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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

                            
000
FXUS66 KMTR 222124
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
224 PM PDT Sat Jun 22 2019

.SYNOPSIS...High pressure will bring a diminished marine layer,
above average temperatures, light offshore flow away from the
coast, and drier conditions this weekend. 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 03:00 PM PDT Saturday...Forecast is playing
out as expected early this afternoon, with a tangibly warmer/drier
airmass inland, clear skies away from the immediate coast, and 
gradually weakening winds aloft as the synoptic scale pattern 
transitions towards favoring a high pressure ridge currently 
located just off the coast. Offshore winds peaked overnight, with
both Mt St Helena and Mt Diablo reaching gusts of at least 50 mph.
Downsloping northerly winds across the North/East Bay have led 
to dramatically warmer early afternoon temperatures versus 24 
hours, for example, the Santa Rosa airport is currently at 93F, 
which is 20F warmer than the same time yesterday (73F). Elsewhere,
afternoon temperatures are coming in near anticipated levels, 
with upper 60s to 70s along the coast, upper 70s to 80s for most 
urban locations near the shorelines, and upper 80s to mid 90s 
deeper inland. 

For tonight, the marine layer should experience increased
subsidence aloft, compressing the layer and forcing it closer to
the surface as it struggles to remain banked along portions of 
the San Mateo, Santa Cruz, and Monterey coastlines. 

Zooming out, water vapor imagery shows a 553dm low centered in the
northern Gulf of Alaska that is positioned atop an amplified 
594dm high pressure centered around 135W. Forecast model data
indicates that both of these features will shift downstream
through the coming days, with the high pressure ridge continuing
to dominate the local weather through the remainder of this
weekend. Forecast models also agree that the low pressure will
suppress the ridge beginning later tomorrow, weakening it, and
forcing it towards the southeast. As a result of this and
alignment of the ridge axis, temperatures across the North Bay 
are expected to reach their highest points this afternoon while
most other areas will peak slightly warmer tomorrow. 

Looking ahead, Monday will be a return to near seasonal conditions
as the synoptic scale pattern will transition from favoring the
warming ridge to the arriving cool trough from the Gulf of Alaska.
Monday should also herald the return of the marine layer though a
more robust onshore regime/marine layer is expected Tuesday. 

The upper low settles in near the coastal Pacific Northwest on
Tuesday where it will remain quasistationary through the remainder
of the week. As a consequence, temperatures across the entire 
region are anticipated to fall below seasonal normals on Tuesday, 
with further cooling due to the trough influence on Wednesday, the
coolest day of the upcoming week. Afternoon highs on Wednesday 
are currently forecast to run 8 to 12 degrees below normal for 
this time of the year, with coasts/shorelines peaking from the 
upper 50s to upper 60s, somewhat inland communities from the upper
60s to mid 70s, and the more remote inland spots from the upper 
70s to mid 80s. While the upper circulation lifts out of the
Pacific Northwest through the day on Friday, the broader parent 
trough will remain in place across the western CONUS. 

Neutral conditions are then anticipated through next weekend 
until a four corners ridge backbuilds towards California by July 
1st. 

&&

.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...
     .Tngt...SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm

&&

$$

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

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