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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

FXUS66 KMTR 132206

Area Forecast Discussion
National Weather Service San Francisco Bay Area
206 PM PST Tue Nov 13 2018

.SYNOPSIS...Smoke over the region will continue to result in poor
air quality through at least midweek with dry conditions and 
light offshore winds. Seasonably mild, dry conditions will then 
persist through the upcoming weekend. A pattern change will 
potentially bring rainfall to the region around Thanksgiving.


.DISCUSSION...as of 02:00 PM PST Tuesday...Synoptic pattern
features an amplified upper level ridge overhead with a closed 
low over the Southern Plains and a broad trough in the eastern 
Pacific. A weak shortwave trough on the eastern side of this broad
trough is advecting high clouds overhead. These clouds have kept 
temperatures cooler this afternoon areawide. Highs so far have 
only been in the low to mid 60s in the Bay Area and points 
northward, while Santa Cruz southward has seen temperatures in the
mid 60s to lower 70s. The difference in temperature between the 
northern and southern half of the CWA is caused primarily by 
smoke coverage. The Bay area is still socked in by smoke and poor 
air quality, with visibilities around 2 1/2 miles this afternoon 
is several locations such as Santa Rosa, SFO, and Oakland. 
Meanwhile, the southern half of the CWA has been spared the worst 
of the air quality issues. Expect smoke to persist across the Bay
Area through this week as winds remain weak, limiting mixing. 
Surface winds will turn onshore Wednesday afternoon along the 
coast, but this may not offer much relief as a large area of smoke
extends several hundred miles off the coast.

An upper level ridge and a weak surface pressure gradient will 
keep winds light through the rest of the week. Winds will be 
generally 10 mph or less with gusts below 15 mph. Relative 
humidity remains low in the higher terrain, with relative humidity
mainly in the 10-30 percent range above 1000 feet. Poor overnight
humidity recoveries are expected through the rest of the week. 
Due to the dry air and very dry fuels, near critical fire weather
conditions will continue. An increasing onshore component in the 
surface flow will allow humidity to gradually increase along the 
coast. Highs this week will be seasonably warm with 60s near the 
coast and low to mid 70s inland. A limiting factor, but less 
certain, will be aerial smoke coverage, with more smoke reducing 
daytime heating potential. Lows will not be quite as cold as 
recent nights with the increase in moisture in the valleys. 

Looking ahead to next week, the advertised pattern change remains
likely but details are still far from certain. Models are at least
in agreement that a low will move to our south on Monday and
Tuesday. This does not look to bring any precipitation, but it
opens the door for a broad trough to potentially drop far enough
south to bring rain to our area as it moves eastward from the 
Pacific. Models are not consistent run to run on the timing or 
strength of the trough and precipitation, but both the GFS and 
Euro show precip sometime in the Wednesday to Friday timeframe 
next week. As always with forecasts this far in advance, details 
are likely to change. Bottom line is that a pattern change is 
likely next week, and the potential for precipitation around 
Thanksgiving is increasing.


.AVIATION...As of 9:36 AM PST Tuesday...For 18z TAFs. Smoke
continues to be the main impact to terminals today with reduced
vis across the region. Satellite imagery shows high clouds
moving overhead and obscuring any view of the wildfire smoke. 
Latest HRRR smoke forecast suggests that smoke will remain an 
issue through the period likely reducing slantwise as well as
surface vis. Winds will stay generally light through the period.

Vicinity of KSFO...Smoke continues to reduce vis to 3SM-4SM at the
terminal. Expecting low vis to continue with possible minor
improvements for a brief time tonight. Light winds.

SFO Bridge Approach...Similar to KSFO. Smoke will continue to
impact slantwise vsbys along the approach. Light winds.

Monterey Bay Terminals...Smoke will continue to reduce
visibilities and bring in possible low cigs. Gusty SE winds
through late this morning/early this afternoon again in the
Salinas Valley with light winds elsewhere.


.MARINE...as of 08:12 AM PST Tuesday...Light southerly winds will
persist today and tonight as an upper low passes by well to the 
north. Winds will shift back out of the northwest tomorrow as high
pressure builds back over the eastern Pacific. Light mixed swell 
will continue through mid-week before a northwest swell arrives.





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