356  
FXUS10 KWNH 051653  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1152 AM EST FRI JAN 05 2018  
 
VALID JAN 05/1200 UTC THRU JAN 09/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z MODEL EVALUATION INCLUDING PREFERENCES AND FORECAST CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
...BROAD CYCLONIC FLOW AROUND LARGE FILLING LOW OVER EASTERN  
CANADA THROUGH SATURDAY NIGHT...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: ABOVE AVERAGE  
 
MODELS SHOW A HIGH DEGREE OF SIMILARITY OVER EASTERN CANADA AND  
THE NORTHEAST CONUS OVER THE NEXT COUPLE DAYS...SO A GENERAL MODEL  
BLEND IS PREFERRED WITH HIGH CONFIDENCE.  
 
   
..SOUTHERN BRANCH OF SPLITTING TROUGH ALONG NOAM WEST COAST
 
   
..MOVING INTO WASHINGTON AND OREGON TONIGHT
 
 
...DIGGING INTO SRN PLAINS BY SUNDAY WITH AN UPPER LEVEL LOW  
POTENTIALLY CLOSING OFF ALONG CENTRAL GULF COAST MON...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF 12Z NAM / 12Z GFS / 00Z ECMWF  
CONFIDENCE: AVERAGE  
 
DIFFERENCES WITH THIS TROUGH ARE RELATIVELY MINOR THROUGH SUNDAY  
MORNING (07/12Z) ALTHOUGH INITIAL DIFFERENCES WITH THE CMC AND  
UKMET GROW OVER TIME. THE 00Z CMC HAS HIGHER HEIGHTS AT THE BASE  
OF THE TROUGH INITIALLY RELATIVE TO OTHER MODELS...WHICH  
EVENTUALLY LEADS TO A LESS AMPLIFIED AND FASTER SOLUTION AS THE  
TROUGH PUSHES INTO THE SOUTHERN STATES. THE 00Z UKMET IS MORE  
SIMILAR TO THE OTHER MODELS INITIALLY...BUT IS ABOUT 6 HOURS  
SLOWER WITH THE PHASING OF SEVERAL SHORTWAVES NEAR THE PACIFIC  
NORTHWEST COAST...AND EVENTUALLY ENDS UP LAGGING THE TROUGH AXIS  
MORE SUBSTANTIALLY.  
 
THE TREND OVER THE PAST 3-4 MODEL CYCLES HAS BEEN TOWARD A SLOWER  
AND STRONGER WAVE AS IT EMERGES FROM THE ROCKIES AND MOVES TOWARD  
THE GULF COAST. THIS IS CURRENTLY BEST EXEMPLIFIED BY THE 12Z  
NAM...12Z GFS...AND 00Z ECMWF...WHICH ARE ALSO IN FAIRLY GOOD  
AGREEMENT OVERALL. THEREFORE A BLEND OF THE THREE MODELS IS  
PREFERRED AT THIS TIME.  
 
DIFFERENCES IN BOUNDARY LAYER TEMPERATURES ARE ALSO LIKELY TO  
PROVE IMPORTANT IN PRECIPITATION TYPE FORECASTS FROM PORTIONS OF  
THE CENTRAL U.S. INTO THE SOUTHEAST. DPROG/DT FOR THE 12Z NAM AND  
12Z GFS SHOW A TREND TOWARD SLIGHTLY COLDER SURFACE TEMPERATURES  
OVER THE PAST SEVERAL MODEL CYCLES ON SUNDAY AND MONDAY IN TWO  
MAIN REGIONS: (1) OVER THE SNOW PACK REGION IN PORTIONS OF THE  
MIDWEST...AND (2) IN THE LEE OF THE APPALACHIAN RIDGES FROM NRN GA  
INTO THE CAROLINAS. THE NAM IS GENERALLY MORE RESILIENT WITH COLD  
AIR IN THESE AREAS THAN THE GFS/ECMWF...BUT WOULD NOT BE SURPRISED  
TO SEE FURTHER TRENDS IN THIS DIRECTION. MODELS TEND TO HAVE A  
BIAS WITH ERODING SHALLOW ARCTIC AIR MASSES TOO QUICKLY...AND THE  
AIR MASS CURRENTLY IN PLACE IS RELATIVELY ANOMALOUS.  
 
   
..NORTHERN BRANCH OF SPLITTING TROUGH ALONG NOAM WEST COAST
 
 
...MOVING THROUGH THE CANADIAN ROCKIES THIS WEEKEND AND THEN  
DIGGING TO NEAR THE GREAT LAKES BY MONDAY...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF 12Z NAM / 12Z GFS / 00Z ECMWF  
CONFIDENCE: AVERAGE  
 
IN CONTRAST WITH HOW IT HANDLES THE SOUTHERN STREAM WAVE...THE 00Z  
CMC IS SLOWER AND MORE AMPLIFIED WITH THIS NORTHERN STREAM WAVE  
THAT WILL PASS NEAR THE US-CANADA BORDER ON SUNDAY AND MONDAY.  
THIS IS DIFFERENCE FROM THE OTHER MODELS AND DOES AFFECT SENSIBLE  
WEATHER IN THE GREAT LAKES REGION IN THE MODEL. THE 00Z UKMET IS  
MORE SIMILAR TO THE OTHER AVAILABLE MODELS...BUT IT IS FASTER WITH  
A LEAD SHORTWAVE...LEADING TO LOWER HEIGHTS ALOFT IN THE NORTHEAST  
REGION BY MONDAY. FOR THESE REASONS...THESE TWO MODELS ARE NOT  
INCLUDED IN THE PREFERENCE. THE NAM...GFS...AND ECMWF LOOK MORE  
SIMILAR AND A BLEND OF THE THREE IS PREFERRED.  
 
...NEXT UPPER LEVEL TROUGH OVER THE EASTERN PACIFIC APPROACHING  
THE WEST COAST SUNDAY NIGHT INTO MONDAY...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF 12Z GFS / 00Z ECMWF  
CONFIDENCE: AVERAGE  
 
FOR A 60+ HOUR FORECAST...THE MODELS SHOW A FAIR DEGREE OF  
SIMILARITY WITH HOW THEY HANDLE THIS TROUGH APPROACHING THE WEST  
COAST. THERE ARE SOME DETAIL DIFFERENCES. BY 09/00Z THE 12Z GFS  
AND 00Z ECMWF DEVELOP A MORE CONCENTRATED SURFACE LOW OFFSHORE OF  
NRN CALIFORNIA...WHILE THE 12Z NAM AND 00Z UKMET HAVE A MORE  
ELONGATED (AND WEAKER) AREA OF LOW PRESSURE OFFSHORE OF THE WEST  
COAST. MOST MODELS DO SHOW SOME DEGREE OF JET COUPLING OFF THE  
WEST COAST AND A MORE CONCENTRATED AREA OF PVA...SO THE GFS/ECMWF  
IDEA OF A MORE CONCENTRATED LOW IS PREFERRED. THE 00Z CMC DEVELOPS  
A MUCH STRONGER CLOSED LOW ALOFT FURTHER WEST THAN THE TROUGH AXIS  
ON OTHER MODELS...BETWEEN 30-35N LATITUDE. THIS IS NOT SUPPORTED  
BY OTHER AVAILABLE MODELS SO IT WAS ALSO EXCLUDED FROM THE  
PREFERENCE.  
 
MODEL TRENDS AT WWW.WPC.NCEP.NOAA.GOV/HTML/MODEL2.SHTML  
500 MB FORECASTS AT WWW.WPC.NCEP.NOAA.GOV/H5PREF/H5PREF.SHTML  
 
LAMERS  
 

 
 
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