Area Forecast Discussion
Issued by NWS San Diego, CA

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000 FXUS66 KSGX 240439 AAA AFDSGX Area Forecast Discussion National Weather Service San Diego CA 938 PM PDT Sun Apr 23 2017 Updated aviation, marine, and beaches sections. .SYNOPSIS... Increasing onshore flow through Monday will bring cooling well inland, more nocturnal low clouds and fog as the marine layer builds, and strong westerly winds across the mountains and deserts. Fair, dry, and a little warmer midweek as the marine layer shrinks and onshore flow weakens. Then, windy and cooler again Thursday and Friday as a strong low pressure center develops over the Great Basin. Fair, with a warming trend next weekend as the wind lets up and high pressure rebuilds aloft. && .DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE... SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES... Satellite imagery midday showed the thicker high clouds had cleared the forecast area, with patchy low clouds over the coastal waters. Despite the strong April sun, temperatures at noon were running some 9 to 15 degrees F below values observed at noon yesterday. Westerly winds were stronger too, with peak gusts of 35 to 45 MPH reported at a few remote wind-prone sites in the mts. Surface pressure gradients continue to gain momentum at 6-7 MBS onshore KSAN to the deserts. Near term...The main weather concern over the next couple of days is wind. Westerly winds are increasing today and will be quite gusty over the mts/deserts this evening. A few gusts to 50 mph or more are possible in wind-prone mtn and desert foothill areas. Even stronger winds will develop late Mon through Mon night and should easily reach Advisory strength over the mts/deserts at times. Local high wind conditions are possible on the remote desert slopes Mon night into early Tue where peak gusts could exceed 65 MPH. Expect a Wind Advisory to be issued for these areas within 24 hours. It will be cooler Mon with more morning marine clouds, but max temps will still be within a few degrees of average, then warmer into Thu. The pattern... Strong high pressure aloft, centered over the EastPac along 135W, and a broad trough over the Rockies and Great Plains will continue a NW flow aloft along the West Coast this week. Shortwaves embedded in that flow will bring periods of enhanced westerly winds across SoCal, strongest over the mts/deserts and outer coastal waters late Mon, and possibly again late in the week. The latest GFS/ECMWF runs are coalescing around a solution that strongly digs the Rockies trough on Fri/Sat, cutting off a low pressure center over northern AZ on Sat. The GFS runs remain the most aggressive. Depending on how this plays out, we will see more strong onshore wind later this week, becoming northerly behind a cold front at some point late Fri into Sat. Besides a period of strong winds over the deserts/mts, it would be a little cooler than previously forecast inland late in the week. Any cold advection west of the mts would be mitigated by the downsloping, offshore flow. Nights should be cooler though, with drier, continental air in place and no cloud cover. && .AVIATION...
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240350Z...Coast/Valleys...Areas of low clouds currently impacting TAF sites in San Diego County with BKN-OVC cigs around 1500 ft MSL and tops to 2000 ft MSL. Low clouds will spread inland and northward, impacting KSNA by 07Z and KONT/KSBD by 10Z. Expect areas of vis reduced to 1-3SM where clouds and terrain intersect. By about 15Z, bases could lift to around 2000 ft MSL with tops to 2500 ft MSL. Low clouds and fog likely to clear to the coast by 18Z. Mountains/Deserts...West winds with gusts 25-45 kt will produce mod- stg up/downdrafts and LLWS over and east of the mountains through Monday.
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&& .MARINE...
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Periods of northwest winds with gusts of 25-30 kt and combined seas of 10-12 ft are possible in the outer coastal waters Monday evening through Friday evening. This would result in conditions hazardous to small craft through much of the week in the outer waters. Meanwhile for the inner waters, gusts of 20 kt and seas near 8 to 9 feet will be possible at times, but Small Craft Advisory conditions are not currently anticipated.
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&& .BEACHES...
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A southwest swell of 3 ft from 200 degrees at 15 seconds combined with elevated west swells will produce surf of 3-6 ft with occasional 7 ft sets through Monday. The above normal surf will also create conditions for strong rip currents and longshore currents. A Beach Hazards Statement remains in effect through Monday evening. A series of large northwest swells with periods of 7-14 seconds will maintain elevated to locally high surf and strong rip currents through most of the workweek.
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&& .SKYWARN... Skywarn activation is not requested. However weather spotters are encouraged to report significant weather conditions. && .SGX WATCHES/WARNINGS/ADVISORIES... CA...Beach Hazards Statement through Monday evening for Orange County Coastal Areas-San Diego County Coastal Areas. PZ...None. && $$ PUBLIC...JAD/JMB AVIATION/MARINE/BEACHES...PG

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