Area Forecast Discussion
Issued by NWS San Diego, CA

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000
FXUS66 KSGX 222052
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
200 PM PDT Wed Mar 22 2017

.SYNOPSIS...
Precipitation will increase for awhile this evening as the trough
moves overhead and combines with instability, and there is a slight
chance of a thunderstorm this afternoon and evening. Areas of fog
will continue in the mountains tonight, and gusty winds will
continue on the desert mountain slopes through Thursday. Fair
weather will return by Thursday afternoon and continue Friday, then
a weak trough of low pressure could bring a few showers Saturday.
Another system could bring a few showers next Monday or Tuesday.
Temperatures will average a little below normal through early next
week.

&&

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

A few showers continued this afternoon, especially in San Bernardino
County. Some lightning was upstream over Santa Barbara County as
well as about 300 miles WSW of San Diego. As cold air moves in from
the west, instability will increase with surface based CAPE values
of up to 800 J/kg, so there could be an isolated thunderstorm over
our forecast area this afternoon/evening. The main vort max with the
low pressure trough will mostly go into northern Baja, and its
southerly position will be the main reason for only modest precip
amounts here, mostly under 1/10 inch since Tuesday except local
amounts over one inch in the San Bernardino County Mountains. The
main trough will move through this evening, and enough moisture and
dynamics, including a 110-kt jet just to our south, should bring a
little increase in the precip, with San Diego County likely
receiving more than areas further north. Precip should gradually end
late tonight and early Thursday. Additional precip amounts should be
mostly less than 1/4 inch west of the mountains, but with variable
amounts due to convection, and locally over one-half inch in the
mountains. Snow levels will lower to around 6000 feet tonight, but
only 1-2 inches will occur especially with the northern higher
mountains having less available moisture. Winds should gust around
50 MPH on the normally wind-favored desert mountain slopes at times
through Thursday.

The trough will move to the southern Rockies by Thursday night with
a weak ridge moving through So-Cal. An open trough will move through
California Saturday with most of the moisture staying to our north,
so only light showers should occur here. After weak ridging again
Sunday, models are showing another trough from the northwest around
Monday or Tuesday, and a few showers were put in the afternoon
forecast package for Monday. GFS is much wetter than ECMWF. The
ridging will likely stay over the East Pacific and not build into
California through at least the middle of next week, so temperatures
should stay mostly a little below normal through mid-week,
especially daytime maximums.

&&

.AVIATION...
222010Z...BKN cloud decks within the 1500-10000 ft msl layer, local
vis 5 sm, and isolated showers over and west of the mountains, SCT
clouds in the 5000-10000 ft msl layer and P6SM vis in the deserts,
and mountain obscuration of coastal mountain slopes will continue
through 23/0000 UTC. 23/0000-0900 UTC, an increase in shower
activity over and west of the mountains and in the high deserts will
occur, with BKN-OVC cloud decks within the 1500-10000 ft msl layer
and areas of 2-5 sm vis over and west of the mountains, SCT-BKN
clouds in the 5000-10000 ft msl layer and mainly P6SM vis in the
deserts, and mountain obscuration of coastal mountain slopes.
There`s a chance that vis/cigs could reach down to 1 sm/500-700 ft
AGL in heavier showers.

There is also a slight chance of thunderstorms through 23/0600 UTC,
with CB tops to 25000 ft msl.

After 2100 UTC, west winds 20-30 kt with gusts 35-45 kt along the
ridges and desert slopes with mod-stg up/downdrafts and LLWS over
and east of the mountains.

&&

.MARINE...
A low pressure trough moving through the region will bring scattered
showers through tonight, with isolated thunderstorms possible this
afternoon and evening. Winds will be on the increase and become
hazardous to small craft out of the west-northwest tonight at speeds
of 20 to 25 knots with gusts to around 30 knots. In addition to the
winds, seas will rise to near or above 10 feet over portions of the
coastal waters late tonight. A Small Craft Advisory goes into effect
for the coastal waters late this evening and continues through
Thursday afternoon. Winds and seas will diminish by Thursday evening.

&&

.BEACHES...
There is a slight chance for thunderstorms at the beaches this
afternoon and evening, resulting in a slight risk of lightning for
beach goers/swimmers.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory through 5 PM PDT Thursday for Coastal
Waters from San Mateo Point to the Mexican Border and out to 30 nm-
Waters from San Mateo point to the Mexican Border Extending 30 to 60
nm out including San Clemente Island.

&&

$$

PUBLIC...Maxwell
AVIATION/MARINE/BEACHES...Harrison


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