday morning with broken to overcast high clouds moving
across area terminals. Light onshore winds will become light
and variable overnight. IFR cigs will return early Wednesday


.FIRE of 10:35 PM PDT Monday...A pattern change is
on the way, with a general cooling, moistening trend and the
possibility of wetting rains arriving this week. Winds will begin
to shift and become more onshore tonight, helping to bring the
cooler temps, moister air mass, and improved air quality for many
areas. Temperatures will decrease a few to several degrees each of
the next 3 days, returning to near normal temperatures by mid
week. Humidities will also increase given the cooler temperatures
and onshore flow. Some uncertainty regarding the upcoming rains.

A convective complex off of the Baja California coast located on
the southwestern flank of the exiting ridge will bring a slight
chance of thunderstorms southern Monterey and San Benito counties
from late this morning through the evening.

Model data now indicates light non-wetting warm sector precip
arriving late morning Thursday, while the main band of cold
frontal precipitation arriving Thursday evening through Friday
morning. This system will bring wetting rains to some areas but
not others, with the best chance of wetting rains over the North
Bay mountains and hills, where fires are most active. Much less
precipitation is expected farther southward and/or at lower

&& of 04:53 PM PDT Tuesday...An upper level disturbance
is currently progressing northeastward towards the southern end of
the coastal waters. This system has been associated with light
precipitation and lightning. There is still a chance of
thunderstorms to move over the southern waters this afternoon and
evening. A trough and cold front is still  forecast to move across
the waters late Thursday. This system may bring light rain. A
moderate to long period mixed northwesterly and southerly swell is
expected this afternoon and tonight. Some buoys are already
reporting periods of 18 to 19 seconds. Later this week a very
large northwesterly swell train is forecast to arrive across the
area. This large swell will create hazardous conditions.






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