National Weather Service Forecast Discussion

Dicussion provided by the National Weather Service Forecast Office located in San Diego, California

000
FXUS66 KSGX 221149
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
449 AM PDT Tue Jun 22 2021

.SYNOPSIS...
Night and morning low clouds and fog spreading into portions of
the valleys will continue this week, but not spread as far inland
for the weekend into early next week. Mid and high level moisture
and clouds will spread northward across the area through
Wednesday. This will bring scattered, mostly light showers at
times with light measurable amounts more likely in the mountains.
There is also a slight chance of thunderstorms on Wednesday.
Cooling will spread inland through Wednesday with warming
beginning for inland areas on Thursday. The warming trend will
continue for Friday through Monday as high pressure strengthens to
the north.

&&

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

.SHORT TERM (Today through Thursday)...
A weak upper level low pressure system off the California coast
will move slowly eastward, but with the upper center by late
Wednesday remaining just off the coast to the west of the Bay
Area. This low pressure system will then weaken through Thursday.
This low pressure system will draw mid and high level moisture,
some moisture from once was tropical cyclone Dolores, northward
into the southwest states through Wednesday.

The 12Z Monday ECMWF Extreme Forecast Index for QPF for tonight
and Wednesday had some very large Shift of Tails values over
portions of southern California and adjacent states, highlighting
the potential for a climatological very unusual precipitation
event. The latest 00Z cycle has backed off somewhat, with the
highest values in southern California from around the San
Bernardino County mountains northward across portions of the high
desert. It should be noted that this is a very dry time of the
year for southern California, so it doesn`t take all that much
precipitation to be a very unusual event for this time of year.

The mean of all of the ensemble members of the 00Z cycle of the
ECMWF increases the precipitable water values at NKX with the
peak of 1.5 inches at 12Z Wednesday. That would exceed the
highest on record for that date and time of 1.27 inch, but below
the record of 1.61 inch for the month of June.

Model soundings show the atmosphere saturating initially today
down to around 500 mb with with a layer between 700 mb and 500 mb
becoming more saturated for late tonight into Wednesday morning
with the moisture then decreasing in the afternoon and evening
with the moisture centered around 600 mb. The incoming 12Z NKX
sounding has a nearly saturated layer from around 550 mb to a
little above 300 mb.

Some light showers are possible as early as this afternoon with
the most likely location for any measurable amounts in the higher
elevations of the mountains with virga more likely elsewhere. As
the moist layer lowers late tonight into Wednesday morning, there
would be greater potential for some light measurable amounts, even
at low elevations with the greater amounts still in the
mountains. The atmosphere becomes weakly unstable on Wednesday
with isolated thunderstorms possible in addition to showers. With
a dry subcloud layer below cloud bases around 700 mb or higher,
strong wind gusts with showers or isolated thunderstorms are more
likely at lower elevations.

While the moisture does increase, any forcing is fairly weak, so
timing and amounts are less certain beyond it being more likely
for greater amounts in the mountains and less at lower
elevations. Uncalibrated ECMWF ensemble members have a mean of
around 2 tenths of an inch in the San Bernardino Mountains with
the wettest ensemble members to around 4 tenths of an inch. The
HREF has a local PMM QPF with a max of around one half inch in
the San Bernardino Mountains.

In addition to the mid and high clouds through Wednesday, there
will be areas of night and morning coastal low clouds and fog
spreading into portions of the valleys. There will be a cooling
trend into Wednesday with inland areas beginning to warm on
Thursday.

&&

.LONG TERM (Friday through Monday)...
For the weekend into early next week, high pressure aloft will
strengthen and be centered somewhere in the vicinity of the
Pacific Northwest and northern Rockies with weaker high pressure
extending southward through California.

While a warming trend is expected for Friday through Monday, there
remains greater uncertainty in just how much warming will occur.
The current forecast is close to NBM with the deterministic NBM
near the middle of the guidance spread versus the higher
percentiles (warmer end of the guidance) for inland areas with the
recent heat wave. The marine layer should become shallower with
night and morning coastal low clouds not spreading as far inland,
mainly just into coastal areas by Sunday and Monday.

&&

.AVIATION...
220935Z...Coast/Valleys...Low clouds with bases 1000-1500 ft MSL
have surged about 15-20 miles inland early this morning. Cigs still
should stick southwest of KONT through the morning. MOD-HI
confidence that low clouds will scatter out around 15-16Z inland and
closer to 17-19Z toward the coast. High clouds have come into the
region early this morning and will continue throughout the day AOA
15000 feet MSL.

Mountains/Deserts...Chance of SHRA across the area this afternoon
and evening along with high clouds AOA 15000 feet MSL. Desert slopes
and deserts of SBD county will see wind gusts 25-30 kts at times
this afternoon and evening, which may cause MOD UDDFS. Calmer winds
expected in VCTY KPSP/KTRM today.

&&

.MARINE...
Slight chance of showers for the inner coastal waters this evening
into early Wednesday, along with periodic wind gusts near 20 knots
in the outer waters each afternoon and evening through Friday.
Otherwise, no significant hazardous marine weather is expected
through Saturday.

&&

.BEACHES...
Tides close to 7 feet and elevated surf may produce areas of beach
erosion and coastal tidal overflow in Orange County through
Thursday. See the Beach Hazard Statement for details.

&&

.FIRE WEATHER...
Mid and high level moisture and clouds will move northward across
the area through Wednesday. Scattered mostly light showers are
possible as early as this afternoon, with any measurable amounts
most likely in the mountains, Greater coverage is more likely
from late tonight into Wednesday morning. With the moisture mainly
in the mid levels and above, measurable amounts are more likely
in the mountains and less at lower elevations. Chances for wetting
rainfall remains low, but has been increased somewhat and is
around 30 percent in the San Bernardino Mountains on Wednesday.
Isolated thunderstorms are also possible on Wednesday. Dry
lightning remains a possibility, though the precipitation
potential has increased somewhat in the mountains. There could
also be strong gusty winds with any thunderstorms, especially at
lower elevations.

High pressure to the north is expected to bring drying and warming
for late in the week into early next week. Lowest humidity for the
deserts onto the desert slopes of the mountains will once again
fall to around 10 percent. That low humidity combined with gusty
southwest to west sea breeze winds each afternoon and evening will
result in elevated fire weather conditions for the deserts onto
the desert slopes of the mountains each afternoon. With the
warming, the potential for plume-dominated fire growth for the
mountains and deserts will return for late morning through early
evening each day.

&&

.SKYWARN...
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Beach Hazards Statement from this evening through Thursday
     evening for Orange County Coastal Areas.

PZ...NONE.

&&

$$

PUBLIC/FIRE WEATHER...17
AVIATION/MARINE/BEACHES...APR

NWS SGX Office Area Forecast Discussion