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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

                            
000
FXUS66 KMTR 190455
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
952 PM PDT Wed Sep 18 2019

.SYNOPSIS...A warming and drying trend is forecast for late week 
and weekend as high pressure builds in from the eastern Pacific. 
A low pressure system moving to our east early next week may create
the potential for dry offshore winds and increased fire concerns.

&&

.DISCUSSION...as of 9:52 PM PDT Wednesday...Rain gages over Sonoma
county and San Mateo county as well as over Alameda county generally
had the highest rain totals from this morning's frontal passage. The
highest reports made available to our office include Oak Ridge in
Sonoma county with 0.19" and Half Moon Bay varying between 0.12" and
0.10" and Alameda 0.17" rain. Similar to Monday's trough passage 
Kentfield once again did not receive measurable rain today at the 
official cooperative station. The farther south the front moved the
weaker it became and many places did not receive measurable rain.

From the coastal North Bay along the San Francisco peninsula coast
including the Bay Area mountains highs today were mostly in the 
mid 60s to 70. Along San Francisco Bay to the inland valleys highs
were mostly in the 70s and over interior Monterey and San Benito 
counties highs were as warm as the upper 70s to middle 80s. 

Daytime convective clouds generated from surface warming and residual
muggy air were capped by a subsidence inversion in the lower levels
of the atmosphere; the inversion was based near 6,400 feet on the
00z Oakland upper air sounding. Patchy low clouds are redeveloping
from the South Bay to the Central Coast, otherwise a drying trend
is underway for the overnight hours. Thursday is shaping up to be
sunny to mostly sunny with highs in the 60s at the coast to the 70s
to lower 80s inland.

500 mb height troughing over the West Coast will slowly advance to
the east over the Intermountain West through late week. A closed 
500 mb 591 decameter high pressure center over the eastern Pacific
will weaken and retrograde through Saturday. A return to 500 mb 
ridging will return to NorCal Thursday, but a temporary retrogression
of the eastern Pacific high will offset rising heights over the 
cwa to some extent into the weekend. Another upstream trough over 
the Pacific will pass over the Gulf of Alaska and the Pacific 
Northwest over the weekend, upon reaching the Intermountain West 
it will tend to move southeastward and slow as the mid-latitude 
flow over the western CONUS buckles under amplification. This may 
bring a period of offshore wind flow to our cwa early next week as
surface pressures build over the Great Basin with implications mostly
on rising fire weather concerns as well as rising temperatures thus 
it bears watching in subsequent model runs. Current ECMX MOS 
guidance trends temperatures warmer to hotter peaking on Tuesday of 
next week with highs in the 90s inland. The ECMWF and GFS are 
showing good model agreement in the large scale 500 mb height 
pattern tonight into early next week, the pattern then becoming 
noisy with diverging solutions by the middle of next week. 

For additional details please see previous forecast discussion.

&&

.PREV DISCUSSION...as of 02:13 PM PDT Wednesday...As skies clear
tonight, cool overnight lows are expected in the valleys with 
upper 40s to lower 50s with mid to upper 50s near the coast and 
bays. The base of the upper trough responsible for bringing us 
scattered showers and cooler temperatures will shift through our 
forecast area on Thursday. Highs are expected to be similar to 
today, ranging from the upper 60s at the coast to the 70s and 
lower 80s inland, which are near to slightly below normal for the 
date. 

High pressure over the eastern Pacific will then begin to build
towards the West Coast on Friday and Saturday, resulting in a 
warming trend for the region. The strongest warming will be felt
inland, with highs on Friday reaching the mid to upper 80s and
upper 80s to mid 90s on Saturday. Light offshore flow and a lack 
of a strong marine layer will also allow temperatures to warm at 
the coast, where temperatures in the 70s to low 80s are
anticipated.

Temperatures trend slightly lower Sunday into Monday as another 
upper level trough drops southward into the Pacific Northwest and
into the Intermountain West. There remains some uncertainty with 
the track, but at this time, dry conditions look most likely with 
temperatures near to slightly above seasonal averages. Depending 
on the exact track of the aforementioned system, the development 
of offshore flow looks possible early next week which may result 
in an increase in temperatures and much drier conditions. This 
pattern will be closely monitored in the coming days as it may 
create the potential for increased fire concerns.

&&

.AVIATION...As of 4:46 PM PDT Wednesday...Upper level trough and 
weak cold front moved through the area today bringing low to mid 
level moisture to the area. SCT-BKN stratocumulus clouds prevail 
with cigs 3000-5000 feet. These clouds will decrease after 04Z 
when nighttime cooling takes effect. No inversion is present as 
the airmass aloft has cooled. Models indicate decreasing low-level
moisture tonight so latest forecast leaves out cigs for the SFO 
Bay Area. More clouds are present in the MRY Bay Area this 
afternoon and could linger through the night. However confidence 
is not high and it could clear up.

Vicinity of KSFO...SCT-BKN through 04Z with bases around 5000
feet. VFR after 04Z with FEW-SCT clouds. West winds to 15 kt
diminishing after 04Z. 

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Cigs above 3000 feet dropping into MVFR
after 03Z. Confidence is not high as there could be less clouds. 

&&

.MARINE...as of 04:29 PM PDT Wednesday...Generally light west to
northwest winds will continue overnight with locally gusty winds
along and south of Point Sur. Northwest  winds gradually
strengthen across the waters by Thursday  afternoon becoming
moderate for most of the waters. Mixed seas  will continue with a
moderate period northwest swell and a light  southerly swell.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm from 9 PM

&&

$$

PUBLIC FORECAST: Canepa/ST
AVIATION: W Pi
MARINE: Rowe

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