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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

FXUS66 KMTR 221146 CCA

Area Forecast Discussion
National Weather Service San Francisco Bay Area
342 AM PST Fri Feb 22 2019

.SYNOPSIS...Cool and dry weather conditions will continue across
most of the forecast area through the weekend, except for rain
chances in the North Bay from Saturday afternoon through Sunday. 
Rain will become likely across the North Bay early next week, and
rain may shift southward across the rest of the San Francisco Bay
Area as well. Warmer temperatures are forecast next week, 
especially during the overnight hours. 


.DISCUSSION...As of 3:30 AM PST Friday...Overnight lows are
running somewhat warmer than previously anticipated despite a
cold, dry air mass settling into the region. Breezy northerly
winds are responsible for the discrepancy, and this is on display
particularly well at Napa airport. KAPC (Napa airport) is running
9 degrees warmer (45 vs 36) than this same time yesterday, despite
a dew point 14 points lower (19 vs 33), due to 16 mph winds out of
the north. On the flip side, KMRY (Monterey airport) is running 13
degrees cooler (35 vs 48) than the same time last night given 
much lighter winds. With calm winds, radiational cooling at the
surface brings continual heat loss and cooler temperatures.
However, with even light surface winds, some turbulent mixing 
occurs which blends the relatively warmer air aloft with the
cooling air near the surface, effectively reducing the heat loss 
felt at the surface and keeping overnight temperatures higher 
than they otherwise would be. Latest HRRR/WRF show winds weakening
away from the hills between 4-8am, which should provide a few
hours of peak diurnal cooling rates. This may be sufficient in
dropping many areas into the 30s to around 40 just after sunrise.

The low pressure system responsible for the cool air mass and 
recent showery activity is forecast to exit southern California 
into Arizona today. Weak ridging will develop in the wake of the 
this system closer to home but will quickly transition into mostly
benign westerly flow for all but extreme northern California 
through at least most of Saturday. 

Farther north this weekend, a 535dm 500mb upper low is forecast to
gradually slide southward along the coastline of British Columbia.
This feature will interact with a 0.85" TPW plume being advected
towards the West Coast by a different low pressure system deeper 
in the Pacific Ocean. As a result, wet weather is anticipated to 
return to the northern periphery of the North Bay sometime late 
Saturday into early Sunday. The EURO is the most aggressive with 
bringing rain into the region, pushing light rain as far southward
as coastal San Mateo County by Saturday evening. The GFS on the 
other hand keeps the bulk of the moisture north of Sonoma into 
Mendocino County through the weekend. For now, have leaned more 
towards the EURO output, with a slight chance to chance of rain 
across the North Bay on Sunday. 

Models then show a higher likelihood of rain by early next week 
when the aforementioned well offshore Pacific low closes in on the
California coastline, while at the same time lugging along a 
wetter, more expansive precipitable water plume for the nearby 
storm systems to work with. TPW values jump up as high as 1.25" 
later Monday into Tuesday but the GFS and EURO disagree on where 
exactly this plume of moisture will be aimed. The GFS aims it 
directly at the SF Bay Area while the EURO and NAM push it 
slightly farther northward into the North Bay. For now, will 
continue to side with the EURO/NAM output and the current forecast
reflects this, with the bulk of the QPF residing across Sonoma, 
Marin, and Napa counties. The heaviest rain will likely occur from
late Monday throughout the day Tuesday, with lingering showers 
possible as late as Thursday morning. Storm total rain between 
Monday and Wednesday from this storm system will be entirely 
dependent on where the narrow band of deeper moisture is focused,
however, given the latest data we are expecting 3.5-5" in the 
North Bay mountains and coastal ranges, with 1.5-3.5" in the North
Bay valleys. Farther south, San Francisco and Oakland may see up 
to around an inch, while the South Bay and inland East Bay may see
up to around half an inch. Farther south, the Monterey Bay region
may only see around a quarter of an inch as the bulk of the 
moisture remains farther northward. All that said, these numbers 
may change significantly in favor of the SF Bay Area if it turns 
out the GFS solution becomes favored by the other models. 

In addition to rain, breezy to gusty southerly winds should be 
expected. Winds are currently not expected to be as big of an 
impact as with previous storm systems so far this year. 
Temperatures should be relatively mild when compared to recent 
days as well given the warmer, moister origin of this air mass. 
Afternoon highs in the 60s to low 70s are possible by midweek 
while lows will struggle to drop below the upper 40s under mostly 
cloudy skies.

The region will briefly dry out late Thursday into early Friday of
next week as weak ridging develops across the southwestern CONUS.
Models show general agreement that a different storm system will
bring another round of rain and wind to California late next week
through next weekend. But for now, the focus will be on the
upcoming storm system set for early next week.  Stay tuned. 


.AVIATION...As of 3:24 AM PST Friday...for 12Z TAFS...VFR 
conditions through the period with light to moderate winds. A few
cu build up clouds are possible this afternoon, but impacts will 
be minimal. High level clouds increase on Saturday.

Vicinity of KSFO...VFR..

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR.

&& of 03:35 AM PST Friday...Breezy to gusty northerly
winds will persist through early this morning before diminishing.
Moderate northwest swell will  continue into next week. Southerly
winds will develop later this weekend and into early next weekend.


     .Tday...SCA...Pigeon Pt to Pt Pinos 0-10 nm until 9 AM
             SCA...Pt Arena to Pigeon Pt 10-60 nm until 9 AM
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm until 9 AM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Rough Bar Advisory for SF Bar until 9 AM




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