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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

                            
000
FXUS66 KMTR 180107
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
607 PM PDT Tue Sep 17 2019

.SYNOPSIS...A weather system approaching from the northwest will
bring isolated to scattered showers to northern California late
tonight and across the San Francisco Bay Area early Wednesday.
Temperatures will remain slightly below seasonal averages as well
through Thursday. A warming trend is then forecast for Friday and
into the upcoming weekend as high pressure builds in from the
eastern Pacific.

&&

.DISCUSSION...as of 01:23 PM PDT Tuesday...A mid/upper level low 
currently sliding down the British Columbia coast is pushing a 
frontal boundary into the Pacific Northwest. Meanwhile, mostly 
sunny conditions and mild temperatures persist across the region 
with temperatures generally in the 70s. Overnight, generally quiet
weather conditions will persist with the potential for a slight 
increase in low-level clouds in advance of the approaching cold 
front. 

Light rain showers will be possible over the North Bay late in 
the night into early Wednesday morning as the cold front advances 
southward. The boundary will then push across the San Francisco 
Bay Area around sunrise and during the morning commute with 
isolated to scattered rain showers possible. However, the frontal 
boundary will be weakening as it approaches the Central Coast with
lesser potential for showers south of Santa Cruz and Santa Clara 
Counties. With this said, for those that do see rain showers, 
rainfall amounts will generally be around 0.10" of an inch or less
with isolated amounts upwards of 0.25" in the North Bay Mountains
and/or coastal ranges. Cannot rule out a few sprinkles early in 
the morning for Monterey and San Benito Counties, yet widespread 
measurable rainfall is unlikely. Other than wet roadways during 
the Wednesday morning commute, impacts from this system will be 
minimal with generally light northerly winds. 

Dry conditions then develop by Wednesday afternoon and evening as
the front dissipates over the region. Temperatures will also 
change little with this frontal passage with widespread 70s inland
to upper 60s coast. A gradual warming trend is then forecast for 
late week in response to high pressure building over the eastern 
Pacific. As the ridge shifts toward the coast Friday and Saturday,
widespread 80s are likely inland with even a few of the warmer 
inland locations reaching into the lower 90s. 

In the extended, model differences result in lower confidence with
respect to the overall synoptic pattern and forecast weather
conditions. The GFS and ECMWF both show another mid/upper level
trough dropping down the West Coast early next week. However, the
ECMWF is much further inland over the Great Basin while the GFS
tracks the system closer to the coastline. Meanwhile, high
pressure will continue to dominate over the eastern Pacific early
next week. Until we see a bit more convergence in the model
solutions, will not make many chances to the extended forecast
period. Thus, will maintain dry weather conditions and
temperatures near to slightly above seasonal averages. 

&&

.AVIATION...As of 6:07 PM PDT Tuesday...Widespread VFR per metar
observations and satellite imagery; Monday's and last night's newly
arrived air mass and drying had pretty good momentum to it shaping
up for a nice /VFR/ day today. Onshore winds are locally gusting
to the 20 knots range, similarly gusty winds in the hills. Overall
at this point it looks like decent chances VFR will hold into the 
evening. Advancing frontal boundary from the northwest still expected
to bring clouds and showery weather early Wednesday morning, tending
to diminish as the band of showers moves south and east.

Vicinity of KSFO...VFR, gusty west wind to the 20 knots range
until 04z, lighter wind tonight with increasing cloud coverage
overnight and Wednesday morning. A few showers possible Wednesday
morning.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR, west winds 10 knots diminishing and
becoming light southeast tonight. Few to scattered clouds Wednesday.

&&

.MARINE...as of 5:50 PM PDT Tuesday...Light to locally gusty west
to northwest winds will prevail over the waters through the 
middle of the week. The strongest winds will be generally confined
along and south of the coastal points, as well as through the 
coastal gaps and over the bays. A front will move through tonight 
and Wednesday bringing a slight chance of showers to the northern 
waters. Northwesterly winds will increase at the end of the week. 
Mixed seas will continue with a moderate period northwest swell 
and a light southerly swell.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...SCA...Mry Bay until 9 PM
             SCA...Pigeon Pt to Pt Pinos 0-10 nm until 9 PM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm

&&

$$

PUBLIC FORECAST: RGass
AVIATION: Canepa
MARINE: Rowe

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