Quantitative Precipitation Forecast
Issued by NWS

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000
FOUS30 KWBC 230755
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
351 AM EDT SAT MAY 23 2015

...VALID 12Z SAT MAY 23 2015 - 12Z SUN MAY 24 2015...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
30 SSW CDR 25 S AIA 35 SSE SNY 10 NNE AKO 35 SW IBM 15 NNW LAR
30 ESE PAT 10 NE DRC 40 NE SIB 30 SSW CDR.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
35 ENE FST 30 ESE LBB 15 E BGD 20 SSW PTT 30 SSW FNB 20 NE MCI
40 SW SZL SLG 30 NNE PRX 25 ESE ACT 15 S BYY 25 SE BKS
45 SW MMNL 45 NNE MMMV 65 WSW MMPG 25 ESE 6R6 35 ENE FST.

MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 NNW LRD 45 S MMPG 20 WSW MMPG DLF 15 SSE JCT 20 S BMQ
20 W 3T5 30 NW VCT 50 N ALI 20 NNW LRD.

MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 ESE UKL 20 ENE CNU 20 SE CFV 30 NNW MLC 30 E AQR 15 ESE GLE
15 E SEP 35 NW 6R9 15 ENE E29 30 W SJT 25 SW SWW 50 SSE CDS
35 SSE HHF 25 SSE WWR 30 NNW WDG 30 SSW EMP 10 ESE UKL.


...SOUTHERN PLAINS INTO THE LOWER MISSOURI VALLEY...

...SIGNIFICANT HEAVY RAINFALL EVENT REMAINS ON TRACK ACROSS MUCH
OF THE SOUTHERN PLAINS INTO PORTIONS OF THE MIDWEST...

THE LATEST SYNOPTIC AND MESOSCALE MODEL GUIDANCE CONTINUES TO
SUPPORT A SIGNIFICANT...HIGH-IMPACT MULTI-DAY HEAVY RAIN EVENT
SPANNING THROUGH THE WEEKEND AND INTO EARLY NEXT WEEK...BEGINNING
IN EARNEST DURING THE LATTER HALF OF THE DAY 1 PERIOD (SATURDAY
AFTERNOON INTO SUNDAY MORNING)...HIGHLIGHTED BY THE EXPECTATION OF
A LARGE SCALE...SLOW MOVING NOCTURNAL QLCS THIS EVENING AND
OVERNIGHT. CONSIDERING THE ANTECEDENT SATURATED SOILS AND LOW FFG
ON BOTH SIDES OF THE RED RIVER ACROSS SOUTHERN-CENTRAL OK AND
NORTHERN TX...THE POTENTIAL FOR ADDITIONAL FLASH FLOODING OVER
THESE AREAS WILL BE ENHANCED.

DURING THE DAY 1 PERIOD...THE MODELS ARE IN REASONABLY GOOD
AGREEMENT WITH THE MASS FIELDS...AS THE MAIN (BROAD) UPPER LOW
PIVOTS SLOWLY ACROSS THE 4 CORNERS REGION AND CENTRAL ROCKIES. AS
THE NEXT SUBTROPICAL JET STREAK AND ASSOCIATED SHORTWAVE ENERGY
TRAVERSES THE BASE OF THE LONGWAVE TROUGH...THE ENSUING UPSTREAM
HEIGHT FALLS WILL INCREASE THE BROAD-SCALE UPPER DIFFLUENCE
ACROSS THE SOUTHERN PLAINS NORTHEAST INTO THE MIDWEST AS THE
TROUGH-RIDGE ORIENTATION AMPLIFIES (UPPER TROUGH BECOMES MORE
NEGATIVELY TILTED). FROM A DYNAMICAL STANDPOINT...THE LARGE SCALE
UPPER DIVERGENCE WILL INCREASE SUBSTANTIALLY ALONG THE STREAM OF
THE BACKED SW-S UPPER LEVEL FLOW BETWEEN THE TROUGH-RIDGE AXES.
MEANWHILE...INCREASE S-N UPPER CONFLUENCE EAST OF THE UPPER TROUGH
WILL INDUCE AN ANOMALOUS 90-100 KT SOUTHERLY 250 MB JET STREAK
ACROSS THE SOUTHERN-CENTRAL HIGH PLAINS...ONE WHICH BY LATE IN THE
PERIOD WOULD PROVIDE AN ADDITIONAL BOOST TO THE ALREADY ROBUST
UPPER LEVEL DIVERGENCE AND DEEP-LAYER ASCENT WITHIN ITS RIGHT
ENTRANCE REGION ACROSS THE SOUTHERN PLAINS.

THE ANOMALOUS COOL SEASON DYNAMICAL SETUP WILL COUPLE WITH THE
EQUALLY-ROBUST WARM SEASON THERMODYNAMIC PROFILE IN GENERATING
WIDESPREAD HEAVY RAIN TO THIS REGION...PARTICULARLY LATE TODAY
THROUGH EARLY SUN MORNING AS THE MESOSCALE SIMULATED REFLECTIVITY
GUIDANCE SUGGESTS MULTIPLE ROUNDS OF CLUSTERED CONVECTION PRIOR TO
THE ONSET OF ONE MAIN (SIGNIFICANT) QLCS AFTER 00Z. THE
STRENGTHENING LOW LEVEL INFLOW (45-50 KT SOUTHERLY LLJ ~850 MB)
AND ANOMALOUS 850-700 MB MOISTURE TRANSPORT FROM THE WESTERN GULF
WILL BOOST PW VALUES BETWEEN 1.75-2.00+ INCHES...OR 2-2.5 STANDARD
DEVIATIONS ABOVE THE NORM PER THE SREF/GEFS ENSEMBLES. MODERATE
DEEP-LAYER INSTABILITY (MUCAPES 1000-2000 J/KG) WILL BE IN PLACE
THIS EVENING AND OVERNIGHT ACROSS THE SOUTHERN PLAINS...SOUTH OF
THE SURFACE WARM FRONT WHICH WILL LIFT NORTH INTO THE CENTRAL
PLAINS BY 12Z SUN.

THE MAIN CONCERN FOR THE OVERNIGHT PERIOD...BOTH IN TERMS OF QPF
AND THE AREAL-EXTENT OF THE ELEVATED EXCESSIVE RAINFALL
POTENTIAL...WILL BE HOW PROGRESSIVE THE DEVELOPING QLCS WILL BE
LATE IN THE PERIOD...ESPECIALLY OVER SOUTH TEXAS. THE
AFOREMENTIONED LLJ WILL BECOME NEARLY PARALLEL AND OF THE SAME
MAGNITUDE (IF NOT HIGHER) THAN THE MEAN 850-300 MB WIND ACROSS
NORTH TX INTO OK AND KANSAS...THEREBY LEADING TO INCREASING UPWIND
PROPAGATION AND LIGHT (IN SOME AREAS EASTERLY) CORFIDI VECTORS.
THIS WILL LIKELY LEAD TO A SLOW EASTWARD PROGRESSION OF THE
CONVECTION...OR AT THE VERY LEAST MULTIPLE LINE SEGMENTS LIFTING
NNE. HOWEVER...ACROSS SOUTH-CENTRAL AND SOUTHERN TEXAS...MORE
DIRECTIONAL SHEAR IS NOTED BETWEEN THE LOW LEVEL INFLOW AND MEAN
WIND...WITH THE LOW LEVEL INFLOW MARKEDLY HIGHER THAN THE MEAN
FLOW. THIS IS ALLOWING FOR A MORE SIGNIFICANT SOUTHEASTERLY MBE OR
CORFIDI VECTOR (15-20+ KTS)...WHICH WOULD EXPLAIN WHY MULTIPLE
HIGH RESOLUTION MODELS (NAM
CONEST...WRF-ARW...WRF-NMMB...NSSL-WRF) BRING HEAVIER RAINFALL
AMOUNTS INTO THE CRP-VIC CORRIDOR OF SOUTHEAST TX (THOUGH NOT ALL
THE WAY TO BRO AND THE LOWER RGV). MOREOVER...AS WITH ANY MADDOX
SYNOPTIC-TYPE HEAVY RAINFALL EVENTS SUCH AS THIS...THERE IS ALWAYS
A CONCERN THAT THE STRONGER CONVECTION FARTHER SOUTH ALONG THE
AXIS OF GREATER DEEP-LAYER INSTABILITY WILL AT LEAST TO SOME
DEGREE DISRUPT THE RAINFALL AMOUNTS FARTHER NORTH. FOR THIS
REASON...FOR NOW WPC WILL HOLD OFF ON HOISTING A "HIGH" RISK AREA
ACROSS PORTIONS OF SOUTHERN OK/NORTHERN TX ALONG THE RED RIVER
WHERE THE CURRENT FFG IS LOW. IN ADDITION...IN AN EFFORT TO
MAINTAIN CONTINUITY FROM THE PREVIOUS FORECAST AND REMAIN IN LINE
WITH THE LATEST GLOBAL GUIDANCE/TRENDS...WPC OPTED TO KEEP THE
LOWER DETERMINISTIC QPF AMOUNTS ALONG THE TX GULF COAST...WHILE AT
THE SAME TIME EXTENDING THE SLIGHT RISK FOR EXCESSIVE RAINFALL
INTO THAT SAME AREA.


...HIGH PLAINS OF NORTHEAST CO/WESTERN NEBRASKA/SOUTHEAST WY...

MAINTAINED THE SLIGHT RISK AREA FROM THE PREVIOUS DAY 2 EXCESSIVE
RAINFALL OUTLOOK. STRONG AND RATHER PERSISTENT UPPER LEVEL
DYNAMICS WILL MARK THE DAY 1 PERIOD ACROSS THE OUTLOOK AREA AS THE
AMPLIFYING UPPER TROUGH-RIDGE PATTERN INCREASES THE UPPER
DIFFLUENCE. MEANWHILE...THE DEVELOPING S-N UPPER LEVEL JET STREAK
ACROSS THE SOUTHERN-CENTRAL HIGH PLAINS ON THE EASTERN EDGE OF THE
TROUGH WILL ENHANCE THE UPPER DIVERGENCE AND DEEP-LAYER ASCENT
ACROSS THE OUTLOOK AREA IN THE LFQ OR LEFT EXIT REGION. THE
FAVORABLE DYNAMICAL PROFILE ALONG WITH THE 20-30 KT SOUTHEASTERLY
(UPSLOPE) LOW-LEVEL INFLOW...ANOMALOUS MOISTURE FLUX/PW
VALUES...AND AVAILABLE DEEP-LAYER INSTABILITY (MUCAPES 500-1500
J/KG) WILL LEAD TO AREAS OF HEAVY TO POTENTIALLY EXCESSIVE
RAINFALL. AREAL-AVERAGE AMOUNTS ARE EXPECTED TO RANGE AROUND
1-1.25 INCH ON A 20 KM SCALE...HOWEVER...PER THE HIGHER RESOLUTION
GUIDANCE...ISOLATED HIGHER AMOUNTS OF 2-3+ INCHES WITHIN A 6-12
HOUR PERIOD WOULD LEAD TO LOCALIZED RUNOFF ISSUES CONSIDERING THE
RELATIVELY LOW FFG VALUES IN THE AREA.

HURLEY
$$




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