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Serra Mesa Post Office, California Weather Forecast Discussion

800
FXUS66 KSGX 300426 CCA
AFDSGX

Area Forecast Discussion...CORRECTED National Weather Service San Diego CA 926 PM PDT Mon Sep 29 2025

.SYNOPSIS... Patchy drizzle possible in the coastal basin overnight with a deeper marine layer. High temperatures remain well below normal Tuesday. Gradual warming expected Wednesday through Saturday with a shallower marine layer. Weak offshore flow may develop Saturday morning. Cooler and cloudier conditions look to return for early next week.

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

Update: Currently, there is a major shortwave trough advancing into northern and central California. Although we are at the "tail end" of this system as it transitions just north of the region, it may still allow for some spotty areas of drizzle overnight and in the early morning hours, especially for the foothills of the inland areas. It will also allow for some increase gusty winds during the afternoon, as well as slightly cooler temperatures, and there could be a few passing mid to high level clouds during the daytime hours tomorrow. The marine layer should also fill in much broader and further inland as ceilings slightly lift. There could also be an actual shower or two developing over the mountains during the afternoon hours. The forecast beyond tomorrow shows some slight slight ridging that will help to gradual warm things towards the end of the week, which will be followed by another deepening trough over the region for this upcoming weekend, which may become cut off, and keep conditions cooler and unsettled going into the early part of next week.

(Previous discussion submitted at 156 PM):

A trough of low pressure will approach the west coast and begin to move inland this afternoon and overnight. This will increase onshore flow across the area, resulting in more widespread and persistent cloud coverage this evening and overnight. Some high resolution guidance is indicating the possibility of patchy drizzle in the coastal basin, so that has been added to tonight`s forecast. Additionally there will be a minor uptick in westerly winds over the mountains and deserts each afternoon and evening today and Tuesday. In addition to increased clouds and winds, there will be minor cooling across much of the area into Tuesday.

A few degrees of warming is expected for Wednesday and Thursday as the low departs to the east, allowing weak high pressure to build in from the south. Ensemble guidance begins to diverge Friday with the progression of an upper level trough that is expected to approach the US West Coast. There remains good consistency with the timing of the trough, but the amplitude of the trough varies quite a bit. Currently, around 60% of the ensemble space doesn`t dig as far south, resulting in less cooling, while the other 40% is more amplified and therefore cooler on Friday. The spread between the NBM 10th and 90th percentiles on Friday is around 8-10 degrees for inland areas.

Most guidance shows the trough moving off to the east on Saturday. This will allow for cooler air to settle into the Great Basin, setting up surface high pressure, and driving offshore flow across Southern California. Current guidance indicates the offshore will be mostly gradient driven, with no upper level support. The CW3E ensemble currently shows a 20% chance of a weak Santa Ana on Saturday. This will mainly generate warming conditions west of the mountains on Saturday. The offshore flow will be brief, as another low digs through the Pacific Northwest and somewhere into the Great Basin Sunday and Monday for more cooling and a much deeper marine layer. Given the inside track of this low, the system will be on the drier side. Both the GEFS and EPS ensembles are showing at least light precipitation west of the mountains sometime Mon-Tue, likely the result of the deep marine layer and stronger onshore flow. For now precipitation chances are below 15% for Monday.

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.AVIATION... 300330Z....Coasts/Valleys...Low clouds based 2000-3000 ft MSL are quickly developing over San Diego/Orange County coasts/valleys. Clouds should quickly expand to cover the coastal basin by 07-08Z. 30-40% chance for -DZ after 11z Tue, with greatest chances north of SD county. Cigs may intermittently drop below 2000 ft MSL in patchy -DZ. Scatter out 16-18z Tuesday morning. SKC through the afternoon with clouds struggling to redevelop Tuesday evening. Patchy coverage of low clouds for the coasts and western valleys after 06Z Wednesday.

Mountains/Deserts...VFR conditions through Tuesday. More mid and high level clouds will move through the region between 05-14z Tue.

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.MARINE... No hazardous marine conditions are expected through Friday.

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.BEACHES... South swell at 4-5 feet with a period of 12-13 seconds will continue to produce elevated surf and a high rip current risk through Tuesday. Surf of 4-7 feet with sets 8 feet expected on south facing beaches. Strong longshore currents will impact all beaches. Surf will slowly begin to decrease Tuesday with strong rip currents lingering into Thursday. More information can be found in the Beach Hazards Statement.

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

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.SGX WATCHES/WARNINGS/ADVISORIES... CA...Beach Hazards Statement through Tuesday evening for Orange County Coastal Areas-San Diego County Coastal Areas.

PZ...None.

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UPDATE...Stewey PUBLIC...CO AVIATION/MARINE/BEACHES...KW

NWS SGX Office Area Forecast Discussion

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