262  
FXUS10 KWNH 051833  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
132 PM 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  
 
---18Z UPDATE---  
NO CHANGE TO THE PRELIMINARY PREFERENCE AS MODELS REMAIN IN GOOD  
AGREEMENT OVERALL WITH THIS SYSTEM.  
 
---PREV. DISCUSSION---  
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: NON-CMC BLEND  
CONFIDENCE: AVERAGE  
 
---18Z UPDATE---  
AS THE 12Z RUNS OF THE UKMET...CMC...AND ECMWF HAVE ARRIVED...THE  
TRENDS TOWARD A SLOWER AND STRONGER WAVE ALOFT AS IT DEPARTS THE  
ROCKIES HAVE CONTINUED. THE 12Z CMC HAS SLOWED DOWN THE WAVE  
OVERALL BUT IT STILL HAS THE TROUGH AXIS MUCH FURTHER EAST OF  
OTHER MODELS...SO IT REMAINS EXCLUDED FROM THE PREFERENCE. THE 12Z  
UKMET HAS MOVED INTO MUCH BETTER ALIGNMENT ON THE STRENGTH AND  
TIMING OF THE WAVE SO IT IS NOW INCORPORATED IN THE PREFERENCE.  
AND THE 12Z ECMWF IS NOW EVEN SLOWER...WITH A STRONGER CLOSED LOW  
OVER THE NWRN GULF OF MEXICO AT A LOWER LATITUDE THAN BEFORE.  
DESPITE THE NON-CMC MODELS SHOWING A SIMILAR IDEA AND  
TIMING...THEY DO VARY IN THEIR PLACEMENT OF THE UPPER LEVEL LOW.  
THE 12Z GFS/NAM ARE FURTHEST NORTH...THE UKMET IS IN THE  
MIDDLE...AND THE ECMWF IS FURTHEST SOUTH. GIVEN THE TRENDS AND THE  
DIGGING NATURE OF THE WAVE...IT MAY END UP CLOSER TO THE  
ECMWF/UKMET...BUT WILL PREFER A BLEND OF ALL FOUR DUE TO  
UNCERTAINTY.  
 
---PREV. DISCUSSION---  
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: NON-CMC BLEND  
CONFIDENCE: AVERAGE  
 
---18Z UPDATE---  
THE 12Z CMC AND 12Z UKMET SHOW MORE SIMILARITY WITH THE GFS...NAM  
AND ECMWF NOW IN THE OVERALL STRUCTURE OF THIS SHORTWAVE.  
HOWEVER...THE CMC CONTINUES TO HAVE LOWER HEIGHTS THROUGH THE  
GREAT LAKES REGION BY MONDAY AND THUS STILL HAS THE WAVE DIGGING  
MORE THAN OTHER MODELS. THE UKMET HAS BACKED OFF WITH ITS STRENGTH  
OF THE LEAD SHORTWAVE AND THUS MORE CLOSELY MATCHES THE OTHER  
MODELS. THEREFORE...THE UKMET IS NOW INCLUDED IN THE BLEND  
PREFERENCE...BUT THE CMC IS NOT.  
 
---PREV. DISCUSSION---  
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 / 12Z ECMWF  
CONFIDENCE: AVERAGE  
 
---18Z UPDATE---  
THE 12Z GFS AND 12Z ECMWF CONTINUE TO SHOW THE PREFERRED DEPICTION  
OF A CONCENTRATED SURFACE LOW ASSOCIATED WITH THE APPROACHING  
TROUGH. THE 12Z CMC HAS ALSO TRENDED CLOSER IN POSITION OF THE  
SURFACE LOW BUT REMAINS STRONGER WITH THE WAVE ALOFT. THE CHANGES  
ON THE UKMET WERE MINIMAL. THEREFORE...THE PREFERENCE IS TO  
MAINTAIN A BLEND OF THE GFS/ECMWF.  
 
---PREV. DISCUSSION---  
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  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page