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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

FXUS66 KMTR 220415

Area Forecast Discussion
National Weather Service San Francisco Bay Area
915 PM PDT Fri Jun 21 2019

.SYNOPSIS...Dry north winds will persist in the hills through
Saturday morning. The marine layer will continue to erode and
compress through Saturday as an upper level ridge builds over
California. The result will be a continued warming trend this
weekend, especially inland, along with mostly clear skies. A 
distinct cooling trend is forecast from Monday through Wednesday,
resulting in much cooler temperatures by the middle of next week. 


.DISCUSSION...As of 9:15 PM PDT Friday...A warming trend got 
underway across most of our area today as northerly flow aloft 
mixed drier air into the boundary layer. Satellite imagery early 
this evening showed that stratus/fog had cleared from all coastal 
areas in the San Franciso Bay Area, with only patchy low clouds 
persisting along the coasts of Santa Cruz and Monterey Counties. 
Given that light onshore flow currently persists near sea level, 
it's likely we will see at least areas of low clouds and fog 
redevelop in coastal areas overnight, mainly south of the Golden 
Gate, and also locally inland from Monterey Bay. Fort Ord Profiler
data shows that the marine layer is rapidly compressing, so it's 
unlikely that low clouds and fog will make it very far inland 
tonight. And, the current 6 mb surface pressure gradient from ACV 
to SFO is another factor that will inhibit inland development of 
marine layer clouds/fog in the SF Bay Area overnight.

Main concern in the short-term is the likely redevelopment of
gusty and dry north winds across the higher hills of the North 
Bay overnight, and also locally in the East Bay Hills. These 
offshore winds in the hills will continue to bring a drier 
airmass into our region which will mean poor overnight humidity 
recoveries in the hills through the weekend, as well as warmer 
temperatures. These factors mean heightened fire weather concerns 
in the North and East Bay Hills this weekend (see Fire Weather 
Discussion below for more details).

An upper level ridge centered offshore along 135W is forecast to 
build eastward and across California into the weekend. Thus, the 
warming trend that got off to a slow start today will become more 
pronounced on Saturday. High temperatures tomorrow are forecast to
be anywhere from 5 to 12 degrees warmer than today, with the most
significant warming expected inland. Light onshore flow is
expected to persist at the surface and this will likely prevent 
temperatures near the immediate coast from warming past the low to
mid 70s. Low to mid 90s are likely in the warmest inland valleys 
tomorrow. Little change is expected as we move into Sunday, except
perhaps slight cooling in the North Bay Valleys where an earlier 
seabreeze is forecast to develop compared to Saturday.

A significant change is forecast to occur as we move through the 
first half of next week. The models agree that an upper trough 
currently in the western Gulf of Alaska will move to the southeast
and then dig south along the West Coast from Monday through 
Wednesday. This will result in robust cooling and temperatures by 
midweek are forecast to be as much as 15-20 degrees cooler than 
this weekend and from 5 to 10 degrees cooler than normal. The 
upper trough is forecast to linger through the end of next week, 
keeping temperatures on the cool side of normal through the end of
the extended forecast period. 


.AVIATION...As of 4:53 PM PDT Friday...Offshore flow will keep 
VFR conditions over the SFO Bay Area tonight through Saturday. 
Even the MRY Bay Area has cleared out this afternoon so low clouds
will be later to form tonight. 

Vicinity of KSFO...VFR. West to northwest winds up to 20 kt with 
gusts to 25 kt through 04Z.

SFO Bridge Approach...Similar to SFO but lighter winds are 
anticipated away from the San Bruno gap. 

Monterey Bay Terminals...MVFR after 05Z. 


.FIRE WEATHER...As of 8:50 PM Friday...A second round of moderate
and gusty north winds are forecast for the higher hills in the
North and East Bay tonight and Saturday morning, with gusts up to
30-35 mph expected over the exposed ridges above 2000 feet. These
north winds are ushering a drier airmass into the region and 
relative humidity recoveries in the higher hills both tonight and
Saturday night will be poor. In terms of offshore wind events, 
this one looks to be on the weak/light side in terms of winds, 
coverage, and humidity. Would need to see more widespread coverage
and extreme winds/lowering humidity to put our region into 
critical fire weather conditions. Nonetheless, a very warm and dry
weekend will mean increased concerns for fire spread of any new 
fire starts. Winds in the hills are expected to be lighter on 
Saturday night and Sunday morning. Much cooler conditions, and 
greatly improved nighttime humidity recoveries, are forecast to 
develop from Monday through Wednesday of next week. 


.MARINE...As of 8:49 PM PDT Friday...High pressure over the 
eastern Pacific extends to the Pacific Northwest coast. This will 
keep the strongest winds confined to the northern waters through 
Sunday. Winds will decrease early next week as the high weakens. 
These winds will generate steep fresh swell that will be the 
predominate wave state through the coming days. 


     .Tngt...SCA...SF Bay until 9 PM
             SCA...Pigeon Pt to Pt Pinos 0-10 nm until 9 PM
             GLW...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm until 3 AM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm until 3 AM
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Rough Bar Advisory for SF Bar until 3 AM




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