Quantitative Precipitation Forecast
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FOUS30 KWBC 261500
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1059 AM EDT TUE SEP 26 2017

...VALID 15Z TUE SEP 26 2017 - 12Z WED SEP 27 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
95 NW MMCU 115 SE DUG 70 SSW DMN 40 SW SVC 35 NNE SAD 40 S SJN
45 ESE SJN 45 WSW FMN 35 WSW TEX 35 NNW CPW 20 ENE E33
20 WSW LVS 40 WNW CVS 15 S PYX 25 WSW ICT 25 SE OJC 45 S SZL
15 WNW AQR NFW 30 W 05F 10 S BMQ 15 SSW 3T5 20 NE VCT 20 ENE KRP
15 SSE 2R8 20 SE NQI 15 W BKS 35 SSW HBV 10 NNE MMMY 45 WSW MMIO.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
105 NW MMMV 75 S 6R6 35 SSW FST MRF 100 W MRF 50 S MMCS
25 E PVW 10 NE PNC TUL 25 WSW MWL 30 NW JCT 10 NNW HDO 35 E COT
20 ESE LRD 35 SE MMMV.

MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
40 W DRT 30 ESE 6R6 40 E 6R6 35 W ECU 10 NNW UVA 20 W COT
35 NNW LRD 35 NW LRD 45 S MMPG.


1500 UTC UPDATE...

EXPANDED THE EASTERN FLANK OF MARGINAL RISK AREA TO ENCOMPASS MORE
OF SOUTH AND SOUTH-CENTRAL TX...INCLUDING THE HILL COUNTRY WHILE
ALSO CAPTURING SOUTHERN PORTIONS OF THE MIDDLE TX COAST. THE
GUIDANCE HAS STRUGGLED THUS FAR WITH THE SLOW-MOVING BANDS OF
CONVECTION ALONG THE EASTERN PERIPHERY -- LIKELY FEEDING OFF OF
THE DIFFERENTIAL HEATING (ENHANCING THE LOW-LEVEL FRONTOGENESIS
ALREADY IN PLACE VIA THE UPPER JET STREAK TO THE NORTH) AND
ATTENDANT RIBBON OF GREATER DEEP-LAYER INSTABILITY (POSITIVE
dCAPE/dT). SOME OF THE RECENT CAMS ARE DOING A BETTER JOB
DEPICTING THESE SHORT TERM TRENDS...INCLUDING RECENT HRRR RUNS AS
WELL AS THE 12Z NAM CONEST NEST.

HURLEY

ORIGINAL DISCUSSION BELOW...


...RIO GRANDE / EDWARDS PLATEAU AND SOUTH-CENTRAL TEXAS...

AN UPPER LOW IS SET TO CLOSE OFF OVER ARIZONA...WHILE THE
DOWNSTREAM UPPER JET STRENGTHENS. IN RESPONSE...LOW LEVEL EASTERLY
INFLOW WILL INCREASE ALONG THE RIO GRANDE...CONVERGING INTO A
TROUGH / INVERTED TROUGH FEATURE SITTING OUT IN THE PRE-FRONTAL
ENVIRONMENT. WITH SOME INVOLVEMENT OF HIGH LEVEL MOISTURE FROM THE
REMNANT OF PACIFIC TROPICAL CYCLONE PILAR...AS WELL AS ENHANCED
INFLOW OFF THE WESTERN GULF...PRECIPITABLE WATER WAS ALREADY
NEARING 2.00 INCHES...AND IS FORECAST TO EXCEED THAT VALUE. THE
SETUP HAD ALREADY PRODUCED A SLOW MOVING MCS LATE MONDAY...WITH
RADAR ESTIMATES AND UNOFFICIAL GAUGE READINGS SHOWING LOCAL 5-PLUS
INCH RAINFALL WEST OF COTULLA. LIFT AND INFLOW INCREASE ON TUESDAY
SUCH THAT A REPEAT...OR EVEN BETTER ORGANIZED MCS...IS POSSIBLE.

USING THE NAM CAPE FORECAST TO SHAPE EXPECTATIONS...AND ADVECTING
THE CONVECTION ALONG DURING EACH 6-HOUR PERIOD...WPC CAME UP WITH
A FORECAST RESEMBLING THE 00Z WRF-ARW...NSSL WRF...AND NAM CONUS
NEST. WE LEANED MORE HEAVILY TO THE WRF-ARW...GIVEN ITS TENDENCY
TO PUSH OUTFLOW FARTHER EAST AND SOUTH...WHICH HAD BEEN THE MODE
ON MONDAY. THIS MEANS WE EXPECT VERY HEAVY RAIN ON THE MEXICAN
SIDE OF THE RIVER...AND HEAVY RAIN WILL ALSO BLEED OVER ONTO THE
U.S. SIDE AS IT DID ON MONDAY. THE DETERMINISTIC WPC QPF PUSHES 3
TO 5 INCH AMOUNTS INTO A SMALL PART OF TEXAS FROM MAVERICK COUNTY
INTO ADJACENT COUNTIES. NOTE THAT THE HI-RES MODELS UNANIMOUSLY
PRODUCE SPOT TOTALS OF SEVEN INCHES OR GREATER. IT IS SOMEWHAT
MORE LIKELY THAT SUCH AMOUNTS WOULD OCCUR IN MEXICO...BUT ALL OF
THIS RAIN WILL FIND ITS WAY INTO THE RIVER...AFFECTING WEST GULF
RFC OPERATIONS.

WPC WILL MAINTAIN A SMALL MODERATE RISK OF EXCESSIVE RAINFALL FOR
THE DAY 1 PERIOD.


...WEST TEXAS TO SOUTHERN AND CENTRAL OKLAHOMA...

EARLY TUESDAY MORNING A SLOW-MOVING FRONT STRETCHED FROM EASTERN
AND SOUTHERN OKLAHOMA TO CENTRAL NEW MEXICO.
CONVECTION HAD STRUGGLED WHEN MOVING OFF THE FRONT INTO THE WARM
SECTOR...WHERE LAPSE RATES WERE RELATIVELY POOR. CONTINUED
SYNOPTIC ASCENT...HOWEVER...INCLUDING A STRENGTHENING UPPER JET
CIRCULATION...SHOULD REPLENISH INSTABILITY TODAY ALONG AND TO
EITHER SIDE OF THE SURFACE FRONT. THE SETUP WILL SUPPORT RENEWED
CONVECTION...AND A SWATH OF GENERALLY 1 TO 3 INCH RAINFALL FROM
THE TEXAS SOUTH PLAINS TO WESTERN NORTH TEXAS AND SOUTHWEST
OKLAHOMA. WPC ALSO STRETCHED MODERATE AREAL AVERAGE TOTALS DOWN
TOWARD EL PASO. THE HI-RES MODELS HAVE GENERALLY BEEN
OVER-FORECASTING HEAVY RAINFALL IN THIS PATTERN...BUT THERE IS
DECENT POTENTIAL FOR TRAINING TODAY...WITH CELLS STRADDLING THE
FRONTAL ZONE AND MOVING SOUTHWEST TO NORTHEAST. THE MAIN LIMITING
FACTORS WILL LIKELY BE THE LACK OF A WELL ESTABLISHED POOL OF CAPE
IN THE INFLOW REGION OVER CENTRAL TEXAS...AS WELL AS RELATIVELY
WEAK LOW LEVEL INFLOW WIND SPEEDS. STILL...ISOLATED TOTALS
EXCEEDING 3 INCHES MAY OCCUR WHEREVER THE TRACKS OF HEAVIER CELLS
HAPPEN TO OVERLAP...AND THE INTENSITY COULD BE SUFFICIENT TO
EXCEED FLASH FLOOD GUIDANCE. THIS IS DESCRIBED BY A SLIGHT RISK.


...NEW MEXICO...

WITH A CLOSED LOW FORMING IN ARIZONA...THE LOW LEVEL RESPONSE WILL
DRAW INCREASED MOISTURE CONTENT OUT AS FAR WEST AS WESTERN NEW
MEXICO...WHILE INCREASED FORCING / DIVERGENCE EAST OF THE LOW
CENTER SHOULD AIDE IN PRODUCTION OF STEEPER LAPSE RATES AND
GREATER INSTABILITY COMPARED TO RECENT DAYS. THE ATMOSPHERE MAY
STILL BE DRY ENOUGH...AND THE SHEAR STRONG ENOUGH...TO YIELD
MAINLY ISOLATED OR SCATTERED CONVECTION WITH LITTLE UPSCALE
GROWTH. INDIVIDUAL CELL MOTIONS ARE LIKELY TO BE NORTH TO
NORTHEAST AT A GOOD CLIP AS WELL...WITH MEAN 0-6 KM WINDS OF 15 TO
20 KNOTS. THESE LIMITING FACTORS WILL KEEP THE EXCESSIVE RAINFALL
RISK IN THE MARGINAL CATEGORY. THERE IS POTENTIAL FOR ISOLATED
FLASH FLOODING...HOWEVER...AS PRECIPITABLE WATER VALUES SURGE UP
TO AROUND 1.00 INCH OR GREATER IN WESTERN AND CENTRAL NEW
MEXICO...AND STORM SCALE BOUNDARIES AND TERRAIN INFLUENCES COULD
ALLOW CELLS TO BECOME ANCHORED OR TRAIN FOR BRIEF PERIODS.

BURKE
$$




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