196  
FXUS02 KWBC 120618  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
217 AM EDT THU OCT 12 2017  
 
VALID 12Z SUN OCT 15 2017 - 12Z THU OCT 19 2017  
   
..OVERVIEW
 
 
A PROGRESSIVE FLOW PATTERN IS EXPECTED ACROSS THE NORTHERN TIER OF  
THE CONUS THROUGH THE MEDIUM RANGE. MEANWHILE, SOUTHERN U.S.  
RIDGING WILL WEAKEN SOME BUT POSITIVE HEIGHT ANOMALIES SHOULD  
PERSIST. A SHORTWAVE AND ASSOCIATED SURFACE CYCLONE ARE FORECAST  
TO TRACK FROM THE GREAT LAKES ON DAY 3 (THU) TO THE CANADIAN  
MARITIMES ON DAY 4 (FRI) WITH A TRAILING COLD FRONT CROSSING THE  
MS VALLEY AND EASTERN U.S. BY EARLY NEXT WEEK THE FOCUS SHIFTS TO  
A SERIES OF FAST-MOVING SHORTWAVES AND SURFACE FRONTAL SYSTEMS  
ORIGINATING IN THE NORTH PACIFIC THAT MOVE INTO BRITISH COLUMBIA  
AND THE U.S. NORTHWEST BEFORE TRACKING EAST ACROSS THE NORTHERN  
ROCKIES/NORTHERN PLAINS. THE MOST SIGNIFICANT OF THESE SYSTEMS IS  
EXPECTED TO REACH THE NORTHWEST LATE ON DAY 6 (WED) INTO DAY 7  
(THU).  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
DETERMINISTIC MODEL SPREAD HAS CONTINUED TO GRADUALLY REDUCE  
SURROUNDING THE SYSTEM CROSSING THE MIDWEST ON DAY 3. THE ECMWF  
HAS BEEN VERY CONSISTENT OVER THE PAST FEW RUNS IN ITS SOLUTION AT  
500 HPA IN SHOWING AN AMPLIFIED AND SOMEWHAT ELONGATED WAVE, AND  
THE GFS SEEMS TO BE TRENDING TOWARD THE ECMWF. THE CMC HAS ALSO  
BEEN FAIRLY CONSISTENT ON DAY 3 AND ITS SOLUTION FALLS WELL WITHIN  
RANGE OF CONSENSUS. BY DAY 4, HOWEVER, THE CMC APPEARS TO BE TOO  
WEAK AND SUPPRESSED WITH THE SURFACE LOW OVER/NEAR THE  
NORTHEASTERN STATES. THE 12Z ECMWF/GFS WERE THE PREFERRED  
SOLUTIONS FOR THIS SYSTEM ON DAYS 3-4, COMPRISING A MAJORITY OF  
THE WPC FORECAST BLEND.  
 
FOR THE QUICK SUCCESSION OF PACIFIC SYSTEMS MOVING ACROSS THE  
NORTHWEST/WESTERN CANADA, SPREAD INCREASES ESPECIALLY FROM DAY 5  
(TUE) ONWARD. IN GENERAL, THE ECMWF AND ECMWF ENSEMBLE MEAN HAVE  
SHOWN THE GREATEST DEGREE OF RUN-TO-RUN CONSISTENCY SURROUNDING  
THESE WAVES. THESE SOLUTIONS WOULD SHOW A SHORTWAVE CROSSING THE  
CANADIAN PRAIRIES/U.S. NORTHERN PLAINS ON WED AND SKIRTING THE  
GREAT LAKES ON THU. THE GFS IS SLOWER AND ALSO STRONGER WITH  
EASTERN U.S. RIDGING WHICH CAUSES THE SHORTWAVE TO TAKE A MORE  
NORTHERLY TRACK TOWARD JAMES BAY. THE CMC HAS BEEN FAIRLY  
INCONSISTENT FROM RUN TO RUN BUT SEEMS TO FALL MORE IN THE SPIRIT  
OF THE ECMWF WITH WEAKER RIDGING AND A WAVE SKIRTING THE NORTHERN  
PLAINS/GREAT LAKES. FARTHER WEST, A SECOND STRONGER SHORTWAVE IS  
FORECAST TO REACH THE NORTHWEST BY LATE WED/EARLY THU. THE ECMWF  
AND CMC HAVE BOTH BEEN SOMEWHAT CONSISTENT IN SHOWING A SLIGHTLY  
SLOWER PROGRESSION WHILE THE GFS IS MUCH QUICKER. ENSEMBLES SHOW A  
RELATIVELY LARGE SPREAD BY THIS TIME BUT IN GENERAL THE 12Z  
ECENS/NAEFS/18Z GEFS MEANS ALL SUPPORT A PROGRESSION SLOWER THAN  
THE GFS. THUS, FOR DAYS 5-7 THE GFS WAS WEIGHTED SIGNIFICANTLY  
LESS IN THE FORECAST RELATIVE TO THE ECMWF, WITH WEIGHTING OF THE  
12Z ECENS AND NAEFS MEANS INCREASED THROUGH THE END OF THE PERIOD.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE COLD FRONT MOVING FROM THE MS VALLEY TO THE EASTERN SEABOARD  
SUN-MON WILL SUPPORT SCATTERED AREAS OF PRECIPITATION. THE  
HEAVIEST AMOUNTS SHOULD REMAIN MOSTLY NORTH OF THE U.S. CANADIAN  
BORDER ACROSS ONTARIO/QUEBEC CLOSEST TO THE BEST LOW/MID-LEVEL  
FORCING. THE BEST POTENTIAL FOR ANY HEAVY RAIN ASSOCIATED WITH  
THIS SYSTEM WOULD BE ACROSS NORTHERN NEW ENGLAND, NORTHERN MAINE  
IN PARTICULAR AS THE WARM FRONT WILL BE IN THE VICINITY ON THU.  
BEHIND THE FRONT DRY CONDITIONS ARE EXPECTED TO PREVAIL ACROSS  
MOST OF THE CENTRAL AND EASTERN U.S. FOR THE DURATION OF THE  
MEDIUM RANGE PERIOD WITH THE CYCLONE TRACK GENERALLY ALONG/NORTH  
OF THE CANADIAN BORDER. SCATTERED TO NUMEROUS SHOWERS AND STORMS  
ARE EXPECTED TO LINGER ACROSS FLORIDA, HOWEVER AS THE TRAILING END  
OF THE SURFACE COLD FRONT ENTERS THE STATE AND THEN STALLS ACROSS  
SOUTH FLORIDA BY TUE-WED NEXT WEEK. FARTHER WEST, DRY CONDITIONS  
WILL ALSO PREVAIL UNTIL A SERIES OF PACIFIC FRONTAL SYSTEMS  
IMPACTS THE NORTHWEST TUE-THU. RAIN SHOULD BECOME WIDESPREAD BY  
THE MIDDLE OF NEXT WEEK ACROSS THE NORTHWEST WITH HEAVY RAIN  
POSSIBLE ESPECIALLY IN FAVORED TERRAIN AREAS. SNOW LEVELS SHOULD  
BE RELATIVELY HIGH GIVEN THE PERSISTENT ONSHORE FLOW THROUGH THU,  
BUT SOME SNOWS WILL BE POSSIBLE AT THE HIGHEST ELEVATIONS OF THE  
CASCADES.  
 
TEMPERATURES ARE EXPECTED TO BE WELL ABOVE AVERAGE ACROSS MUCH OF  
THE EASTERN THIRD OF THE NATION AHEAD OF THE COLD FRONT SUN-MON,  
WITH HIGH TEMPERATURES 5 TO 15 DEG F ABOVE AVERAGE COMMON. AS THE  
FRONT PASSES, TEMPERATURES WILL DROP TO SLIGHTLY BELOW SEASONAL  
NORMS. IN THE WEST, TEMPERATURES SHOULD GRADUALLY WARM THROUGH THE  
PERIOD TO NEAR OR SLIGHTLY ABOVE NORMAL VALUES. BY MID-WEEK CLOUDS  
AND PRECIPITATION SHOULD LOWER MAX TEMPS TO 5 TO 10 DEG BELOW  
NORMAL ACROSS THE NORTHWEST.  
 
RYAN  
 

 
 
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