Scientific Forecaster Discussion
fxus66 kmtr 131757
Area forecast discussion
National Weather Service San Francisco Bay area
957 am PST Wed Nov 13 2019
Synopsis...increasing clouds today ahead of an incoming dry upper
trough that will bring a cooling trend into Thursday. By Friday
temperatures start to rebound leading to another warming and
drying trend this weekend with some light offshore winds possible.
Dry pattern continues through the middle of next week.
Discussion...as of 08:29 am PST Wednesday...near term weather
impacts to the Bay area are focused around the fog being reported
this morning. Hard to actually see the fog this morning on
satellite as widespread mid-high clouds stream over the region.
Cams, automated sensors and spotter reports do indicate fog is
lowering visibility to only a few miles with two sensors showing
dense fog along the coast. Did a quick update to increase fog
coverage this morning, especially along the coast. Given the high
clouds clearing of fog will be slow going this morning.
Temperatures will also be cooler today given the foggy start, high
clouds overhead and increased onshore flow.
Previous discussion...as of 3:26 am PST Wednesday...satellite shows
extensive mid and high level clouds over the Pacific that will
pass over the Bay area today. Onshore winds will gradually
increase through the day but winds will remain light with weak
surface pressure gradients. The clouds and onshore flow will lead
to a few degrees of cooling with highs in the upper 60s to mid
70s. Still above normal but not as warm as Tuesday.
The incoming trough will bring some higher dewpoint air to the
region and overnight lows tonight will remain mild with plenty of
cloud cover in place as well.
Thursday will be the coolest day we've seen in a while, especially
in the hills as the upper trough passes over the region. There's
no real strong cold advection with surface winds staying from the
southwest. Daytime highs on Thursday in the 60s most areas with
perhaps some lower 70s for the warmest interior valleys.
The upper trough passes to our east by Friday. There will be left
over clouds Friday morning but a clearing trend by Friday
afternoon with temperatures rebounding by a few degrees back
towards seasonable norms.
High pressure builds this weekend with a noted warming and drying
trend. There will be some periods of light offshore winds as high
pressure noses into the region. There will be stronger jet energy
pointed in the Pacific northwest with a cut-off well west of baja. Expect
widespread readings in the 70s this weekend with plenty of
The ridge pattern slowly breaks down early next week with some
modest cooling Monday and Tuesday but still dry. Models diverge on
details with some type of system dropping into the Great Basin
and trying to carve out a trough in the long range over socal. As
this occurs the 06z GFS hints at some dry north wind potential by
the middle of next week that will need to be watched given the
lack of rain and continued dry fuels. The pna remains positive for
another week while a weak mjo signal is forecast to hold together
through the end of the month offering some hope for organized
precip as we approach the Thanksgiving time frame.
Aviation...as of 09:45 am PST Wednesday...for 18z tafs. Broken-overcast
high clouds at 15000-20000 ft obscured The View of the marine
layer this morning, however, now able to see it banked along the
coast with fingers of stratus over kmry, kwvi, and through the
Golden Gate gap towards Berkeley/Oakland Hills. The marine layer
is expected to mix out and pull back to the shoreline by 19z. An
approaching weak, generally dry trough will bring a few hundredths
of precipitation to the North Bay with an increase in low to mid
level moisture overnight tonight into early Thursday morning. This
will bring generally scattered-overcast IFR to MVFR cigs primarily from
02-18z Thursday. Winds will also shift to become more southerly with
the frontal passage but remain generally light overall.
Vicinity of ksfo...VFR through the day today, then increasing low
level clouds after 04z thru Thursday morning. Low clouds will then
left to mid levels after 16-18z Thursday with impacts to visuals
possible through midday Thursday. Onshore winds today will shift
to become more southerly early Thursday morning.
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...fingers of stratus across the lip of the
Monterey Bay from the Monterey peninsula through kwvi, including
kmry. This should retreat by 1900z bringing widespread VFR to the
region. Low clouds return by 02z and persist through mid morning
Thursday if not beyond. Light winds near kmry but occasionally breezy
northwest winds near ksns.
Marine...as of 09:37 am PST Wednesday...light to moderate
southerly winds will develop today over the coastal waters.
Locally breezy southerly winds are forecast along the coast. Winds
will switch to northwest late on Thursday night into Friday
morning. A light northwest swell will continue through Thursday
morning. A larger, longer period northwest swell will arrive
Thursday evening and continue through Friday and Saturday.
Public forecast: mm/rww
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