446  
FXUS10 KWNH 021846  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
145 PM EST TUE JAN 02 2018  
 
VALID JAN 02/1200 UTC THRU JAN 06/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z MODEL EVALUATION INCLUDING PREFERENCES AND FORECAST CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
   
..SEPARATE WAVES DIGGING FROM THE CENTRAL TO EASTERN CONUS
 
 
...PHASING OF THE NORTHERN AND SOUTHERN STREAM WAVES WITH RAPID  
DEEPENING OF A COASTAL CYCLONE ALONG THE EASTERN SEABOARD  
WEDNESDAY INTO THURSDAY...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
GREATEST WEIGHT ON 12Z GEFS MEAN / 00Z EC ENS MEAN  
LEAST WEIGHT ON THE 12Z NAM  
CONFIDENCE: SLIGHTLY BELOW AVERAGE  
 
---18Z UPDATE---  
THE OVERALL TREND ON THE 12Z MODEL CYCLE WAS FOR A FASTER  
DEEPENING RATE OF THE SURFACE LOW OFF THE EAST COAST BETWEEN  
03/18Z AND 04/18Z...WHICH HAS ALSO TRENDED THE SURFACE LOW TRACKS  
FURTHER TO THE WEST IN THAT TIME FRAME. AS THE LOW OCCLUDES OFF  
THE NORTHEAST COAST BEYOND 04/18Z...THE CHANGES IN THE SURFACE LOW  
TRACK ARE SMALLER THAN FURTHER DOWN THE COAST. DESPITE THE OVERALL  
TREND...DIFFERENCES DO REMAIN. THE 12Z GFS IS FAIRLY CLOSE TO THE  
12Z GEFS MEAN...BUT THE OPERATIONAL RUN DOES REMAIN SLIGHTLY  
FASTER AND FURTHER EAST. THEREFORE...THE SUGGESTION FROM ENSEMBLES  
APPEARS TO BE THAT A CONTINUATION OF THE SLIGHT/GRADUAL WESTWARD  
TREND COULD CONTINUE.  
 
THE PREFERENCE WAS TO MAINTAIN A BROADER BLEND FOR THE MASS FIELDS  
AS THE BIG PICTURE AND EVOLUTION ARE SIMILAR ON ALL THE  
MODELS...THE DIFFERENCES ARE MAINLY IN PLACEMENT AND EXACT TIMING.  
LESS WEIGHT PLACED ON THE 12Z NAM...WITH GREATER WEIGHT TOWARD THE  
12Z GEFS MEAN AND 00Z ECMWF ENSEMBLE MEAN.  
 
---PREV. DISCUSSION---  
MODELS ARE IN AGREEMENT THAT SEVERAL SHORTWAVES WILL DIG INTO THE  
EASTERN CONUS...SPLIT INTO A NORTHERN STREAM WAVE TRACKING FROM  
THE CANADIAN PRAIRIES TO THE MID ATLANTIC...AND A SOUTHERN STREAM  
WAVE TRACKING FROM THE CENTRAL PLAINS TO THE GULF COAST AND THEN  
KICKING ENE TO THE COASTAL CAROLINAS. THEY ARE ALSO IN AGREEMENT  
THAT THE NORTHERN AND SOUTHERN STREAMS SHOULD PHASE AS THE BROADER  
TROUGH APPROACHES THE EAST COAST...SETTING UP EXPLOSIVE  
CYCLOGENESIS AIDED BY A PRE-EXISTING BAROCLINIC ZONE...AND  
REINFORCED BY THE PROXIMITY OF THE GULF STREAM.  
 
ALTHOUGH THE GENERAL PROGRESSION IS WELL AGREED UPON...THERE  
REMAINS A FAIR DEGREE OF UNCERTAINTY INTO THE EXACT TRACK OF THE  
EVOLVING CYCLONE...AND GIVEN PROXIMITY TO THE EAST COAST THIS IN  
TURN PROVIDES UNCERTAINTY IN SENSIBLE WEATHER IMPACTS.  
 
MOST OF THE MODELS HAVE SHOWN A GRADUAL TREND TOWARD A SURFACE LOW  
TRACK THAT IS FURTHER WEST WITH A FASTER DEEPENING RATE. SUCH A  
TREND IS NOT UNCOMMON AS A NOR'EASTER EVENT APPROACHES. THE 12Z  
NAM CONTINUES TO BE ON THE WESTERN EDGE OF THE OPERATIONAL  
MODELS...WHICH MAY GROW PARTIALLY OUT OF LOWER SURFACE PRESSURES  
IT IS DEPICTING GENERALLY ACROSS THE SOUTHEAST OVER THE FIRST  
24-36 HOURS OF THE FORECAST. THE 12Z UKMET JUST CAME IN WITH A  
SOLUTION THAT IS SLOWER AND CLOSER TO THE COAST AS WELL...BUT  
STILL NOT QUITE AS FAR WEST AS THE NAM.  
 
FOR THE 00Z/06Z CYCLE...THE GFS AND ECMWF WERE BOTH EAST OF THEIR  
ENSEMBLE MEANS...AND THE 12Z GFS HAS TRENDED SLIGHTLY WEST SUCH  
THAT IT IS MUCH CLOSER TO THE 06Z GEFS MEAN. THAT THE OPERATIONAL  
RUNS WERE EAST OF THEIR ENSEMBLE MEANS SUGGESTS THERE IS STILL A  
REASONABLE EXPECTATIONS THAT THE WESTWARD TREND MAY CONTINUE FOR  
ANOTHER MODEL CYCLE OR TWO.  
 
DESPITE THE NAM BEING ON THE WESTERN CUSP OF ENSEMBLE SPREAD AND  
NOTABLY FURTHER WEST THAN THE GLOBAL MODELS...GIVEN THE TRENDS IT  
HAS NOT BEEN TOTALLY DISCOUNTED. THE PREFERENCE ON THE MASS FIELDS  
FOR THIS SYSTEM IS CURRENTLY TO USE A GENERAL MODEL BLEND. THIS  
WILL MORE FULLY CAPTURE THE LINGERING DEGREE OF UNCERTAINTY THAN  
LOCKING IN ON A PARTICULAR MODEL OR TWO. HOWEVER...LESS WEIGHT IS  
PLACED ON THE 12Z NAM WITH GREATER WEIGHT ON THE 06Z GEFS MEAN AND  
00Z ECMWF ENSEMBLE MEAN TO BETTER REPRESENT AN APPROXIMATE  
COMPROMISE SOLUTION AT THIS TIME.  
 
...CLOSED UPPER LEVEL LOW NEAR 30N/133W BECOMING ABSORBED BY AN  
APPROACHING EASTERN PACIFIC TROUGH...  
...UPPER LEVEL TROUGH AND ASSOCIATED SURFACE LOW APPROACHING THE  
WEST COAST BY FRIDAY...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: AVERAGE  
 
---18Z UPDATE---  
THE 12Z CMC HAS COME INTO MUCH BETTER ALIGNMENT WITH THE OTHER  
GLOBAL MODELS IN HOW THE CLOSED LOW INTERACTS WITH THE APPROACHING  
TROUGH. IT NO LONGER SHOWS SUBSTANTIALLY MORE AMPLIFICATION TO THE  
TROUGH THROUGH THE NEXT 24 HOURS. DETAIL DIFFERENCES REMAIN. THE  
12Z ECMWF STILL SHOWS TWO DISTINCT WAVES WITH THE NORTHERN ONE  
(OFFSHORE BRITISH COLUMBIA) STRONGER...AND THE 12Z CMC AND UKMET  
ALSO SHOW TWO WAVES BUT THE EVOLUTION IS DIFFERENT FROM THE ECMWF.  
AFTER INCORPORATING THE NEW CMC...THE PREFERENCE CONTINUES TO BE  
FOR A GENERAL BLEND WHICH WILL BEST ACCOUNT FOR THE RANGE OF  
SOLUTIONS BEING PRESENTED BY MODELS AT THIS TIME.  
 
---PREV. DISCUSSION---  
THE 00Z CMC BEGINS SHOWING MORE SUBSTANTIAL DEVIATIONS FROM OTHER  
GLOBAL MODELS WITH THE APPROACHING TROUGH BY TONIGHT AROUND  
150W...WITH GREATER AMPLIFICATION IN PARTICULAR BETWEEN 30N AND  
40N. THE 00Z CMC ENSEMBLE MEAN WAS MUCH CLOSER TO THE OTHER  
ENSEMBLE MEANS AND GLOBAL MODELS WITH ITS DEPICTION OF THE MASS  
FIELDS. THIS LEADS THE OPERATIONAL CMC INTO SHOWING MUCH GREATER  
DIFFERENCES BY THURSDAY 04/12Z WITH A SIGNIFICANTLY SLOWER AND  
MORE AMPLIFIED TROUGH. GIVEN THE LACK OF SUPPORT FROM OTHER  
OPERATIONAL GLOBAL MODELS OR EVEN ITS ENSEMBLE MEAN...THE 00Z CMC  
IS EXCLUDED FROM THE PREFERENCE FOR THIS SYSTEM.  
 
FURTHER DIFFERENCES EXIST IN THE OTHER AVAILABLE OPERATIONAL  
MODELS. AS THE CLOSED LOW GETS ABSORBED BY THE APPROACHING  
TROUGH...THE 00Z ECMWF AND UKMET SHOW THE SOUTHERN AND NORTHERN  
STREAM WAVES TENDING TO SPLIT MORE INTO TWO DISTINCT WAVES BY  
FRIDAY 05/12Z...WHILE THE 12Z NAM AND GFS SHOW A MORE CONTINUOUS  
TROUGH OVERALL. THE RESULT IS A SLOWER ARRIVAL OF THE MORE  
SIGNIFICANT HEIGHT FALLS ON THE ECMWF AND UKMET...WHICH MAY LEAD  
TO SOME DIFFERENCES IN SENSIBLE WEATHER IMPACTS. HOWEVER...THE  
PREFERENCE IS FOR A GENERAL BLEND OF THESE TWO IDEAS TO BETTER  
INCORPORATE THE TIMING UNCERTAINTIES RELATED TO THE SPECIFIC  
EVOLUTION OF THE TROUGH.  
 
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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