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

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099
FXUS66 KMTR 202346
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
446 PM PDT Sun Oct 20 2019

.SYNOPSIS...A warming and drying trend will begin today and
continue through Thursday as high pressure strengthens over
California. In addition, offshore flow will develop by Monday, and
increase by midweek, which will result in warm temperatures
reaching all the way to the coast. Locally gusty offshore winds
may develop at times in the hills, especially in the North Bay
Hills on Wednesday night and Thursday morning. In the longer
range, cooling is forecast next weekend though another round of
offshore winds look increasingly likely by late next weekend to
early next week.

&&

.DISCUSSION...As of 02:30 PM PDT Sunday... California is
positioned between a 593dm 500mb high pressure ridge 800 miles to
the SSW and a 540dm 500mb low pressure trough 1000 miles to the
east. Satellite imagery shows clear skies across all but the
northern periphery of the state along the Oregon border early this
afternoon. Temperatures are up today while humidity is down across
the region as the aforementioned high pressure ridge begins a
multi day encroachment of the Golden State. Sonoma county is
seeing the largest day over day change as of 2PM, with
temperatures at Santa Rosa +19 (60 vs 79) over yesterday and
humidity down to 29% from 95%, primarily due to clear skies today
versus a weak cold front moving through yesterday.

Otherwise, regionwide temperatures are up generally 3 to 6 degrees
over yesterday, while dew points are down 5 to 15 degrees. This
warming/drying trend is forecast to intensify through the upcoming
work week, with a tangibly warmer/drier air mass advecting into
the region by tomorrow as the high pressure begins to gain a
foothold in the Golden State. Tomorrow is forecast to be generally
5 to 9 degrees warmer than today, with the largest degree of
warmest expected for the inland valley locations of the
North/East/South Bay. In addition, the offshore gradient will
steepen and consequently cause winds inland to veer towards the N/NE
and become increasingly offshore, especially inland. Dry to very dry
conditions during the days with poor humidity recoveries overnight
will take hold as these offshore winds amplify through the coming
week.

The high pressure ridge is forecast to continue a slow march
towards the California coast and anchor itself just off the San
Francisco Bay area shoreline from Wednesday into Thursday. As a
result, Wednesday into Thursday is expected to see the strongest
offshore winds as the SFO-WMC gradient steepens further, in
addition to unseasonably warm temperatures (generally 12 to 16
degrees above seasonal normal), and extremely dry conditions.

The latest forecast package made several updates to this upcoming
workweeks temperatures, winds, and humidity forecast,
specifically by increasing daytime highs Wednesday through Friday
(ie downtown San Francisco is now forecast to push into the mid
80s on Thursday), overnight lows in the higher elevations (thermal
banding), and a focus on downsloping wind(gusts)s impacting
downwind locations.

Near critical to critical fire weather conditions are anticipated
to develop no later than Tuesday night and persist through at
least Friday morning -- with current thinking that with continued
model support, a fire weather watch for the higher terrain of the
North Bay may likely be issued during the day on Monday, with some
possibility of also including the East Bay hills and Santa Cruz
mountains, though the North Bay hills/mountains will see the
highest localized fire risk overall.

Looking ahead -- the high pressure system will make a break
towards Southern California and then Baja California into this
upcoming weekend. This will result in a cooling trend for the
region, especially by Sunday -- with a return to near normal
temperatures. That said -- fire weather concerns will extend from
the late weekend into early next week as another energetic "inside
slider" pattern looks to bring yet another round of dry offshore
winds to the broader region. Thus, we are still squarely in the
prime of our fire weather season, so continue to remain mindful
about these dry conditions and take precautions to prevent
unintended fire starts.

&&

.AVIATION...as of 4:46 PM PDT Sunday...Developing offshore flow
over the area will keep VFR conditions over the SFO Bay Area.
Models suggest some increasing low level moisture over the coastal
waters tonight with patchy cigs developing along the coast and
around MRY Bay Area.

Vicinity of KSFO...VFR. West wind 15-20 kt decreasing after 04Z.

SFO Bridge Approach...Same as KSFO.

Monterey Bay Terminals...IFR cigs expected after 08Z but
confidence is not high as it could stay clear.

&&

.FIRE WEATHER...As of 4:10 PM PDT Sunday...Heightened fire
weather concerns are expected through the week as warm and very
dry conditions combine with gusty offshore winds at times to
create critical to near critical fire weather conditions. A
warming and drying trend will continue through Thursday as high
pressure builds over California and offshore flow develops.
Offshore flow will be mostly light except locally moderate and
gusty at times in the hills. Relative humidity values will drop
into the teens by Tuesday afternoon. Very poor overnight humidity
recoveries are expected in the hills starting Monday night and
continuing for the rest of the week. The period of greatest
concern is Wednesday night and Thursday morning when some models
indicate the potential for stronger north to northeast winds in
the hills, mainly across the higher elevations of the North Bay.
Fire Weather Watches and/or Red Flag Warnings may be issued over
the next few days if critical fire weather conditions are deemed
likely to occur.

&&

.MARINE...as of 04:24 PM PDT Sunday...Moderate to locally gusty
northerly winds will prevail over the coastal waters resulting in
hazardous conditions. Winds will be  locally stronger south of
Monterey Bay along the coast. Northerly  winds will increase this
evening north of Point Reyes and over  the outer waters. A
moderate northwest swell will also continue  to moves through the
coastal waters.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...SCA...Pigeon Pt to Pt Pinos 0-10 nm until 11 PM
             SCA...Pt Arena to Pigeon Pt 10-60 nm from 6 PM
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm

&&

$$

PUBLIC FORECAST: DRP
AVIATION: W Pi
MARINE: MM
FIRE WEATHER: Dykema

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