305
FOUS30 KWBC 212023
QPFERD
EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
423 PM 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...
..2030Z UPDATE
..SOUTHWEST
ONCE AGAIN NO SIGNIFICANT CHANGES WERE MADE AS FAR AS THE BROAD
SCALE FORECAST. HOWEVER, A HIGHER-END MODERATE RISK WAS INTRODUCED
FOR MOST OF THE MODERATE RISK AREA IN SOUTHWEST COLORADO, INCLUDING
THE SAN JUAN MOUNTAINS. IN ADDITION TO A NOMINAL INCREASE IN THE
FORECAST RAINFALL FOR THAT AREA, ANALYSIS OF THE LATEST 12Z RUNS OF
THE GUIDANCE SHOW THERE ARE NUMEROUS CONCERNING THRESHOLDS THAT
WILL BE THREATENED WITH THIS UPCOMING EVENT. THESE THRESHOLDS
INCLUDE:
1)AN OVER 90% CHANCE OF 5 INCHES OR MORE OF RAIN IN 24 HOURS TO
OCCUR 2) SIMILAR CHANCES OF SEEING 100 YEAR ANNUAL-EXCEEDENCE-
INTERVALS MET 3) 1-2 DAY RAINFALL RECORDS IN AREAS SUCH AS DURANGO
AND TELLURIDE BROKEN 4) PWATS OF NEAR 1 INCH AMOUNTING TO 4-5 SIGMA
ABOVE NORMAL 5) INSTABILITY BETWEEN 500-1,000 J/KG, WHICH FOR MOST
AREAS IS NOT A LOT, BUT FOR THIS AREA OF THE SOUTHWEST, AMOUNTS TO
ENOUGH TO SUPPORT PERSISTENT CONVECTION 6)A SNOWMELT COMPONENT WITH
MUCH OF THE EARLY SEASON SNOWPACK ON THE SUMMITS OF THE SAN JUANS
LIKELY MELTING AS SNOW LEVELS RISE TO 12,000 TO 13,000 FT AND 7) A
CLIMATOLOGICAL COMPONENT SUCH THAT THE LAST TIME THE DURANGO AREA
SAW 4-5 INCHES OF RAIN IN 24 HOURS, MAJOR RIVER FLOODING RESULTED,
AND THERE WAS NO SNOWMELT TO ASSIST IN THE LAST EVENT.
ALL OF THAT IS TO SAY THAT THE FLOODING CONCERNS CONTINUE TO
INCREASE, AND THERE IS SOME POTENTIAL THAT A HIGH RISK UPGRADE MAY
BE NEEDED, ESPECIALLY IF RAINFALL FORECASTS CONTINUE TO INCREASE
AND CAMS SUPPORT CONTINUED ROUNDS OF CONVECTION.
ELSEWHERE ACROSS THE SOUTHWEST, SIGNIFICANT RAINS WILL ALSO IMPACT
WESTERN NEW MEXICO, THE MOGOLLON RIM AREA OF ARIZONA, AND MUCH OF
THE NATIONAL PARKS AND SLOT CANYONS FROM SOUTHERN UTAH NORTH AND
EAST TO THE UINTA MOUNTAINS IN THE NORTH. A LARGE SLIGHT RISK AREA
REMAINS FOR MUCH OF THE REST OF THE 4 CORNERS REGION, WITH A HIGHER
END SLIGHT FOR THE SOUTHERN TIER OF UTAH AS WELL AS THE MOGOLLON
RIM OF EAST-CENTRAL ARIZONA.
LINGERING RAINFALL AS THE "BOWLING BALL" LOW IN THE WEST CONTINUES
PROGRESSING EAST INTO SOUTHERN CALIFORNIA COULD CONTINUE ANY
FLOODING THAT OCCURS THIS AFTERNOON INTO TONIGHT. NEARLY ALL OF THE
RAIN WITH THIS EVENT FOR SOCAL WILL BE OVER BY THE START OF THE
PERIOD, SO A MARGINAL RISK WAS MAINTAINED.
..SOUTHEAST FLORIDA
ONSHORE FLOW INTO THE URBAN CORRIDOR FROM WEST PALM BEACH DOWN TO
MIAMI MAY ALLOW ROBUST CONVECTION OCCURRING OFF THE COAST OVER THE
GULF STREAM TO TRY TO MOVE INTO COASTAL COMMUNITIES. THESE
OCCURRENCES WILL BE FEW AND FAR BETWEEN. THE MARGINAL RISK WAS
MAINTAINED DUE TO THE URBAN INFLUENCE POTENTIALLY ALLOWING FOR AN
ISOLATED INSTANCE OF FLASH FLOODING.
WEGMAN
..PREVIOUS DISCUSSION
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 CONJUNCTION 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 SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE SACRAMENTO
MOUNTAINS SOUTH THROUGH THE EL PASO METRO...
..2030Z UPDATE
A SLIGHT RISK UPDATE WAS INTRODUCED FOR THE SACRAMENTO MOUNTAINS
SOUTH THROUGH THE EL PASO METRO. PERSISTENT RAINFALL WITH AFTERNOON
INSTABILITY VALUES BETWEEN 500 AND 1,000 J/KG WILL SUPPORT
SCATTERED CONVECTION CAPABLE OF SUPPORTING UP TO 1.5 IN/HOUR
RAINFALL RATES. THE BURN SCARS/TOPOGRAPHY OF THE SACRAMENTO
MOUNTAINS AND THE URBAN FACTOR IN THE EL PASO METRO WILL BOTH LOWER
THE THRESHOLDS NEEDED TO PRODUCE FLASH FLOODING. THIS UPGRADE WAS
COORDINATED WITH THE EPZ/EL PASO, TX FORECAST OFFICE.
..SOUTH FLORIDA
FEW CHANGES WERE NEEDED ONCE AGAIN HIGHLIGHTING THE URBAN I-95
CORRIDOR FROM WEST PALM THROUGH MIAMI. CONDITIONS APPEAR A BIT MORE
FAVORABLE FOR HEAVY RAINFALL IN THIS AREA ON FRIDAY AS COMPARED
WITH THURSDAY, BUT EXPECTED FLASH FLOODING IS LIKELY TO REMAIN
ISOLATED.
WEGMAN
..PREVIOUS DISCUSSION
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 INTEGRATED 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...
20Z UPDATE: THE GENERAL FORECAST WITH SLGT RISKS ACROSS THE
SOUTHWEST U.S. AND SOUTHEAST FL ON D4 MAINTAIN RELEVANT CONTINUITY
AS THE OVERALL SETUP REMAINS CONSISTENT WITHIN BOTH ENSEMBLE MEANS
AND GLOBAL DETERMINISTIC OUTPUTS. A MINOR ADJUSTMENT WEST WITH THE
SLGT/MRGL RISKS ACROSS THE SOUTHWEST TO SOUTHERN HIGH PLAINS WAS
MADE AS THE CONSENSUS FOR A BIT OF A SLOWER 500MB EVOLUTION WAS
EVIDENT IN THE 12Z SUITE WHICH CORRELATED WITH THE HEAVIER PRECIP
FOOTPRINT SHIFTING SLIGHTLY FURTHER WEST THAN THE ORIGINAL
FORECAST. THIS WILL STILL HAVE LIMITED BEARING ON THE EXPECTED
FLASH FLOOD PROSPECTS AS MUCH OF THE SAME AREAS FROM PRIOR FORECAST
ISSUANCE WILL BE IMPACTED. THE ONE AREA WHERE THIS DID SHIFT BACK
WAS OVER THE TX PANHANDLE WHERE THE EASTERN FRINGES OF THE SLGT AND
MRGL RISK WERE ADJUSTED TO REFLECT THE DIFFERENCES IN THE TIMING OF
THE ULL EJECTION. SOUTHEASTERN FL HEAVY RAIN THREAT REMAINS
PREVALENT WITH THE MEAN QPF OUTPUT STILL FAVORING THE IMMEDIATE
SOUTHEAST FL COASTAL PLAIN, OVERLAPPING THE URBAN CENTERS FROM PORT
ST. LUCIE TO MIAMI. TOTALS BETWEEN 1-3" ARE FORECAST IN THIS ZONE
FOR AN AREAL AVERAGE, HOWEVER THIS SETUP WILL BE CONDUCIVE FOR
MAXIMA CLOSER TO 4-6", PENDING THE POSITIONING OF EXPECTED HEAVIER
CONVECTION ANCHORING NEAR THE COAST AS EASTERLY FLOW WILL ALLOW
FOR ENHANCED LOW-LEVEL CONVERGENCE INTO THE COAST. THIS WAS
SUFFICIENT FOR A CONTINUATION OF THE SLGT RISK WITH ONLY MINOR
ADJUSTMENTS ON THE WESTERN EDGE OF THE RISK.
D5 REMAINS CONSISTENT FOR BOTH THE SOUTHERN AND CENTRAL HIGH
PLAINS, AS WELL AS THE SOUTHEASTERN FL COAST. THE LATTER IS MORE OF
A CONTINUED APPROACH OF HEAVY RAIN POTENTIAL INTO THE COASTAL AREAS
OF SOUTHEAST FL WHICH IS DEPICTED IN SOME OF THE GUIDANCE. THAT
SAID, SOME MODELS ARE LESS ENTHUSIASTIC FOR THIS TIME FRAME
COMPARATIVELY WITH A SMALLER FOOTPRINT FOR HEAVY RAIN POTENTIAL
WHICH COULD ALLOW FOR EITHER A SMALLER SLGT RISK IN THE FUTURE, OR
A DOWNGRADE TO A STANDARD MRGL RISK. THIS WILL BE SOMETHING TO
MONITOR OVER TIME, BUT WITH THE SIGNAL STILL PRESENT IN ENOUGH
GUIDANCE AND ENSEMBLE MEANS, DECIDED TO KEEP THE SLGT RISK AND WILL
ASSESS MORE IN THE FUTURE.
THE HIGH PLAINS WILL SEE THE NEXT HEAVY RAIN DISTRIBUTION FROM OUR
EVOLVING MID AND UPPER LEVEL PATTERN OUT WEST COME THE SECOND HALF
OF THE WEEKEND. THIS SIGNAL IS PRETTY LOCKED IN AMONG GUIDANCE
WHICH HAS PRODUCED SOME RESPECTABLE QPF TOTALS WHEN ASSESSING THE
MEANS, AND ESPECIALLY THE 75/90TH PERCENTILE OUTPUTS OF THE BOTH
THE NATIONAL BLEND AND ECENS/GEFS, RESPECTIVELY. THE CONSENSUS FOR
RAINFALL >2" IS POSITIONED OVER SOUTHEAST CO DOWN THROUGH THE TX
PANHANDLE AND ADJACENT NORTHEASTERN NM HIGH PLAINS. THIS SETUP IS
TEXTBOOK FOR HEAVY RAIN PROSPECTS IN THIS PARTICULAR ZONE AS THE
DISTURBANCE OVER THE FOUR CORNERS FINALLY OPENS UP MORE AND EJECTS
INTO THE CENTRAL ROCKIES, AND EVENTUALLY THE PLAINS THE END OF THE
FORECAST CYCLE AND BEYOND. THE SLGT WAS MORE THAN SUFFICIENT FOR
THE D5 PERIOD ACROSS THE AREA WITH A POTENTIAL FOR A HIGHER RISK,
PENDING THE EVENTUAL EVOLUTION OF THE PATTERN UPSTREAM.
KLEEBAUER
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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