335
FXUS66 KSGX 190345
AFDSGX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN DIEGO CA
845 PM PDT SUN MAY 18 2025
SYNOPSIS
AREAS OF LOW CLOUDS TONIGHT WITH WEAK OFFSHORE FLOW BRIEFLY
DEVELOPING ALONG THE FOOTHILLS MONDAY MORNING. WARMER THIS WEEK
WITH THE PEAK OF THE HEAT EXPECTED TO BE WEDNESDAY CLOSER TO THE
COAST AND THURSDAY FOR INLAND AREAS. THE MARINE LAYER WILL BECOME
MUCH SHALLOWER THIS WEEK WITH PATCHIER COVERAGE NEAR THE COAST.
BRIEF COOLING OVER THE UPCOMING WEEKEND IS POSSIBLE.
DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...
EVENING UPDATE...
AREAS OF LOW CLOUDS REMAIN BANKED UP AGAINST THE MOUNTAINS AND
OVER THE INLAND VALLEYS THIS EVENING, WITH THE GREATEST COVERAGE
IN SAN DIEGO COUNTY. EXPECT THIS BATCH OF CLOUDS TO GRADUALLY
CLEAR OUT TONIGHT AS WEAK OFFSHORE FLOW DEVELOPS ALONG THE
MOUNTAINS AND FOOTHILLS. ALONG THE COAST, HI-RES GUIDANCE STILL
SUGGESTS THE DEVELOPMENT OF A WEAK COASTAL EDDY, WHICH WILL HELP
GENERATE A SEPARATE BATCH OF LOW CLOUDS ALONG THE COAST AND
POTENTIALLY INTO THE FAR WESTERN VALLEYS BY MONDAY MORNING.
OTHERWISE GUSTY WEST WINDS THROUGH THE PASSES AND LOCALLY INTO THE
DESERTS THIS EVENING WILL WEAKEN OVERNIGHT.
PREVIOUS DISCUSSION...
A WEAK RIDGE OF HIGH PRESSURE ALOFT WILL BUILD OVER THE US WEST
COAST. THE MARINE LAYER IS EXPECTED TO BECOME MUCH SHALLOWER THIS
WEEK WITH PATCHY COVERAGE ALONG THE COAST.
LATEST ENSEMBLE GUIDANCE IS INDICATING THE BUILDING RIDGE OF HIGH
PRESSURE WILL BE LESS AMPLIFIED THAN PREVIOUSLY SHOWN. THE WARMING
TREND IS STILL ON TRACK WITH THE PEAK WEDNESDAY AND THURSDAY, BUT
HIGH TEMPERATURES MAY BE LOWER THAN PREVIOUSLY THOUGHT. THAT BEING
SAID, HIGH TEMPERATURES ALONG THE COAST AND IN THE DESERTS ARE STILL
EXPECTED TO BE UP TO 10 DEGREES ABOVE AVERAGE WITH TEMPERATURES 10
TO 15 DEGREES ABOVE AVERAGE FOR THE VALLEYS. NBM CHANCES FOR
EXCEEDING 100 DEGREES IN THE INLAND EMPIRE WEDNESDAY AND THURSDAY
ARE 20 TO 30 PERCENT WITH A 10 PERCENT CHANCE IN THE SAN DIEGO
COUNTY VALLEYS. FOR INLAND ORANGE COUNTY CHANCES OF EXCEEDING 95
DEGREES ARE LESS THAN 10 PERCENT. EVEN THE 99TH PERCENTILE OF THE
ECMWF (ONLY ONE PERCENT OF ENSEMBLE MEMBERS WILL EXCEED THE LISTED
TEMPERATURE) KEEP HIGH TEMPERATURES WEST OF THE MOUNTAINS UNDER 100
DEGREES, ALTHOUGH IT DOES HAVE LOCATIONS LIKE RIVERSIDE REACHING THE
UPPER 90S. FOR THE LOW DESERT, CHANCES OF EXCEEDING 110 DEGREES HAVE
DECREASED, WITH CHANCES FOR WEDNESDAY NEAR ZERO AND CHANCES FOR
THURSDAY 5 TO 10 PERCENT. HEATRISK FOR THE VALLEYS IS STILL EXPECTED
TO BE MODERATE, WITH LOCALLY MAJOR HEATRISK IN THE LOW DESERT. THIS
IS LIKELY DUE TO ELEVATED LOW TEMPERATURES, WHICH WILL LIMIT
OVERNIGHT RELIEF FROM THE HEAT.
BY FRIDAY, ENSEMBLE SOLUTIONS BEGIN TO SHOW AN UPPER LEVEL LOW
PRESSURE SYSTEM MOVING INTO THE PACIFIC NORTHWEST. THERE REMAINS
UNCERTAINTY IN HOW FAR SOUTH THIS TROUGH WILL DIG OVER THE WEEKEND
BUT THERE IS GOOD AGREEMENT IN IT WEAKENING THE RIDGE. HOW MUCH THE
RIDGE WEAKENS WILL HAVE AN INFLUENCE ON HOW MUCH COOLING WE'LL SEE,
HOW WELL THE MARINE LAYER WILL REBOUND, AND IF THERE WILL BE ANY
INCREASE IN WINDS FOR THE MOUNTAINS AND DESERTS. CURRENT FORECAST
FOR NEXT WEEKEND FOLLOWS THE NBM WITH A GRADUAL COOLING (BUT
TEMPERATURES REMAINING SLIGHTLY ABOVE NORMAL) AND INCREASED CLOUD
COVERAGE ALONG THE COAST.
AVIATION
190345Z...COAST/VALLEYS...CLOUDS (BASED AROUND 3500-4500FT MSL)
CONTINUE TO PERSIST ACROSS THE COASTAL MOUNTAIN SLOPES AND ACROSS
THE INLAND VALLEYS OF SAN DIEGO COUNTY BUT THE COASTS AND INLAND
EMPIRE REMAIN CLEAR. WITH THE AID OF A WEAK COASTAL EDDY SPINNING UP
TONIGHT, EXPECTING CLOUDS TO DEVELOP NEAR THE COAST AFTER 09Z
MONDAY, BASED CLOSER TO 1200-2000FT MSL. THESE CLOUDS WILL BE
FOCUSED MORE ACROSS THE NEARSHORE WATERS AND IMMEDIATE COASTS, WITH A
60-75% CHANCE OF BRINING CIGS TO THE COASTAL TAF SITES. ANY CLOUDS
THAT FORM AND MOVE ASHORE WILL CLEAR BY 15-16Z MONDAY MORNING.
MOUNTAINS/DESERTS...MOSTLY CLEAR THROUGH MONDAY EVENING. WEST TO
NORTHWEST GUSTS OF 20-25KTS WILL GRADUALLY SUBSIDE THROUGH 06-09Z
THIS EVENING. WINDS TURN NORTH-NORTHEAST MONDAY AFTERNOON WITH A
WEAK OFFSHORE PUSH OCCURRING.
MARINE
NO HAZARDOUS MARINE CONDITIONS EXPECTED THROUGH SATURDAY.
SKYWARN
SKYWARN ACTIVATION IS NOT REQUESTED. HOWEVER WEATHER SPOTTERS ARE
ENCOURAGED TO REPORT SIGNIFICANT WEATHER CONDITIONS.
SGX WATCHES/WARNINGS/ADVISORIES
CA...NONE.
PZ...NONE.
PUBLIC...SS/CO
AVIATION/MARINE...MUNYAN
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