Scientific Forecaster Discussion
fxus66 ksgx 202110
Area forecast discussion
National Weather Service San Diego California
110 PM PST Mon Feb 20 2017
it will be mostly dry with cooler than average days due to
continued onshore flow. A storm system moving by to the north will
spread some clouds and a slight chance for light precipitation into
northern portions of the forecast area through Tuesday, otherwise
partly cloudy. Cooler and breezy for Wednesday and Thursday, then a
bit warmer again on Friday as flow aloft turns more westerly. Cooler
with increasing clouds into the weekend, with a chance of
precipitation breaking out Sunday through early next week.
Discussion...for extreme southwestern California including Orange...
San Diego...western Riverside and southwestern San Bernardino
Skies were sunny over the deserts, and partly cloudy to mostly cloudy
elsewhere early this afternoon. No remote rain gages indicated any
accumulating rainfall over the last few hours. Surface pressure
gradients were running 8 mbs onshore ksan-SW Nevada and 2 mbs onshore to
the lower deserts. At midday, winds were generally light, except
over portions of the high deserts where some gusts around 25 miles per hour were
A cold, low pressure trough over the NE Pacific will be feeding
moisture into central and northern California through Wednesday. Northern
portions of the sgx forecast area will be on the fringes of this
activity with small probability of precipitation. The final shortwave moves inland on Wed
and drops far enough south to bring strong onshore flow across
socal, but the models keep it dry this far south. Winds will likely
be advisory strength or better along the desert slopes of the
mountains late Wed/Wed night.
Once that shortwave moves east, a dry, northwest flow aloft prevails through
Fri, turning more zonal into the weekend as low pressure redevelops
over the far NE Pacific. Daytime temperatures are forecast to remain
a few degrees below average with moderate onshore flow into the
Extended forecast discussion...
The hemispheric flow is undergoing a longwave shift as a strong
ridge bubbles up over the eastpac near 160w and a shallow, broad
trough develops over North America. In addition, the ridge over the
eastpac is forecast to cut-off for a time over the Gulf of Alaska
late in the week, which frequently promotes under-cutting low
pressure systems dropping out of the Arctic. The West Coast is
downstream of this developing Rex block, and in an unstable northerly
flow pattern where shortwaves can amplify and be cut-off from the
This instability has been exhibited in recent ensemble and
deterministic global model runs, which show large solution spreads
beyond about 120 hours. This translates into a low confidence
forecast from late in the weekend into next week across socal.
With that in mind...the 12z GFS has backed off of the more aggressive
storm system it had for the weekend on the 12z sun run, but instead
now shows a larger trough developing farther up the West Coast. This
steers higher precipitable water into socal on sun/Mon (with precip) a bit later than
previously forecast. Oddly enough, the operational European model (ecmwf) run this
morning showed something similar for early next week. Both now show
precip sun through Tue. But due to the model instability, a blend of
ensemble and operational guidance was used in the forecast for next
weekend and beyond. This has produced chance pops early next week,
but it could be another substantial precip event for socal.
202047z...bkn-sct clouds in the 2500-7000 ft msl layer over and west
of the mountains with prevailing p6sm vis through 21/0300 UTC except
for mountain obscuration over portions of the coastal slopes.
Elsewhere, p6sm vis and few-scattered clouds in the 5000-7000 ft msl layer
will continue. 21/0300-1500 UTC, broken clouds over the coast and
valleys, with variable bases 1500-3500 ft msl and variable tops up
to 4000 ft msl. Isolated -dz/-shra possible in Orange and San
Bernardino counties, producing local vis 2-5 sm. Elsewhere, scattered-few
clouds at or above 8000 ft msl and p6sm vis.
northwesterly winds and seas will increase Wednesday through
Thursday, with winds reaching 15-25 kt with gusts to 30 kt, and seas
reaching near or above 10 feet. This will result in conditions
hazardous to small craft during that time.
a large, moderate period west-northwest swell varying from the 280
to 290 direction will build early Wednesday, peak late Wednesday
through early Thursday at around 12 ft/14 seconds, then lower the
rest of Thursday. This will likely result in another round of high
surf of around 6-9 feet. Before that time, moderate swells will
continue to bring elevated surf of 3-6 feet. Strong rip current
activity is likely through most of this week.
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.