Scientific Forecaster Discussion
fxus66 ksgx 261009
Area forecast discussion
National Weather Service San Diego California
309 am PDT Tue Sep 26 2017
high pressure over the Great Basin will bring offshore flow through
Wednesday with sunny, warm days and cool nights. Winds will be
gusty at times in the mountains, foothills and the inland Empire.
Warmer Thursday and Friday with lighter winds. Later today, a weak
eddy circulation offshore may bring a return of coastal low clouds
and fog each night and morning through Thursday. Cooler by the
weekend as onshore flow returns.
Discussion...for extreme southwestern California including Orange...
San Diego...western Riverside and southwestern San Bernardino
Skies were clear over the County Warning Area at 3 am PDT. Some low, marine clouds
were observed along the Baja Coast, well south of San Diego, and
high clouds were thickening and streaming rapidly south over eastern
California/western Nevada. These clouds mark the shortwave that will amplify the
trough over US later today and tonight. Surface pressure gradients
were holding very weakly onshore ksan to the lower deserts, and
about 6 mbs from SW Nevada. North-NE winds continued in some mtn areas at 3
am PDT with local peak gusts to 25 mph, but in general, winds were
Gusty north-NE winds are expected again this morning over, mainly
northern portions of the County Warning Area with a moderate sfc pressure gradient
in place. The gradient will be weakening later today as a shortwave
trough digs south, forming a cut-off upper-level low pressure center
over southeast California. The low may have a hand in spinning up a coastal eddy.
Numerous hires model runs, and even dew-point guidance in MOS,
suggests this may happen today or tonight. The eddy would draw a
moist marine layer north along the coast, beneath the dry,
Continental air, possibly resulting in areas of low clouds and fog
for Wed/Thu mornings along the coast.
Aside from that little event, weak offshore flow prevails through
Friday as the longwave trough, which has been over the west since
last week, finally lifts out. That keeps inland areas quite dry and
warm through Friday with daytime temperatures several degrees above
average. Global model solutions begin to diverge over the weekend,
but are showing another, weaker amplitude trough digging into the
west. This pattern will weaken the sprawling sfc high over the west
and replace it with sfc low pressure and an onshore flow pattern.
Look for cooler weather and more marine influence with periods of
low clouds and fog developing inland.
GFS/European model (ecmwf) 00z solutions diverge widely next week lowering forecast
250900z...mostly clear skies today through tonight. Pockets of
breezy northeast winds will produce intermittent and isolated low level wind shear
in the foothills of the Santa Ana Mountains and south of the Cajon
Pass through 21z today. Kont and ksbd may be impacted at times.
no hazardous marine weather is expected through Friday.
north-northeast winds are currently only gusting to around 20-25 mph
at a few locations, but should strengthen again this morning.
Strongest winds will be in and below the Cajon Pass, and over the
Santa Ana mts/canyons with local gusts 30-35 mph. Winds will be
lighter over San Diego County. Daytime minimum relative humidity will be near 10-15
percent. This could result in a few hours of near critical fire
weather conditions in the wind prone areas. The duration of this
condition will be too short and isolated to support the issuance of
any fire weather products.
Relative humidity is forecast to increase Wednesday through Friday over coastal and
western valley areas, but minimum relative humidity will still fall to 15-25
percent across the far inland valleys. Overnight recovery will be
better, and northeast winds will be weaker as well during this time.
A general onshore flow pattern will return this weekend for cooler
conditions and higher daytime humidity.
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.