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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1400Z Aug 20, 2014)
 
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Abbreviations and acronyms used in this product
 
Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1000 AM EDT WED AUG 20 2014

...VALID 15Z WED AUG 20 2014 - 12Z THU AUG 21 2014...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
SYR 35 NNE IPT 35 WNW CXY CBE 20 NNW W99 15 WNW W99 10 E EKN
EKN 15 N EKN 15 NW MGW 15 ESE AFJ AGC FKL 10 NNW JHW 20 N ROC
SYR.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
PNM 10 NE TOB CCY 15 W VTI 10 N GBG 20 SE PIA MTO 15 N OLY
10 WSW FOA 15 WNW ENL 15 ESE ALN 30 WSW PPQ 35 SW UIN 10 WSW DNS
15 WNW PIR 35 NE HEI KD50 35 SE CWIK CYBR 15 SW KGAF 15 N ADC
PNM.


1400 UTC UPDATE

CHANGES TO THE PREVIOUS EXCESSIVE RAINFALL POTENTIAL OUTLOOK WERE
TO EXTEND THE SLIGHT RISK AREA OVER THE NORTHERN PLAINS/UPPER MS
VALLEY FARTHER TO THE SOUTHEAST.  THIS IS TO CAPTURE THE CURRENT
CONVECTION THAT IS MOVING FROM THE NORTHWEST TO THE SOUTHEAST
ACROSS PORTIONS OF CENTRAL IA INTO WEST CENTRAL IL.  OVER THE NEXT
SEVERAL HOURS---TRAINING IN A NORTHWEST TO SOUTHEASTERLY DIRECTION
LIKELY TO CONTINUE WITH ISOLATED 1-2"+ AMOUNTS IN AN HOUR OR TWO
POSSIBLE.    THE LATEST HRRR SEEMS TO HAVE THE BEST HANDLE ON THIS
ACTIVITY---SUGGESTING HEAVY RAIN POTENTIAL IN THE 6 HOUR PERIOD
ENDING 1800 UTC IN THE APPROX CURRENT OBSERVED AXIS---WITH A
DECREASE IN ACTIVITY AFTER THIS.  THE EMPHASIS FOR CONVECTION
SHOULD THEN SHIFT BACK TO THE NORTHWEST AHEAD OF THE HEIGHT FALLS
EJECTING INTO THE NORTHERN PLAINS LATE THIS AFTERNOON.

OVER THE NORTHERN MID ATLANTIC---NO SIGNIFICANT CHANGES MADE TO
THE PREVIOUS OUTLOOK AREA.


ORAVEC


PREVIOUS DISCUSSION


...NORTHERN PLAINS...

THE WELL DEFINED HEIGHT FALLS AND ASSOCIATED AXIS OF ABOVE AVERAGE
PW VALUES WILL BE PRESSING INTO THE NORTHERN PLAINS WEDNESDAY AND
WED NIGHT.  MODEL CONSENSUS IS FOR THE HEAVY PRECIPITATION
POTENTIAL TO DEVELOP TODAY OVER THE NORTHERN PLAINS WITH THE
ORGANIZING CONVECTION WEDNESDAY AFTERNOON IN A REGION OF FAIRLY
UPPER DIFFLUENT FLOW AND STRENGTHENING LOW LEVEL INFLOW INTO THE
WARM FRONTAL BOUNDARY FORECAST OVER THE NORTHERN PLAINS. FROM A
SYNOPTIC SCALE STANDPOINT...THIS AREA WILL BE THE BENEFICIARY OF
FAVORABLE DYNAMICAL FORCING FOR DEEP-LAYER ASCENT AS THE UPPER
DIVERGENCE INCREASES IN THE DIFFLUENT FLOW PATTERN ALOFT WEST OF
THE H5 RIDGE AXIS AND DOWNSTREAM OF THE TROUGH. MOREOVER...THE
ACTIVITY DURING THE DAY WED WILL BE ENHANCED BY A VORT MAX
CURRENTLY ACROSS WY THAT WILL BE LIFTING NE INTO THE NRN
PLAINS...ONE THAT WILL BECOMING CONVECTIVELY-ENHANCED UPON MOVING
INTO THE NRN HIGH PLAINS. THERMODYNAMICALLY...INCREASING/DEEPENING
S-SW FLOW WILL MAKE FOR EFFICIENT MOISTURE TRANSPORT INTO THE
AREA...AS PWATS CLIMB TO NEAR 1.75 INCHES WHICH IS BETWEEN 2.5 AND
3 STANDARD DEVIATIONS ABOVE NORMAL FOR LATE AUGUST (NEAR THE 99TH
PERCENTILE). THESE HIGH PWATS AND DEEP WARM CLOUD LAYERS (WET BULB
ZERO HEIGHTS ABOVE 10KFT) WILL MAKE FOR HIGH RAINFALL EFFICIENCY
LATER THIS AFTERNOON INTO THE EVENING WHEN DEEP LAYER INSTBY WILL
BE MOST ROBUST (MUCAPES GENERALLY 1000-2000 J/KG...EXPECT LOCALLY
UP TO 3000 J/KG ACROSS ND PER THE LATEST ARW).

DESPITE THE FACTORS FAVORING HEAVY RAINFALL...THERE IS A FAIRLY
LARGE SPREAD IN THE MODELS WITH RESPECT TO WHERE THE HEAVIEST
RAINS OVER THE NORTHERN PLAINS MAY BE TOWARD THE END OF THE
FORECAST PERIOD...ONCE AGAIN LEADING TO LOW CONFIDENCE IN QPF
DETAILS WITH A CONTINUED LARGE SLIGHT RISK AREA NOTED.  AT THE
SAME TIME...THERE IS A SIGNAL IN MOST OF THE GUIDANCE FOR THE
POTENTIAL FOR HEAVY TO ISOLATED EXCESSIVE AMOUNTS ACROSS THESE
AREAS...WITH LOCALIZED AMOUNTS OF 3+ INCHES NOTED FROM THE HIGHER
RESOLUTION GUIDANCE. A BLEND OF THE 00 UTC HIGH RES
GUIDANCE...ALONG WITH THE IN-HOUSE PSEUDO BIAS CORRECTED MEANS
WERE USED FOR QPF DETAILS.


...EASTERN GREAT LAKES INTO THE UPPER OHIO VALLEY AND NORTHERN
APPALACHIANS...

A SURFACE FRONTAL BOUNDARY WILL SLOWLY PUSH EASTWARD FROM THE
EASTERN LAKES AND UPPER OH VALLEY INTO THE NORTHERN MID-ATLANTIC
AND NORTHERN APPALACHIANS DURING THIS PERIOD.  AN AXIS OF
1.5-1.75" PWATS ALONG AND AHEAD OF THIS FRONT IN A REGION OF UPPER
LEVEL DIFFLUENCE/UPPER LEVEL DIVERGENCE EAST OF THE UPPER TROUGH
AXIS AND IN THE VICINITY OF THIS FRONT WILL SUPPORT POTENTIAL FOR
ORGANIZED FRONTAL CONVECTION ACROSS THESE AREAS. THE ACTIVITY WILL
BECOME ENHANCED IN TERMS OF BOTH AREAL-COVERAGE AND INTENSITY
DURING THE PEAK HEATING HOURS TODAY AS MUCAPES CLIMB TO AROUND
1000 J/KG. MODERATE TO HEAVY RAINFALL TOTALS ARE DEPICTED...WITH
AREAL AVERAGE AMOUNTS FROM .75 TO 1.25 INCHES...HOWEVER LOCALLY
HIGHER AMOUNTS (2-4+ INCHES PER THE HIGH RES GUIDANCE) WILL LIKELY
CAUSE SOME RUNOFF ISSUES GIVEN THE RELATIVELY LOW FFG OVER THIS
REGION.

====
...LOWER MISSOURI VALLEY...

SOME CONCERN HERE THROUGH WED MORNING ACROSS PARTS OF NEBRASKA
INTO IA/NRN MO AS LOW LEVEL JET INCREASES TO OVER 30 KTS INTO E/W
FRONTAL BOUNDARY NEAR THE KS/NEB BORDER (MADDOX "FRONTAL" TYPE
HEAVY RAINFALL SIGNATURE). THE MODELS CONTINUE TO SHOW MSTR
DEEPENING ALONG AND NORTH OF THE LLVL BNDRY OVERNIGHT WITH PWATS
INCREASING TO OVER 1.50 INCHES IN A NW/SE AXIS ACRS ERN
NEBRASKA...NEAR 5H RIDGE CREST AXIS. THE DIVERGENCE ALOFT...WHICH
TYPICALLY INCREASES JUST WEST OF THE UPPER RIDGE AXIS...SHOULD GET
A BIT OF A BOOST WITH THE INCREASED UPPER LEVEL CONFLUENCE TO THE
N-NE AND THUS ADDED FORCING WITHING THE RIGHT ENTRANCE REGION OF A
SHARPENING (ALBEIT SUBTLE) UPPER JET STREAK. THE EXPECTATION IS A
RATHER QUICK EXPANSION OF ELEVATED TSTMS IN A NW-SE FASHION
PARALLELING THE ELEVATED WARM FRONT...WITH THE LOW-LEVEL INFLOW
AND 850-300 MB MEAN (STEERING) FLOW BECOMING MORE ALIGNED WITH
TIME EARLY THIS MORNING... FAVORING MORE UPWIND PROPAGATION WITH
TIME WITH CELL REGENERATION/TRAINING A GOOD BET. SOME OF THE
LATEST HIGH RESOLUTION GUIDANCE (INCLUDING THE LAST SEVERAL HRRR
RUNS AND MOST RECENT WRF-ARW AND WRF-NMMB) CONTINUE TO SUPPORT THE
POTENTIAL OF LOCALIZED AMOUNTS BETWEEN 3 AND 5 INCHES ACROSS THIS
AREA. FFG VALUES REMAIN GENERALLY HIGH ACROSS THIS AREA...SO
BELIEVE ANY EXCESSIVE RAINFALL ISSUES THAT MIGHT DEVELOP HERE
WOULD BE RATHER ISOLATED.

HURLEY