645   
FXUS10 KWNH 191644  
PMDHMD  
  
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1244 PM EDT SUN APR 19 2020  
  
VALID APR 19/1200 UTC THRU APR 23/0000 UTC  
  
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
  
12Z MODEL EVALUATION WITH INITIAL PREFERENCES AND CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
  
STORM TRACKING ACROSS THE GULF COAST REGION  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z NAM/00Z ECMWF BLEND  
CONFIDENCE: MODERATE  
  
THE SOUTHERN STREAM SHORTWAVE CURRENTLY OVER THE SOUTHERN PLAINS  
WILL CONTINUE TRACKING EASTWARD ACROSS THE DEEP SOUTH THROUGH  
TONIGHT AND THEN EXIT THE EAST COAST BY MONDAY AFTERNOON,  
SUSTAINING A WELL DEVELOPED SURFACE LOW THAT WILL RESULT IN  
WIDESPREAD HEAVY RAIN AND SEVERE WEATHER.  A DUAL-LOW SURFACE LOW  
STRUCTURE IS APPEARING LIKELY BY MONDAY MORNING AS A SECOND LOW  
DEVELOPS OFF THE MID-ATLANTIC COAST, AND PASSES WELL SOUTH OF NEW  
ENGLAND BASED ON CURRENT GUIDANCE.  
  
THE 12Z GFS BECOMES FASTER AS EARLY AS MONDAY MORNING, AND THIS  
TREND CONTINUES AS THE LOW EXITS THE COAST.  THIS IS SIMILAR TO  
WHAT THE 6Z GFS WAS PORTRAYING.  THE SURFACE LOW TRACKS ARE  
SIMILAR IN POSITION AMONG THE NON-GFS GUIDANCE THROUGH MONDAY  
AFTERNOON, SO THE MAIN DIFFERENCES BETWEEN THE MODELS REMAINS THE  
PLACEMENT OF THE AXIS OF HIGHEST RAINFALL, WHICH HAS MORE TO DO  
WITH MESOSCALE INTERACTIONS AND THE OVERLAY OF THE INSTABILITY AND  
MID LEVEL LIFT.  THE 00Z UKMET REMAINS SLOWER AND LEFT OF THE  
CONSENSUS WITH THE SHORTWAVE AND THE SURFACE LOW ONCE IT EXITS THE  
COAST, LIKELY DUE TO THE SLOWER SHORT WAVE ENERGY IN THE  
DEVELOPING TROUGH.  BECAUSE OF THIS, THIS MODEL WAS ALSO EXCLUDED  
FROM THE PREFERRED BLEND.  BECAUSE OF THE DIFFERENCES WITH THE QPF  
PLACEMENT AND THE TRACK OF THE SURFACE LOW OFF THE COAST, FORECAST  
CONFIDENCE REMAINS AVERAGE.  
  
BUILDING TROUGH OVER THE GREAT LAKES AND THE NORTHEAST  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z GFS/00Z ECMWF BLEND  
CONFIDENCE: MODERATE  
  
THE LARGE SYNOPTIC SCALE TROUGH OVER CENTRAL/EASTERN CANADA AND  
THE NORTHERN TIER OF THE U.S. GETS REINFORCED WITH A STRONG  
SHORTWAVE TRACKING SOUTHWARD ACROSS MANITOBA, AND THEN EVOLVING  
INTO A COLD UPPER LOW OVER SOUTHERN ONTARIO AND QUEBEC.  THIS WILL  
SUSTAIN A COLD FRONT CROSSING THE GREAT LAKES AND OHIO VALLEY, AND  
THEN REACHING THE EAST COAST BY TUESDAY EVENING.  
  
THE 12Z GFS/NAM ARE CLOSE TO THE CONSENSUS TRACKING SHORT WAVE  
ENERGY IN THE PROGRESSIVE MID LEVEL FLOW FROM MANITOBA ACROSS THE  
UPPER GREAT LAKES THROUGH TUESDAY AFTERNOON.  BEYOND THAT TIME,  
THE 12Z NAM IS SLIGHTLY SLOWER AND A LITTLE MORE AMPLIFIED WITH  
THE DEVELOPING LONG WAVE TROUGH APPROACHING NORTHERN NEW ENGLAND.   
THE 00Z CMC BECOMES EVEN MORE AMPLIFIED ACROSS NEW ENGLAND BY  
WEDNESDAY MORNING AS THE TROUGH ACQUIRES NEGATIVE TILT.  SINCE THE  
GFS/ECMWF HAVE THE SUPPORT OF THEIR ENSEMBLE MEANS, THESE  
SOLUTIONS COMPRISE THE PREFERRED BLEND.   
  
TROUGH ACROSS THE SOUTHWEST AND THEN THE SOUTHERN PLAINS  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z NAM/00Z UKMET/00Z ECMWF BLEND  
CONFIDENCE: LIMITED-MODERATE  
  
THE NEXT SHORT WAVE IN THE FLOW TRACKS TOWARD THE CENTRAL  
CALIFORNIA COAST AFTER MONDAY MORNING. THERE IS STILL SOME MODEL  
SPREAD CONCERNING THE TIMING OF THIS SHORT WAVE, AS IT UNDERCUTS  
MID LEVEL RIDGING ACROSS THE PACIFIC NORTHWEST.  THOUGH THERE IS A  
LOOSE CONSENSUS TAKING THE MID LEVEL SYSTEM OVER NEVADA AND  
ARIZONA ON TUESDAY, MORE NOTEWORTHY DIFFERENCES BECOME APPARENT BY  
TUESDAY NIGHT AND INTO WEDNESDAY AS THE SYSTEM CROSSES THE  
SOUTHERN ROCKIES AND EVENTUALLY THE SOUTHERN PLAINS.  SIMILAR TO  
YESTERDAY, THE GFS REMAINS MORE PROGRESSIVE AND ALSO AHEAD OF ITS  
ENSEMBLE MEAN, AND THE 00Z CMC IS ON THE OTHER END OF THE SPECTRUM  
WITH A SLOWER SOLUTION AND WEAKER WITH THE SURFACE LOW THAT  
EMERGES OVER THE PLAINS.  THE ECMWF IS SLIGHTLY WEAKER WITH THE  
TROUGH BUT IS CLOSE TO THE CONSENSUS REGARDING TIMING, WHILST THE  
UKMET IS SLIGHTLY STRONGER.  THE 12Z NAM IS ALSO CLOSE TO TO  
ECMWF/UKMET REGARDING TIMING, SO A BLEND OF THOSE THREE SOLUTIONS  
SHOULD SERVE WELL AS A STARTING POINT IN THE FORECAST.  
  
CLIPPER SYSTEM REACHING NORTHERN PLAINS BY WEDNESDAY MORNING  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z GFS/00Z UKMET/00Z ECMWF BLEND  
CONFIDENCE: MODERATE  
  
A SUBTLE MID-LEVEL SHORTWAVE TRACKING AROUND THE UPPER RIDGE  
ACROSS SOUTHWESTERN CANADA WILL PROGRESS QUICKLY TOWARDS THE  
NORTHERN PLAINS IN STRONG NORTHWEST FLOW, ALONG WITH A  
CLIPPER-TYPE SURFACE LOW REACHING NORTH DAKOTA AND THEN MINNESOTA  
BY EARLY WEDNESDAY ALONG AN EXISTING QUASI-STATIONARY FRONT.  THE  
12Z NAM IS DISPLACED TO THE NORTHEAST WITH THE TRACK OF THIS LOW  
AND THE 00Z CMC HAS A SLIGHTLY WEAKER LOW.  
  
MODEL TRENDS AT WWW.WPC.NCEP.NOAA.GOV/HTML/MODEL2.SHTML  
500 MB FORECASTS AT WWW.WPC.NCEP.NOAA.GOV/H5PREF/H5PREF.SHTML  
  
HAMRICK  
  
  
 
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