Flash Flood Watch 1, NWS San Diego, CA

Area Forecast Discussion
National Weather Service San Diego CA
128 PM PST Sat Jan 6 2018

.SYNOPSIS...
Partly cloudy and mild through the weekend. The season's first
significant precipitation event is expected Monday and Tuesday as a
Pacific storm system arrives from the northwest. In the meantime,
temperatures will remain above average and patchy fog will continue
over the coastal waters and develop locally inland each night and
morning. High pressure and dry weather will return for the remainder
of next week.

&&

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

A veil of high clouds was drifting east across SoCal this afternoon
with a few lenticular clouds near the mountains. At 1 PM PST...radar
continued to show weak echoes drifting east off the coastal waters
which have resulted in some sprinkles over northern portions of the
CWA. These may affect San Diego County this afternoon but will not
be of consequence except perhaps to gather dust on vehicles. Lower
clouds were barely visible over the coastal waters. Surface pressure
gradients were weak onshore KSAN to the deserts at around 3 MBS but
peak winds were still 30 to 40 MPH on the desert slope and near some
passes. Surface dewpoints were well into the 50s across much of the
coastal plain.

Forecast issues, if any, over the next 24 to 48 hours will involve
fog. With the very high sfc dewpoints in place and light sfc winds
expected, it is only a question of the amount of high clouds that
may or may not inhibit radiative cooling overnight.

A weak shortwave trough will be passing by to our NE tonight,
allowing heights to rebound on Sunday, there is an offshore trend
that develops, but it should remain quite weak. In any case, expect
another mild to warm day on Sunday, perhaps with less high clouds.
By Monday, the high clouds should begin increasing again ahead of
the incoming low pressure center from the NW. This may keep max
temps a little lower, but there could still be some patchy fog,
mainly over coastal areas.

Both 12Z ECMWF and GFS runs are in sync, digging a cold upper-level
low pressure center SE across southern and central CA on Tuesday and
pushing it rapidly east on Wednesday. Ahead of it, southerly winds
will increase and become gusty by evening. Some rain will break out
and become increasingly prevalent Monday night on the coastal slopes
of the San Bernardino Mts. Snow levels will be quite high...above
8000 FT ahead of the front, limiting snowfall potential for many of
the resort towns. As the front nears on Tue morning, southerly winds
will be quite strong over San Bernardino County where an Advisory or
High wind Warning may be needed. There will also be a threat of
thunderstorms with brief heavy rainfall rates with and behind the
front on Tuesday, that could set off debris flows in recently burned
areas. There will be an abrupt wind shift to westerly behind the
front and precip should become more showery. Colder air will filter
in, lowering snow-levels before the precip ends entirely later Tue
night or early Wed. Dry and mild weather returns for the latter half
of the week as high pressure rebuilds aloft.

&&

.QPF/Hydrology... All areas should see the first significant rain/
snow of the season when all is said and done on Wednesday. A
moderate PW inflow and fast movement of the storm should help keep
amounts within reasonable limits for a winter storm.  Preliminary
rainfall totals are for 3-5 inches on the slopes of the San
Bernardino Mts, with 1.5 to 3 inches for the coastal slopes of the
other mts. Across the coastal basin...Orange County/Inland Empire
could see 1 to 1.5 inches, with 0.75 to 1.50 inches estimated for
western San Diego County. For the deserts...0.25 to locally 1 inch
is possible, biggest amounts over the high deserts or near
thunderstorms. For snowfall...some heavy snowfall is possible on the
highest mtn peaks, but below to around 5000 FT, it will mainly be
convective showers behind the front that provide any accumulation.
This makes it more difficult to estimate, but several inches are not
out of the question in some spots, which could result in some travel
impacts, especially with the gusty westerly winds.

Weather advisories and/or watches for this event will likley be
formulated and issued later this evening. Forecast rainfall rate
potential would produce debris flows near the fresh burn scars in
Orange and western Riverside Counties, producing moderate impacts
below more steeply sloped areas. Less impact is expected for the
Lilac burn (due to modest terrain influence), and older burn scars
in San Diego County. Flooding of mainstem rivers is not expected.

&&

.AVIATION...
052100Z...Coast...Patches of low clouds and fog within are lingering
at the beaches this afternoon. CIGS are around 500 ft MSL. Impacts
not likely at KCRQ, KSAN and KSNA before 01Z Sunday. Low clouds and
patchy dense fog returning to the coastal airports between 03Z and
06Z Sunday with bases 400-600 ft MSL and vis locally blo 1 mile at
times. The timing of impacts at coastal TAF sites is uncertain. Low
clouds and fog should clear by 17Z Sunday.

Valleys/Mountains/Deserts...BKN-OVC150-200 and unrestricted
visibility through Sunday.below 1000 ft

&&

.MARINE...
A shallow marine layer will result in patchy fog during the nights
and mornings through Monday. Visibilities could be less than 1 mile
at times.

From Tuesday through Wednesday morning a storm will bring rain,
gusty west northwest winds (up to 30 KT gusts) and combined seas of
6-10 ft with steep waves. Small Craft Advisory conditions are
probable.

&&

.BEACHES...
A short-period swell of up to 11 ft from 285 degrees is expected
next Tuesday through Thursday, with high surf and strong rip
currents at the beaches.
&&

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

&&

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

PZ...NONE.

&&

PUBLIC...jad
AVIATION/MARINE/BEACHES...PG