Area Forecast Discussion
Issued by NWS San Diego, CA

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193 FXUS66 KSGX 271552 AFDSGX AREA FORECAST DISCUSSION National Weather Service San Diego CA 852 AM PDT MON JUN 27 2016 .SYNOPSIS... High pressure aloft will continue hot weather for inland areas through Wednesday followed by slow cooling into next weekend as high pressure aloft weakens and a weak trough of low pressure develops over the Pacific Northwest. Mid and high level moisture associated with a disturbance moving through the region will bring a slight chance of afternoon and early evening thunderstorms with gusty winds through Wednesday, mainly near the mountains. The marine layer and onshore flow will keep coastal high temperatures cooler with areas of night and morning low clouds and fog. && .DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE... SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES... Visible and radar imagery is showing an easterly upper level wave moving through the region, bringing mid and high level clouds and some light radar returns which are likely producing only virga or sprinkles. The 12Z Miramar sounding shows precipitable water having increased to 1.48 inches, although the majority of the moisture appears to be above 700 mb, with a dry layer noted below 700 mb until you reach the 10 deg C marine layer inversion near 1000 ft MSL. It is due to the dry layer below 700 mb that most of the precipitation may evaporate before reaching the ground, which is why most of it will appear as virga or maybe some sprinkles. These clouds will continue to move through Southern California through the day, although the moisture field from the NAM12 shows clearing skies intruding in from the south in the afternoon. The amount of cloud cover we have today, and which locations receive more sunlight than others, will play a factor in not only how hot we get today, but also the amount of day-time heating induced instability. Due to the mid and high clouds, temperatures may be reduced slightly compared to yesterday. However, with some higher surface dewpoints in the area (low 60s shown currently over most coast, valley and desert areas), it may feel more muggy and uncomfortable. Despite the increase in moisture and the wave moving overhead, the elevated instability parameters shown by the forecast models for this afternoon are not impressive. Also, the strong easterly flow is not conducive for thunderstorms that develop over the mountains to maintain their structure. Thus, the latest HRRR run doesn`t show much shower or thunderstorm activity this afternoon. Nevertheless, a slight chance for thunderstorms remains in the forecast today for the mountains, with some activity possibly being blown west into the valleys. Meanwhile, marine layer stratus was able to develop over coastal areas today. Stratus should be able to clear out of inland areas by late morning, although the decently strong inversion could keep some low clouds hanging around at the coast. Models show drier air intruding into Southern California Tuesday as flow turns southerly in the wake of the easterly wave, resulting in less of a chance for afternoon mountain thunderstorms. However, the clearer skies and strong ridge over the four corners region should result in hotter conditions, with highs 5-10 degrees above normal. Continued hot for inland areas on Wednesday, with 850 mb temperatures in the deserts reaching 27-29 deg C, and near 26-27 deg C in the valleys. A Heat Advisory continues for the deserts, mountains and valleys through Wednesday. Despite the ridge weakening on Thursday, looks like the 850 mb temperatures do not decrease much, if at all, so hot conditions may continue. Cooler Friday into the weekend as a trough moves into the Pacific Northwest, with a deepening of the marine layer as well, resulting in night and morning stratus into the coast and portions of the valleys. && .AVIATION... 271530Z...Coast/Valleys...Areas of sct-bkn clouds based 800-1200 feet MSL with tops around 1800 ft MSL will burn back to out over the coastal waters. Local vis 2-4 miles along higher coastal terrain/mesas. After 28/06z this evening low clouds bkn-ovc based 800-1200 feet MSL with tops around 1800 ft MSL will push inland about 15 miles with local visibilities 2-4 miles along higher coastal terrain/mesas. There is a moderate risk that the coastal low clouds will again be patchy so the confidence is low on the timing and duration of the bkn low clouds at KSAN tonight. Mountains and Deserts...Sct-bkn clouds based around 10000 feet MSL with unrestricted visibilities. This afternoon between 27/19Z and 28/00Z isolated SHRA/TSRA with cloud bases 9000 ft MSL and tops to 35000 ft MSL will be possible over the mountains. && .MARINE... 830 am...No hazardous marine weather is expected through Friday. && .BEACHES... 830 am...The long period south swell will continue to decrease, but 3-5 foot surf will continue on the southwest facing beaches today. The surf will lower to 2 to 4 feet Tuesday. Another long-period south swell from 200 degrees will bring 4-7 foot surf to southwest facing beaches late Wednesday and Thursday along with dangerous rip currents. && .FIRE WEATHER... Slight chance of dry lightning over the mountains and portions of the valleys this afternoon and evening, as an easterly wave moves through the region. Drying will take place Tuesday through Wednesday, with maybe just enough moisture leftover on Tuesday to produce isolated afternoon thunderstorms over the mountains. && .SKYWARN... Skywarn activation is not requested. However weather spotters are encouraged to report significant weather conditions. && .SGX Watches/Warnings/Advisories... CA...Heat Advisory until 8 PM PDT Wednesday for Apple and Lucerne Valleys-Coachella Valley-Riverside County Mountains-San Bernardino County Mountains-San Bernardino and Riverside County Valleys-The Inland Empire-San Diego County Deserts- San Diego County Mountains-San Gorgonio Pass Near Banning. PZ...None. && $$ PUBLIC/FIRE WEATHER...Harrison AVIATION/MARINE/BEACHES...Small is the U.S. government's official web portal to all federal, state and local government web resources and services.