Scientific Forecaster Discussion
fxus66 ksgx 232040
Area forecast discussion
National Weather Service San Diego California
140 PM PDT sun Jul 23 2017
isolated thunderstorms are possible near the mountains this
afternoon. A greater influx of moisture tonight and Monday will
bring slight inland cooling, more clouds, higher humidity, and a
better chance for showers and thunderstorms, with the chances
greatest near the mountains, and in the afternoon. Moisture will
decrease Tuesday into Wednesday, but a few thunderstorms are still
possible over the mountains on Tuesday. Dry for the rest of the week
and seasonally warm.
Discussion...for extreme southwestern California including Orange...
San Diego...western Riverside and southwestern San Bernardino
Marine clouds were limited over the inner waters this afternoon.
Satellite imagery identified only a couple of spots along the coast
with low clouds. At 1 PM PDT...higher clouds were spreading slowly
westward from the deserts, and cumulus were bubbling over the mts
and High Desert areas. Surface pressure gradients were weak to
moderate with peak wind gusts below 25 miles per hour.
The ridge over the SW will weaken through Monday as a trough inches
closer to the West Coast, and a weak easterly wave is drawn
northwestward from northwest Mexico into the area. Monsoonal moisture
accompanying the wave will trigger thunderstorm activity over the
lower deserts of Arizona/California tonight, sending more clouds and moisture our
way on Mon. The added clouds and humidity will keep Max temps on
Monday lower throughout the region, and may result in scattered
showers and thunderstorms. The wave will rapidly weaken and move out
of the area on Tue, but residual moisture and instability may still
pop a few thunderstorms in the afternoon over the mts/high deserts.
By Wednesday the subtropical ridge will be rebuilding westward,
further crimping any thunderstorm threat here, and possibly
threatening desert areas again with extreme heat late in the week.
Onshore flow is projected to continue west of the mts, maintaining a
shallow marine layer and more moderate Summer conditions. Both the
GFS/European model (ecmwf) 12z operational runs show the high reaching near 600 dm at
500 mb over Nevada early next week. This position is far enough north to
allow easterly flow aloft over socal, which may eventually lead to
more monsoonal activity.
In the near term...given the build-ups we have at midday, expect
isolated thunderstorms to develop, despite weak instability and
moderate mid-level shearing flow. Debris from activity farther east
late today/tonight will likely drift westward over the County Warning Area overnight
and on Mon.
Some showers are possible west of the mts Mon, but weakening
easterly flow and the lack of instability aloft should keep the
thunderstorm threat confined to areas nearer the mts. Instability is
marginal, but provided there is sufficient heating along the higher
terrain, ample moisture and weak winds should allow updrafts to grow
into thunderstorms. Given the high values of precipitable water expected (2+ inches
over the lower deserts and adjacent mtn slopes) stronger, slow-
moving storms could produce local flash flooding.
Expect activity to diminish overnight on Mon into Tue, with more
activity possible Tue afternoon/evening along the higher terrain due
to residual moisture and instability. If any storms do develop, they
should drift into the high deserts (based on forecast steering
flow). The main threats will be lightning and heavy rain leading to
possible flash flooding.
232000z...coast/valleys...predominantly sky clear-scattered at or above 10000 ft msl
will prevail through 00z Monday. Scattered-broken low clouds with bases near
1200 ft msl will filter back along the coast after 00z, likely
spreading inland 15-25 miles by 14z, then clearing by 17z Monday. An
isolated rain showers or -tsra is possible between 09z and 18z Monday, with
the best potential over San Diego County. Any precipitation should
have little to no impact on the taf sites. Few-scattered at or above 7000 ft msl
and distant thunderstorms and rain to the east will prevail between 18z Monday and 00z
Mountains/deserts...cumulonimbus and isolated thunderstorms and rain over the mountain peaks
through 02z Monday. Any storms that form will produce locally strong
up/downdrafts and gusty surface winds. Cloud tops and base this
afternoon should range from 9000 ft msl to 35000 ft msl. An isolated
rain showers or thunderstorms and rain remains possible overnight. More widespread +tsra are
anticipated between 19z Monday and 03z Tuesday, with isolated tops
near 45000 ft msl and strong up/downdrafts.
an isolated shower is possible through Monday, though thunderstorms
are looking less likely. No hazardous marine weather is expected
Monday night through Thursday night. Starting Friday, a building
south swell from what is now Tropical Storm Hilary may arrive in the
Southern California bight. This has the potential to produce 5-8 ft
a high tide near 7 ft this evening could result in minor tidal
overflow in low-lying beach areas, therefore a beach hazard
statement remains in effect through midnight. Surf heights will
mostly be 1-3 ft, and this should limit the potential for tidal
overflow to some degree.
Though not entirely out of the question, thunderstorms are looking
less likely along the beaches Monday morning. Forecaster consensus
at time favors only a slight chance of very light showers.
Skywarn activation is not requested. However activation may be
needed on Monday. Weather spotters are encouraged to report
significant weather conditions.
California...beach hazards statement through this evening for Orange County
coastal areas-San Diego County coastal areas.