NWS Forecast Area Discussion - San Francisco Area

FXUS66 KMTR 171226

Area Forecast Discussion
National Weather Service San Francisco Bay Area
426 AM PST Sat Nov 17 2018

.SYNOPSIS...Widespread smoke and haze at times combining with
coastal fog resulting in extended poor air quality and low
visibilities over the San Francisco Bay Area should show some
improvement first along the immediate coast today then inland
Sunday. A mid to upper level ridge has weakened and troughing is
already slowly moving in from the west this morning. Additionally
forecast models continue to indicate an active Pacific weather
pattern reaching California next week with the potential for
widespread periodic rainfall starting Wednesday and continuing
through the latter part of the work week.


.DISCUSSION...As of 4:12 AM PST Saturday...Smoke, haze and
coastal fog, some of it patchy dense fog with visibilities under
1/4 mile, will combine to keep visibilities low this morning; air
quality remains poorest in the Bay Area. Forecast highs were
lowered a few degrees for today from the South Bay to the north
Central Coast due to a chilly start, increasing cloud cover,
lingering smoke and haze, thus in these areas temperatures may
struggle to reach 60F. Stagnant weather continues as a well
entrenched albeit much weaker offshore wind pattern remains in
place unable to break down as a weaker mid November sun angle and
feeble diurnal warming over land just can`t get a foot hold of any
kind to generate a decent sea-breeze. Awaiting changes in the mid
latitude westerly flow is what is needed to dislodge this pattern,
and the early stages of it is already on our door step.

A weak 500 mb trough is approx 400 miles west of the cwa with a
925 mb elevated cool front approaching our coastal waters today;
elevated fronts typically are weak, but overall cooling and its
proximity could cause patch or two of drizzle along the Big Sur
Coast today based on the HRRR output which is the only model
showing any qpf. Otherwise this feature remains out of reach to
generate anything more than cloud cover and advancing higher
dewpoint air eastward generating morning coastal fog as we are
seeing in the observations and on satellite imagery early this
morning. Wind flow remains light today.

The next system upstream over the eastern Pacific is a 500 mb closed
low which weakens as it reaches SoCal Tuesday. Much earlier model
output had this spun up as a leading early week rain generator for
us, but this is not the case anymore. However, it`ll give traction
to a wind reversal which will greatly help clear up the air and
improve air quality if not already begun so first on the coastline
by later today and then inland Sunday. The GFS and ECMWF have been
otherwise consistent that this system will dry out by the time it
reaches SoCal.

As maritime polar air settles southward to vicinity of 40 degrees
latitude over the North Pacific next week the door will generously
open to wet weather here in California. Increasing moisture from
the subtropical latitudes may get ingested into these low pressure
systems based on the 00z output (and earlier output) GFS integrated
water vapor transport forecasts. Much unlike winter season 2017-2018
with large, ongoing meridional displacement, next week`s weather at
least shows characteristics more closely to zonal flow. An ongoing
relatively large northern hemispheric wavenumber and somewhat
distorted polar vortex remain favorable for storm generation with
winter solstice still a bit more than 1 month away. Recent GFS and
ECMWF 240 hour forecast rain accumulation totals have suggested
totals could for the most part reverse early season rain deficits
in our cwa and this of course would greatly alleviate fire weather


.AVIATION...As of 04:26 PM PST Saturday...For 12Z tafs. A
combination of widespread smoke and haze with patchy coastal fog
and stratus is affecting the region this morning bringing MVFR/IFR
vis and/or cigs. Expect patchy fog/stratus to continue into late
this morning for the Monterey taf sites and possibly in the North
Bay. Latest run of the HRRR smoke model suggests some improvement
in smoke late this afternoon and into this evening. This may
result in somewhat improved visibilities tonight. However,
forecast models suggest more widespread stratus development
tonight into Sunday. Generally light winds through the period.

Vicinity of KSFO...IFR/MVFR vis due to wildfire smoke. Slant range
vis issues will persist. Latest HRRR smoke model suggests some
improvement late this afternoon and tonight. More widespread low
clouds are expected tonight. Winds will remain light through the

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...MVFR vis with MVFR/IFR cigs due to a
combination of smoke and stratus/fog. LIFR vis from fog will be
possible until around 16Z. An early return of the marine layer is
forecast for tonight. Generally light winds.

&& of 02:14 AM PST Saturday...Generally light winds
will continue through the weekend and into early next week.
Southerly winds will increase somewhat by mid- week as an upper
trough approaches the region. Mixed moderate northwest swell and a
light southerly swell will persist.





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NWS MTR Office Area Forecast Discussion