Scientific Forecaster Discussion
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Area forecast discussion
National Weather Service San Diego California
950 PM PDT sun Sep 22 2019
a trough of low pressure will bring slightly cooler weather Monday,
but it will still be quite warm. The trough will stall over
northwest Mexico Tuesday and Wednesday, resulting in gusty offshore
winds in the mountains and inland valleys. Wrap around moisture may
bring a few showers to the mountains and deserts, mainly on
Wednesday. It will turn cooler toward the end of the week as a large
trough settles over the west.
Discussion...for extreme southwestern California including Orange...
San Diego...western Riverside and southwestern San Bernardino
No major changes were made this evening. The marine layer induced
clouds are patchy attm but expected to fill in overnight and push
along the coastal mesas late tonight.
Previous discussion, issued at 205 PM pdt:
Today was a very warm day with highs west of the mountains in the
80s and 90s except within a mile of the coast. It would have been
even warmer if not for a layer of high cirrus clouds that muted the
suns heating. Tomorrow will be another warm day, just not quite as
warm as today, with lower heights and somewhat stronger onshore flow
in the afternoon ahead of a trough moving down the California
interior. The exception will be the lower deserts where the highs
tomorrow will be nearly the same as they were today.
The marine layer will remain shallow through Wednesday morning, and
there may be areas of dense fog in the coastal zones or far western
San Diego valleys the next three mornings.
The trough evolves into a closed low over the lower Colorado River
valley Monday night before drifting into northwest Mexico Tuesday. Remnant
tropical moisture from Lorena surges into Arizona tonight and will bring
heavy rain to eastern neighbors Monday and Monday night. The western
edge of deep moisture will clip eastern Imperial County and eastern
Riverside County, and Storm Prediction Center has a risk of thunderstorms in these
areas. At this time the chances for showers are small in the San
Diego County deserts and the Coachella Valley as the rapidly digging
shortwave forces most of the deep moisture east of the area.
The greatest impacts from this system will likely be the gusty
offshore winds in the mountains and inland valleys on the backside
of the low on Tuesday. Downslope flow on gusty northeast winds will
send a dry, warm airmass into the inland Empire for what will be a
warm afternoon. Confidence is lower for potential wrap around
moisture reach the mountains and deserts on Wednesday for possible
On Thursday the upper low slowly fills and drifts east as a longwave
trough begins settling over the western states. Most of its
shortwave energy will stay to the north, but lower heights and
stronger onshore pressure gradients will bring cooler weather and a
deeper marine layer Friday and Saturday, perhaps deep enough for
scattered light showers along and west of the mountains.
230300z...coast...moderate to high confidence in ceilings tonight at all
coastal terminals, with low to moderate confidence in onset time.
Ksan/kcrq expected to see ceilings after 06-07z, with ksna likely
delayed a few additional hours. Bases 500-1000 feet above ground level. Visible
restrictions 3-5 sm likely 10-16z on elevated terrain and mesas,
locally below 3 sm at times. Terminals clearing near 16z with VFR
conditions prevailing thereafter.
Valleys/mountains/deserts...sct-bkn high clouds at or above 20000 ft mean sea level and
unrestricted vis through Monday evening.
no hazardous marine weather is expected through Friday.
Monday won't be quite as hot as today and the humidities will be
slightly higher as the onshore flow strengthens ahead of a low
pressure system moving down the California interior. This trough
will stall over northwest Mexico and set up a period of gusty
northeast winds of 20 to 30 mph over the mountains and into the
foothills and inland valleys Tuesday. There will be several hours of
near critical fire weather conditions Tuesday afternoon when the
warm, downslope flow pushes temperatures into the 90s and low 100s
in the inland Empire, and the relative humidity falls to between 7 and 12%. By
early evening this airmass will modify and the dry offshore flow
weakens and the sea breeze advances inland. Weaker offshore flow may
persist through Wednesday morning, but it will be cooler and relative humidity
A large, cool, early season trough will develop over the west after
Wednesday with unseasonably high relative humidity and possible showers along and
west of the mountains. Westerly winds will be strong and gusty at
times over the mountains and deserts.