525  
FXUS04 KWBC 221027  
QPFPFD  
 
QUANTITATIVE PRECIPITATION FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
626 AM EDT SUN APR 22 2018  
 
FINAL DAY 1, DAY 2 AND DAY 3 QPF DISCUSSION  
VALID APR 22/1200 UTC THRU APR 25/1200 UTC  
REFERENCE AWIPS GRAPHICS UNDER...PRECIP ACCUM - 24HR  
 
DAY 1  
   
..SOUTHEAST U.S
 
 
A CLOSED LOW OF MODERATE STRENGTH AND MAINTAINING ITSELF ALONG ITS  
SLOW EASTWARD MARCH - CHARACTERISTICALLY A VERY APRIL SYSTEM -  
WILL SPREAD RAIN AND THUNDERSTORMS THROUGHOUT AREAS FROM THE LOWER  
OHIO VALLEY TO THE GULF COAST. IN FLORIDA EASTERLY ONSHORE FLOW,  
HEATING, AND GRADUALLY INCREASING DEEP LAYER ASCENT WILL ALSO  
PROMOTE WIDESPREAD COVERAGE OF RAINFALL.  
 
MODEL GUIDANCE IS IN DECENT AGREEMENT, AND THE NOTION OF  
WIDESPREAD 1 TO 3 INCH RAINFALL IS SUPPORTED EVEN BY THE GEFS  
REFORECAST DATA BASED ON ANALOG EVENTS. THE MAIN QUESTION IS  
WHETHER RAIN RATES WILL BE INTENSE ENOUGH OR PERSISTENT ENOUGH  
OVER ANY GIVEN LOCATION TO YIELD MUCH IN THE WAY OF FLASH  
FLOODING. GENERALLY THE MOST PERSISTENT FORCING AND BETTER  
ALIGNMENT OF LOWER AND UPPER LEVEL FLOW SHOULD OCCUR DURING THE  
DAY FROM NORTHERN MISSISSIPPI INTO WESTERN TENNESSEE. THIS IS  
PERHAPS WHERE TRAINING WILL BE MOST PREVALENT, BUT INSTABILITY  
WILL BE MORE MARGINAL. AREAS FARTHER SOUTH SHOULD SEE SOMEWHAT  
GREATER INSTABILITY, BUT WITH MID LEVEL FLOW CUTTING ACROSS FROM  
WEST TO EAST, THE MEAN 0-6 KM WINDS ARE AT A SHARP ANGLE TO  
CORFIDI VECTORS, SUGGESTING TRAINING WILL NOT BE TOO COMMON. GIVEN  
FLASH FLOOD GUIDANCE VALUES ARE ALSO MUCH HIGHER, AND WILL BE  
DIFFICULT TO SURPASS WITH SOUTHWARD EXTENT, WPC TRIMMED BACK A BIT  
ON THE INHERITED SLIGHT RISK AREA FOR THE 12Z SUN- 12Z MON PERIOD.  
OTHERWISE, SLIGHT RISK IS MAINTAINED IN AREAS OF LOWER FFG ACROSS  
NORTHERN MS/AL INTO SOUTHERN TN, AND INCLUDING THE ATLANTA METRO  
TO THE EAST, AS WELL AS THE MORE FAVORABLE TRAINING ENVIRONMENT IN  
WESTERN TN.  
 
THROUGHOUT THESE AREAS AREAL AVERAGE PRECIPITATION IS FORECAST AT  
1.5 TO AROUND 3.0 INCHES FOR THE 24-HOUR PERIOD, AND JUDGING BY  
MOISTURE AVAILABILITY AND HI-RES MODEL SOLUTIONS, HOURLY RATES  
COULD PEAK AROUND 1.50 INCHES, LEADING TO AT LEAST SOME RISK OF  
FLASH FLOODING. FOR QPF THERE WAS NO IDEAL SOLUTION, BUT THE  
MODELS WERE WELL CLUSTERED. WPC BEGAN WITH A BLEND OF THE 00Z GFS,  
WRF-NMM, AND THE NON BIAS CORRECTED VERSION OF OUR IN-HOUSE  
ENSEMBLE (MORE GENEROUS WITH AREAL COVERAGE). WE EXPANDED COVERAGE  
NORTHWARD, RESULTING IN QUITE A BIT MORE QPF OVER NORTHERN TN INTO  
KY COMPARED TO THE PREVIOUS CYCLE, AND TONED DOWN QPF ALONG THE  
EAST COAST OF FLORIDA, AS LOW LEVEL CONVERGENCE AND CELL MOTIONS  
APPEAR ERRATIC ENOUGH TO SPREAD THE CONVECTION OUT.  
 
   
..NORTHERN ROCKIES
 
 
IN THE IMMEDIATE WAKE OF A LEAD SHORTWAVE - WHICH DEPOSITED A  
FRONTAL ZONE INTO THE NORTHERN ROCKIES - A STRONGER SHORTWAVE  
TROUGH WILL AMPLIFY SUNDAY OVER IDAHO AND MONTANA, BRINGING A  
PERIOD OF STRENGTHENED FRONTOGENESIS AND WELL DEFINED UPPER JET  
FORCING. WEAKLY CONVECTIVE RAIN/SNOW WILL LIKELY RESULT. THE  
PREVIOUS WPC FORECAST HAD A GREAT HANDLE ON THIS EVENT, AND WE  
MADE ONLY MINOR TWEAKS USING THE 00Z GFS AND OUR IN-HOUSE ENSEMBLE  
TOOLS.  
 
DAYS 2/3...  
   
..TENNESSEE AND OHIO VALLEYS TO THE EASTERN U.S.
 
 
A NORTHERN STREAM TROUGH MOVING OFF OF THE NORTHEAST COAST MONDAY  
MORNING IS FORECAST TO GIVE WAY TO AN AMPLIFYING RIDGE ALONG THE  
EASTERN SEABOARD. THIS WILL SLOW THE FORWARD PROGRESSION OF A  
COMPACT MID-UPPER LEVEL LOW MOVING FROM THE LOWER MISSISSIPPI INTO  
THE TENNESSEE VALLEY ON MON. THIS WILL SET THE STAGE FOR  
PROLONGED PERIOD OF MODERATE TO HEAVY RAINS ALONG THE SOUTHEAST  
COAST BACK INTO THE SOUTHERN APPALACHIANS MON INTO EARLY TUE.  
HEAVIEST ACCUMULATIONS DURING THE DAY 2 PERIOD (ENDING 12 UTC TUE)  
ARE EXPECTED TO CENTER FROM THE SOUTH CAROLINA SHORES AND THE CAPE  
FEAR REGION OF NORTH CAROLINA BACK INTO THE SOUTHERN BLUE RIDGE.  
DEEP MOISTURE, ON THE ORDER OF 2 STANDARD DEVIATIONS ABOVE NORMAL  
AFFORDED BY 30-40 KT SOUTHEASTERLY LOW LEVEL INFLOW, INTERACTING  
WITH FAVORABLE UPPER JET FORCING, IS EXPECTED TO SUPPORT HEAVY  
TOTALS ON THE ORDER OF 1 TO 3 INCHES, WITH POTENTIALLY HEAVIER  
AMOUNTS, ALONG THE COAST. ALTHOUGH REMOVED FROM THE DEEPEST  
MOISTURE AND BETTER INSTABILITY, THERE REMAINS A GOOD MODEL SIGNAL  
FOR A SECONDARY MAX FURTHER WEST, WHERE GOOD OROGRAPHIC ASCENT  
SHOULD SUPPORT HEAVIER TOTALS FROM THE PIEDMONT BACK INTO THE  
SOUTHERN BLUE RIDGE. FOR THE DAY 2 EXCESSIVE OUTLOOK, A 'SLIGHT'  
RISK WAS MAINTAINED ACROSS THESE AREAS.  
 
FOR DAY 3 (12 UTC TUE TO 12 UTC WED), THE HEAVY RAINFALL THREAT IS  
EXPECTED TO WANE SOME AS THE LOW BEGINS TO WEAKEN AND LIFT OUT TO  
THE NORTHEAST. LIGHT TO MODERATE RAINS ARE EXPECTED TO SPREAD  
NORTH THROUGH THE MID-ATLANTIC REGION INTO THE NORTHEAST. AGAIN,  
THE POTENTIAL FOR HEAVY AMOUNTS IS EXPECTED TO LESSEN AS THE  
ONSHORE FLOW AND THE UPPER FORCING WEAKENS.  
 
WPC QPF GAVE SIGNIFICANT WEIGHT TO THE GFS, NAM AND RECENT RUNS OF  
THE ECMWF THROUGH THE DAY 2 PERIOD. BY DAY 3 HOWEVER, LESS WEIGHT  
WAS GIVEN TO THE ECMWF, RELYING MORE HEAVILY ON THE GFS AND NAM AS  
THE 00 UTC ECMWF BECOMES A MORE SOUTHERLY AND A RELATIVE OUTLIER  
WITH THE POSITION OF THE LOW.  
   
..NORTHERN ROCKIES TO THE GREAT PLAINS
 
 
A WELL-DEFINED SHORTWAVE TROUGH IS EXPECTED TO SUPPORT LIGHT TO  
MODERATE PRECIPITATION, INCLUDING MOUNTAIN SNOWS, FROM THE  
NORTHERN AND CENTRAL ROCKIES TO THE ADJACENT PLAINS MON INTO EARLY  
TUE. WHILE THE MODELS SHOW FAIRLY GOOD CLUSTERING THROUGH DAY 2,  
THERE REMAINS SIGNIFICANT SPREAD THEREAFTER, WITH THE GFS SHOWING  
MORE INTERACTION WITH A SHORTWAVE TROUGH MOVING ACROSS CENTRAL  
CANADA, CARRYING A MORE AMPLIFIED TROUGH FURTHER EAST INTO THE  
CENTRAL GREAT PLAINS AND MID MISSISSIPPI VALLEY WED MORNING. WITH  
MORE SEPARATION IN THE FLOW, THE ECMWF DROPS THIS ENERGY FURTHER  
SOUTH ALONG THE CENTRAL HIGH PLAINS. WITH SUPPORT FROM THE UKMET  
AND THE CANADIAN GLOBAL, LEANED CLOSER TO THE ECMWF SOLUTION, WITH  
WPC QPF HOLDING DEVELOPING PRECIPITATION ACROSS THE CENTRAL AND  
SOUTHERN GREAT PLAINS FURTHER WEST LATE TUE INTO EARLY WED.  
 
BURKE/PEREIRA  
 
GRAPHICS AVAILABLE AT WWW.WPC.NCEP.NOAA.GOV/QPF/QPF2.SHTML  
 

 
 
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