textproduct: San Diego

SYNOPSIS

Another round of monsoon thunderstorms is expected for the mountains on Tuesday with locally heavy rainfall rates. An isolated storm or two may pop up over the mountains again Wednesday, but otherwise drier conditions are expected for late week into the weekend. A cooling trend will bring below normal temperatures to the region from Wednesday onward along with breezy conditions for the mountains and deserts. For the coastal basin, the marine layer is expected to deepen each day through the weekend.

DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE

SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES...

Monsoon thunderstorms put on a show over the mountains Monday afternoon, particularly in Riverside and San Diego Counties. A similar set up today will likely allow for another round of thunderstorms over the mountains this afternoon. The ridge over the Four Corners region and the weak trough to the northwest remain locked in place. Another surge of monsoon moisture should provide PWATs around 1.5" or slightly higher, similar to what was seen yesterday. The latest CAM guidance suggests rainfall rates of up to 1" per hour once again with the highest rainfall rates over San Diego and Riverside Counties where the moisture is best. Further north, rainfall rates of 0.5-0.75" remain possible over the eastern San Bernardino County mountains and locally into the High Desert. Lower PWATs further west should limit storm activity closer to the LA County line.

The only notable difference in today's set up is that the mean flow is expected to weaken slightly and shift more out of the south by this afternoon. With the flow more parallel to ridge lines in San Diego and Riverside Counties, storms may be able to maintain themselves for longer and there may even be a few cases of training storms. This theoretically increases the risk of flash flooding today, but the outcome will depend on exactly where convection eventually sets up. Regardless, rainfall rates up to 1" per hour will be enough for a marginal risk of flash flooding in itself. Additionally, the southerly orientation of flow will all but eliminate any chance of a storm drifting into the eastern valleys, but a stray storm in the low deserts remains possible. As always with any thunderstorm, frequent lightning, gusty winds, and small hail will be possible. Ensure you are prepared to take shelter in a sturdy and enclosed structure should you hear thunder.

By Wednesday, the ridge over the Four Corners weakens some and retreats eastward and the trough to the west deepens. Increasing trough influence will turn the flow out of the southwest, which will cut off the monsoon moisture. Exactly when this occurs will determine whether or not a storm or two can develop over the mountains on Wednesday. If anything develops, rainfall rates should be lower than today with the most likely location for convection being near Big Bear Lake.

As the trough continues to deepen just offshore, the cooling trend will become more pronounced from Wednesday onward. Many areas will see their coolest temperatures since early July from Wednesday through the weekend with high temperatures ranging 3 to 12 degrees below normal along with slightly below normal low temperatures. Also, the marine layer is expected to deepen as upper heights fall through the week. Marine layer clouds may fill most of the coastal basin Thu-Fri mornings. Increased onshore flow will create breezy conditions for the mountains and deserts with the strongest winds expected Wednesday and Thursday afternoons and evenings.

From this weekend into early next week, model guidance deals with a battle between the rebuilding ridge to the east and a reinforcing trough diving out of the northwest. The timing, strength, and locations of these features creates minor differences in model solutions heading into next week. However, the ensemble mean suggests slightly below normal temperatures and little to no chance for a return of the monsoon through at least mid-next week.

AVIATION

110600Z....Coast...Patchy low clouds to gradually develop overnight, increasing in coverage and duration, with best chances of cigs between 11-15Z Tuesday. Bases around 700-1000 feet MSL. Scatter out of any cigs Tuesday by 16-17Z. Low clouds around 1000 feet MSL to develop after 04Z Wednesday with greater coverage and uniformity.

Valleys/Mountains/Deserts...Scattered TSRA over mountains and locally in deserts after 18Z Tuesday. Bases around 9000 feet MSL with tops to 50000 feet. Any TSRA could produce lightning, strong up/downdrafts and associated vis reductions. Otherwise, SCT-BKN clouds mostly above 15000 feet MSL through Tuesday.

MARINE

No hazardous marine conditions are expected through Saturday.

BEACHES

High tides will exceed 7 feet this evening and again Wednesday evening between 800-930 PM, resulting in minor tidal overflow of low- lying areas along the beach. Surf will be low and is not expected to add much to the flooding potential. Check the Coastal Flood Advisory for details.

SKYWARN

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

SGX WATCHES/WARNINGS/ADVISORIES

CA...Coastal Flood Advisory until 11 PM PDT Wednesday for Orange County Coastal Areas-San Diego County Coastal Areas.

PZ...None.