Area Forecast Discussion
Issued by NWS San Diego, CA

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000
FXUS66 KSGX 162041
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
145 PM PDT Mon Oct 16 2017

.SYNOPSIS...
Weakening high pressure aloft will bring another hot day to the
region on Tuesday. A weak trough may trigger an isolated
thunderstorm on Wednesday, but the chances are small. The trough
will bring cooling Thursday and Friday along with a deeper marine
layer and gusty west winds in the mountains and deserts. Dry and hot
again early next week with another Santa Ana weather pattern setting
up over the West.

&&

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

It was another very hot day today with a record high of 98 (so far)
at 1 PM, which broke the old record of 96 set in 1997. High pressure
and a very dry airmass contributed to a rapid warmup this morning. 1
PM temperatures were 5 degrees higher than 24 hours ago in the
Valleys, Mountains and Deserts, and relative humidities in the
single digits and teens. The coastal zones were a few degrees lower
due to the return of the onshore sea breeze.

A 500 mb ridge axis was directly over SoCal this morning and it will
drift slowly eastward through tonight. On Tuesday the flow aloft
becomes more zonal, but surface cooling will be minor, and tomorrow
will be another very warm day with temperatures 10-15 degrees above
average west of the mountains, and 5-10 degrees above average in the
mountains and deserts.

On Tuesday night and Wednesday a couple of weak Pacific shortwaves
traverse central and southern CA. The first wave passes over Santa
Barbara and San Bernardino County Tuesday night. Moisture advection
is not in phase with this first shortwave and that will limit the
potential for elevated convection. Deeper moisture is available with
the second wave on Wednesday. The WRF point sounding forecast shows
near saturation in the 400-600 mb layer resulting in a PW of 1.57"
Wednesday afternoon. Most unstable CAPE values range from 200-400
J/kg Wednesday, and the NAM High Level Total Totals reach 31-33,
which has historically been the trigger values for isolated to
scattered thunderstorms. Have introduced a slight chance of
thunderstorms on Wednesday to the mountains and parts of the deserts
and valleys. This will be likely be adjusted over the next few
shifts as more hi-rez model data becomes available. Precip amounts,
if any, will be light with most of the precip evaporating in the dry
subcloud layer. Best chances for measurable rain will be in the
mountains.

On Thursday and Friday a broad longwave trough moves into the
Pacific Northwest where rain is likely. Farther south in our area
the precip chances are minimal, but the trough will bring cooling, a
deeper marine layer, and stronger onshore flow with gusty afternoon
and evening west winds in the mountains and deserts.

On Saturday the trough moves east and a strong ridge of high
pressure will build over the West. This Santa Ana weather pattern
will bring another round of hot, dry weather and elevated fire
weather concerns early next week.

&&

.AVIATION...
161500Z...FEW-SCT AOA 20,000 ft MSL, unrestricted visibility and
light winds will continue through Tuesday.

&&

.MARINE...
No hazardous marine weather is expected through Thursday. Winds will
increase Friday as a front traverses the region, potentially
bringing wind gusts of 20 kt or more to the Southern California
Bight. A short period northwest swell will also increase Friday and
Saturday.

&&

.BEACHES...
A south southwest swell from 200 degrees will build today and peak
Tuesday-Wednesday at 3-4 ft/16-18 seconds. This will produce 3-6 ft
surf with sets to 7 ft north of Del Mar. Highest surf will occur
along southwest facing beaches. Strong rip and longshore currents
will accompany the elevated surf. Swell and surf will begin to lower
on Thursday. A Beach Hazard Statement is in effect for elevated surf
and strong rip currents Tuesday and Wednesday.

&&

.FIRE WEATHER...

Light winds, hot weather and low humidities will continue through
Tuesday. The RH will start to increase and temperatures start to
cool Wednesday and Thursday as the ridge aloft weakens and onshore
flow and the marine layer slowly return. A weak disturbance aloft
interacting with increasing midlevel moisture may trigger an
isolated thunderstorm Wednesday. Measurable rain is unlikely except
in the mountains, where 0.01-0.10" are possible with any storms that
do form.

Thursday and Friday...Onshore winds will be increasing over
mtn/desert areas and become strong and gusty from the west with
increasing RH driving well inland.

Saturday through next Tuesday...Strong high pressure aloft and a
return of offshore flow will once again raise wildfire concerns with
very low RH, heat, and locally gusty offshore winds.

&&

.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 Tuesday morning through Wednesday
     evening for Orange County Coastal Areas-San Diego County Coastal
     Areas.

PZ...NONE.

&&

$$

PUBLIC...Moede
FIRE WEATHER...Moede
AVIATION/MARINE...Albright



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