Wildfire in California, September 25 to 27, 2017

Area Forecast Discussion
National Weather Service San Diego CA
120 PM PDT Mon Sep 25 2017

.SYNOPSIS...
High pressure over the Great Basin will bring Santa Ana conditions
through Wednesday with sunny, warm days and clear nights. Winds
will be gusty at times in the mountains, foothills and the Inland
Empire. Warmer Thursday and Friday with lighter offshore winds.

&&

.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...

No changes in the outlook for the ongoing Santa Ana weather pattern
with warm days, clear nights, and gusty winds in the mountains
passes, foothills and I.E. at times through Wednesday morning. Winds
at Noon today were blustery at the usual offshore wind locations
including Corona (18G24 mph), Riverside (13G23 mph), and Campo
(15G24 mph). It was cool in the mountains (Big Bear 54 degrees), and
warm in the coastal and valley zones (87 at Corona, 86 at Fullerton,
and 85 at Escondido). Palm Springs matched Corona with 87 degrees.
The winds will diminish through evening, then a second burst of
similar intensity offshore winds will pick up again early Tuesday
morning.

Tonight the WRF is advertising a moderately strong coastal eddy
spinning up over the coastal waters, but the HRRR is much weaker
with the eddy circulation. Considering the model disparity, have
used a model blend and projected some low clouds/patchy fog over the
coastal waters later tonight into Tuesday morning, but with
uncertainty as to how widespread it will be. Coverage will likely be
limited to the coastal waters tonight or the immediate coast
tonight. The eddy continues Tuesday night into Wednesday morning
when the coverage should be a little greater, and the marine
layer a couple hundred feet deeper.

*From Previous Discussion*

Santa Ana conditions produced locally gusty winds in the mountains,
foothills and passes this morning, strongest San Bernardino and
Riverside Counties, and the Santa Ana Mountains where 10-20 mph
winds were gusting to 30-35 mph. Winds were lighter in San Diego
County. The Santa Ana airmass has reached all the way to the coast
this morning where dewpoints were in the 30s and low 40s. Farther
inland the dewpoints were lower, ranging from the teens to the low
30s.

Both the ECMWF and GFS model are consistent with the Santa Ana
weather pattern this week. A longwave trough over the West will be
reinforced by a shortwave dropping south over NV tomorrow. This
shortwave will develop into a cut-off low over the Lower Colorado
River Valley Wednesday. The trough then lifts NE on Thursday and
Friday with higher heights resulting in even hotter days to end the
week. The coastal areas may not see a significant temperature change
Thu or Fri though, as the marine layer returns in response to a
coastal eddy that will form on Wednesday. Increasing boundary layer
RH will likely bring some marine layer clouds and patchy fog back
into coastal areas during that time.

Long Range outlook for the weekend and beyond: The ECMWF and GFS are
at odds with each other. The GFS is forecasting a mean ridge pattern
through early next week, while the ECMWF is suggesting a trough for
stronger onshore flow and a hint of cooling. In either case,
precipitation is unlikely, and the main challenge will be if
temperatures will be average or above average.

&&

.AVIATION...
251645Z...Mostly clear skies through Tuesday. Local northeast winds
in foothills and below canyons into valleys with Local mdt
up/downdrafts and LLWS west of the mtns. KONT and KSBD are most
likely TAF sites to be impacted by these winds and LLWS. A similar
offshore wind pattern will continue Tuesday.

The southern coast of San Diego county could see periods of SCT-BKN
low clouds with bases 1000 ft MSL or lower and vis 3SM or less
between 09Z and 16Z Tue. KSAN could be impacted but confidence is
low.

&&

.MARINE...
No hazardous marine weather is expected through Friday.

&&

.FIRE WEATHER...
North-northeast winds in the mountains and foothills ranged from 10
to 20 mph through early afternoon with gusts between 20-30 mph. The
windiest site had gusts above 35 mph including Fremont Canyon (45
mph at 9 AM), Crestline Ridge (41 mph at 2 AM), and Santiago Peak
(40 mph at 1 AM). Campo had a wind gust of 24 mph at 1 PM. The
winds will drop off through the evening, then increase again
Tuesday morning. Strongest winds will be in and below the Cajon Pass
and over the Santa Ana Mts/canyons, with lighter winds over
San Diego County. On Wednesday the winds will a little weaker.
Afternoon humidities will bottom out between 10-15% tomorrow and for
increased wildfire risk, but limited areal coverage of these
conditions will be too small to require any fire weather products.

The will be a small increase in RH Wednesday through Friday, but
overall it will remain fairly dry with afternoon humidities below
25%. It will be dry and very warm days through next weekend,
creating an elevated wildfire risk region-wide. Winds may be gusty
both off and onshore at times, but should be localized and not over
any extended period.

&&

.SKYWARN...
Skywarn activation will not be needed today.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...NONE.
&&

PUBLIC/FIRE WEATHER...Moede
AVIATION/MARINE...PG