Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Diego California
900 PM PDT Monday Apr 20 2015
Synopsis...low pressure aloft will move over Southern California on
Tuesday...and remain through Thursday. Daytime heating will create
instability and scattered showers...mainly in the mountains.
Isolated thunderstorms are possible as well. West of the
mountains...the marine layer will deepen significantly through
Wednesday...accompanied by more extensive cloudiness which will hold
daytime temperatures below average. Dry and warmer early next week
under building high pressure aloft.
Discussion...for extreme southwestern California including Orange...
San Diego...western Riverside and southwestern San Bernardino
Marine clouds were gathering over the coastal waters and spreading
inland while inland areas remained mostly clear. By 8 PM PDT...surface
onshore pressure gradients had reached 6 to 9 mbs ksan-deserts
supporting westerly winds of 15-25 miles per hour with locally higher gusts in
the mts/deserts. The 00z Miramar sounding had a 3 degree c inversion
based near 2400 feet. This was about 1300 feet deeper than yesterday at
It was a few degrees cooler again inland today and this will be the
case again tomorrow as well...especially well inland as the marine
Weak low pressure aloft off the socal coast will drift east over the
area on Tuesday. The system already managed to spark numerous
thunderstorms over the Sierra...even as close as northwest Ventura County
before sunset. A similar situation is likely tomorrow...except the
instability will extend farther south across San Bernardino and
Riverside counties. This has prompted a chance of showers in these
areas. West of the mountains...there is a smaller chance for a few
showers as the marine layer builds...but little accumulation is
expected based on model guidance.
A colder...more dynamic low pressure center will develop off the
central coast Tuesday night and dive south well offshore...turning east
across northwest Mexico late Thursday. This will keep a cool and conditionally
unstable airmass over socal through Thursday which could result in more
diurnally driven scattered showers/thunderstorms...especially in the
mts. The forcing and moisture availability does not support any
widespread precipitation over the area...but a few showers could produce
moderate amounts of rain...especially in the higher elevations on
Thursday. The 00z NAM is hitting the precipitation harder on Thursday afternoon from
the crest of the Santa Rosa mts northwest across the higher elevations of
the San Bernardino mts.
From previous discussion....
A final weaker open short wave trough is expected on Friday...though
weak as it carves into Arizona under northwest flow. This scenario
is dryer for Friday but if the upper low tracks further south as
prognosticated by a few ensemble members its still a scattered shower
threat for eastern areas...thus have retained some weather over
eastern areas for Friday and Friday night. The mean upper trough
axis does not cross until Saturday morning.
Overall the scenario for Tuesday and Thursday is showery and
isolated to scattered in nature with isolated thunderstorms
producing the main convective rainfall. Have lower chances along the
coast on Thursday and depicted convective quantitative precipitation forecast fields favored over
the inland mountain spine...and some showers in the bight for
Tuesday morning. Lingering isolated shower in northern areas and
High Desert on Wednesday near the first upper low center.
For the weekend into next Monday...dry with a warming trend under
building high pressure ahead of the next Pacific upper trough.
Aviation... 210345z...coast/valleys...stratus with bases 2000-2500
feet mean sea level and tops to 4000 feet mean sea level developing along the coast will move
into the inland valleys overnight. Lower coastal slopes of the
mountains will be obscured through Tuesday morning. Low clouds becoming
scattered-broken with bases rising to 3000-3500 feet mean sea level around 18z.Stratus
becoming broken-overcast with bases 2500-3000 feet mean sea level after 22/03-06z.
Mountains and deserts...few-sct clouds at or above 20000 feet and unrestricted
visible through 18z Tuesday. After 18z...scattered cumulus and isolated
showers/thunderstorm are possible over the mountain crests...with
the highest chances in San Bernardino and Riverside counties. Any
storms that develop will be capable of producing gusty winds and
small hail...and have bases near 8 kft with tops to 20 kft. Showers
and thunderstorms dissipating by sunset.
850 PM...no hazardous marine weather is forecast through Saturday
morning. Northwest winds gusting to 20 knots are possible over portions
of the outer coastal waters Saturday afternoon and evening.
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.