697  
FXUS10 KWNH 241608  
PMDHMD  
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1100 AM EST SUN DEC 24 2017  
 
VALID DEC 24/1200 UTC THRU DEC 28/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z MODEL EVALUATION INCLUDING PREFERENCES AND FORECAST CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
...S/W IN CENTRAL PLAINS SWINGS NEGATIVE TILT THROUGH MID-ATLANTIC  
LATE TONIGHT AND SPURRING DUAL SFC LOWS ALONG COAST AND SPINE OF  
APPALACHIANS INTO NEW ENGLAND BY LATE MON...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: SLIGHTLY ABOVE TO ABOVE AVERAGE  
 
GOES-WV MOSAIC SHOWS A FAST MOVING AMPLIFYING S/W ACROSS THE  
CENTER OF THE US THAT WILL CONTINUE TO AMPLIFY WITH SLIGHT  
NEGATIVE TILT AS IT ROUNDS THE LARGER SCALE TROF BASE LATER TODAY.  
THIS WILL SPUR DEVELOPMENT OF SURFACE LOW IN THE UPPER TN VALLEY  
THAT WILL RIDE THE WESTERN SLOPES OF THE APPALACHIANS TOWARD  
CENTRAL NY BY 25/12Z BEFORE WEAKENING INTO THE RAPIDLY DEVELOPING  
COASTAL LOW ALONG DOWN EAST MAINE WHICH FORMED FROM THE SAME S/W  
HEIGHT FALLS ALONG THE LINGER FRONT NEAR THE MOUTH OF THE  
CHESAPEAKE AROUND 00Z TONIGHT. OVERALL THE MASS FIELD FOR BOTH  
STORMS ARE VERY SIMILAR THOUGH OBVIOUSLY WITH A RAPIDLY DEVELOPING  
SITUATION...MESOSCALE UPSCALE INFLUENCES WILL BE PRESENT BUT  
SYNOPTICALLY A GENERAL MODEL BLEND CAN BE SUPPORTED AT SLIGHTLY  
ABOVE TO ABOVE AVERAGE CONFIDENCE.  
 
   
..DEVOLUTION OF GLOBAL VORTEX OVER CANADA
 
EMBEDDED S/W ACROSS  
GREAT LAKES/NEW ENGLAND FROM WED/THURS...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NON-CMC BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
AFTER THE PASSAGE OF THE MAIN MID-STREAM S/W (SEE SECTION  
ABOVE)...STRONG JET WILL CONTINUE TO TRACK MORE SUBTLE S/W  
FEATURES ACROSS THE NORTHERN TIER INTO THE GREAT LAKES EVENTUALLY  
DEFORMING THE LARGER VORTEX TO A MORE FLAT/ZONAL FLOW IN THE  
NORTHERN US BY TUES. GUIDANCE IS VERY TIGHT WITH THIS EVOLUTION  
INCLUDING SUPPORTING ELONGATED SURFACE TROF FROM THE COASTAL LOW  
BACK TO THE GREAT LAKES WITH OCCASIONAL CLOSED SURFACE LOWS ALONG  
THE ARCTIC FRONT IN THE GREAT LAKES. BY WED HOWEVER...IN  
COMBINATION WITH S/W ELONGATING INTO THE CONFLUENT FLOW OVER THE  
NORTHERN ROCKIES FROM THE NW PACIFIC ON TUES (SEE SECTION BELOW) A  
FAST MOVING CLIPPER WILL SLIGHTLY AMPLIFY ACROSS THE GREAT LAKES  
INTO MID-ATLANTIC. THE 00Z CMC IS MOST AGGRESSIVE WITH THIS  
SCENARIO AND ALSO SUPPORTS DISTORTING THE INNER CORE OF THE  
FILLING GLOBAL VORTEX IN CANADA TOWARD THE EAST. WHILE THIS IS  
SUPPORTED SLIGHTLY BY A FASTER ECMWF...THE 12Z GFS/NAM ARE A BIT  
SLOWER AND LESS PHASED AS WELL MAKING THE CMC SEEM A BIT OUT OF  
ALIGNMENT FROM AN OTHERWISE NICE SPREAD IN THE MASS FIELDS  
(ESPECIALLY AT 5 THROUGH 7H). AS SUCH A NON-CMC BLEND IS  
SUGGESTED BUT IMPACTS ARE LOW AND SPREAD IS MILD TO HAVE SLIGHTLY  
ABOVE AVERAGE CONFIDENCE IN THE BLEND.  
 
...SE PERIPHERY OF S/W TROF CLIPPING PAC NW EARLY MON WITH WEAK  
SFC REFLECTION...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
A NW TO SE TROF EXTENSION FROM GULF OF AK AS WELL AS S/W ENERGY  
EMERGING FROM THE SUBTROPICS TRIES TO BREAK THROUGH THE MEAN RIDGE  
AXIS THAT HAS BEEN DOMINATING THE WEST COAST FOR AWHILE NOW. GIVEN  
THE STRENGTH OF THE HIGH TO THE NORTHER...THE TROF SHEARS SOME OF  
THE SE PERIPHERY OF THE TROF INTO THE WEAKNESS AT THE BASE OF THE  
OMEGA BLOCK NEAR THE MOUTH OF TH COLUMBIA RIVER 00Z TONIGHT.  
THESE HEIGHT FALLS BRINGING MOISTURE/WEAK SURFACE FRONT INTO THE  
WEST COAST. AT THE EDGES OF THESE TROFS INTERSECTING MEAN  
RIDGES...THERE IS NORMALLY MODEST TO HIGH SPREAD BUT RECENT 12Z  
GFS/NAM CONTINUE TO MATCH THE TIMING/SHAPE WITH PRIOR 00Z RUNS  
(UKMET/CMC/ECMWF) AND ENSEMBLE SUITE. ENOUGH TO SUPPORT A GENERAL  
MODEL BLEND. THERE REMAIN SOME TIMING DIFFERENCES BUT SMALL  
ENOUGH TO HAVE SLIGHTLY ABOVE AVERAGE CONFIDENCE IN THIS BLEND.  
NOTE PLEASE SEE SECTION ABOVE FOR MORE ON THIS ENERGY AFTER IT  
UNDERCUTS THE RIDGE INTO THE CONFLUENT FLOW AROUND THE SOUTHWEST  
PERIPHERY OF THE GLOBAL TROF LATE MONDAY AND BEYOND.  
 
   
..NEXT PACIFIC S/W ENTERING PAC NW LATE TUES/EARLY WED
 
ROLLING  
THROUGH NORTHERN ROCKIES TO WY BY 00Z THURS...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z GFS AND 00Z ECMWF/CMC BLEND  
CONFIDENCE: AVERAGE  
 
EVEN THOUGH S/W DESCRIBED ABOVE TAMPED THE WESTERN RIDGE DOWN...IT  
BUILDS BACK SLIGHTLY BEFORE A SIMILAR EASTERN EXTENSION S/W TROF  
FROM A WESTERN GULF OF AK WAVE SHOWS UP ON THE PACIFIC NW DOORSTEP  
LATE TUES. GIVEN PROXIMITY FROM THE MAIN WAVE...THIS FEATURE IS A  
BIT MORE COMPACT AND SHOWS A BIT MORE SPREAD BUT GIVEN TIME OUT IN  
THE FUTURE AND SOURCE REGION...THIS SPREAD IS FAIRLY GOOD. THE  
TIMING SEEMS THE MAJOR DIFFERENCE THOUGH THE LATITUDE IN WHICH IT  
INTERSECTS THE MEAN RIDGE IS A BIT DIFFERENT TOO. THE 00Z  
UKMET/NAM ARE BOTH A BIT FURTHER NORTH... BUT THE UKMET IS A BIT  
FASTER ENTERING THE PAC NW. THE GFS/ECMWF/CMC ARE ALL A BIT SOUTH  
RELATIVELY SPEAKING BUT THE ECMWF/CMC MATCH THE TIMING OF THE  
UKMET AND THE GFS IS A TAD SLOWER. AT THIS POINT WILL FAVOR THE  
SOUTHERN TRACK A BIT MORE GIVEN ENSEMBLE SUPPORT THOUGH AGAIN THE  
SPREAD IS MINOR TO HAVE AVERAGE CONFIDENCE AT THIS POINT IN THIS  
BLEND.  
 
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