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000
FXUS66 KMTR 121132
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
432 AM PDT Wed Sep 12 2018

.SYNOPSIS...A deepening upper level trough over the West Coast will
bring temperatures to below seasonal averages region-wide through at
least late week if not into the upcoming weekend as well. Breezy
onshore winds are also expected to persist along with dry weather
conditions.

&&

.DISCUSSION...as of 03:22 AM PDT Wednesday...Mainly clear sky
conditions across the region early this morning aside from
widespread high clouds over the greater San Francisco Bay Area.
While a few patches of low clouds have been detected by satellite
imagery overnight, widespread stratus has had a difficult time
developing. This is the result of cooler, drier air advecting in
over the region in response to a deepening mid/upper level trough
over the West Coast. With this system, breezy conditions persist
in the higher ridges and peaks across the region where
northwesterly winds are gusting to 25 to 45 MPH. While relative
humidity values are also in the 20-30% range in these areas, these
conditions remain confined to the highest ridges/peaks and are
not widespread. With all this said, may see a bit of stratus try
to develop through sunrise as the marine layer continues to re-
establish itself.

In wake of a dry frontal passage and cold air advection over the
region, temperatures this afternoon will be a few to as much as 20
deg F cooler than yesterday. The most noted cool-down will be in the
interior where daytime temperatures will only warm into the 60s to
middle 70s compared to 80s and lower 90s yesterday. Meanwhile, at
the coast, look for widespread 60s and 70s under mainly clear sky
conditions thanks to the mixed out marine layer. Breezy northwest
winds will also continue today through Thursday, especially near the
coast and in the higher elevations across the region.

Look for temperatures to remain below seasonal averages through late
week and potentially into the upcoming weekend as a broad trough
remains the dominate weather feature along the West Coast. While
some of the warmest inland areas and far southern portion of the
Central Coast may warm into the lower 80s, most areas will see 60s
and 70s given the cooler air mass aloft. The tricky part of the
forecast remains whether (or not) the marine layer will support
stratus development through late week. For now, will keep low clouds
during the night and early morning hours giving way to mostly sunny
conditions each afternoon. However, if the marine layer remains well
mixed, mainly clear sky conditions may prevail.

A slight warming trend appears possible early next week as the
mid/upper level trough lifts northward away from the region and high
pressure builds to our south. However, longer range guidance
indicates temperatures will likely remain at or below seasonal
averages late through much of next week as well. Dry weather
conditions are also likely to persist through the forecast period.

&&

.AVIATION...as of 04:32 AM PDT Wednesday...For 12z tafs. The
northerly gradient has weakened overnight while the marine layer
deepens. Satellite imagery showing passing high clouds along with
patches of stratus attempting to form near KOAK and over the
Monterey Bay. Fairly low confidence forecast for this morning in
terms of how much these patches of low clouds will expand. It is
possible some locations will experience MVFR/IFR cigs at some
point this morning, although confidence is low. Conditions will be
monitored and the tafs will be adjusted as needed. VFR expected by
late this morning. Models are still suggesting that low clouds
will redevelop tonight.

Onshore winds this afternoon 10-15 kt with locally higher winds
and gusts at KSFO.

Vicinity of KSFO...VFR. Westerly winds gusting to 25 kt this
afternoon and early evening.

SFO Bridge Approach...A few patches of cigs may form around the
approach but mainly VFR.

Monterey Bay Terminals...A patch of stratus has begun to form in
the Monterey Bay. It is possible this could spread into KMRY and
KSNS this morning. VFR by the afternoon.

&&

.MARINE...as of 03:22 AM PDT Wednesday...Moderate to strong
northwest winds will continue across the coastal waters through
tonight. Strongest winds are expected over the inner coastal
waters, especially south of Point Sur where near gale-force gusts
will be possible through tonight. Additionally, expect breezy
winds over the bays this afternoon and evening. Winds will lessen
some on Thursday as high pressure over the eastern Pacific
weakens. Mixed northwest and a long period southerly swell will
continue through the week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...SCA...Mry Bay from 11 AM
             SCA...Pigeon Pt to Pt Pinos 0-10 nm from 11 AM
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm until 3 PM
             SCA...SF Bay from 1 PM

&&

$$

PUBLIC FORECAST: RGass
AVIATION: AS
MARINE: AS

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