Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

000 
FXUS66 KMTR 192200
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
200 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 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 09:50 AM PST Tuesday...for 18z TAFs. A dry cold
front is sweeping through the region today and bringing MVFR-VFR
cigs, gusty northwest winds, and will bring local low level wind 
shear concerns later today and into tonight. In addition, and
increase in low to mid level cloud coverage is possible in the 
post frontal environment later this afternoon to evening, though
heights, coverage, and times are low confidence. 

Wind gusts will be strongest along the coast and near coastal
gaps, including near KSFO, where gusts 35kt to 40kt are possible
through the day today, while sustained speeds there will push to
30kt. Elsewhere, breezy to gusty winds are possible at different
times today, see TAFs. 

Models have tremendously underrepresented this weather system with
winds coming in stronger than expected, as such, confidence on
winds is only moderate at best.

Localized SCT-BKN MVFR level cigs are possible from 22Z TUE - 04Z
WED today due to the cumulus/stratocumulus development in the 
post frontal environment. 

Low level wind shear will predominately occur from 340 to 020
degrees with speeds 30-40kt at FL020 AT KSTS, KSFO, KLVK at 
different times but mainly overnight tonight, see TAFs.

Vicinity of KSFO...Low clouds in approach have dissipated for now,
however except to see more cumulus development later this
afternoon that may impact visuals. Very gusty west to northwest
winds today, 30 to 40kt, with sustained speeds to 30kt through
03-04Z WED. Low level wind shear possible overnight as winds turn
light at surface but remain NNE 30-35kt just above the surface.
Improving conditions all around after sunrise WED.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Gusty W-NW winds today on the order of
15-25kt, possibly higher. Clouds have dissipated at KMRY but will
linger at KSNS through at 19Z. MVFR cigs return around 00Z. Winds
weaken overnight and skies gradually clear through sunrise to mid
Wednesday morning.

&&

.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 09:38 AM PST Tuesday...The cold front continues 
to move southward over the coastal waters this morning. Northwest 
winds are forecast to increase today with gale force winds 
developing over the northern outer waters Tuesday evening and into
Wednesday. Northwest swell builds and as the northwest winds 
increase, wind generated waves will also be generated creating 
large and hazardous seas Tuesday night into Wednesday. Winds and 
waves will subside Thursday.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...Red Flag Warning...CAZ507-511
             SCA...Pigeon Pt to Pt Pinos 0-10 nm until 2 AM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm until 2 AM
             SCA...Pt Arena to Pigeon Pt 10-60 nm until 2 PM
             GLW...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Mry Bay until 8 PM
             SCA...Pt Arena to Pt Reyes 0-10 nm until 8 PM
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm until 8 PM
             GLW...Pt Arena to Pt Reyes 0-10 nm from 8 PM
             GLW...Pigeon Pt to Pt Piedras Blancas 10-60 nm from 8 PM
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...SF Bay until 9 PM

&&

$$

PUBLIC FORECAST: RGass
AVIATION: DRP
MARINE: DRP/DK
FIRE WEATHER: RGass/MM

Visit us at www.weather.gov/sanfrancisco

National Weather Service Glossary of Abbreviations