Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
834 PM PDT Thursday Sep 18 2014
Synopsis...the weather system that brought light rain to the
region last night and earlier today will push southward overnight.
Precipitation chances will end in all areas by late Thursday evening.
Temperatures are forecast to remain near normal through the
upcoming weekend and dry weather conditions are expected through
early next week. Another Pacific storm system may bring rainfall
to our region starting around the middle part of next week.
Discussion...as of 8:30 PM PDT Thursday...an early season weather
system brought widespread light rainfall to our region last night
and today. In general...rainfall totals were greatest in the North
Bay where as much as two-thirds of an inch fell. But isolated
portions of the coastal hills south of the Golden Gate saw as
much rainfall...or even a bit more...as the North Bay due to
significant low level moisture being lifted up the west and
southwest slopes of the the coastal terrain by onshore flow. Rain
totals were greatest in the Santa Cruz Mountains where as much as
three-quarters of an inch fell. Up to a third of an inch accumulated
in the hills above Big Sur and isolated spots in the East Bay
hills picked up a quarter of an inch. Most lower elevation
locations south of the Golden Gate accumulated about a tenth of an
inch or less. A public information statement with region-wide
24 rainfall totals was issued earlier this evening.
The upper trough responsible for our rainfall is digging south
along the central California coast this evening. Precipitation has
long since ended in the North Bay...but a few rain gages in the
hills above Big Sur indicate ongoing light rain there. A recent
forecast update included removal of rain chances from the North
Bay and adding slight rain chances to the Western Hills of
Monterey County through late evening. All precipitation is expected to
end by late tonight as the trough continues dig to our
south...forming a cutoff low west of Point Conception by Friday
One concern in the short-term is the possibility of fog later
tonight as the low levels become more stable. Surface dewpoints
remain relatively high this evening...in the upper 50s and lower
60s...and so patchy late night fog is certainly possible...especially
in the North Bay valleys. The recent forecast update included the
addition of patchy fog to coastal areas and the North Bay valleys
for late tonight and early Friday morning.
The 00z Oak sounding shows that the airmass has dried out some
since this morning...particularly in the 850-950mb layer.
Thus...we should see better radiational cooling overnight and
low temperatures tonight at least a few degrees below last night's warm
nighttime readings. However...coastal water temperatures remain well
above normal and so we can expect continued warmer than normal
nighttime temperatures...especially near the coast.
The upper low is forecast to remain off the Southern California coast
through Friday night and then lift to the northeast and across
south-central California on Saturday and Saturday night. The
models agree pretty well in confining convective precipitation to the
higher mountains well to our east and southeast as the low lifts
out. However...can't entirely rule out isolated
showers/thunderstorms over the far southeastern portion of our
forecast area...particularly on Saturday afternoon and evening.
Daytime temperatures are expected to drift a bit higher for inland
areas the next few days...but may cool a bit in coastal areas as a
marine inversion redevelops. In general...daytime temperatures are
expected to be near normal over the upcoming weekend.
The weather pattern in the northeast Pacific is expected to remain
unusually active for this time of year well into next week. A
longwave trough is forecast to remain anchored in the Gulf of
Alaska. The medium range models show a series of increasingly
strong shortwave troughs moving through the longwave
trough...eventually bringing rain into our region starting around
the middle part of next week.
Aviation...as of 5:38 PM PDT Thursday...the airmass has dried out
just a little in the middle to upper levels but humidity levels are
still fairly elevated at the ground level. Surface dewpoint
temperatures are presently running in the middle 50s to lower 60s.
Not entirely sure that we'll get through tonight without some
significantly reduced visibilities in at least patchy fog over the
waters and land. Plus...nights are just a little longer supporting
greater nocturnal cooling to saturation. We've already seen some
late night/early morning radiational fog a couple times a week or
two ago...can't recall the exact dates. Fairly decent mixing
hanging around through the early evening should help maintain VFR
or MVFR at area terminals. Any IFR/LIFR tonight should be
primarily late... beginning closer to midnight. Air mass will
become increasingly stable tonight through Friday. VFR is likely
at area terminals by late Friday morning.
Vicinity of ksfo...VFR with west winds under 20 knots forecast for
the evening. MVFR ceiling forecast to develop by 07z. Patchy fog potential
tonight but not confident enough to place in taf forecast quite yet.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...MVFR ceilings by 03z although kmry
already reporting bkn012 ceiling. Low cloud ceilings forecast tonight
but same uncertainty regarding potential for IFR/LIFR. Similar to
the Bay area...believe any IFR/LIFR will hold off until late
tonight...midnight or thereafter.
Marine...as of 2:30 PM PDT Thursday...a slow moving storm system centered
off of the Mendocino coast will maintain generally light southerly
winds across the coastal waters into the overnight hours. Winds
will become northwest by Friday morning as the storm system pushes
southeast. A large...long period southwesterly swell will continue
to impact the waters through this evening but is expected to
slowly subside over the next two days.
Public forecast: dykema
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