519
FOUS30 KWBC 180059
QPFERD
EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
759 PM EST MON NOV 17 2025
DAY 1
VALID 01Z TUE NOV 18 2025 - 12Z TUE NOV 18 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF
SOUTHERN CALIFORNIA...
..TRANSVERSE RANGES OF SOUTHERN CALIFORNIA
A SLIGHT RISK REMAINS, THOUGH THE WESTERN END HAS BEEN CUT BACK
GIVEN THE CURRENT PLACEMENT OF A PACIFIC COLD FRONT JUST WEST OF
LOS ANGELES. THE PLUME OF MAX IVT (250 KG/M/S FOR INLAND LOCATIONS
TO NEAR 300 KG/M/S AT THE COAST) EXTENDED AHEAD OF A COLD FRONT
INTO LOS ANGELES COUNTY AT 00Z WITH A RAINFALL HISTORY OF 0.25 TO
0.50 INCHES IN 15 MINUTES WITHIN THE UPSLOPE REGIONS OF THE
TRANSVERSE RANGES DATING BACK TO EARLIER THIS AFTERNOON. THE IVT
PLUME IS FORECAST TO WEAKEN OVERNIGHT AS THE COLD FRONT CONTINUES
TO SINK SOUTH AND EAST, CLEARING SAN DIEGO COUNTY BETWEEN 06-09Z,
WITH WEAKENING AND VEERING OF LOW LEVEL FLOW. A NARROW WINDOW OF
TIME REMAINS WHERE HOURLY RAINFALL OF 0.5 TO 1.0 INCHES WILL BE
POSSIBLE (SHOULD BRIEF TRAINING DEVELOP DESPITE THE OVERALL
PROGRESSIVE NATURE OF THE FRONT), BUT THIS THREAT LOOKS TO DIMINISH
WITH TIME OVERNIGHT. LOCALIZED/SPOTTY ADDITIONAL TOTALS UP TO 1.5
INCHES WILL BE POSSIBLE.
..CENTRAL CALIFORNIA
AS THE MID-LEVEL CLOSED LOW SLOWLY PARALLELS THE CENTRAL CALIFORNIA
COAST TOWARD THE SOUTH, STEEP MID-LEVEL LAPSE RATES OF ABOUT 7-8
C/KM WILL SUPPORT WEAK INSTABILITY UP TO A COUPLE OF HUNDRED J/KG.
MODEST PWS OF 0.6 TO 0.8 INCHES AND ONSHORE FLOW MAY SUPPORT SOME
BRIEF BURSTS OF HEAVY RAIN FROM SHOWERS/THUNDERSTORMS ALONG THE
COAST. FOR INLAND LOCATIONS, SLOW MOVING SHOWERS/THUNDERSTORMS WILL
LINGER FOR ANOTHER FEW HOURS BEFORE THE LOSS OF SURFACE HEATING
REDUCES INSTABILITY AND AVAILABLE FUEL TO SUPPORT HIGHER RAINFALL
RATES.
THE MAIN CONCERN FOR ANY ISOLATED FLASH FLOODING ACROSS CALIFORNIA
STEMS FROM VERY WET ANTECEDENT CONDITIONS GIVEN THE PAST FEW DAYS
OF HEAVY RAINFALL AND AREAS OF FLOODING.
..LOWER COLORADO RIVER VALLEY INTO WEST-CENTRAL ARIZONA
AS THE CLOSED LOW ALONG THE WEST COAST EDGES CLOSER TO THE DESERT
SOUTHWEST AND HEIGHT FALLS BEGIN TO OVERSPREAD THE REGION, LOW
LEVEL MOISTURE TRANSPORT FROM THE SOUTH WILL INCREASE AND EXPAND
WEAK INSTABILITY FROM EASTERN CALIFORNIA INTO SOUTHERN NEVADA AND
WESTERN ARIZONA. THE LEFT-EXIT REGION OF A 90-100 KT JET STREAK
MAY AID WITH ASCENT LATER IN THE PERIOD OVER THE LOWER COLORADO
RIVER VALLEY. SHOWERS AND EMBEDDED THUNDERSTORMS ARE EXPECTED TO
DEVELOP AND EXPAND IN COVERAGE THROUGH THE 06-12Z PERIOD.
UNIDIRECTIONAL SOUTHERLY WINDS SHOULD PROMOTE REPEATING AND
POSSIBLE TRAINING WITH PEAK HOURLY RAINFALL NEAR 1 INCH ALONG WITH
THE POTENTIAL FOR ISOLATED FLASH FLOODING WITH THE THREAT EXTENDING
INTO THE D2 ERO PERIOD / 12Z TUESDAY.
OTTO
DAY 2
VALID 12Z TUE NOV 18 2025 - 12Z WED NOV 19 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
WEST CENTRAL ARIZONA...
20Z UPDATE:
UPGRADED TO A SLIGHT RISK ACROSS PORTIONS OF WEST CENTRAL AZ WITH
THIS UPDATE. STRONG FORCING JUST AHEAD OF THE UPPER LOW WILL
SUPPORT REPEATED ROUNDS OF SHOWERS AND EMBEDDED HEAVIER CONVECTIVE
ELEMENTS WITHIN THE DEEP LAYERED SOUTHERLY FLOW. NOT A LOT OF
INSTABILITY FORECAST, BUT VALUES UPWARDS OF 500-1000 J/KG SHOULD
BE ENOUGH TO SUPPORT LOCALLY HEAVIER RAINFALL RATES...OVER 0.5"/HR
BUT GENERALLY UNDER 1". NONETHELESS THE REPEATED ROUNDS AND SOME
UPSLOPE COMPONENT SHOULD RESULT IN RAINFALL LOCALLY EXCEEDING
2-3" OVER THE DAY. HREF NEIGHBORHOOD PROBABILITIES OF EXCEEDING 2"
ARE 50-70% AND 3" ARE 40-50%. THIS RAINFALL SHOULD BE ENOUGH TO
RESULT IN ISOLATED TO SCATTERED FLASH FLOOD IMPACTS, ESPECIALLY
OVER ANY MORE SENSITIVE AREAS.
WE ALSO ADDED A SMALL MARGINAL RISK CENTERED OVER KY. A LOW MOVING
ACROSS THE OH VALLEY WILL PLACE KY IN THE WARM SECTOR, WITH
INSTABILITY FORECAST TO INCREASE OVER 1000 J/KG BY TUESDAY
AFTERNOON. AN INITIAL BATCH OF SHOWERS SHOULD MOVE ACROSS DURING
THE MORNING WITH THE WARM FRONT, WITH ANOTHER MORE ROBUST
CONVECTIVE ROUND EXPECTED DURING THE AFTERNOON AND EVENING AS
INSTABILITY INCREASES AHEAD OF THE COLD FRONT. BOTH THE HREF AND
REFS INDICATE 1"/HR NEIGHBORHOOD PROBABILITIES OF 20-40% AND THE
HREF NEIGHBORHOOD PROBABILITIES OF EXCEEDING 3" DURING THE PERIOD
ARE LOCALLY OVER 15%. IT'S A PRETTY MARGINAL SITUATION, BUT GIVEN
THE DEGREE OF MOISTURE TRANSPORT AND FORECAST UPTICK IN
INSTABILITY IT DOES SEEM LIKE AN EVENT THAT COULD LOCALLY EXCEED
MODEL QPF FORECASTS...THUS CAN NOT RULE OUT SOME ISOLATED, MAINLY
URBAN, FLASH FLOOD CONCERNS.
CHENARD
..PREVIOUS DISCUSSION
MAINTAINED THE PREVIOUS;Y ISSUED MARGINAL RISK AREA OVER PORTIONS
OF ARIZONA WITH LITTLE NEED FOR MORE THAN COSMETIC CHANGES IN
DEFERENCE TO THE MODEL GUIDANCE. A CLOSED LOW WITHIN A BROADER
TROUGH DIGS SOUTHWARD ALONG THE THE WEST COAST OF THE US AND THEN
STARTS TO MOVE INLAND LATE. THE OVERALL SYNOPTIC PATTERN SEEMS TO
BE HANDLED COMPARABLY BY THE ECMWF/GFS AND UK MODELS FOR MUCH OF
DAY 2 BUT SOME OF THE SUBTLE DIFFERENCES IN THE DETAILS RESULT IN
NON-NEGLIGIBLE DIFFERENCES IN QPF AND THE ATTENDANT RISK OF
EXCESSIVE RAINFALL.
PRECIPITABLE WATER VALUES GET NEAR 2 STANDARD DEVIATIONS ABOVE NORMAL
FOR THIS TIME OF YEAR IN SOUTHERN-WESTERN AZ AND MOST-UNSTABLE
CAPE VALUES CLIMB ABOVE 500 J/KG SOUTH OF THE MOGOLLON RIM AND INTO
THE DESERTS AND PHOENIX METRO. THIS WILL MAINTAIN THE RISK OF
ELEVATED (0.50"+) HOURLY RAINFALL RATES TUE-TUE NIGHT AS NOTED FROM
THE RRFS EXCEEDANCE PROBABILITIES (THROUGH 00Z WED), WHILE ALSO
IMPLIED FROM THE 16/12Z RRFS 3-HOURLY QPFS. MUCH LIKE THE MODELS
FROM SUNDAY AFTERNOON...THE 24HR QPF GUIDANCE FROM THE 17/00Z MODEL
CYCLE HAS COME UP. AT THIS POINT THERE IS STILL NOT ENOUGH
CONFIDENCE FROM A DURATION PERSPECTIVE TO UPGRADE TO A SLIGHT RISK
OVER THE LOWER TERRAIN SOUTH OF THE MOGOLLON RIM, WHICH WOULD
CONCEIVABLY INCLUDE THE PHOENIX AREA, BUT THE POTENTIAL TO DO SO
WILL EXIST OVER THE NEXT DAY OR SO. THE MACHINE-LEARNING FIRST
GUESS FIELDS ALSO HIGHLIGHT PART OF THE MARGINAL RISK AREA WITH
SOME SPOTTY 5 TO 10 PERCENT OF EXCEEDING FLASH FLOOD GUIDANCE. AT
THIS POINT...THE MACHINE-LEARNING COVERS ONLY THE FIRST HALF OF DAY
2...SO THE UPCOMING DAY SHIFT WILL HAVE A COMPLETE LOOK AT THE
OBJECTIVE GUIDANCE FOR THE DAY 2 PERIOD.
BANN
DAY 3
VALID 12Z WED NOV 19 2025 - 12Z THU NOV 20 2025
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
SOUTH CENTRAL TEXAS TO A PORTION OF THE ARKANSAS RIVER VALLEY AND
OVER THE SOUTHWEST...
20Z UPDATE:
ADDED A MARGINAL RISK ACROSS PORTIONS OF NM AND AZ WITH THIS
UPDATE. WITH THE CLOSED LOW TRENDING SLOWER, SCATTERED SHOWERS AND
EMBEDDED THUNDERSTORMS ARE LIKELY TO CONTINUE INTO WEDNESDAY
ACROSS THESE AREAS. OVERALL IT LOOKS LIKE LESS OF A FOCUS THAN DAY
2, BUT NONETHELESS STILL ENOUGH OF A CONVECTIVE SIGNAL TO SUGGEST
LOCALIZED FLASH FLOODING COULD CONTINUE TO BE A CONCERN.
CHENARD
..PREVIOUS DISCUSSION
A MID- AND UPPER-LEVEL TROUGH WILL BE MAKING ITS WAY EASTWARD
ACROSS PORTIONS OF THE SOUTHERN GREAT BASIN ON WEDNESDAY AND INTO
EARLY THURSDAY. EAST OF THAT SYSTEM...LOW LEVEL MOISTURE WILL START
TO BE DRAWN NORTHWARD WITH DEWPOINTS IN THE 60S BY WEDNESDAY AFTERNOON
WHICH SHOULD HELP FEED SHOWERS AND THUNDERSTORMS FROM THE AFTERNOON
INTO WEDNESDAY NIGHT/EARLY THURSDAY MORNING. MOST UNSTABLE CAPE
PUSHES 1000 J PER KG AND PRECIPITABLE WATER VALUES IN EXCESS OF
1.5 INCHES BECOMES ESTABLISHED FROM PARTS OF SOUTH-CENTRSL TEXAS
INTO EASTERN OKLAHOMA AND WESTERN ARKANSAS. BASED ON THE
GEFS...THE PRECIPITABLE WATER AXIS IS ROUGHLY BETWEEN 2 AND 2.5
STANDARDIZED ANOMALIES ABOVE CLIMATOLOGY FOR THIS TIME OF YEAR.
BASED ON THAT COMBINATION...SUSPECT LOCALLY HEAVY RAINFALL RATES
ARE POSSIBLE. THERE HAS BEEN A TENDENCY FOR THE UPPER TROUGH TO
SLOW ITS FORWARD SPEED WITH A SIMILAR DELAY IN CONVECTION
INITIATION. WITH THE EVENT SPANNING INTO THURSDAY WITH BROADER
AREAL COVERAGE AND AN INCREASING RISK OF EXCESSIVE RAINFALL (SEE
WPC MEDIUM RANGE GRAPHICS AND DISCUSSIONS FOR FURTHER
DETAILS)...OPTED TO MAINTAIN THE MARGINAL RISK ON THE UNCERTAINTY
OF TIMING.
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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