Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
440 am PDT Tuesday Sep 27 2016
Synopsis...continued hot today with only minor cooling near the
coast later this afternoon. A more noted cooling trend will start
Wednesday and continue into Thursday as a Pacific trough
approaches the region. Near to below normal temperatures Friday
and Saturday. Slight chance of light rain in the North Bay by
Sunday and Monday with continued seasonably cool weather.
Discussion...as of 4:23 am PDT Tuesday...skies are clear across
the district with satellite showing strong heat signature from the
Loma fire that has remained active all night. Nearby weather
stations suggest that overnight temperatures stayed in the upper
70s to around 80 over the fire with humidity only in the teens.
Only saving Grace has been light winds so far. (See fire weather
Monday was another record breaking day across the Bay area. In
general temperatures will be near or only slightly cooler than
yesterday for inland areas with highs once again into the 90s and
lower 100s. There is some evidence of coastal cooling with the
marine layer now hugging the Sonoma coast. This in combination
with west winds off the ocean will bring some noted cooling to the
beaches and within a few miles of the ocean...but temperatures
will still be above normal near the coast. Its the type of set-up
where the city will likely see its high temperature around noon
with the afternoon seabreeze then ushering in some relief. But the
initial surge wont penetrate too far inland this afternoon and
evening. More significant cooling is still on track for Wednesday
with 60s for the beaches and 80s well inland. Further cooling
trend will continue into Thursday as upper trough approaches far
northern California with increased onshore flow and synoptic
By Thursday and Friday this heat will be a distant memory with
autumn like highs in the 60s and 70s along with brisk NW winds off
The long range forecast is getting a lot more interesting. The 00z
ECMWF trended much wetter with an initial system on Sunday that
could bring some light rain to the North Bay. The ECMWF solution
suggests a second reinforcing system on Monday. About four model
runs ago it showed a similar solution though not as wet. The GFS
solutions look more reasonable at this time with a slight chance
of light rain for the North Bay on Sunday followed by another
chance Monday as a shortwave passes over Cape Mendocino. Given the
early time of year and transitional period have opted to keep pops
low until better model consistency is established. High confidence
for cooler temps but low confidence on precipitation at this time.
Will be watching the next few runs of the ECMWF to gage better but
its a scenario where the long range forecast could change rapidly
to a wet and or dry solution.
Aviation...as of 4:30 am PDT Tuesday...stratus approaching pt
Reyes from the northwest this morning and will likely spread along
the coast during the day today. But VFR conditions will prevail
today in the sfo and mry Bay area. Low clouds likely to return to
the mry Bay area tonight but still uncertainty on whether it will
spread locally into the sfo Bay area late tonight.
Vicinity of ksfo...VFR.
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...VFR. Some smoke from the Loma fire in the
Santa Cruz Mountains may produce a smoke layer at sns this
Fire weather...as of 4:23 am PDT Tuesday...5 known fires burning
in the district at this time. Sawmill fire in northern Sonoma is
showing good progress as well as the Cambridge in Marin County.
Ruins fire in Santa Cruz was reportedly contained to just a few
acres. Main interest will be in the Loma fire while the soberanes
fire continues to slowly increase containment.
In terms of weather for the Loma...the main fire is burning from
2000 feet and bumping the ridgelines around 3500 feet near Loma
Prieta. Infrared data shows significant spots at around 1500 feet
elevation on the northeast flank in what appears to be heavy brush
with Timber fuel. At this elevation the fire will see little or no
relief from any marine air today, especially with it all burning
on the Santa Clara County side of the Santa Cruz mtns. Highs
today about 90-100 across the fire with relative humidity in the teens. Only bit
of good news is winds look to remain pretty light, perhaps 5-10
mph out of the west/northwest on the ridges this afternoon. Will
need to watch out for the northwest seabreeze down the Santa Clara
Valley if the fire creeps closer to the valley bottom. Expect poor
humidity recovery again tonight. Should see some noted cooling on
weds with winds looking to turn southwest as the marine air surges
in. This will be a potentially dangerous period as local areas of
strong winds can sometimes be found on top of these inversion
layers. The long range trends look promising in term of cooler
temps and higher rh but those will arrive with stronger northwest
winds. Fire history in this area includes the 2008 Summit and 2003
Croy fire (as well as the 2009 Loma fire). Lessons from those is
the fuels will take a while to recover and moderate/strong NW
winds can greatly increase fire behavior even with higher
humidity...but those scenarios are a few days off. Today the fire
will be fuels and topography driven. Not seeing much evidence for
any instability of note aloft today.
Soberanes...continued hot and dry today. Early morning RAWS show
temps still in the 70s with rh in the teens. Another day of
triple digit heat and low humidity. General winds look to be light
from the west/northwest this afternoon after the morning offshore
ends with terrain driven for the interior of the fire. Some subtle
cooling weds and more noted by Thursday across interior of the
fire. Would not get too excited about the ECMWF solutions this far
south at this time.
Climate...here are the record highs for September 27.
Monterey Bay area
Marine...as of 3:21 am PDT Tuesday...high pressure covers the
eastern Pacific and the Great Basin while thermal trough lies
along the California coast. The trough will move into the
California interior today allowing for northwest winds to develop.
Strongest winds will be over the northern outer waters.
Public forecast: rww
aviation: west pi
marine: west pi
fire weather: rww
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