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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

FXUS66 KMTR 200503

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1003 PM PDT Thu Sep 19 2019

.SYNOPSIS...A warming trend will begin on Friday and continue 
through Saturday. A low pressure system moving southward to our 
east early next week may create the potential for continued 
warming temperatures, dry offshore flow and increased fire weather

&& of 10:03 PM PDT Thursday...After a generally cooler
start this morning compared to Wednesday morning the mid September
sun helped recover temperatures to the mid 60s coastside through the
70s bayside to lower 80s inland valleys today; average temperatures
varied up to a few degrees cooler inland locations to a couple degrees
warmer coastal locations compared to the daily normals.

The upper level trough is continuing to move slowly eastward over the
Intermountain West. Clear skies tonight continue with exception of a
few patches of coastal low clouds developing by early Friday morning.
Sunny to mostly sunny skies return Friday with highs reaching the mid
to upper 60s on the coastline to the 80s inland valleys. Large scale
downward vertical motion under increasing 500 mb height ridging will
result in steady lower level warming beginning Friday thus we should
see the return of a marine temperature inversion on the profilers by
Friday afternoon or evening. Patchy coastal stratus may return either
late Friday night or early Saturday morning. Away from the coast skies
will continue clear with more or less steady warming over the weekend
and early next week with daytime highs in the 90s much more common.

With the passage of an upper level trough to our east early next week
offshore winds increase fire weather and heat risks. For late next
week, today's 12z medium range models showed much greater agreement
on a change to unseasonably cooler weather. The 00z GFS still leans
in this direction with 500 mb height falls to the mid 550s decameters
over the Bay Area late next week, close but not quite as low as the
earlier 18z run. For reference, 550 decameters is the minimum for all
September Oakland upper soundings for 1948-2014 period of record.

For additional details please see previous forecast discussion.


.PREV of 02:00 PM PDT Thursday...An upper ridge 
centered over the eastern Pacific will begin to build over the 
West Coast on Friday and continue to strengthen through Saturday. 
Temperatures aloft will warm, with 850 mb temps reaching 
approximately 15C on Friday and 17C on Saturday. The warming aloft
will translate to warming at the surface, especially away from 
the coast. Inland valley locations look to warm well into the 80s 
on Friday, with mid 80s to lower 90s common on Saturday. Onshore 
winds will keep temperatures from rising too much at the coast, 
but an increased offshore component on Saturday will help 
temperatures reach the 70s to lower 80s. North to northeast winds 
will be breezy at times in the North Bay Hills on Saturday 
morning, but not windy or dry enough for major fire weather 

On Sunday, the ridge will begin to flatten as another trough
approaches the Pacific Northwest. In response high temperatures 
should cool a few degrees. The trough will then take a northwest
to southeast trajectory and is likely to follow an "inside slider"
type path, with the core of the trough moving just east of the 
Sierra crest. The past several runs of the GFS deterministic have 
kept the trough over Utah and Colorado, but this appears to be an 
outlier compared to GFS and ECMWF ensemble means. As the trough 
moves into the Desert Southwest, offshore flow will develop as 
surface high pressure strengthens over the Intermountain West. 
This will likely result in a significant warming and drying trend 
for our area Monday through Wednesday. The exact track and 
strength of the trough early next week will determine the strength
of the offshore flow and amount of drying/warming we see. This 
pattern will need to continue to be closely monitored as it could 
lead to increased fire weather concerns. 

Warm and dry conditions look to persist through the middle of next
week. The long range ensembles are starting to hint at a trough
dropping down the West Coast next weekend, which would result in
cooler conditions for the region. 


.AVIATION...As of 4:40 PM PDT Thursday...A few afternoon cumulus
clouds over the area. Drier air moves into the low levels tonight
so VFR conditions expected through Friday. 

Vicinity of KSFO...A few cumulus clouds at 3000-4000 ft through
04Z then clear skies expected. West winds 15 to 20 kt with gusts to
25 kt through 04Z. 

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR conditions are forecast to prevail
through Friday. Could see some localized stratus try to develop 
overnight along the MRY Bay shoreline but confidence of occurrence
is rather low.

&& of 04:32 PM PDT Thursday...Moderate and locally gusty
northwest winds winds will continue into the evening hours over
the waters, with the strongest winds  in the outer waters north of
Point Reyes and locally south of Point Sur along the Big Sur 
coast. Winds will then gradually subside overnight. An 11 second 
northwest swell will be the predominate wave in the waters today, 
bringing the potential of hazardous conditions from squared seas 
to portion of the waters. A much lighter southerly swell will also
be mixed in.


     .Tngt...SCA...Mry Bay until 9 PM
             SCA...Pt Arena to Pt Reyes 0-10 nm until 9 PM
             SCA...Pigeon Pt to Pt Pinos 0-10 nm until 9 PM
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm until 3 AM
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm until 3 AM




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