235  
FXUS02 KWBC 140627  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
127 AM EST TUE NOV 14 2017  
 
VALID 12Z FRI NOV 17 2017 - 12Z TUE NOV 21 2017  
   
..PATTERN OVERVIEW
 
 
A GENERALLY PROGRESSIVE FLOW PATTERN WILL PERSIST ACROSS THE CONUS  
THROUGH THE MEDIUM RANGE DOWNSTREAM OF HIGHLY AMPLIFIED FLOW  
ACROSS THE NORTH/CENTRAL PACIFIC. THE FLOW PATTERN WILL ALLOW FOR  
SHORTWAVE ENERGY ORIGINATING IN THE ARCTIC TO DIVE SOUTHEASTWARD  
AROUND THE EASTERN SIDE OF A STRONG NORTH PAC/BERING SEA RIDGE  
BEFORE AMPLIFYING IN THE WESTERLY FLOW ACROSS THE CONUS. SOME  
GUIDANCE CONTINUES TO SUGGEST THE POTENTIAL FOR RIDGING TO BUILD  
INTO THE WESTERN U.S. BY EARLY NEXT WEEK, SETTING THE STAGE FOR A  
MORE AMPLIFIED FLOW PATTERN ACROSS NORTH AMERICA WITH EASTERN U.S.  
TROUGHING, ALTHOUGH MODELS SHOW A FAIRLY LARGE SPREAD WITH RESPECT  
TO EVEN THE LARGE SCALE PATTERN BY THAT TIME (SEE BELOW). IN  
GENERAL, THE PATTERN FAVORS CONTINUED POTENTIAL FOR OCCASIONAL  
ROUNDS OF COLD AIR MOVING SOUTH FROM CANADA INTO THE GREAT  
LAKES/EASTERN U.S., AND WET CONDITIONS FOR MUCH OF THE WEST COAST.  
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT AND PREFERENCES...
 
 
MODELS SHOW GENERAL AGREEMENT ON DEEPENING A SURFACE CYCLONE AS IT  
EXITS NEW ENGLAND THU-FRI, WITH ONLY THE 12Z UKMET DIFFERING AND  
KEEPING THE LOW CLOSER TO THE COAST FOR LONGER. OTHERWISE,  
ATTENTION SHIFTS WESTWARD TO AN AMPLIFYING TROUGH MOVING ONSHORE  
ALONG THE WEST COAST AND EVENTUALLY THE ROCKIES THU-FRI. THE ECMWF  
HAS BEEN RELENTLESS ON ITS SLOWER SOLUTION WITH THIS WAVE, AND THE  
MAJORITY OF OTHER GUIDANCE (INCLUDING THE GFS) HAS GRADUALLY  
TRENDED TOWARD THE ECMWF. AS A SURFACE CYCLONE DEVELOPS ACROSS THE  
CENTRAL PLAINS ON FRI AND QUICKLY MOVES TOWARD THE GREAT LAKES,  
THE SAME TRENDS HOLD WITH THE ECMWF ON THE SLOW SIDE OF THE SPREAD  
AND THE GFS ON THE FAST SIDE. THE 12Z UKMET WAS GENERALLY IN THE  
MIDDLE ALTHOUGH A BIT MORE TOWARD SLOWER SIDE. AS THE CYCLONE  
BEGINS TO MATURE ACROSS THE LOWER GREAT LAKES/ONTARIO ON SAT, THE  
ECMWF/GFS ARE REASONABLY WELL CLUSTERED AROUND THE  
ECENS/NAEFS/GEFS MEANS WITH RESPECT TO THE SURFACE LOW POSITION  
AND INTENSITY, ALTHOUGH SOME MODEST SPREAD REMAINS. THUS, A BLEND  
OF THE DETERMINISTIC 12Z ECMWF/UKMET AND 18Z GFS SERVED AS A  
STARTING POINT FOR THE FORECAST DURING DAYS 3-5, WITH MOST WEIGHT  
PLACED ON THE ECMWF. THIS TRENDS THE FORECAST TOWARD A SLOWER  
PROGRESSION OF THE UPPER WAVE AND SURFACE FRONT.  
 
FROM SUN ONWARD, RATHER SIGNIFICANT UNCERTAINTY EXISTS ACROSS THE  
WESTERN U.S. AN ARCTIC SHORTWAVE ORIGINATING ACROSS SIBERIA IS  
FORECAST TO DIVE SOUTHWARD ALONG THE EASTERN PERIPHERY OF THE  
NORTH PACIFIC RIDGE, APPROACHING THE WEST COAST. AT THE SAME TIME,  
GUIDANCE SUGGESTS RISING HEIGHTS ACROSS THE WEST AS A RIDGE BUILDS  
OVERHEAD. WHILE THE GFS SHOWS THESE SHORTWAVES QUICKLY  
DEAMPLIFYING AS THEY MOVE INTO THE ANTICYCLONIC FLOW, THE 12Z  
ECMWF/CMC SHOW A STRONGER SHORTWAVE THAT UNDERCUTS THE RIDGE AND  
MOVES INTO THE FOUR CORNERS REGION BY MON-TUE. THE ECMWF HAS SHOWN  
TO HAVE A BETTER HANDLE AS OF LATE ON ENERGY ORIGINATING IN THE  
ARCTIC (ESPECIALLY WHEN MATCHED BY THE CMC). FURTHER,  
TELECONNECTIONS ASSOCIATED WITH THE STRONGEST HEIGHT ANOMALY  
CENTERS WITHIN THE PERSISTENT PACIFIC REX BLOCK DO NOT SUPPORT A  
STRONG WESTERN U.S. RIDGE (WHICH SHOULD IN THEORY ALLOW FOR ENERGY  
TO UNDERCUT THE RIDGE AS SHOWN BY THE EC). WHILE THE EC ENSEMBLE  
SHOWS A LARGE AMOUNT OF SPREAD, A NOTABLE TREND WAS OBSERVED IN  
THE DIRECTION OF THE 12Z DETERMINISTIC EC. THUS, THE FORECAST WAS  
TRENDED STRONGLY IN THIS DIRECTION FOR DAYS 6-7 (MON-TUE), WITH  
THE 12Z ECENS MEAN AND DETERMINISTIC ECMWF COMPRISING A MAJORITY  
OF THE FORECAST, AND LESSER WEIGHTING PLACE ON THE NAEFS TO  
ACCOUNT FOR SOME UNCERTAINTY.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
LOW PRESSURE DEVELOPING OFF THE NEW ENGLAND COULD SHOULD GENERALLY  
BE FAR ENOUGH OFFSHORE TO NOT HAVE A SIGNIFICANT IMPACT IN TERMS  
OF SENSIBLE WEATHER, ALTHOUGH SOME LIGHT SNOW IS POSSIBLE ACROSS  
EASTERN NEW ENGLAND EARLY ON FRI. THE BIGGER WEATHER MAKER WILL BE  
THE SYSTEM DEVELOPING ACROSS THE CENTRAL U.S. FRI AND MOVING  
QUICKLY TOWARD THE GREAT LAKES/NORTHEAST WITH A COLD FRONT  
TRAILING THROUGH THE MS/OH VALLEYS AND EASTERN U.S. THIS SYSTEM IS  
EXPECTED TO PRODUCE SHOWERS AND EVEN SOME THUNDERSTORMS FROM THE  
MS VALLEY EASTWARD. THE ENERGETIC NATURE OF THE SYSTEM ALONG WITH  
AN IMPRESSIVE INFLOW OF MOISTURE AND INSTABILITY AHEAD OF IT COULD  
PRODUCE SOME AREAS OF HEAVY RAIN AS WELL AS THE POTENTIAL FOR SOME  
SEVERE THUNDERSTORMS. THE STORM PREDICTION CENTER IS HIGHLIGHTING  
A CHANCE FOR SEVERE WEATHER FRI/FRI NIGHT ACROSS PORTIONS OF THE  
SOUTHERN PLAINS AND MS/OH VALLEYS. PLEASE REFER TO PRODUCTS ISSUED  
BY THE SPC FOR FURTHER DETAILS ON THE SEVERE WEATHER THREAT. COLD  
AIR IN PLACE ON THE NORTH AND WEST SIDE OF THE SYSTEM COULD RESULT  
IN AREAS OF SNOWFALL ACROSS PORTIONS OF THE UPPER MIDWEST, GREAT  
LAKES, AND NORTHERN NEW ENGLAND FRI-SUN. FARTHER WEST, A FRONTAL  
SYSTEM REACHING THE WEST COAST ON SAT IS EXPECTED TO PRODUCE HEAVY  
RAIN AND MOUNTAIN SNOW ACROSS THE PAC NW. THE WEST COAST WILL  
REMAIN WET THROUGH EARLY NEXT WEEK AS THE FIRST FRONTAL SYSTEM  
MOVES DOWN THE COAST AND WEAKENS AND A SECOND, STRONGER FRONTAL  
SYSTEM APPROACHES BY MON-TUE, PRODUCING ANOTHER ROUND OF HEAVY  
PRECIPITATION. RISING SNOW LEVELS BY EARLY NEXT WEEK SHOULD KEEP  
ANY HEAVY SNOWS CONFINED TO THE HIGHEST ELEVATIONS OF THE NORTHERN  
WA CASCADES.  
 
TEMPERATURES WILL GENERALLY BE BELOW AVERAGE FROM THE UPPER  
MIDWEST/GREAT LAKES TO THE EAST COAST AS TROUGHING REMAINS  
PREVALENT AND A SERIES OF FRONTAL SYSTEMS BRING REINFORCING ROUNDS  
OF POLAR AIR. FARTHER WEST, TEMPERATURES WILL REMAIN ABOVE AVERAGE  
AND MAY RISE FURTHER BY EARLY NEXT WEEK AS SOME DEGREE OF RIDGING  
BUILDS OVERHEAD.  
 
RYAN  
 

 
 
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