522   
FXUS04 KWBC 020957  
QPFPFD  
  
QUANTITATIVE PRECIPITATION FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
557 AM EDT SAT JUN 02 2018  
  
FINAL DAY 1, DAY 2 AND DAY 3 QPF DISCUSSION  
VALID JUN 02/1200 UTC THRU JUN 05/1200 UTC  
REFERENCE AWIPS GRAPHICS UNDER...PRECIP ACCUM - 24HR  
  
   
..EASTERN U.S.  
  
SLOW-MOVING UPPER TROUGH WILL NUDGE EASTWARD TOWARD THE ATLANTIC  
COAST TODAY. WILL CONTINUE TO SEE LITTLE CHANGE IN THE OVERALL  
PATTERN UNTIL THIS DEEP, COMPACT TROUGH PUSHES OFF THE COAST.  
GIVEN THE ORIENTATION, THE MODELS CONTINUE TO INDICATE THE MORE  
FOCUSED AREA OF FORCING (DPVA, UPPER DIVERGENCE) ACROSS THE MID  
ATLANTIC REGION DURING THE PEAK HEATING HOURS THIS AFTERNOON INTO  
THE EVENING. THUS EXPECT MORE WIDESPREAD CONVECTIVE COVERAGE TODAY  
COMPARED TO YESTERDAY ACROSS THIS REGION, WHERE MUCH RAIN HAS  
FALLEN OVER THE PAST SEVERAL WEEKS AND FFG VALUES ARE QUITE LOW AS  
A RESULT OF THE WET ANTECEDENT CONDITIONS. LIGHT DEEP-LAYER  
FLOW/LOW 0-6 KM BULK SHEAR (GENERALLY AOB 10 KTS) WOULD AGAIN BE A  
HUGE DETERRENT FOR MORE ORGANIZED, PROLONGED CONVECTION; HOWEVER,  
GIVEN THE LOW FFG VALUES (ESPECIALLY WEST OF I-95), PERSISTENT  
BROAD-SCALE FORCING, AMPLE/DEEP SUBTROPICAL MOISTURE, AND  
SURFACE-BASED CAPES LIKELY PEAKING BETWEEN 1000-2000 J/KG PRIOR TO  
THE ONSET OF CONVECTION, BELIEVE THE HOURLY OR EVEN SUB-HOURLY  
RAINFALL RATES WILL EXCEED THE 1 HOURLY FFGS OVER MANY AREAS, AS  
DEPICTED BY THE SSEO AND HREF PROBABILITIES. THUS PER  
COLLABORATION WITH WFOS AKQ, LWX, AND PHI, HAVE HOISTED A MODERATE  
RISK FOR FLASH FLOODING ACROSS A LARGE PORTION OF THE MID  
ATLANTIC, ENCOMPASSED BY SLIGHT AND MARGINAL AREAS. IN TERMS OF  
THE QPF, WPC AGAIN RELIED HEAVILY ON THE HIGH-RES MEANS IN THIS  
AREA, ESPECIALLY THE HREF.  
  
   
..CENTRAL-NORTHERN PLAINS AND UPPER MIDWEST  
  
MID/UPPER LEVEL TROUGH ACROSS THE NORTHERN PLAINS EARLY THIS  
MORNING WILL BECOME MORE NEGATIVELY TILTED DURING THE DAY 1  
PERIOD, AS THE MORE CONFLUENT UPPER FLOW ACROSS THE CENTRAL  
PLAINS-MID MS VALLEY CAUSES THE SOUTHERN PERIPHERY OF THE VORT  
LOBE (TROUGH BASE) TO TAKE OFF A BIT FASTER COMPARED TO THE  
REMAINDER OF THE TROUGH. UPPER LEVEL FORCING WILL BE BOLSTERED BY  
THE NEGATIVE TILT (INCREASED DIVERGENCE ALOFT AND MORE FOCUSED  
LOW-MID LAYER QS CONVERGENCE), HOWEVER THE MODEST-AT-BEST DEEP  
LAYER INSTABILITY WILL PRECLUDE THE EXCESSIVE RAINFALL THREAT TO  
SOME DEGREE OVER AREAS THAT ARE EXPECTED TO BENEFIT FROM THE  
STRONGEST DYNAMICS (INCLUDING MN-WI). FARTHER SOUTH, ONGOING MCS  
ACROSS THE LOWER MO VALLEY WILL REMAIN PROGRESSIVE EARLY THIS  
MORNING, THE UNDERGO DECAY AFTER 12Z FOLLOWING THE DIMINISHING  
NOCTURNAL LLJ. RAINFALL AMOUNTS INTO THE MID MS VALLEY AND WESTERN  
OH VALLEY WILL BE MORE MODEST ON AVERAGE, I.E. AVERAGING UNDER  
1.0", THOUGH PER THE INDIVIDUAL HIGH-RES CAMS, LOCALIZED TOTALS OF  
2-3+ INCHES WILL REMAIN POSSIBLE AS THE AXIS OF FAVORABLE  
THERMODYNAMICS (CAPES OF AT LEAST 1000-2000 J/KG AND ANOMALOUS PWS  
OF 1.5-1.75") ALONG AND AHEAD OF THE SURFACE FRONT PUSHES W-E  
ACROSS THE AREA. WPC QPF WAS BASED LARGELY FROM THE HIGH-RES  
MEANS, ESPECIALLY THE HREF PROBABILITY-MATCHED MEAN IN PROVIDING  
MORE DETAIL WITH THE AREAS MORE LIKELY TO SEE HEAVIER AMOUNTS.  
  
   
..LOWER MISSISSIPPI VALLEY  
  
ANOTHER MORE FOCUSED AREA OF MODERATE-HEAVY RAINFALL WILL ALIGN  
GENERALLY N-S/NNE-SSW ALONG THE PROGRESSIVE COLD FRONT, WITH  
VIGOROUS DEEP-LAYER INSTABILITY OWING TO THE STEEP MID-LEVEL LAPSE  
RATES AND PW VALUES MAXING OUT BETWEEN 1.75-2.0". WPC ALSO  
UTILIZED A BLEND OF THE HIGH-RES CAM GUIDANCE IN THIS REGION WITH  
THE QPF, ESPECIALLY WITH THE HREF MEAN, IN GENERATING  
AREAL-AVERAGE TOTALS UNDER 1.0" FOR MOST. WOULD ALSO EXPECT  
LOCALIZED TOTALS OF 2-3+ INCHES IN THIS AREA AS WELL; HOWEVER,  
GIVEN THE SWIFT PROGRESSION (AIDED BY THE STRONG DOWNDRAFT CAPE  
AND RESULTANT CONVECTIVE OUTFLOWS), THE EXCESSIVE RAINFALL THREAT  
WILL REMAIN SOMEWHAT MITIGATED (I.E. MARGINAL).   
  
   
..SOUTHEAST  
  
THE MODELS, ESPECIALLY THE HIGH-RES CAMS, SHOW A HEAVY RAINFALL  
SIGNAL ACROSS PORTIONS OF THE GULF AND SOUTHEAST COASTS FROM  
SOUTHEAST MS EASTWARD ALONG AND NORTH OF I-10 THROUGH FAR NORTHERN  
FL, SOUTHERN AL-GA, AND FAR SOUTHERN SC. THIS AREA WILL LIE WITHIN  
A COL REGION TODAY, AS A MID LEVEL SHEAR AXIS (FAR SOUTHERN  
PERIPHERY OF THE EAST COAST TROUGH) TRAVERSES EVER SO GRADUALLY  
ACROSS THE AREA IN BETWEEN THE UPPER RIDGE AXES W AND E. THE HIGH  
CAPE/LOW SHEAR ENVIRONMENT WILL ONCE AGAIN FAVOR PULSE-TYPE  
CONVECTION DURING THE PEAK HEATING HOURS THIS AFTERNOON INTO EARLY  
EVENING; HOWEVER THE WEAK UPPER LEVEL FORCING DOES IMPLY MORE  
COVERAGE EARLIER AND LATER IN THE DAY COMPARED TO CLIMATOLOGY,  
ESPECIALLY GIVEN THE LOW CONVECTIVE TEMPERATURES. SIMULATED  
REFLECTIVITY FORECASTS PER THE CAMS SHOW THE MOST WIDESPREAD  
CONVECTIVE EXTENT BETWEEN 17-23Z, BOLSTERED BY THE DAYTIME HEATING  
ALONG WITH DEVELOPING GULF AND ATLANTIC BREEZES WHICH WILL HELP  
FOCUS THE LOW-LEVEL MOISTURE CONVERGENCE. ONE-HOURLY FFG ACROSS  
THIS REGION IS BETWEEN 2.5-3.0"; GIVEN THE DEEP SURFACE-BASED  
INSTABILITY (CAPES IN UNTAPPED AIRMASS AVERAGING 2500-3500 J/KG  
DURING PEAK HEATING) ALONG WITH THE SUBTROPICAL MOISTURE (PWS  
1.75-2.0"), LOCALIZED HOURLY RATES APPROACHING OR EVEN EXCEEDING  
FFG ARE ANTICIPATED. AS A RESULT, A MARGINAL RISK WAS INCLUDING  
OVER THIS REGION IN THE DAY 1 ERO.  
  
DAY 2  
  
   
..GREAT LAKES TO CENTRAL APPALACHIANS  
  
STRENGTH AND TIMING OF A NEGATIVELY TILTED TROUGH ARE SLIGHTLY OFF  
AMONG THE 00Z GUIDANCE, THOUGH CLOSE ENOUGH TO ALLOW QPF TO BE  
BASED ON THE 00Z GFS/NAM/ECMWF. THE HIGHEST DAY 2 QPF IS OVER  
ONTARIO.  
   
..GULF COAST  
  
THE FRONT PROGRESSES SOUTH SUNDAY UNTIL REACHING THE GULF OF  
MEXICO. DYNAMICS ARE NOT AS STRONG ALONG THIS PORTION OF THE FRONT  
BUT INSTABILITY IS MUCH GREATER. QPF WAS RAISED FOR COASTAL GULF  
WITH ISOLATED HEAVY AMOUNTS STILL POSSIBLE WHERE CONVECTION  
BECOMES ORGANIZED PER A BLEND OF THE 00Z GFS/NAM/ECMWF.  
  
...SOUTHERN TO CENTRAL ROCKIES..  
  
AN UPPER TROUGH MOVES EAST FROM THE FOUR CORNERS TO THE CO/KS  
BORDER SUNDAY. THIS SYSTEM WILL HELP DRAW INCREASING PW VALUES  
NORTHWARD THROUGH THE SOUTHERN TO CENTRAL ROCKIES AND SOUTHERN  
HIGH PLAINS. THIS WILL SUPPORT INCREASING CONVECTION ACROSS THESE  
AREA SUNDAY AFTERNOON IN A REGION OF FAIRLY WELL DEFINED UPPER  
DIFLUENCE AHEAD OF THESE HEIGHT FALLS. QPF WAS BASED ON A BLEND OF  
THE 00Z NAM/GFS/ECMWF.  
  
   
..MID-ATLANTIC  
  
A MID-LEVEL SHORTWAVE TROUGH CROSSING THE SOUTHERN GREAT LAKES  
SUNDAY WILL DRAW IN A REMNANT TROUGH OVER THE MID-ATLANTIC. THIS  
WILL MAINTAIN SURFACE LOW PRESSURE OVER EASTERN NC AND MAINTAIN  
ONSHORE FLOW OVER THE CENTRAL MID-ATLANTIC UNTIL THE COLD FRONT  
FROM THE GREAT LAKES CROSSES SUNDAY NIGHT. STABLE ONSHORE FLOW  
WILL BE IN A TROPICAL PLUME OF MOISTURE MAKING FOR LOCALLY HEAVY  
RAIN. QPF BASED A BLEND OF THE 00Z NAM/GFS/ECMWF WHICH IS A LITTLE  
FASTER THAN THE FRIDAY EVENING GUIDANCE. THE MARGINAL RISK OF  
EXCESSIVE RAINFALL WAS MAINTAINED OVER THE ENTIRE MID-ATLANTIC.  
  
DAY 3  
   
..SOUTHERN NEW ENGLAND  
  
MID-LEVEL LOW PRESSURE SPINS OVER ONTARIO/QUEBEC MONDAY. TROPICAL  
MOISTURE IS LIFTED INTO NEW ENGLAND WITH GOOD DYNAMICS. LITTLE TO  
NO INSTABILITY LIMITS EXCESSIVE RAIN RISK TO A DAY 3 MARGINAL RISK  
OF EXCESSIVE RAINFALL. QPF BASED ON A BLEND OF THE 00Z  
NAM/GFS/ECMWF WHICH WERE IN GOOD AGREEMENT FOR SOUTHERN NEW  
ENGLAND.  
   
  
HURLEY/JACKSON  
  
GRAPHICS AVAILABLE AT WWW.WPC.NCEP.NOAA.GOV/QPF/QPF2.SHTML  
  
  
 
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