661
FXUS65 KTWC 161833
AFDTWC
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TUCSON AZ
1133 AM MST WED SEP 16 2026
SYNOPSIS
A WEATHER SYSTEM MOVING IN FROM NORTHERN MEXICO
TODAY WILL BRING WIDESPREAD SHOWERS AND THUNDERSTORMS INTO
THURSDAY MORNING WHICH WILL BE CAPABLE OF PRODUCING HEAVY
RAINFALL. DRIER CONDITIONS MOVE IN FRIDAY LIMITING THE DAILY
CHANCE THROUGH THIS WEEKEND INTO EARLY NEXT WEEK. HIGH
TEMPERATURES WILL BE BELOW NORMAL TODAY AND THURSDAY, THEN BE
AROUND NORMAL FRIDAY INTO EARLY NEXT WEEK.
UPDATE...10 AM WEDNESDAY
FORECAST REMAINS ON TRACK AND THE PREVIOUS AFD IS VALID FOR THE
OVERALL SITUATION. THIS UPDATE WILL FOCUS ON THE LATEST CAMS
AND CURRENT MESOA.
A TROPICAL LOW HAS MOVED INLAND INTO MEXICO SW OF HERMOSILLO
WITH AN UPPER LEVEL LOW WEST OF CENTRAL BAJA. WE ARE ALSO SEEING
MULTIPLE EMBEDDED MCVS MOVING THROUGH GRAHAM/GREENLEE, WHICH IS
MAINTAINING CONVECTION BEING ENHANCED BY UPSLOPE FLOW INTO THE
WHITE MTNS.
THESE FEATURES ARE SETTING THE STAGE FOR A RATHER ROBUST AND
POTENTIALLY SIGNIFICANT DAY ACROSS ARIZONA. LATEST CAMS HAVE
VARYING SOLUTIONS, BUT THEY ALL HAVE SOME FLAVOR OF A BROAD
AREAL EXPANSION OF CURRENT SHOWERS/STORMS WITH INTENSIFICATION
THROUGH THE LATE MORNING. WHILE WE ARE SEEING SOME DEEPER LEVEL
CONVECTION, MUCH OF THIS IS FAIRLY SHALLOW AS OF NOW. HOWEVER
THESE ARE VERY EFFICIENT RAINFALL PRODUCERS DUE TO THE HIGHLY
ANOMALOUS PWATS AND FAVORABLE TALL SKINNY CAPE SOUNDINGS
SUPPORTING A TROPICAL REGIME. THIS ENVIRONMENT, IN CONJUNCTION
WITH DEEP WARM CLOUD RAIN PROCESSES, IS PROVIDING 1" TO NEARLY
1.5" RAINFALL RATES IN 15-30 MINS SO FAR THIS MORNING. WOULD
EXPECT THIS TO CONTINUE AND MAXIMIZE AS DEEPER CONVECTION
BEGINS TO DEVELOP AROUND NOON. IN FACT, THE LOW LEVEL
CIRCULATION THAT HAS MOVED INLAND INTO MEXICO IS SHOWING RENEWED
SIGNS OF RAPIDLY COOLING CLOUD TOPS ON SATELLITE.
AS THIS LOW PRESSURE APPROACHES SOUTHERN AZ BY LATE MORNING,
EXPECT COVERAGE TO INCREASE WITH AN INCREASING FLASH FLOOD
RISK. GIVEN WHAT WE HAVE SEEN THIS MORNING, HIGH PWATS,
SATURATED GROUNDS, AND THE MODERATE RISK ERO, WE COULD SEE SOME
ISOLATED INSTANCES OF SIGNIFICANT FLASH FLOODING. DECIPHERING ON
EXACTLY WHERE THAT COULD BE IS TRICKY, BUT THERE IS SOME
INDICATION IT WILL FOLLOW THE LOW PRESSURE CIRCULATION AND JUST
NORTH, NE AND EAST OF THE LOW. THAT WOULD PLACE PORTIONS OF
SANTA CRUZ, CENTRAL AND EASTERN PIMA, AND PINAL COS IN THE
TRACK. THIS IS LINING UP WELL WITH THE INITIAL WOFS RUNS SHOWING
A HIGH PROBABILITY OF SEEING 2" OF RAIN WEST OF TUCSON IN THE
NEXT FEW HRS. 90TH PERCENTILE IS SHOWING WIDESPREAD 1-2" WITH
ISOLATED TO SCATTERED AREAS OF 2-3".
THERE COULD ALSO BE A TRAILING BAND OF CONVECTION,
WHICH COULD TRAIN OVER SOME AREAS THIS EVENING. THIS IS
SUPPORTED BY 12Z HREF NOW SHOWING A 30% CONTOUR OF GREATER THAN
3 INCHES OF RAIN IN 3 HRS. THIS SIGNAL IS JUST WEST OF THE
TUCSON METRO ALIGNED WELL WITH RADAR TRENDS AND WOFS RUNS, BUT
THE 10% CONTOURS IS COVERING MOST OF THE CWA FROM TUCSON WEST
AND NORTH INTO CENTRAL AZ.
WHILE THE SEVERE RISK IS LOW, WE CANNOT RULE OUT A WET
MICROBURST OR TWO WITH A FEW ISOLATED GUSTS APPROACHING 60 MPH.
OTHERWISE, VERY HEAVY RAIN AND FLASH FLOODING IS THE MAIN
THREAT.
PLEASE SEE THE PREVIOUS DISCUSSION FOR THE EXTENDED AS THIS WILL
SERVE AS THE PRIMARY AFD TODAY. WE MAY UPDATE AND INCLUDE A
HYDROLOGY SECTION LATER TODAY AS WE MONITOR TRENDS.
PREVIOUS DISCUSSION
A VERY MOIST ATMOSPHERE FOR MID-
SEPTEMBER IS IN PLACE AWAITING THE DYNAMICS FROM A SMALL UPPER
LOW THAT WAS SOUTH OF HERMOSILLO, MX THIS MORNING. THIS FEATURE
WILL SWING NORTH ACROSS THE AREA TODAY BRINGING MODERATE TO
HEAVY RAINFALL TO PORTIONS OF SOUTHEAST ARIZONA AND THE
INCREASED CHANCES FOR FLASH FLOODING DUE TO MOIST ANTECEDENT
CONDITIONS. A FLASH FLOOD WATCH IS IN EFFECT UNTIL 11 PM
TONIGHT.
SATELLITE IMAGERY SHOWED SOUTHERLY FLOW ACROSS THE AREA WHICH
CONTINUES TO SLOWLY INCREASE PWAT VALUES INTO THE 1.3" TO 1.8"
RANGE WHICH IS 175%-200% OF NORMAL. RADAR SHOWING INITIAL AREA
OF SHOWERS/STORMS MOVING INTO SOUTHERN COCHISE COUNTY. EXPECT
AREAL COVERAGE OF SHOWERS/STORMS TO SLOWLY INCREASE OVER THE
NEXT 2-5 HOURS BEFORE MORE RAPID DEVELOPMENT OCCURS LATER THIS
MORNING AS THE UPPER LOW EDGES CLOSER TO THE INTL BORDER. HOURLY
RAINFALL RATES OF 1"-2.5" WITH SOME OF THE HEAVY SHOWERS WITH
SOME STORMS OVER 3". THE WEATHER PREDICTION CENTER HAS A
MODERATE RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF PIMA,
PINAL, AND GRAHAM COUNTIES TODAY.
THURSDAY: PRECIPITABLE WATER REMAINS ELEVATED FOR THIS TIME OF
YEAR WITH A SLIGHT RISK FOR EXCESSIVE RAINFALL CONTINUING EAST
AND SOUTH OF TUCSON. SHOWERS AND STORMS THAT DEVELOP WILL STILL
HAVE THE POTENTIAL FOR LOCALLY HEAVY RAINFALL, BUT WITHOUT THE
DISTURBANCE NEARBY THERE WILL BE A LACK OF FORCING TO HELP AID
IN HIGHER RAINFALL RATES LIKE WEDNESDAY.
FRIDAY THROUGH TUESDAY: DRIER AIR FINALLY BEGINS TO FILTER INTO
SOUTHEAST ARIZONA AS FLOW ALOFT SHIFTS MORE
WESTERLY/SOUTHWESTERLY. THIS COULD HELP LIMIT THUNDERSTORM
ACTIVITY SOMEWHAT AND KEEP ACTIVITY MAINLY EAST AND SOUTH OF
TUCSON. HIGH TEMPERATURES RETURN TO AT OR SLIGHTLY ABOVE NORMAL.
8 TO 14 DAY OUTLOOKS (WED SEP 23 TO TUE SEP 29)
TEMPERATURE: LEANING ABOVE NORMAL, 39-42% CHANCE
PRECIPITATION: LEANING ABOVE NORMAL, 39-42% CHANCE
AVIATION /18Z WEDNESDAY THROUGH 18Z THURSDAY/
VALID THRU 17/12Z.
SCT-BKN CLOUDS 7K-10K FT AGL WITH ISOLD -SHRA/-TSRA BCMG THRU
MUCH OF THE VALID PD. AFTER 16/17Z, SCT-NUM SHRA/TSRA THRU END
OF VALID PD. VSBYS NR 1 SM AT TIMES WITH SHRA+/TS. GUSTY AND
ERRATIC OUTFLOW WINDS TO AROUND 35-45 KTS NR STRONGEST TS.
OUTSIDE OF TS, SFC WINDS SLY/SWLY AT 7-12 KTS. AVIATION
DISCUSSION NOT UPDATED FOR TAF AMENDMENTS.
FIRE WEATHER
A WEATHER SYSTEM MOVING IN FROM NORTHERN MEXICO
TODAY WILL BRING WIDESPREAD SHOWERS AND THUNDERSTORMS INTO
THURSDAY MORNING WHICH WILL BE CAPABLE OF PRODUCING HEAVY
RAINFALL. DRIER CONDITIONS MOVE IN FRIDAY LIMITING THE DAILY
CHANCE THROUGH THIS WEEKEND INTO EARLY NEXT WEEK. HIGH
TEMPERATURES WILL BE BELOW NORMAL TODAY AND THURSDAY, THEN BE
AROUND NORMAL FRIDAY INTO EARLY NEXT WEEK. MIN RH VALUES MAINLY
20 TO 45 PERCENT IN THE VALLEYS AND 35 TO 65 PERCENT IN THE
MOUNTAINS OVER THE NEXT WEEK. RH RECOVERIES WILL CONTINUE TO BE
EXCELLENT OVERNIGHT. WINDS WILL GENERALLY REMAIN LESS THAN 15
MPH AND TERRAIN DRIVEN. GUSTY AND ERRATIC WINDS WILL ACCOMPANY
THUNDERSTORMS.
TWC WATCHES/WARNINGS/ADVISORIES
FLOOD WATCH THROUGH THIS EVENING FOR AZZ501>515.
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