037  
FXUS04 KWBC 300957  
QPFPFD  
 
QUANTITATIVE PRECIPITATION FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
556 AM EDT WED MAY 30 2018  
 
FINAL DAY 1, DAY 2 AND DAY 3 QPF DISCUSSION  
VALID MAY 30/1200 UTC THRU JUN 02/1200 UTC  
REFERENCE AWIPS GRAPHICS UNDER...PRECIP ACCUM - 24HR  
 
DAY 1  
   
..OVERVIEW
 
 
GUIDANCE WAS ALL GENERALLY WELL CLUSTERED WITH QPF OUTPUT FOR  
LARGE SCALE SYNOPTIC SYSTEMS IN THE MIDWEST (ABSORBING TROPICAL  
DEPRESSION ALBERTO) AND IN THE GREAT BASIN / NORTHERN ROCKIES.  
SOMEWHAT MORE TEDIOUS CONVECTIVE FORECASTS WERE SEEN OVER THE  
SOUTHERN PLAINS INTO THE SOUTHEAST, INCLUDING FLORIDA. WPC QPF  
LEANED TOWARD DETAILS IN THE WRF-NMMB, WHICH HAS PERFORMED BETTER  
THIS YEAR THAN IN ANY PREVIOUS YEAR, AND SEEMS A BETTER CHOICE  
THAN THE OTHER HI-RES OPTIONS MUCH OF THE LAST FEW MONTHS. WE ALSO  
USED THE HREF PROBABILITY MATCHED MEAN - WHICH BOOSTS THE SIGNAL  
FOR EMBEDDED HEAVIER AMOUNTS. OVERALL THIS APPROACH LED TO AN  
INCREASE OF QPF ALONG THE PAST OF T.D. ALBERTO, AND ALSO IN  
ASSOCIATION WITH THE NEXT MCS EXPECTED TO TOP THE THERMAL RIDGE  
OVER OKLAHOMA/KANSAS WEDNESDAY EVENING.  
 
PLEASE SEE ALSO WPC TROPICAL CYCLONE ADVISORIES AND STORM  
SUMMARIES FOR ALBERTO.  
 
DAYS 2/3...  
   
..EASTERN SEABOARD FROM THE MID ATLANTIC SOUTH TO FLORIDA
 
 
THE DAY 2 PERIOD (THURSDAY) WILL BEGIN WITH ABUNDANT MOISTURE IN  
PLACE FOR MOST OF THE EAST COAST EXTENDING WEST TO THE  
APPALACHIANS. PRECIPITABLE WATER VALUES SHOULD BE ABOVE 1.6 TO 1.7  
INCHES IN MOST PLACES, WITH SOME VALUES APPROACHING 2 INCHES  
LIKELY. AN UPPER LEVEL TROUGH OVER THE GREAT LAKES ON THURSDAY  
WILL STEADILY ADVANCE TO THE EAST, AND PERHAPS DIG SLIGHTLY,  
THROUGH THE DAY 2-3 PERIOD WITH AN ASSOCIATED LOW-LEVEL COLD FRONT  
ALSO PUSHING INTO NORTHERN PORTIONS OF THE REGION. THE ANOMALOUSLY  
HIGH PRECIPITABLE WATER VALUES ALONG THE ENTIRE EAST COAST AND  
GENERAL UPPER LEVEL SUPPORT FOR VERTICAL MOTION SHOULD CONTRIBUTE  
TO SCATTERED CONVECTION THROUGH MUCH OF THE REGION.  
 
WPC QPF WAS BASED ON A BLEND OF THE 00Z ECMWF, UKMET, AND GFS,  
WITH THE GREATEST WEIGHT PLACED ON THE ECMWF. THE GFS APPEARED TO  
BE ADVANCING THE FRONT TOO QUICKLY, AND THIS AFFECTS ITS  
DISTRIBUTION OF RAINFALL, PARTICULARLY IN THE MID ATLANTIC. THE  
DIFFERENCES BETWEEN THE NBM AND WPC QPF ARE NOT TOO SIGNIFICANT  
OVERALL, BUT MAY BE LARGER IN AREAS WHERE THE NAM IS INDICATING  
HEAVY RAINFALL. THE NAM WAS GENERALLY EXCLUDED FROM WPC QPF DUE TO  
POTENTIAL ISSUES WITH CONVECTIVELY INDUCED VORTICITY MAXIMA OVER  
THE PLAINS PROPAGATING DOWNSTREAM.  
 
A SLIGHT RISK WAS CONTINUED ON DAY 2 OVER THE MID ATLANTIC, AND  
INTRODUCED ON DAY 3 FOR SIMILAR AREAS. SEVERAL ROUNDS OF SCATTERED  
CONVECTION MAY NOT PRODUCE SIGNIFICANT AREAL AVERAGED RAINFALL  
TOTALS, BUT THE CONVECTIVE NATURE OF THE RAIN SHOULD LEAD TO MORE  
SUBSTANTIAL LOCALIZED TOTALS. WITH PWATS AROUND THE 99TH  
PERCENTILE ON THURSDAY (SLIGHTLY LOWER BUT STILL ABOVE AVERAGE ON  
FRIDAY), THE ENVIRONMENT WOULD BE SUPPORTIVE OF RAIN RATES AS HIGH  
AS 2 IN/HR. MANY PARTS OF THE AREA ALSO HAVE ABOVE NORMAL  
STREAMFLOW GIVEN A WET PATTERN IN RECENT MONTHS, SO SOME ISOLATED  
TO SCATTERED EPISODES OF FLASH FLOODING APPEAR POSSIBLE BOTH DAYS.  
 
   
..OZARKS INTO THE TENNESSEE VALLEY
 
 
THIS IS THE REGION WITH THE GREATEST FORECAST UNCERTAINTY. THE  
INFLUENCE OF THE TROUGH SHOULD BE RESTRICTED TO THE NORTH, AND AS  
SUCH THE DOMINANT FORCING FOR CONVECTION MAY BE CONVECTIVE  
PROCESSES (OUTFLOW, COLD POOLS) AS WELL AS LOW-LEVEL BOUNDARIES  
AND THE LOW-LEVEL JET. THUS, THE PRECIPITATION FORECAST IS HIGHLY  
LIKELY TO BE INFLUENCED BY CONVECTIVE EVOLUTION IN PREVIOUS DAYS,  
AND THIS LEADS TO UNCERTAINTY IN PLACING THE SWATH OF HEAVIEST  
RAINFALL. IT ALSO APPEARS LIKELY THAT ONE OR MORE MCSS MAY EVOLVE  
GIVEN THAT THE REGION WILL BE SITUATED ON THE N/NE PERIPHERY OF A  
LOW-LEVEL THERMAL RIDGE AND ELEVATED MIXED LAYER. ONE MCS MAY BE  
ONGOING AT THE BEGINNING OF THE PERIOD IN THE OZARKS, WITH ANOTHER  
ONE POTENTIALLY INITIATING THURSDAY EVENING WHERE LOW-LEVEL INFLOW  
INTERCEPTS A RESIDUAL OUTFLOW BOUNDARY. FOR NOW, A BLEND OF THE  
ECMWF AND GFS WAS GENERALLY FOLLOWED, BUT SEVERAL CHANGES WERE  
MADE: (1) TO NUDGE THE QPF AXIS SOUTH, AS MCS PROPAGATION TENDS TO  
BEND A BIT MORE TOWARD THE HIGHER THETAE VALUES THAN MODELS  
INDICATE; (2) SHOW MORE OF A NATURAL PROGRESSION, AS THE MODELS  
TEND TO SHOW THE HIGHER QPF MOVING SLOWLY OR SITTING IN SIMILAR  
LOCATIONS FOR CONSECUTIVE 6-HR PERIODS; AND (3) TO INCREASE THE  
QPF MAXIMA ABOVE WHAT A SIMPLE BLEND WOULD YIELD. SLIGHT RISKS  
WERE INCLUDED ON THE DAY 2 AND 3 EXCESSIVE RAINFALL OUTLOOKS TO  
ACCOUNT FOR THE POTENTIAL FOR ORGANIZED CONVECTIVE CLUSTERS/LINES  
TO PRODUCE FLASH FLOODING.  
 
   
..NORTHERN ROCKIES INTO NORTHERN PLAINS
 
 
A POTENT SHORTWAVE WILL FOCUS DCVA AND MID-UPPER LEVEL HEIGHT  
FALLS ACROSS THE REGION, SPREADING FROM THE NORTHERN ROCKIES ON  
THURSDAY, INTO THE NORTHERN PLAINS ON FRIDAY. GIVEN THE STRONG  
FORCING AND ABOVE AVERAGE PWATS, IT IS UNSURPRISING THAT THE  
MODELS ALSO SHOW A CLEAR SIGNAL FOR HEAVY RAIN. THIS IS  
PARTICULARLY TRUE ON FRIDAY (DAY 3), AS THE HEIGHT FALLS ARRIVE IN  
THE PLAINS AND REACH A CORRIDOR OF SIGNIFICANT CONVECTIVE  
INSTABILITY. MODELS DEVELOP WIDESPREAD CONVECTION DURING THE  
AFTERNOON AND THEN SPREAD IT EAST, LIKELY IN THE FORM OF ONE OR  
MORE CONVECTIVE LINES. FOLLOWED A GENERAL BLEND OF THE ECMWF, GFS,  
AND UKMET, BUT QPF MAXIMA WERE INCREASED TO MATCH OUR INTERNAL  
BIAS-CORRECTED ENSEMBLE. GLOBAL MODELS CAN TEND TO UNDERESTIMATE  
QPF IN CASES OF WIDESPREAD CONVECTION OR SIGNIFICANT CONVECTIVE  
LINES. THE GFS AND ECMWF SHOW A LARGE QPF MAXIMUM IN NEBRASKA, AND  
THIS MAKES SENSE. ON THE SOUTHERN PERIPHERY OF THE ADVANCING  
TROUGH, FLOW AND STORM MOTIONS WILL BE WEAKER. ANY CONVECTIVE  
CLUSTERS OR LINES MAY PROPAGATE MORE SLOWLY THAN FURTHER NORTH,  
AND THIS MAY ALSO ALLOW THE CONVECTION TO TAKE ON AN INCREASING  
WEST-EAST ORIENTATION WITH TIME (WHICH WOULD FAVOR HEAVIER RAIN).  
A SLIGHT RISK WAS MAINTAINED IN MONTANA ON DAY 2, AND INTRODUCED  
IN THE PLAINS ON DAY 3.  
 
BURKE/LAMERS  
 
GRAPHICS AVAILABLE AT WWW.WPC.NCEP.NOAA.GOV/QPF/QPF2.SHTML  
 

 
 
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