950  
FXUS10 KWNH 211846  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
246 PM EDT TUE APR 21 2020  
 
VALID APR 21/1200 UTC THRU APR 25/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z MODEL EVALUATION WITH FINAL PREFERENCES AND CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
...DEVELOPING LONG WAVE TROUGH OVER NEW ENGLAND WITH AN ASSOCIATED  
SURFACE LOW...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: ABOVE AVERAGE  
 
THE 12Z NAM AND GFS CONTINUE TO REMAIN ON TRACK/CONSISTENT WITH  
THE TIMING AND DEPTH OF THE SHORTWAVE HELPING TO SWING THE LARGER  
SCALE TROF NEGATIVE TILT THROUGH THE NORTHEAST WED. THE RESULTANT  
TRIPLE POINT WILL TRACK NEAR THE GULF OF MAINE AND QUICKLY INTO  
SOUTHEAST CANADA. AS SUCH A GENERAL MODEL BLEND WILL CONTINUE TO  
BE PREFERRED AT ABOVE AVERAGE CONFIDENCE.  
 
...MID LEVEL TROUGH TRACKING FROM ACROSS THE SOUTHWEST AND  
SOUTHERN PLAINS TO THE MID ATLANTIC...   
..SURFACE LOW ASSOCIATED WITH THE MID LEVEL TROUGH
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z ECMWF/00Z ECENS BLEND WITH 12Z GEFS  
CONFIDENCE: AVERAGE  
 
19Z UPDATE: THE 12Z UKMET AND CMC BOTH TRENDED FASTER; WHILE  
STILL LAGGING THE ECMWF/ECENS/GEFS MEAN, THERE IS BETTER OVERALL  
TIMING/PLACEMENT RELATIVE TO THE ECMWF. THIS INCREASES CONFIDENCE  
OVERALL; YET NOT ENOUGH FOR INCLUSION THAT WOULD SMOOTH THE SIGNAL  
OF THE WAVE OUT TOO MUCH AT THIS POINT. AS SUCH GREATER  
CONFIDENCE IS GIVEN TO THE 12Z ECMWF SOLUTION. AGAIN, IT IS PAIRED  
WELL WITH THE 12Z GEFS SOLUTION. SO WILL SUPPORT A 12Z ECMWF/00Z  
ECENS BLEND WITH INCLUSION OF THE 12Z GEFS AS A COMPROMISE.  
CONFIDENCE IS INCREASED TO AVERAGE  
 
---PRIOR DISCUSSION---  
MEDIUM-SIZED SHORTWAVE IS STARTING TO TAKE BETTER SHAPE ACROSS THE  
SOUTHWEST TODAY, SUPPORTING LEE-CYCLOGENESIS ACROSS EASTERN NM.  
THERE IS SOLID MODEL AGREEMENT INITIALLY, THOUGH THE CMC IS A TAD  
SOUTH. THIS SMALL DIFFERENCE WITH THE 00Z CMC GROWS GREATER  
THROUGH THE MID-MS VALLEY THURSDAY AND IS CLEARLY LAGGING. THE  
WAVE IS WEAKENING SLIGHTLY THROUGH THE OHIO VALLEY INTO  
INCREASINGLY CONFLUENT FLOW...THIS LEADS TO INCREASED SPREAD IN  
TIMING OF THE SURFACE WAVE TRACKING THROUGH THE OHIO VALLEY TOWARD  
THE EAST COAST INTO FRIDAY. LIKE THE 00Z CMC, THE 00Z UKMET WILL  
LAG TOO, EVENTUALLY BECOMING LESS FAVORABLE. THE 12Z NAM  
CONTINUES A TREND OF STRONGER BUT ALSO FASTER INTO THIS CONFLUENT  
FLOW, WHILE THE GFS IS A BIT SLOWER BUT EQUALLY COMPACT AND  
FAVORING STRONGER FLUX CONVERGENCE INTO THE DEVELOPING DEFORMATION  
ZONE, OVER STRONGER CONVECTION; EVENTUALLY SLOWING ITS PROGRESS  
THROUGH THE OHIO VALLEY INTO THE EAST. THESE ARE KNOWN NEGATIVE  
BIASES OF THE GFS WOULD FAVOR THE 06Z GFS TIMING/INFLUENCE OVER  
THE OPERATIONAL 12Z GFS. THE NAM APPEARS TOO FAST, GIVEN ITS  
STRENGTH RELATIVE TO THE ENSEMBLE SUITE, SO A BIT  
COUNTER-INTUITIVE, STILL IT MAY NOT BE TOO OUT OF PHASE FOR SOME  
INCLUSION, ESPECIALLY THROUGH THE OHIO VALLEY.  
 
THE 00Z ECMWF IS TIMED WELL INITIALLY WITH THE GFS AND NAM AND  
ALSO FAVORS STRONGER CONVECTIVE DEVELOPMENT SOUTH, OPPOSITE TO THE  
GFS, AND MORE IN LINE WITH TYPICAL SPRING (RECENT) PATTERN. THIS  
HELPS TO ELONGATE/SHEAR THE LARGER SCALE SHORTWAVE, MAKING IT  
GENERALLY WEAKER THROUGH THE EAST COAST, PERHAPS A TAD TOO MUCH  
GIVEN INTER-COMPARISON WITH ECENS MEMBERS. AS SUCH A ECMWF HEAVY  
BLEND WITH THE 06Z GEFS AND 12Z NAM AND 00Z ECENS MEAN.  
CONFIDENCE IS SLIGHTLY BELOW AVERAGE IN THIS BLEND GIVEN THE LOWER  
PREDICTABILITY GIVEN IMPORTANCE OF CONVECTIVE SCALE INFLUENCE ON  
THE HIGHER LEVEL MASS FIELDS.  
 
...SHORT WAVE AND SURFACE LOW TRACKING FROM WESTERN CANADA TO THE  
NORTHERN PLAINS...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND-  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
19Z UPDATE: THE CMC MASS FIELD LOOK MUCH CLOSER OVERALL TO THE  
SUITE TO SUPPORT A GENERAL MODEL BLEND. HOWEVER, THE QPF (LIKELY  
DUE TO SLIGHTLY LAGGED MID-LEVEL MOISTURE FEED FROM THE SLOWER  
SOLUTION IN THE SOUTHERN STREAM) RESULTS IN QPF EXTENDING FURTHER  
WEST ACROSS N IL RELATIVE TO THE REMAINING SOLUTIONS, THAT REMOVAL  
OF THE UKMET MAY BE MORE PRUDENT ACROSS THIS AREA WITH RESPECT TO  
THIS ONE ASPECT OF THE SYSTEM.  
 
---PRIOR DISCUSSION---  
A SUBTLE SHORTWAVE/SPEED MAX IN NW FLOW OUT OF NW CANADA WILL DROP  
INTO THE NORTHERN PLAINS BY EARLY WED WITH SOLID MODEL AGREEMENT.  
THE 12Z NAM/GFS CONTINUE STRONG AGREEMENT WITH THE EVOLUTION OF  
THE THIS WAVE BEFORE IT WEAKENS ACROSS IA INTO THE GREAT LAKES,  
UNDER CONFLUENT FLOW REGIME AND APPROACHING STRONGER  
SHORTWAVE/CYCLONE FROM THE SOUTHERN PLAINS. THE 00Z ECMWF/UKMET  
ARE ALSO IN GOOD AGREEMENT WITH THE MASS FIELDS, AND MUCH LIKE THE  
GFS AND NAM CAN TAP SOME OF THE OVERRUNNING MOISTURE FROM THE  
PACIFIC/GULF MOISTURE TO PRODUCE A SOLID/NARROW SWATH OF QPF  
ACROSS THE SOUTHWEST-SOUTHERN GREAT LAKES REGION INTO THURSDAY.  
THE CMC MASS FIELDS ARE A BIT OFF WITH THE SURFACE LOW TRACK BUT  
THE PLACEMENT OF THE FRONTOGENESIS/QPF MAY BE TOLERABLE WITHIN A  
BLEND; YET ITS REMOVAL FOR MASS FIELDS WOULD LIKELY PRESENT A  
BETTER BLEND. CONFIDENCE IS SLIGHTLY ABOVE AVERAGE  
 
THE 00Z NAM/GFS ARE CLOSE TO THE CONSENSUS TRACKING SHORT WAVE  
ENERGY FROM WESTERN CANADA IN THE VERY FAST MID LEVEL FLOW ACROSS  
THE NORTHERN PLAINS BEFORE 23/00Z. THE 00Z NAM BECOMES SLOWER THAN  
THE 00Z GFS AFTER THIS TIME, EVENTUALLY PLACING THE TROUGH OVER  
WESTERN ND (AS DOES THE 00Z CMC, THOUGH NOT AS FAR WEST AS THE 00Z  
NAM). THE 00Z GFS IS CLOSE TO THE 00Z ECMWF/UKMET WITH THE TROUGH  
POSITION EXTENDING FROM ND SOUTH THROUGH NE BY 24/12Z. THERE IS  
MORE SPREAD WITH THE PLACEMENT OF THE RESULTING SURFACE LOW FROM  
ND INTO NE. SINCE THERE HAS BEEN SOME CONSISTENCY WITH THE 00Z  
GFS/00Z ECMWF/00Z UKMET WITH THIS FEATURE, THESE MODELS COMPRISE  
THE PREFERRED BLEND. HOWEVER, DUE TO THE SLOWER SOLUTIONS OF THE  
00Z NAM/00Z CMC AND THE SPREAD IN THE PLACEMENT OF THE SURFACE LOW  
AT THE END OF THE PERIOD, FORECAST CONFIDENCE IS ONLY AVERAGE.  
 
...SHORT WAVE ENERGY TRACKING FROM THE PACIFIC NORTHWEST TO THE  
SOUTHERN PLAINS...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND THROUGH 24.00Z  
12Z ECMWF/UKMET/CMC THEREAFTER  
CONFIDENCE: AVERAGE  
 
19Z UPDATE: THE 12Z UKMET/CMC/ECMWF ALL TRENDED A BIT SLOWER WITH  
THE CORE OF THE SHORTWAVE EMERGING INTO THE SOUTHERN PLAINS  
FRIDAY. GIVEN THE ANGLE IN WHICH THE WAVE IS TRACKING AND TIMING  
OF DAY, THINKING THIS MAY BE MORE THAN A SHIFT, THAN A TREND AS  
THE SHORTWAVE FEATURES UPSTREAM START TO RESOLVE BETTER WITH EACH  
RUN ITERATION. WHILE THE 12Z UKMET IS SLOWEST AND STRONGEST BY  
THE END OF DAY 3, THE CMC IS SLIGHTLY WEAKER THAN THE ECMWF AND  
THE 3 PAIR NICELY WITH THE LONGER TERM ENSEMBLE TREND, ANCHORED BY  
THE ECENS MEMBERS. WHILE THE 12Z GEFS DID SHIFT A TAD FASTER THAN  
THE ECENS, IT IS NOT A FAST/AGGRESSIVE AS THE GFS/NAM. AS SUCH, A  
GENERAL MODEL BLEND REMAINS PREFERRED THOUGH 24.00Z BUT BECOMING  
12Z ECMWF/CMC/UKMET BLEND THEREAFTER, WEIGHTING TOWARD THE ECMWF.  
 
---PRIOR DISCUSSION---  
A STRONG ANTICYCLONICALLY CURVED 250MB PACIFIC JET WILL HELP TO  
TAMP DOWN THE LARGER SCALE RIDGE IN THE EASTERN PACIFIC AND USHER  
SMALLER SCALE EMBEDDED SHORTWAVES ACROSS THE PACIFIC NORTHWEST  
STARTING LATE WED, THAT WILL PROGRESS THROUGH THE ROCKIES INTO  
MORE BAGGY, FLATTER/BROAD TROUGHING. THIS WILL ALLOW THE LEAD  
WAVE TO START TO AMPLIFY ACROSS THE BITTERROOTS BEFORE STRETCHING  
VERTICALLY ALONG THE LEE OF THE ROCKIES, SUPPORTING A LEE TROUGH  
AND SURFACE LOW IN THE HIGH PLAINS, THAT BUILDS SOUTH BY 24.00Z,  
WITH A STRONGER/DEEPER SURFACE WAVE IN THE TX PANHANDLE. THE DEEP  
SOUTHERLY RETURN FLOW WILL SUPPORT ACTIVE CONVECTION THROUGHOUT  
THE PLAINS INTO THE MS VALLEY. OVERALL, THIS IS A DIFFICULT  
PATTERN TO LOCK DOWN THE PRECISE PLACEMENT OF THESE CONVECTIVE  
CLUSTERS, THOUGH GIVEN THIS THE MODEL SPREAD IS FAIRLY CONSISTENT  
TO HAVE MODERATE CONFIDENCE IN A GENERAL MODEL BLEND THROUGH ABOUT  
72HRS (24.12Z). BY THIS TIME, THE 12Z NAM IS VERY STRONG AND  
FAST; TYPICAL OF DAY 3 NEGATIVE BIAS. THE 12Z IS A TAD FASTER, IN  
LINE WITH BIAS BUT NOT TERRIBLY SO. THE 06Z GEFS IS BETTER  
TIMED...AND FOR A MORE SOLID BLEND, PERHAPS SHOULD BE USED ON DAY  
3 OVER THE OPERATIONAL GFS. HOWEVER, WPC PREFERENCE AT THIS TIME,  
IS A GENERAL MODEL BLEND THROUGH 24.00Z, SHIFTING MORE NON-NAM  
THEREAFTER. CONFIDENCE IS AVERAGE GIVEN SMALL SPREAD BUT LOWER  
PREDICTABILITY/ FLOW REGIME.  
 
MODEL TRENDS AT WWW.WPC.NCEP.NOAA.GOV/HTML/MODEL2.SHTML  
500 MB FORECASTS AT WWW.WPC.NCEP.NOAA.GOV/H5PREF/H5PREF.SHTML  
 
GALLINA  
 

 
 
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