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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

                            
000
FXUS66 KMTR 221741
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1041 AM PDT Sat Jun 22 2019

.SYNOPSIS...Dry north winds will persist in the hills through
this morning. The marine layer will continue to erode and 
compress through the day 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 then forecast from Monday through 
Wednesday, resulting in much cooler temperatures by the middle of 
next week. 

&&

.DISCUSSION...as of 08:15 AM PDT Saturday...Sunrise satellite 
imagery shows mostly clear skies across the Golden State early 
today. A less expansive marine deck is evident this morning, 
remaining outside of most of the San Francisco Bay area, and 
primarily inundating inland portions of the Monterey Bay, southern
Santa Clara valley, and Salinas Valley, as well as the entire 
coastline from the Marin Headlands southward through Big Sur.

Temperatures today are forecast to be tangibly warmer than 
previous days, particularly inland, as high pressure builds
towards California from the eastern Pacific through the weekend. 
Afternoon highs for the warmest inland locations will push into 
the mid 90s, while most valley communities away from the immediate
coastline will hover in the upper 70s to low 90s depending on 
distance from the coast. Weak onshore flow will subdue the heat 
near the coast, where highs in the mid 60s to the mid to upper 70s
are expected today. These temperatures are generally 5 to 8 
degrees above seasonal readings, with a few more extreme locations
on each end of that spectrum. Sunday will be similar to today, 
though temperatures may be slightly cooler in the North Bay and 
slightly warmer elsewhere as the ridge weakens and shifts inland.

In addition to the warming temperatures, our other primary area 
of focus has been monitoring the locally strong offshore winds in 
the higher terrain of the North/East Bay. Mt St Helena, Mt Diablo,
and other peaks in those areas have reported N/NE gusts overnight
in excess of 45 mph, with sustained winds in excess of 35 mph. 
Overnight recoveries in these areas have been fair with relative 
humidities at these same spots in the upper 30s to low 40s, well 
above the more critical readings that are possible there (ie, 
single digit to low teens). These winds are forecast to taper down
by midday as the pressure gradient loosens in response to the 
ridge pushing inland. Despite warming temperatures, the 
combination of weakening winds and fair humidities should ease 
fire concerns today.

Looking ahead, a broad 554dm 500mb Gulf of Alaska low will 
flatten the ridge moving into Sunday evening before the base of 
the trough digs southward into California by Tuesday morning. 
Models depict this upper low remaining quasistationary over the 
coastal Pacific Northwest/extreme northern California through much
of the upcoming workweek. This low will help reintroduce an 
onshore regime, cooler temperatures, and a deepening marine layer.

See previous discussion for more information on the current
(overnight) forecast package. 

.PREVIOUS DISCUSSION...As of 3:19 AM PDT Saturday...Satellite 
imagery shows mainly clear skies over most of the district. 
Coastal stratus does extend along the Monterey county coast, 
through the Monterey Bay, and northward along the San Francisco 
Peninsula. Inland, low clouds are extending into the Salinas 
Valley. The offshore surface pressure gradient has increased 
somewhat with 5.4 mb from WMC to SFO, but there is still an 
onshore component with 2.1 mb from SFO to SAC. 

Offshore winds have ramped up in the North Bay Mountains, and to 
a lesser extent in the East Bay Hills. Generally, northeast winds
from 25-35 mph with some higher gusts are being observed in these
higher hills. This is resulting in dry conditions this morning,
and warm temperatures are anticipated across inland areas today. 
High temperatures today are forecast to be anywhere from 5 to 12 
degrees warmer than those on Friday, with the most significant 
warming expected inland. Light onshore flow is expected to persist
at the surface and this will likely keep temperatures near the 
immediate coast limited to the lower to middle 70s. The warmest 
inland areas are expected to warm into the lower to middle 90s. 
Little change is expected on Sunday.

A switch back to cooler summer weather is expected to begin early
next week as an upper level trough approaches the coast. This 
will bring much cooler temperatures, with highs 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 10:31 AM PDT Saturday...VFR. Low clouds
impacting the Monterey Bay terminals earlier this morning have
cleared and given way to VFR conditions region-wide. Look for
onshore flow to increase a bit through late morning and persist
into the afternoon. Wind speeds diminish once again late tonight 
with the potential for low clouds to return to the Monterey Bay 
terminals. 

Vicinity of KSFO...VFR. Northwesterly winds of 15-20 kt are 
expected this afternoon through early evening to around 04z.

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

Monterey Bay Terminals...VFR. Onshore flow will increase to 10-15
kt this afternoon. Winds diminish tonight with the return of low
clouds that will likely persist into Sunday morning.  

&&

.FIRE WEATHER...As of 3:50 AM Saturday...Moderate and gusty north
winds are forecast for the higher hills in the North and East Bay
through this 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 this morning 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 tonight and Sunday 
morning. Much cooler conditions, and greatly improved nighttime 
humidity recoveries, are forecast to develop from Monday through 
Wednesday. 

&&

.MARINE...as of 10:31 AM PDT Saturday...High pressure over the
eastern Pacific extends to the Pacific  Northwest coast with low
pressure across central California. Breezy to gusty northwesterly
winds will occur across the northern outer waters through Sunday
with lighter winds elsewhere. Steep fresh swell will result from
gusty winds over the northern outer waters with moderate
northwesterly swell elsewhere.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm

&&

$$

PUBLIC FORECAST: DRP
AVIATION: RGass
MARINE: RGass
FIRE WEATHER: Sims/Dykema

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