Flash Flood and Debris Flows in California, July 11, 2018

Area Forecast Discussion
National Weather Service San Diego CA
938 AM PDT Wed Jul 11 2018

.SYNOPSIS...
Monsoonal moisture remains in place today with mostly sunny skies
and will bring widely scattered thunderstorms during the
this afternoon and Thursday afternoon...mainly for the
mountains...foothills...high deserts...and possible spilling into
the Inland Empire. Slow drying will begin on Thursday with
greater drying for Friday and Saturday as an upper high pressure
builds right over southern California. Inland temperatures will
increase Friday and Saturday to slightly above normal while the
coastal areas are about average or slightly above. For coastal
areas...a coastal eddy will bring a greater return of coastal low
clouds for tonight into Thursday morning. Upper level high
pressure will lift north over California and Nevada on Sunday and
persist through next week. This will keep average but warm July
temperatures over the region with some inland areas going above
average. Monsoonal moisture will increase slightly and bring
isolated mountain thunderstorms.

&&

.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...

Deep monsoonal moisture remains in place with 1.5 to 1.7 inch
precipitable water widespread. The difference today is a short
wave trough (vort max) is located over us (see water vapor
imagery) with the resultant mid clouds over the high deserts and
Inland Empire but clear elsewhere. This is drifting to the west in
deep uniform east to northeast mid-level steering flow. Based on
mesoanalysis we are stable now and limited or no elevated
instability. However, with abundant sunshine and easterly flow
which will collide with afternoon sea breeze penetrating inland,
there should be good upslope forcing to reach the increasing
boundary layer and mid level instability in the wake of the vort
max. Mid level drier air increases instability aloft. The
steering flow supports pushing storms westward and into the
opposing westerly onshore flow sea breeze. Thus isolated cells
should move and develop in the Inland Empire and some cells
drifting down into foothills. It appears all mountains have a
chance for deep convection this afternoon but the earliest
initiation and stronger more persistent activity should be the
eastern San Bernardino mountains and Onyx Peak down to the Santa
Rosa Plateau. The activity in San Diego Mountains will be less but
given the easterly flow Palomar Mountain which is west of the
crest line could see a thunderstorm. The latest high resolution
model data such as ARW, local WRF, NAM3 and HRRR support this
coverage. There is the potential for isolated flash flood
producing storms.

Given the monsoon moisture in place dewpoints are elevated in the
50s and 60s but even higher on the coast due to ocean temperatures
in the low 70s. Temperautures today are below normal in deserts
due to the moisture levels but still above normal on the coastal
plain and mesas...and about normal for inland valleys.

There is a weak coastal eddy tonight and more organized marine low
clouds will develop and portions of it pushing inland to near the
I-15 of San Diego County. Moisture levels slowly decrease on
Thursday and this is due to the mean layer flow or steering flow
at mid levels turning southerly (versus easterly). This allows for
cells to form along the crest line upslope convergence but less
activity on Thursday. There will be more marine influence but
warming air mass so offsetting conditions on the coast into Friday.

Upper level high pressure at 594 DM at 500 mb over Socal builds
back quickly on Friday for a warming air mass. This center is over
our region and 500 mb flow becomes near zero. 850 mb temperatures
rise from 22-24C on Thursday to 23 to 26C. There should be
several degree rise in temperatures for the deserts up to average
values and slight increase for coastal and inland valleys.

The upper high them drifts just north of Socal across Point
Conception and extends over Reno Nevada. This sets the stage for
northeasterly wind flow and a slow increase in monsoon tropical
moisture. The airmass is progged to stay warm but still near July
averages or slightly above Sunday through next week with 850 mb
temperatures 24 to 28C over the entire region. This will hold the
marine layer down. This type of flow can be conducive to mid level
easterly waves and enhanced thunderstorms but right now at least
isolated convection for high terrain. By mid next week the upper
high drifts back over the favored 4-corners region which brings
the classic southeast monsoonal flow over Socal with July like
temperatures at or slightly above normal.

&&

.AVIATION...
111600Z...Coast...VFR this afternoon then areas of stratus
developing tonight with bases 1000-1200 MSL before clearing by 10 AM
Thursday.

Valleys/Mountains/Deserts...Scattered SHRA/TSRA will form over the
mountains this afternoon after 19Z. Tendency of TSRA will be a slow
westward movement, possibly into the Inland Empire, inland valleys
of San Diego County and parts of the deserts. However, probabilities
of impacts at the TAF sites at these locales are very low. Wind
gusts 30+ KT likely in and near TSRA. TSRA will end after 02Z with
another chance of mountain TSRA Thu afternoon.

&&

.MARINE...
No hazardous marine weather is expected through Saturday

&&

.BEACHES...Ocean teamperaturs in the low to mid 70s.

&&

.FIRE WEATHER...The humidity has increased in the warm tropical
air mass but the threat for lightning activity exists today.

&&

.HYDROLOGY...Thunderstorm potential does bring isolated flash
flood potential to mountains and foothills.

&&

.SKYWARN...
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions. Spotters may
be asked to report this afternoon.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...NONE.
&&

PUBLIC/FIRE WEATHER...Tardy
AVIATION/MARINE...Gregoria