Quantitative Precipitation Forecast
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FOUS30 KWBC 241423
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1023 AM EDT SAT SEP 24 2016

...VALID 15Z SAT SEP 24 2016 - 12Z SUN SEP 25 2016...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 NW CYBR 10 NE MIB 15 SW 2WX 30 NE TMH 20 N HLD 45 NW SHR
30 SSW OLF 45 NE CWAQ 30 WNW CWBD.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 N 5R5 15 E IAH 20 E EFD 15 SSE LVJ 10 SW LBX PSX 10 WSW PKV
20 NNW RKP 30 WNW RKP 15 WNW VCT 20 N 5R5.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
80 SSW MRF 15 NNW FST 25 WNW GAG 15 NW HJH 10 ENE FSD AXN
15 WSW MZH 20 WNW RCX 25 SW VOK 20 NE IOW 15 NE IRK 15 NW GMJ
15 ESE ADH 25 S BWD 10 N UVA 65 SW MMNL.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
115 S MRF 50 WSW 6R6 40 ENE FST 45 E LBB 20 WNW WWR 10 SE SLN
10 ESE MHK UKL 15 W BVO 25 E DUC 35 ESE ABI 10 W JCT 25 ENE DLF
55 WSW MMPG.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 SSE MIW 25 W DSM DNS 20 SSE SHL 10 SW ULM 20 ESE LVN
15 SSW ONA 10 W OLZ 15 SSE MIW.


...15Z UPDATE...

...IOWA AND SOUTHERN MINNESOTA...

COORDINATED WITH WFO MPX...ARX...DMX...DVN TO INTRODUCE SLIGHT
RISK OVER CENTRAL IOWA AND SOUTHERN MINNESOTA. PARTS OF THIS
REGION ARE DEALING WITH ONGOING RIVER FLOODING AND SATURATED
SOILS. SOME OF THE HEAVIER QPF IN THE MODELS IS A LITTLE CLOSE FOR
COMFORT...FALLING OVER WESTERN IOWA. THERE ARE SOME
REASONS...THOUGH...TO EXPECT CONVECTION FARTHER EAST MAY EXCEED
EXPECTATIONS...AS A WEDGE OF MODERATE CAPE WILL LIKELY FORM TODAY
IN BETWEEN THE SLOWLY RETREATING WARM FRONT OVER NORTHEAST IOWA
AND THE APPROACHING BROKEN LINES OF CONVECTION FROM THE WEST.
ALTHOUGH STRONGEST FOCUS FOR DEEP LAYER ASCENT LIFTS NORTHWARD
INTO MINNESOTA...THERE WILL BE AN INCREASINGLY HEALTHY FETCH OF
LOW LEVEL SOUTHERLY FLOW ADVECTING MOISTURE AND INSTABILITY INTO
NORTH-CENTRAL AND NORTHEAST IOWA AND SOUTHERN MINNESOTA...AND WE
MAY SEE HEAVIER MODEL QPFS TREND INTO THOSE AREAS.

FLASH FLOOD GUIDANCE IS AROUND ONE INCH IN AN HOUR...AND MAY NOT
BE FULLY REPRESENTATIVE AS PARTS OF THIS REGION RECENTLY RECEIVED
7 TO 14 INCHES OF RAIN...AND SWOLLEN RIVERS AND SATURATED SOILS
WILL CONTRIBUTE TO RAPID RUNOFF. AS NOTED BY THE
WFOS...THERMODYNAMICS FAVOR A DEEP WARM CLOUD LAYER...AND
PRECIPITABLE WATER VALUES RISE TO 3 STANDARD DEVIATIONS ABOVE
CLIMATOLOGY...ALL SUGGESTING AN EFFICIENT RAINFALL PROCESS EVEN IF
IT IS NOT ANCHORED...OR IS RELATIVELY SHORT LIVED AS THE UPPER
FORCING COMES THROUGH THIS EVENING / TONIGHT.


...CENTRAL-SOUTHERN PLAINS...

RISK AREAS INHERITED FROM THE 12Z ISSUANCE APPEAR TO BE NEARLY
IDEALLY PLACED. WE DID INTRODUCE A SMALL MARGINAL RISK OVER
SOUTHEAST TEXAS -SEE TEXT BELOW. WE ALSO EXPANDED THE RISK AREAS
IN NORTHWEST OKLAHOMA AND SOUTHWEST KANSAS A BIT TO CAPTURE
ONGOING ACTIVITY EARLY TODAY. CERTAINLY ANYWHERE ALONG THE LENGTHY
ZONE OF FORCED ASCENT AHEAD OF THE APPROACHING TROUGH THE WIND
PROFILES FAVOR SOME TRAINING AS CELLS LIFT NNE AND MERGE WITH LINE
SEGMENTS MOVING MORE NORTHEASTWARD. THE HRRR...HOWEVER...DOES
SUGGEST SOME OUTFLOW DOMINANCE OVER OKLAHOMA...ESPECIALLY AS THE
MORE PRONOUNCED SHORTWAVE LIFTS NORTHWARD...CAUSING SOME SUBTLE
VEERING IN THE MID LEVELS. THIS WOULD FAVOR SOME COMBINATION OF
THE 00Z NSSL WRF AND WRF-ARW...AND WHEN OVERLAYING A 50/50 BLEND
OF QPF FROM THOSE MODELS...THE EXISTING RISK AREAS LOOKED TO BE ON
TARGET.

...PREVIOUS...

A LINE OF MOD-HEAVY RAINFALL IS STILL ON TRACK ALONG AND AHEAD OF
THE COLD FRONT. THE MORE SIGNIFICANT/PROGRESSIVE HEIGHT FALLS WILL
PUSH ACROSS THE NORTHERN PLAINS INTO THE NORTHERN PLAINS DURING
THE PERIOD. GIVEN THE FASTER W-E FRONTAL PROGRESSION ALONG WITH
WEAKER AVAILABLE DEEP-LAYER INSTABILITY...RAINFALL AMOUNTS ACROSS
THE NORTHERN PLAINS INTO THE UPPER MS VALLEY WILL AVERAGE LESS IN
COMPARISON TO THE CENTRAL AND SOUTHERN PLAINS.

DESPITE THE SLOWER UPPER TROUGH/SURFACE FRONTAL PROGRESSION...WPC
DID LEAN TOWARD THE HIGH-RES GUIDANCE (WRF-ARW...NMMB...AND RGEM)
WITH RESPECT TO THE EASTWARD PROGRESSION OF THE HEAVY RAINFALL
AXIS OVER THE CENTRAL AND SOUTHERN PLAINS THIS AFTERNOON INTO
EARLY EVENING...GIVEN THE MORE ROBUST DEGREE OF PRE-FRONTAL
INSTABILITY (DEVELOPING COLD POOLS/OUTFLOW
BOUNDARIES)...INCREASING VERTICAL WIND SHEAR...AND TIGHTENING PW
GRADIENT. HOWEVER... EXPECT AN UPTICK IN PRE-FRONTAL LOW-LEVEL
INFLOW OVERNIGHT...WHILE BECOMING MORE ALIGNED WITH THE MEAN
850-300 MB FLOW. THIS WILL ALLOW FOR AN INCREASED POTENTIAL A
SLOWER QLCS PROGRESSION AND THUS CELL TRAINING...AS NOTED PER THE
WEAKENING CORFIDI VECTORS WHICH THE MODELS SHOW BECOMING MORE
EASTERLY WITH TIME OVERNIGHT (INDICATIVE OF THE INCREASING UPWIND
PROPAGATION). DESPITE THE WEAKENING DEEP-LAYER INSTABILITY DURING
THE OVERNIGHT HOURS...MUCAPES OF 1000-1500 J/KG ALONG WITH THE
FAVORABLE MOIST PROFILE (PWS OF 1.75-2.00") AND HIGHER LIKELIHOOD
OF CELL TRAINING WILL BE MORE THAN SUFFICIENT TO SUPPORT A SLIGHT
RISK IN THE EXCESSIVE RAINFALL OUTLOOK. IN TERMS OF THE QPF...WPC
UTILIZED A MULTI-MODEL ENSEMBLE WITH THE AREAL-AVERAGE AMOUNTS
(MAX STRIPE OF 1-1.5")...HOWEVER THE HIGH-RES CAMS SHOW POCKETS OF
MUCH HIGHER TOTALS (3-6") GIVEN THE AFOREMENTIONED FAVORABLE
THERMODYNAMIC AND KINEMATIC PROFILE...ESPECIALLY LATE SAT INTO
EARLY SUN.


...SOUTHEAST TEXAS...

THIS REGION REMAINS BENEATH THE CENTER OF A SMALL UPPER LEVEL
ANTICYCLONE...BUT AT LOW LEVELS INFLOW OFF THE GULF WILL CONTINUE
TO GRADUALLY INCREASE IN RESPONSE TO UPSTREAM HEIGHT FALLS...AND
RICH TROPICAL MOISTURE WAS IN PLACE THROUGHOUT THE COLUMN. DIURNAL
THUNDERSTORMS NEAR AND JUST INLAND FROM THE COAST WILL HAVE LITTLE
ORGANIZATION OWING TO WEAK SHEAR...BUT SUFFICIENT INFLOW AND SLOW
MOVEMENT...TO PERHAPS YIELD ISOLATED 2 TO 3 INCH RAIN AMOUNTS OVER
A SHORT PERIOD OF TIME. THOUGH THE REGION HAS BEEN GENERALLY
DRY...FLASH FLOOD GUIDANCE VALUES HAVE COME DOWN TO AROUND 2.5
INCHES PER HOUR WITH SIMILAR DIURNAL CONVECTION HAVING OCCURRED A
FEW DAYS IN A ROW. THE ODDS OF FLASH FLOODING ARE LOW...BUT WORTH
MENTIONING...AS A COUPLE OF THE HEAVIER DOWNPOURS COULD CAUSE
RAPID RUNOFF DEPENDING WHERE THEY FALL.


...NORTHERN ROCKIES...

CLOSED MID-UPPER LEVEL LOW OVER THE CENTRAL ROCKIES EARLY WILL
SLOWLY LIFT NORTHEAST INTO THE NORTHERN PLAINS TODAY AND TONIGHT.
THE N-S ORIENTED JET STREAK ON THE BACK SIDE OF THE TROUGH WILL
ENHANCE THE MID LEVEL DEFORMATION ON THE BACKSIDE OF THE
TROUGH...EVIDENCED BY THE NARROW N-S VORT LOBE (NARROW AXIS) JUST
BEHIND THE MAIN MID LEVEL VORT CENTER. AS A RESULT...WILL CONTINUE
TO SEE A RATHER ABRUPT END TO THE WESTWARD EXTENT OF THE COMMA
HEAD...THOUGH WHILE RELATIVELY NARROW THE ENHANCED FGEN WILL MAKE
FOR A PRODUCTIVE TROWAL IN TERMS OF PCPN
EFFICIENCY/PRODUCTION...PARTICULARLY THROUGH 00Z SUN. WITH PW
ANOMALIES STILL A HEALTHY 3 TO 4 STANDARD DEVIATIONS ABOVE NORMAL
DURING THAT TIME...THE MODELS (BOTH GLOBAL AND HIGH-RES) CONTINUE
TO DEPICT SOME RATHER ROBUST TOTALS ACROSS NORTHEAST WY INTO
EASTERN MT AND WESTERN ND. USING A MULTI-MODEL CONSENSUS...WPC
NOTED AREAL-AVERAGE AMOUNTS BETWEEN 1-2" ACROSS THIS AREA...WHILE
THE HIGH-RES CAMS SHOW ISOLATED POCKETS OF 3-4" OVER NORTHEAST WY
AND SOUTHEAST MT GIVEN THE UPSLOPE ENHANCEMENT WITH THE 850-700 MB
NORTHERLY FLOW. AS WOULD BE EXPECTED...DEEP-LAYER INSTABILITY WILL
BE SORELY LACKING OVER THIS REGION (AS THUS WILL SHORT-TERM
RAINFALL RATES)...AS CURRENT AND MODEL FORECAST CAPES OF ANY
SIGNIFICANCE (ABOVE 200 J/KG) WERE NOTED FARTHER EAST OF THE
TROWAL/MAX PCPN SHIELD...I.E. FARTHER EAST INTO THE DAKOTAS. AS A
RESULT...THE RISK FOR SHORT-TERM RUNOFF ISSUES WILL BE
MARGINAL/ISOLATED AT BEST.


BURKE/HURLEY
$$





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