696  
FXUS10 KWNH 211841  
PMDHMD  
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
141 PM EST THU DEC 21 2017  
 
VALID DEC 21/1200 UTC THRU DEC 25/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z MODEL EVALUATION INCLUDING PREFERENCES AND FORECAST CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
...S/W EMERGING FROM SOUTHERN STREAM ELONGATING/PHASING WITH S/W  
IN BOUNDARY WATERS OF MN WITH ASSOCIATED WEAK SFC WAVE/NORTHERN  
PORTION OF SURFACE TROF ENTERING NEW ENGLAND FRI/EARLY SAT...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
GOES-16 RGB/WV SUITE DEPICT A NORTHERN STREAM S/W CURRENTLY IN THE  
BOUNDARY WATERS OF MN AS WELL A S/W ENERGY EMERGING OUT OF THE CO  
ROCKIES TOWARD NEB...ENHANCING SURFACE WAVE DEVELOPMENT ACROSS  
CENTRAL KS. THE NORTHERN STREAM ACCELERATES EAST WHILE THE CO S/W  
ELONGATES IN ITS WAKE LEADING TO A GENERALLY FLAT MID-LEVEL FLOW  
ACROSS SOUTHEAST CANADA. HOWEVER...THE SWLY LOW LEVEL FLOW  
COUPLED WITH THE BLOCKING HIGH OVER S QUEBEC/NEW BRUNSWICK LEAD TO  
A MESSY WINTER WEATHER SETUP ACROSS NEW ENGLAND. SLIGHT VARIATION  
IN TIMING ESPECIALLY WITH THE DOWNSTREAM S/W ENTERING BY SUN (SEE  
SECTION BELOW) TRY TO PRESS WARM AIR IN...ESPECIALLY THE 12Z  
GFS...PLEASE REFER TO WPC WINTER WX DISCUSSION FOR ADDITIONAL  
DETAIL/MODEL PREFERENCES WITHIN THE QPFHSD. MASS-WISE THE 12Z NAM  
REMAINS A BIT MORE ENHANCED/STRONGER ALOFT BUT ONLY MANIFESTS A MB  
OR TWO DEEPER ACROSS THE THE OH VALLEY THAN THE 12Z GFS/00Z ECWMF.  
OVERALL A GENERAL MODEL BLEND SEEMS APPROPRIATE FOR THE MASS  
FIELDS THROUGH EARLY SAT (BEFORE SOUTHERN STREAM ROLLS IN...SEE  
BELOW). THERMAL DIFFERENCES LEAD TO SOME REDUCTION IN CONFIDENCE  
TO SLIGHTLY ABOVE AVERAGE.  
 
19Z UPDATE: NO SIGNIFICANT CHANGES JUST PERHAPS A BIT TIGHTER  
OVERALL SOLUTIONS INCLUDING THERMAL FIELDS TO GAIN SOME OVERALL  
CONFIDENCE BUT DO NOT SEE A REAL REASON TO ADJUST FROM INITIAL  
PREFERENCE/CONFIDENCE INTERVAL.  
 
...SOUTHERN PORTION OF MAIN TROF OVER GREAT BASIN TODAY EMERGING  
INTO SOUTHERN PLAINS AND SPURRING SFC WAVE THAT CROSSES LOWER MS  
EARLY SAT AND THRU NEW ENGLAND/MID-ATLANTIC SUN...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NON-CMC BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
GOES-WV MOSAIC DEPICTS MAIN/STRONG UPPER LOW OVER THE GREAT BASIN  
CONTINUING TO DROP SOUTHEAST. ADDITIONALLY A STRONG JET  
STREAK/EASTERN PACIFIC MID-LEVEL S/W THAT WILL BE HIGHLY  
INFLUENTIAL IN THE MOISTURE/QPF AXIS ACROSS THE LOWER MS VALLEY IS  
STARTING TO CROSS THE MEXICAN PLATEAU. SPREAD CONTINUES TO  
TIGHTEN AS MANY OF THE LAGGING/DEEPER/SLOWER CONCENTRIC SOLUTIONS  
ACROSS WEST TX ARE REDUCING OR PICKING UP PACE LIKE PRIOR DAYS  
PREFERENCES. HOWEVER THE 00Z CMC CONTINUES TO BE THE HOLD OVER  
AND CAN BE OUTRIGHT REJECTED. THE TIMING OF THE SWLY STREAM S/W  
IS QUITE STRONGLY AGREED UPON WITH THE REMAINING GUIDANCE BUT THE  
MAIN WAVE EMERGING OUT INTO THE SOUTHERN PLAINS CONTINUES TO SHOW  
TIMING DIFFERENCES. THE GFS HAS SHIFTED TOWARD TYPICAL FASTER  
TREND ESPECIALLY TOWARD SUN AS IT LIFTS INTO NEW ENGLAND THOUGH  
THE COASTAL LOW REMAINS CLOSE ENOUGH TO THE 12Z NAM/00Z ECMWF.  
THE 00Z UKMET IS ALSO QUITE STRONGLY AGREED WITH THE TIMING OF THE  
UPPER LOW HOWEVER...SOME FASTER NORTHERLY STREAM S/W INFLUENCES  
(SEE SECTION BELOW) LEADS TO GREATER INTERACTION AND INFLUENCES  
THE SURFACE LOW DEPTH/TRACK BY BENDING IT WEST ACROSS ME INTO NB  
BY SUN MAKING IT A BIT MORE SUSPECT IN NEW ENGLAND (THOUGH QUITE  
GOOD ACROSS MS/TN VALLEY). THE 12Z NAM HAS TRENDED POSITIVELY  
BOTH WEAKER ALOFT AND TRACKING MORE NORTHEAST THAN PRIOR RUNS AND  
MORE ALIGNED WITH PRIOR PREFERENCES. AS SUCH A NON-CMC BLEND IS  
PREFERRED (THOUGH HEDGING AWAY FROM UKMET TOWARD END OF PERIOD)  
AND CONFIDENCE IS SLIGHTLY ABOVE AVERAGE.  
 
19Z UPDATE: FURTHER TIGHTENING OF THE TIMING OF THE WAVE WAS NOTED  
ESPECIALLY GIVEN SMALL FASTER ADJUSTMENT BY THE ECMWF...MORE  
MODERATE SLOWING OF THE 12Z UKMET TOWARD THE ECMWF AND EVEN A  
MODERATE TRANSITION OF THE 12Z CMC TOWARD THE CONSENSUS; HOWEVER  
THE CMC REMAINS A CLEAR SLOW OUTLIER. THE 12Z GFS/UKMET FURTHER  
TIGHTEN THEIR AGREEMENT EVEN THROUGH SUNDAY MORNING WITH TH  
SURFACE WAVE TURNING LEFT COMPARED TO THE ECMWF/NAM. STILL THE  
SPREAD IS MINOR TO SUGGEST KEEPING A NON-CMC THROUGHOUT AT  
SLIGHTLY ABOVE AVERAGE CONFIDENCE.  
 
...LEAD/TOP OF TROF ACROSS N CANADIAN ROCKIES CROSSING CANADIAN  
PRAIRIES FRI WITH SURFACE WAVE/COLD FRONT THEN SLIDING THROUGH  
GREAT LAKES SUN ENHANCING SOUTHERN STREAM...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NON-CMC BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
TOP OF TROF IN NW CANADA HAS CROSSED THE NORTHERN CANADIAN ROCKIES  
INTO ALBERTA/SASKATCHEWAN AND IS SHOWING SIGNS OF VERTICAL  
STRETCHING/TIGHTENING OF THE CIRCULATION IN WV ALREADY...THIS WILL  
SWING THROUGH CENTRAL CANADA WITH A SURFACE WAVE AND USHER IN THE  
FIRST PUNCH OF ARCTIC AIR ACROSS THE NORTHERN TIER BY LATE  
FRI/EARLY SAT. AS IT ROUNDS THE BASE OF THE GROWING LARGER SCALE  
TROF INTO THE GREAT LAKES THE 00 UKMET SHIFTS A BIT TOO FAST  
RELATIVE TO THE OTHER GUIDANCE/ENSEMBLE SUITE. THIS TUGS THE  
SOUTHERN STREAM A BIT NORTH BUT BEING MORE PHASED/INTERACTIVE IS A  
BIT TOO MUCH TO FAVOR IT ESPECIALLY IN THIS STREAM. ON THE OTHER  
SIDE OF THE SPECTRUM...WITHOUT A SOUTHERN STREAM WAVE...THE 00Z  
CMC IS EXTREMELY FLAT ALLOWING THE WAVE TO ACCELERATE INTO QUEBEC  
TOO FAST AS WELL. THE 12Z NAM SHOWS SOME SMALL INTERNAL STRENGTH  
VARIATION FROM THE 00Z RUN BUT NOT DRAMATICALLY SO...LIKEWISE WAS  
NOTED IN THE 12Z GFS. ALL IN ALL A 12Z NAM/GFS AND 00Z ECMWF  
BLEND IS PREFERRED AT SLIGHTLY ABOVE AVERAGE CONFIDENCE.  
 
19Z UPDATE: THE CMC REMAINS FLAT AND GENERALLY SLOWER BUT THE  
UKMET HAS TRENDED FAVORABLY SLOWER TO MATCH THE 12Z GFS/NAM AND  
WITH LITTLE CHANGE THE 12Z ECMWF. ENOUGH SO TO SUPPORT A NON-CMC  
BLEND AT SLIGHTLY ABOVE AVERAGE CONFIDENCE.  
 
...TRAILING PORTION OF TROF IN NW CANADA ENTERS US ROCKIES FRI/SAT  
AND CARVES OUT BASE OF VAST ARCTIC VORTEX ACROSS GREAT LAKES/OHIO  
VALLEY SUN INTO MON...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z GFS/UKMET/ECMWF BLEND  
CONFIDENCE: SLIGHTLY BELOW AVERAGE  
 
THE TAIL END OF THE TROF SLOWLY DROPS INTO SW BC BY FRI/SAT UNDER  
FAIRLY TIGHT AGREEMENT. HOWEVER...AFTER THAT TIME SPREAD RAPIDLY  
INCREASES MAINLY DUE TO DIFFERENCES IN STRENGTH/POSITION OF THE  
UPSTREAM OMEGA BLOCK AND SWLY FLOW TRYING TO UNDER CUT THE BLOCK.  
THE 12Z GFS/NAM AND 00Z UKMET ARE MORE AGGRESSIVE/FAST WITH THE  
SWLY FLOW...BREAKING THE RIDGE INCREASING THE UPPER JET/FLOW AND  
KICKING THE SHORTWAVE SE ACROSS THE US ROCKIES ALLOWING FOR THE  
ARCTIC PLUNGE TO START EARLY RESPECTIVE TO THE ECMWF/CMC. THE 00Z  
CMC EVEN IS WEAKER WITH THE BLOCK THAN THE ECMWF/ECENS MEMBERS AND  
IS EVEN SLIGHTLY FASTER GIVING A BIT MORE CONFIDENCE TO A FASTER  
SOLUTION. THE OVERALL TRENDS FAVOR THIS SLIGHTLY WEAKER OMEGA  
BLOCK AND UNDERCUTTING FLOW. HOWEVER...THE 12Z NAM IS VERY QUICK  
ACCELERATING EVEN OUTPACING MUCH OF THE GEFS MEMBERS. AS A  
COMPROMISE A 12Z GFS/00Z UKMET BLEND IS PREFERRED OVER THE  
ECMWF/ECENS MEAN. CONFIDENCE IS SLIGHTLY BELOW AVERAGE.  
 
19Z UPDATE: NO SIGNIFICANT CHANGE IN TIMING WITH RESPECT TO THE  
UKMET MATCHING THE GFS WELL (THOUGH MORE SMOOTH THROUGH THE BASE  
OF THE TROF/UL JET STRUCTURE IN COMPARISON). THE 12Z ECMWF/CMC  
BOTH SHOWED A FASTER TRENDING THOUGH WITH A GREATER/EARLIER  
INJECTION OF SWLY FLOW UNDER THE OMEGA BLOCK RIDGE BY THE 12Z  
ECMWF ALLOWED IT TO CATCH UP TO THE GFS/UKMET AS WELL AS OUTPACING  
THE 12Z CMC. THE CMC IS SLOW ENOUGH TO KEEP OUT OF THE  
PREFERENCE. OVERALL THE TIMING/DEPTH OF THE WAVE THROUGH THE BASE  
OF THE LARGE SCALE TROF AS WELL AS THE SOUTHWARD DIRECTING OF THE  
POLAR COLD POOL...AND FOR A TIGHTER BLEND WILL INCLUDED THE ECMWF  
BUT KEEP THE CMC OUT. CONFIDENCE IS INCREASED TOO TO AVERAGE IN  
THE 12Z GFS/UKMET/ECMWF BLEND.  
 
...PACIFIC SHORTWAVE ENERGY TRYING TO SLIDE UNDER CANADIAN WEST  
COAST OMEGA BLOCK LATE SUN INTO PACIFIC NW/CALIFORNIA...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z GFS/UKMET BLEND  
CONFIDENCE: SLIGHTLY BELOW AVERAGE  
 
LARGE OMEGA BLOCK SET UP OVER THE EASTERN GULF OF AK/W BC BY LATE  
SAT AND THE DIFFERENCES IN STRENGTH PLAY A SIGNIFICANT ROLE IN THE  
APPROACH OF THE NEXT TROF ENERGY THAT REACHES THE WEST COAST LATE  
SUN INTO MONDAY. THE 00Z ECMWF/ECENS MEAN ARE PARTICULARLY  
STUBBORN WITH THE RIDGE MUCH STRONGER THAN OTHER GUIDANCE AS WELL  
AS DELAYING ENERGY TRYING TO UNDER CUT THE BLOCK INTO WA/OR BY 00Z  
MON...A SUCH THE DELAY ALLOWS ENERGY TO BULK/LOAD IN THE SOUTHERN  
PORTION OF THE TROF IN THE CENTRAL PACIFIC AND LEADS TO A VERY  
DEEP SURFACE WAVE. THIS IS OPPOSED BY INITIALLY STRONGER SWLY  
FLOW UNDERCUTTING THE FLOW PRESENTED BY THE 12Z NAM/GFS AND 06Z  
GEFS MEMBERS...THIS IS FOLLOWED SLIGHTLY IN TIMING WITH THE UKMET  
WITH THE CMC FILLING THE GAP OF THE UKMET/ECMWF SPREAD. IN  
INJECTING STRONGER FLOW EARLIER...THE SURFACE WAVE DOES NOT LIFT  
NORTH AND HANGS BACK AROUND 33N 135W BY 00Z SUNDAY FOR THE  
REMAINING GUIDANCE. AS SUCH THE ECMWF/ECENS IS NOT FAVORED AT  
THIS TIME. THE 12Z NAM IS A BIT TOO QUICK/STRONG INITIALLY  
COMPARED TO THE 12Z GFS/UKMET AND EVEN THE SLIGHTLY SLOWER CMC.  
AT THIS POINT A 12Z GFS/00Z UKMET BLEND SEEMS A GOOD COMPROMISE  
BUT THERE IS VERY LOW CONFIDENCE GIVEN THE EXTREMELY LARGE  
SPREAD...WHICH APPEARS TO BE RELATED TO MANY DOWNSTREAM  
DIFFERENCES ESPECIALLY THE SHAPE/STRENGTH OF THE OMEGA RIDGE.  
 
19Z UPDATE: AS ALLUDED TO IN UPDATE OF SECTION ABOVE THE ECMWF  
DOES REMAIN THE STRONGEST WITH THE OMEGA BLOCK RIDGE...HOWEVER IS  
MUCH QUICKER IN DRAWING CENTRAL PACIFIC ENERGY AT THE SOUTHEAST  
SIDE OF THE LARGER GULF OF AK TROF LEADING TO HEIGHT FALLS  
UNDERNEATH AND A MUCH FLATTER/LESS AMPLIFIED TROF/FASTER COLD  
FRONT WITH NO DEEP CYCLONE NEARING THE INITIAL PREFERENCE BUT  
STILL LAGS THE MORE PREFERABLE FASTER UKMET/GFS. THE CMC  
CONTINUES TO BE SLIGHT SLOWER AND MORE AMPLIFIED THROUGH JUST SW  
OF VANCOUVER ISLAND WITHIN THE MIDDLE/LOWER ATMOSPHERE TO KEEP IN  
OUT OF PREFERENCE AT THIS TIME. STILL WITH BOTH TRENDING/SHIFTING  
THIS DIRECTION...THIS ONLY FURTHER ENHANCES CONFIDENCE IN A 12Z  
GFS/UKMET BLEND. CONFIDENCE IS NOW SLIGHTLY BELOW AVERAGE TO  
AVERAGE...GIVEN THE INHERENT LACK OF CONTINUITY AND PROBABILITY  
FOR FURTHER ADJUSTMENTS.  
 
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