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000 
FXUS66 KMTR 200643
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1043 PM PST Tue Nov 19 2019

.SYNOPSIS...Gusty and dry offshore winds will develop over the 
North and East Bay Hills/Mountains from tonight through early 
Thursday that will consequently result in critical fire weather 
conditions. Dry conditions, along with seasonable temperatures, 
are forecast from Thursday through the upcoming weekend. 

&&

.DISCUSSION...as of 09:28 PM PST Tuesday...An update was made to 
the forecast for this evening and early overnight tonight to 
increase the chance of rain across the area, especially across the
North Bay. KMUX radar detected a small line of convective showers
across Sonoma County this evening moving southwest, associated 
with an area of elevated instability, near the core of a deep 
upper low. Surface reports have indicated brief moderate to heavy 
rain associated with these showers. Behind the line of heavier 
showers, lighter showers have developed across Lake and Mendocino 
Counties. These should move south into the North Bay this evening.
The latest models indicate these showers should largely dissipate
as they move north to south, but a few isolated light showers or 
sprinkles are possible tonight across parts of the forecast area. 
Additionally, upslope northwesterly flow around the Monterey 
Peninsula and the Santa Cruz Mountains is generating some drizzle 
and light rain. Precipitation has been measurable in some places 
with generally only a few hundredths of an inch. However, some of 
the mountains southeast of Monterey have picked up 0.1 to 0.25 
inches of rain so far. Moist upslope flow will continue overnight 
but shut off before sunrise Wednesday.

Despite this evening's scattered showers, critical fire weather
conditions are still expected to develop late tonight and continue
through early Thursday morning. It's unlikely that the showers
this evening were heavy or widespread enough to make much of an
impact on fuel moistures. Offshore winds are starting to ramp up 
across the North Bay, with a gust to 48 mph at 9 pm on Mt. St. 
Helena. Relative humidity remains high but the offshore winds will
bring a much drier airmass into the region late tonight and into 
Wednesday. Gusty winds will spread across the North and East Bay
Hills tonight and continue through early Thursday morning before
tapering off on Thursday. A Red Flag Warning remains in effect 
from 4 am Wednesday morning through 7 am Thursday morning.

A drier airmass will mean colder overnight low temperatures,
especially in sheltered inland valley locations where lows will
drop well into the 30s beginning Wednesday night. Elsewhere, lows
will drop into the 40s with lows around 50 at the mildest coastal
locations.

For additional details on Fire Weather, the potential for an
additional offshore wind event early next week, and rain in the
long term, see the Previous Discussion and Fire Weather sections
below. 

&&

.PREV DISCUSSION...as of 01:56 PM PST Tuesday...In wake of a frontal 
passage earlier in the day, temperatures are running a few to as 
much as 16 deg F cooler (at KSNS - Salinas Airport) compared to 24
hours ago. While sky conditions are mostly sunny over the region,
some clouds persist in the hills as stronger northwest flow 
persists. Locally breezy conditions also continue in wake of the 
frontal passage and will likely persist through the remainder of 
the afternoon and early evening. The short-range forecast models 
continue to indicate the potential for a few post frontal showers 
to develop later in the day across the Central Coast as a short- 
wave disturbance drops southward along the coast. With this said, 
widespread rainfall is unlikely and much of the greater San 
Francisco Bay Area appears to remain dry.

Heading into this evening, northwesterly winds will diminish as 
offshore flow begins to develop and increase. The strongest winds 
will occur in the North and East Bay Hills/Mountains. As offshore 
flow persists overnight, a much drier air mass will usher into the 
region during the day Wednesday. North to northeast winds will 
remain strong and gusty through Wednesday night combining with the 
dry conditions to produce critical fire weather conditions in the 
North and East Bay Hills/Mountains. Thus, a Red Flag Warning is in 
effect from 4 AM PST Wednesday through 7 AM PST Thursday. For 
additional details, please see the fire weather section below. 

Winds will begin to diminish region-wide after sunrise Thursday, yet 
conditions will remain dry for much of the day. The exception will 
be along the coast where the local WRF model continues to indicate 
the potential for a southerly surge creeping up the Big Sur Coast as 
the offshore gradient relaxes. If this occurs, will look for an 
increase in clouds/fog along the Central Coast with cooler 
temperatures. Otherwise, seasonably cool conditions will persist 
into the upcoming weekend with no widespread precipitation. 

The forecast models continue to hit at the potential for another 
short-lived offshore wind event early next week in response to an 
inside slider dropping southward from the Pacific Northwest into the 
Desert Southwest. While confidence is low at this time, will closely 
monitor in the coming days for the potential for increased fire 
weather concerns. Details continue to vary with respect to the 
potential for precipitation around Thanksgiving. However, the 
forecast ensembles do trend toward the possibility of a more active 
pattern developing sometime late week. 

&&

.AVIATION...as of 10:43 PM PST Tuesday...MVFR-VFR, pockets of light
rain to the south especially near higher terrain and isolated to 
widely scattered showers north. Higher resolution models are having
difficulty resolving the strength of a compact, potent low pressure
system moving southward tonight. A few adjustments made to tafs for
the 06z cycle otherwise still expecting north to northeast low level
wind shear KSTS, KSFO, KOAK, KLVK tafs tonight and Wednesday morning.

Vicinity of KSFO...VFR, occasionally gusty west to northwest surface
wind tonight while low level wind shifts to a northeast direction and
increases resulting in low level wind shear tonight and Wednesday
morning.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...West winds gusting to 20 knots at times 
tonight. MVFR cloud ceilings, tempo showers tonight. MVFR-VFR on
Wednesday.

&&

.FIRE WEATHER....as of 01:56 PM PST Tuesday...Red Flag Warning 
still remains in effect for the North and East Bay Hills/Mountains
from 4 AM PST Wednesday through 7 AM PST Thursday. As mentioned 
above, offshore flow will develop and begin to increase late this 
evening and persist through Wednesday. Humidity values will 
initially start out good tonight, but will rapidly dry out after 
sunrise Wednesday. North to northeast winds of 20 to 30 mph with 
gusts of 35 to 45 mph will be likely over the higher terrain of 
the North and East Bay, especially on Wednesday night. The usual 
wind-prone sites (Mt St Helena and Mt Diablo) may possibly 
experience gusts in excess of 60 mph. Winds will then diminish by 
sunrise Thursday region-wide. 

Fire weather concerns will return early next week as medium range 
models suggest another round of offshore flow impacting the region 
and potentially more widespread. Stay tuned - fire season is 
definitely not over yet until the region receives widespread wetting 
rainfall. 

&&

.MARINE...as of 9:16 PM PST Tuesday...A strengthening low 
pressure system will move southward over the coastal waters 
tonight and reach southern California Wednesday morning. Gale 
force northwest winds will continue mainly over the northern and 
outer coastal waters tonight into Wednesday morning. Small craft 
advisories are in effect for the remainder of the coastal waters. 
Northwest swell builds and as the northwest winds increase, wind 
generated waves will also be generated creating large and 
hazardous seas into Wednesday. Winds and waves will subside 
Thursday. A long period northwest swell train will begin to move 
into the waters on Friday.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...Red Flag Warning...CAZ507-511
             SCA...Pigeon Pt to Pt Pinos 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             GLW...Pt Arena to Pigeon Pt 10-60 nm
             GLW...Pt Arena to Pt Reyes 0-10 nm
             GLW...Pt Reyes to Pigeon Pt 0-10 nm
             GLW...Pigeon Pt to Pt Piedras Blancas 10-60 nm

&&

$$

PUBLIC FORECAST: ST/RGass
AVIATION/MARINE: Canepa
FIRE WEATHER: RGass/MM

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