309  
FXUS10 KWNH 031824  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
124 PM EST WED JAN 03 2018  
 
VALID JAN 03/1200 UTC THRU JAN 07/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z MODEL EVALUATION INCLUDING PREFERENCES AND FORECAST CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
...EJECTING NEGATIVE TILT TROUGH ALONG THE EASTERN SEABOARD WITH  
RAPID DEEPENING OF SURFACE CYCLONE TODAY INTO THURSDAY...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF THE 12Z GFS, 12Z ECMWF, 12Z GEFS, 00Z EC ENS  
CONFIDENCE: AVERAGE  
 
---18Z UPDATE---  
NO CHANGE TO THE INITIAL REASONING. THE 12Z RUNS OF THE  
UKMET...CMC...AND ECMWF HAVE ALL TRENDED AS OTHER MODELS  
DID...WITH A MORE WESTWARD POSITIONING OF THE SYSTEM AND SLIGHTLY  
HIGHER QPF ONSHORE ALONG THE EAST COAST. THIS FURTHER INCREASES  
CONFIDENCE IN THE OVERALL TREND. THE INITIAL BLEND IS STILL  
PREFERRED. FURTHER TRENDS TO THE WEST CANNOT BE DISCOUNTED...BUT  
MODELS HAVE CONTINUED TO CONVERGE OVERALL.  
 
---PREV. DISCUSSION---  
MODEL TRENDS CONTINUE TO BE TO GRADUALLY NUDGE THE POSITION OF THE  
EVOLVING CYCLONE FURTHER WEST. THIS HAS BEEN APPARENT ON THE 12Z  
GFS AND NAM...AND IS PRETTY CONSISTENT ACROSS THE SPECTRUM OF  
AVAILABLE MODELS. OVER THE PAST THREE CYCLES (02/00Z TO  
03/00Z)...THE ECMWF ENSEMBLE HAS NUDGED THE POSITION OF THE  
SURFACE LOW ON THURSDAY MORNING ABOUT 60MI TO THE WSW INDICATING A  
WESTWARD AND SLIGHTLY SLOWER TREND OVERALL...AND THIS TREND  
MAGNITUDE IS SIMILAR TO OTHER MODELS.  
 
CONFIDENCE IN THE WESTWARD TREND IS INCREASED SLIGHTLY BY THE  
LATEST GOES-16 WATER VAPOR LOOPS WHICH SHOW THE BASE OF THE  
EJECTING WAVE IN THE SOUTHEAST TODAY GENERALLY STAYING NORTH OF  
THE GULF OF MEXICO...AND THE 12Z NAM AND GFS INITIALIZING A LITTLE  
TOO LOW WITH 500MB HEIGHTS IN NERN NEW ENGLAND AND THE CANADIAN  
MARITIMES (20M TOO LOW MAINE TO NOVA SCOTIA). A SLIGHT NORTHWARD  
TREND IN THE EJECTING WAVE...AND A MORE AMPLIFIED UPSTREAM  
RIDGE...WOULD TEND TO FAVOR A SLIGHTLY SLOWER/DEEPER SYSTEM THAT  
IS IN CLOSER PROXIMITY TO THE COAST. THE 12Z NAM AND GFS DID MAKE  
SOME MINOR ADJUSTMENTS TO THE NORTH WITH THE EJECTING WAVE.  
 
THE 12Z NAM CONTINUES TO PRODUCE THE MOST PRECIPITATION IN THE  
COLD SECTOR AS THE LOW PUSHES UP THE COAST...AND THIS IS A TYPICAL  
BIAS IN THE MODEL. IT IS ALSO FURTHEST WEST WITH THE SURFACE  
LOW...ALTHOUGH THE MODELS OVERALL HAVE CONVERGED OVER THE PAST  
COUPLE MODEL CYCLES. REMAINING DETAIL DIFFERENCES WOULD AFFECT  
SOME SENSIBLE WEATHER IMPACTS ALONG THE PTYPE AND PRECIP  
GRADIENTS...SO TRENDS OVER THE NEXT COUPLE MODEL CYCLES WILL STILL  
BE IMPORTANT TO MONITOR.  
 
A USAF RECONNAISSANCE PLANE IS SCHEDULED TO SAMPLE THE LOW OFF THE  
SOUTHEAST ATLANTIC COAST THIS EVENING AS THE LOW IS LIKELY TO BE  
RAPIDLY DEEPENING. THESE ADDITIONAL OBSERVATIONS MAY HELP NARROW  
THE MODEL SPREAD A LITTLE FURTHER...PARTICULARLY AS ENSEMBLE  
SENSITIVITY ANALYSIS SHOWS THE ULTIMATE LOW TRACK AND TIMING IS  
SENSITIVE TO CONDITIONS OFF THE COASTAL CAROLINAS THIS EVENING.  
 
OTHERWISE...THE 12Z GFS...06Z GEFS...AND 00Z ECMWF AND EC ENS MEAN  
APPEAR TO OFFER SIMILAR SOLUTIONS WITH THEIR MASS FIELDS AND VERY  
SIMILAR LOW TRACKS AND INTENSITIES. THEREFORE A BLEND OF THESE  
MODELS IS PREFERRED FOR THIS SYSTEM. HOWEVER...STORM STRUCTURE  
WILL PLAY A ROLE IN QPF DISTRIBUTION AND BANDING. FOR MORE ON  
THAT...PLEASE REFER TO THE QPF AND HEAVY SNOW DISCUSSIONS FROM WPC.  
 
   
..UPPER LEVEL TROUGH APPROACHING WEST COAST BY FRIDAY
 
   
..DEAMPLIFYING AND PUSHING INTO THE INTERMOUNTAIN WEST SAT
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 06Z GEFS MEAN / 00Z EC ENS MEAN  
CONFIDENCE: BELOW AVERAGE  
 
---18Z UPDATE---  
NO CHANGE TO THE PRELIMINARY PREFERENCE. OPERATIONAL MODELS ON THE  
12Z CYCLE CONTINUE TO SHOW QUITE A BIT OF VARIABILITY...SO THE  
MASS FIELD PREFERENCE WILL CONTINUE TO BE TOWARD THE ENSEMBLE  
MEANS UNTIL BETTER AGREEMENT OCCURS.  
 
---PREV. DISCUSSION---  
MODELS ARE SHOWING FAIRLY SIMILAR SENSIBLE WEATHER IMPACTS OVERALL  
WITH THIS APPROACHING TROUGH...BUT SHOW MARKEDLY DIFFERENT  
EVOLUTIONS OF THE MASS FIELDS ALOFT. THE 00Z ECMWF AND 00Z CMC  
BOTH FAVOR CLOSING OFF A FAIRLY STRONG LOW ALOFT NEAR 50N AS IT  
APPROACHES THE COAST. THE 12Z NAM AND GFS AND 00Z UKMET SHOW AN  
OPEN TROUGH...BUT ALSO DIFFER ON SOME DETAILS. GIVEN THE  
VARIABILITY...PREFER TO BLEND THE ENSEMBLE MEANS (GEFS AND ECMWF)  
TO ACCOUNT FOR THE DIFFERENT POSSIBILITIES. IN GENERAL...THE  
DEVELOPMENT OF A CLOSED LOW SEEMS THE LESS LIKELY SOLUTION.  
DESPITE A RIDGE BUILDING IN ABOVE THE CORE OF THE TROUGH (INTO THE  
GULF OF ALASKA)...THE ECMWF/CMC SOLUTIONS MAY BE BREAKING DOWN THE  
NORTHERN EXTENT OF THE TROUGH A LITTLE TOO READILY. HOWEVER...THE  
SOLUTION CANNOT BE TOTALLY DISCOUNTED.  
 
PREVIOUS MODEL TRENDS AT WWW.WPC.NCEP.NOAA.GOV/HTML/MODEL2.SHTML  
PREVIOUS 500 MB FORECASTS AT  
WWW.WPC.NCEP.NOAA.GOV/H5PREF/H5PREF.SHTML  
 
LAMERS  
 

 
 
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