088
FOUS30 KWBC 211547
QPFERD
EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1147 AM EDT WED OCT 21 2026
DAY 1
VALID 16Z WED OCT 21 2026 - 12Z THU OCT 22 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
SOUTHERN CALIFORNIA AND THE FOUR CORNERS...
..16Z UPDATE
..SOUTHERN CALIFORNIA
LITTLE HAS CHANGED WITH THE OVERALL FLASH FLOODING RISK ACROSS
SOUTHERN CALIFORNIA. AN ALMOST NOMINAL SOUTHWARD SHIFT IN THE
GUIDANCE DID LEAD TO A MINOR ADJUSTMENT TO THE SLIGHT RISK TO
INCLUDE A BIT MORE OF THE SOUTHEASTERN SUBURBS OF L.A. INTO THE
SLIGHT. URBAN AREAS, BURN SCARS AND FLOOD PRONE CANYONS WEST OF
L.A. WILL HAVE THE GREATEST RISKS FOR FLASH FLOODING. AREAL
RAINFALL RATES ONGOING HAVE GENERALLY BEEN STAYING UNDER A HALF
INCH PER HOUR. HOWEVER, INSTABILITY SHOULD RAPIDLY INCREASE THIS
AFTERNOON TO BETWEEN 500 AND 1,000 J/KG OF MUCAPE. THIS WILL ALLOW
FOR MORE ELEVATED CONVECTION TO SPREAD INLAND, ESPECIALLY AS THE
PLUME OF RAIN CURRENTLY OUT TOWARDS SANTA BARBARA PROGRESSES
EASTWARD TOWARDS OXNARD, MALIBU, AND THE BROADER SANTA MONICA
MOUNTAINS REGION THIS AFTERNOON. WITH FEW CHANGES TO THE RAINFALL
FORECAST, FEW CHANGES WERE NEEDED TO THE FLOODING OUTLOOK.
..FOUR CORNERS
THE START OF A SIGNIFICANT RAINFALL EVENT FOR THE FOUR CORNERS
REGION WILL BEGIN TODAY, ESPECIALLY INTO UTAH AND ARIZONA, WITH THE
HEAVIEST RAINS EVENTUALLY SPREADING INTO SOUTHWEST COLORADO
TONIGHT. INSTABILITY WILL BE LIMITED INTO UTAH AND COLORADO TONIGHT
AS MUCH OF IT REMAINS SOUTH INTO ARIZONA, BUT THE MOSTLY STRATIFORM
RAIN THAT DOES IMPACT THESE AREAS WILL ACT TO PRIME THE SOILS IN
ADVANCE OF THE INCOMING UPPER LEVEL LOW-TROUGH THAT WILL PUSH EAST
INTO THE REGION INTO DAY 2/THURSDAY. MUCAPE VALUES THIS
AFTERNOON TO ARIZONA WILL EXCEED 1,000 J/KG ACROSS MUCH OF THE
SATE, WHILE IT REDUCES TO 500 J/KG OR LESS INTO SOUTHWESTERN UTAH.
FOR THIS REGION, EVEN INSTABILITY VALUES AT 500 J/KG COULD SUSTAIN
ELEVATED INSTABILITY CAPABLE OF ALLOWING THUNDERSTORMS THAT COULD
APPROACH 1 INCH/HOUR RAINFALL RATES. PORTIONS OF SOUTHERN UTAH HAVE
FFG VALUES AT 1/2 INCH PER HOUR. FOR ARIZONA, FFG VALUES ARE AROUND
1 INCH PER HOUR. WIDELY SCATTERED INSTANCES OF FLASH FLOODING ARE
LIKELY TODAY THROUGH TONIGHT.
WEGMAN
..PREVIOUS DISCUSSION
..SOUTHWEST
ON WEDNESDAY/WEDNESDAY NIGHT, A DEEP LAYER CYCLONE MOVES TO POINT
CONCEPTION AND SCOOPS UP MOISTURE FROM THE TROPICS, FLINGING IT
INTO THE SOUTHWEST. A BROADENING ZONE OF 300-700 KG/MS OF
INTEGRATED VAPOR TRANSPORT ENVELOPS THE SOUTHWEST. NEAR THE COLD
LOW ITSELF, MU CAPE OF ~500 J/KG AND PRECIPITABLE WATER VALUES OF
1-1.3" WITH UP TO 30 KTS OF ONSHORE FLOW AT 850 HPA IS FORECAST,
WHICH WOULD SUPPORT HOURLY AMOUNTS TO 2" WHERE CELLS MANAGE TO
TRAIN OR BACKBUILD WITHIN THE TERRAIN. THE ELEVATED SLIGHT RISK WAS
MAINTAINED FOR PORTIONS OF SOCAL, INCLUDING THE COASTAL RANGES
NORTH OF POINT CONCEPTION AND THE TRANSVERSE RANGES.
ACROSS PORTIONS OF THE SOUTHWEST, MU/ML CAPE IS FORECAST TO RISE TO
1000-3000 J/KG WITH LESSER AMOUNTS TO THE NORTH ACROSS THE GREAT
BASIN/ROCKIES, WITH PRECIPITABLE WATER VALUES RISING INTO 0.75-1.5"
RANGE. THE INGREDIENTS AVAILABLE ALONG WITH DETERMINISTIC GUIDANCE
QPF OUTPUT APPEAR SUFFICIENT TO MAINTAIN THE EXISTING SLIGHT RISK.
HOURLY RAIN AMOUNTS TO 2.5" WOULD BE POSSIBLE ACROSS PORTIONS OF
AZ, NM, AND THE FOUR CORNERS IN WIDELY SCATTERED TO SCATTERED
SPOTS DUE TO CELL TRAINING OR WHERE CELLS BACKBUILD OR MERGE.
URBAN AREAS, BURN SCARS, SLOT/BOX CANYONS, AND DRY WASHES/ARROYOS
WOULD BE THE MAIN AREAS OF CONCERN WITH THE HEAVY RAINFALL
ANTICIPATED.
ROTH
DAY 2
VALID 12Z THU OCT 22 2026 - 12Z FRI OCT 23 2026
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
SOUTHWEST COLORADO THURSDAY/THURSDAY NIGHT...
SOUTHWEST/FOUR CORNERS...
THE SLIGHT RISK AREA REMAINS FOR PARTS OF THE SOUTHWEST/FOUR
CORNERS ON THURSDAY AS A DEEP MID/UPPER LEVEL LOW AND ACCOMPANYING
AMPLIFIED UPPER TROUGH MOVES FROM CA INTO THE GREAT BASIN. A NEW
MODERATE RISK WAS RAISED IN CONJUNTION WITH THE GRAND JUNCTION
FORECAST OFFICE. HEAVY TO POTENTIALLY EXCESSIVE RAINFALL REMAINS A
CONCERN GIVEN THE MAGNITUDE OF THE TROPICAL MOISTURE BEING
ADVECTED INTO THE REGION, WITH INTEGRATED VAPOR TRANSPORT OF UP TO
600 KG/MS. DEEP SOUTHERLY FLOW AHEAD OF THE APPROACHING TROUGH WILL
ADVECT AN AIRMASS WITH PRECIPITABLE WATER VALUES OF 1-1.5" INTO
THE REGION, VERY HIGH BY OCTOBER STANDARDS. FREEZING LEVELS WILL BE
QUITE HIGH IN SOUTHWEST CO, CLOSE TO THE MOUNTAIN PEAKS. MU CAPE
IS FORECAST TO BE 1000-2000 J/KG NEAR THE AZ/NM BORDER DURING
DAYTIME HEATING. THE INGREDIENTS IN PLACE STILL SUGGEST THAT AT
LEAST LOCALLY SIGNIFICANT IMPACTS ARE A POSSIBILITY. HOURLY RAIN
AMOUNTS TO 2.5" ARE POSSIBLE GIVEN THE INGREDIENTS AVAILABLE. THIS
IS A HIGHER END SLIGHT RISK FOR PORTIONS OF SOUTHEAST AZ AND
SOUTHWEST CO. URBAN AREAS, BURN SCARS, SLOT/BOX CANYONS, AND DRY
WASHES/ARROYOS WOULD BE THE MAIN AREAS OF CONCERN WITH THE HEAVY
RAINFALL ANTICIPATED.
SOUTH FLORIDA...
THE 00Z HREF/REFS ARE EITHER VERY CLOSE, OR JUST ASHORE, WITH A
HEAVY RAIN SIGNAL ACROSS PORTIONS OF THE GOLD COAST AS MOISTURE
RETURNS FROM THE ATLANTIC/BAHAMAS WITH INSTABILITY FROM THE GULF
STREAM. THE CONCERN WOULD BE ANY CONVECTIVE BANDING THAT TRIES TO
DEVELOP IN THE ANTIPARALLEL FLOW WITH HEIGHT SEEN IN THAT AREA. THE
THREAT IS CONDITIONAL, BUT HOURLY AMOUNTS TO 2" ARE POSSIBLE SHOULD
ANY BANDS SET UP IN THIS ENVIRONMENT, WHICH WOULD BE PROBLEMATIC IN
URBAN AREAS.
ROTH
DAY 3
VALID 12Z FRI OCT 23 2026 - 12Z SAT OCT 24 2026
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL IN AND NEAR THE
SOUTHERN HIGH PLAINS...
SOUTHERN HIGH PLAINS...
THE GUIDANCE SHOWS A MODEST HEAVY RAIN SIGNAL IN AND AROUND THE
SOUTHERN HIGH PLAINS AND FAR WEST TX AS GULF INFLOW MOVES UPSLOPE
INTO THE AREA AND PACIFIC MOISTURE FLOWS IN ALOFT AHEAD OF AN
INCOMING UPPER LEVEL LOW/TROUGH AND INTERGRATED VAPOR TRANSPORT HAS
VALUES UP TO 600 KG/MS. PRECIPITABLE WATER VALUES OF 1-1.25" ARE
FORECAST THERE, WHICH ARE HIGH FOR THE REGION AND FOR OCTOBER. MU
CAPE RISES TO ~1000 J/KG. THE FLOW SHOULD BE FAIRLY UNIDIRECTIONAL
FROM THE SOUTH, SO CELL TRAINING IS POSSIBLE SHOULD ENOUGH
CONVECTIVE COVERAGE DEVELOP. THE GUIDANCE QPF REMAINS MODEST, SO
LEFT THE MARGINAL RISK AS IS.
SOUTH FLORIDA...
THE GLOBAL GUIDANCE IS EITHER VERY CLOSE, OR JUST ASHORE, WITH A
HEAVY RAIN SIGNAL ACROSS PORTIONS OF THE GOLD COAST AS MOISTURE
RETURNS FROM THE ATLANTIC/BAHAMAS WITH INSTABILITY FROM THE GULF
STREAM. THE CONCERN WOULD BE ANY CONVECTIVE BANDING THAT TRIES TO
DEVELOP IN THE ANTIPARALLEL FLOW WITH HEIGHT SEEN IN THAT AREA. THE
THREAT IS CONDITIONAL, BUT HOURLY AMOUNTS TO 2" ARE POSSIBLE
SHOULD ANY BANDS SET UP IN THIS ENVIRONMENT, WHICH WOULD BE
PROBLEMATIC IN URBAN AREAS.
ROTH
DAY 4 AND DAY 5
VALID 12Z SAT OCT 24 2026 - 12Z MON OCT 26 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE WESTERN/SOUTHWEST U.S. AND SOUTH FLORIDA...
THE CURRENT SYNOPTIC PATTERN FOR BOTH THE WESTER CONUS AND FOR SOUTHEAST
FL COAST MAINTAIN SIGNALS FOR HEAVY TO POTENTIALLY EXCESSIVE
RAINFALL THIS WEEKEND. IN EACH OUTLOOK AREA...ONLY SMALL
ADJUSTMENTS WERE MADE BASED ON LATEST DETERMINISTIC AND ENSEMBLE
GUIDANCE ON SATURDAY.
GUIDANCE FOR FLORIDA REMAINED FOCUSED ON IMMEDIATE COASTAL AREAS
AS THE PRIMARY POINT OF EMPHASIS, CORRELATING WITH THE URBAN
CORRIDOR THAT TENDS TO BE MORE PRONE TO FLASH FLOODING GIVEN THE
AMOUNT OF IMPERVIOUS SURFACES.
THE POTENTIAL FOR EXCESSIVE RAINFALL FARTHER WEST TAKES SHAPE IN
THE SOUTHWEST U.S. GENERALLY TO THE LEE OF THE SOUTHERN ROCKIES ON
SATURDAY THAT PROPAGATES NORTH AND EAST-WARD ON SUNDAY. THE THREAT
WILL BE TIED TO THE ANOMALOUS MOISTURE ADVECTION INTO THE AREA
WITH THE BEST ASCENT LOCATED WITHIN THE DOWNSTREAM DIFFLUENT FLOW
ALOFT. SPAGHETTI PLOTS OF QPF FROM THE EC ENSEMBLES AND THE GEFS
ON SATURDAY STILL PRIMARILY DEPICT AREAS ALONG AND EAST OF THE
CONTINENTAL DIVIDE AS THE MAIN AREAS OF CONCERN WITH SOME SIGNALS
FOR LOCALLY 2+ INCHES. CONDITIONS SUPPORTING HEAVY TO EXCESSIVE
RAINFALL PERSIST OVER PORTIONS OF THE WESTERN HIGH PLAINS BUT THE
NORTHERN EXTENT OF THE PRECIPITATION SHIELD WILL BE FIGHTING A FEED
OF COOLER AND MORE STABLE AIRMASS BEING FED INTO THE REGION BY
BUILDING SURFACE HIGH PRESSURE.
BANN
DAY 1 THREAT AREA: HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/94EPOINTS.TXT
DAY 2 THREAT AREA: HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/98EPOINTS.TXT
DAY 3 THREAT AREA: HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/99EPOINTS.TXT
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