329  
FXUS02 KWBC 261633  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1232 PM EDT THU OCT 26 2017  
 
VALID 12Z SUN OCT 29 2017 - 12Z THU NOV 02 2017  
 
...HEAVY RAIN AND GUSTY WINDS LIKELY FOR THE NORTHEASTERN U.S.  
SUNDAY AND MONDAY...  
 
   
..OVERVIEW
 
 
ANOMALOUS RIDGING ACROSS THE NORTHEASTERN PACIFIC IS EXPECTED TO  
PERSIST THROUGH THE MEDIUM RANGE AND EXPAND NORTH ACROSS ALASKA  
TOWARD THE NORTH POLE. THIS WILL SUPPORT A HIGHLY BLOCKED NORTHERN  
STREAM FLOW FROM THE NORTH PACIFIC/ALASKA INTO CANADA, AND A  
STRONGLY NEGATIVE PNA PATTERN. THE RESULT WILL BE PERSISTENT  
TROUGHING ACROSS THE CENTRAL/EASTERN U.S. AND A RELATIVELY ACTIVE  
STORM TRACK FROM THE GREAT LAKES TO THE NORTHEAST. RAPID  
CYCLOGENESIS IS EXPECTED TO OCCUR OFF THE EASTERN SEABOARD SUN-MON  
AS A TROPICAL DISTURBANCE UNDERGOING EXTRATROPICAL TRANSITION  
ENTERS A REGION OF HIGHLY FAVORABLE UPPER-LEVEL DYNAMICS,  
PRODUCING A POTENTIAL HEAVY RAIN AND WIND EVENT FOR PORTIONS OF  
THE NORTHEAST.  
 
...GUIDANCE/UNCERTAINTY ASSESSMENT AND WEATHER  
THREATS/HIGHLIGHTS...  
 
MODELS SHOW RELATIVELY GOOD LARGE SCALE AGREEMENT ACROSS THE CONUS  
THROUGH THE MEDIUM RANGE, WITH THE UPSTREAM BLOCKING RIDGE ACROSS  
THE NORTH PACIFIC PROMOTING CONTINUED TROUGHING ACROSS THE CENTRAL  
AND EASTERN STATES. PARTIAL PHASING BETWEEN A SHORTWAVE MOVING  
FROM THE OH VALLEY TO THE MID-ATLANTIC/NORTHEAST AND ANOTHER  
CROSSING THE UPPER MIDWEST/GREAT LAKES DAYS 3-4 (SUN-MON) IS  
EXPECTED TO RESULT IN RAPID CYCLOGENESIS ALONG THE MID-ATLANTIC  
COAST. AFTER TRENDING TOWARD A GREATER DEGREE OF PHASING  
YESTERDAY, MODEL SOLUTIONS SEEM TO HAVE STABILIZED, WITH A  
CONSENSUS BUILDING FOR RELATIVELY STRONG/RAPID SURFACE LOW  
DEVELOPMENT. A TROPICAL DISTURBANCE CURRENTLY ACROSS THE WESTERN  
CARIBBEAN/CENTRAL AMERICA COAST IS EXPECTED TO MOVE QUICKLY  
NORTHWARD OFF THE SOUTHEAST U.S. COAST ON DAY 3 BEFORE MOVING  
ENTERING THE REGION OF RAPID EXTRATROPICAL CYCLOGENESIS. THE  
RESULTANT EXTRATROPICAL CYCLONE IS FORECAST BY ALL GUIDANCE TO  
TRACK INTO THE NORTHEAST BY MONDAY. DETERMINISTIC MODEL  
DIFFERENCES ARE MINOR WITH RESPECT TO TIMING/LOW TRACK, AND A  
RANGE OF DEPTHS IS SHOWN BY DETERMINISTIC MODELS WHEN THE SYSTEM  
REACHES PEAK INTENSITY ACROSS NEW ENGLAND MONDAY MORNING, LIKELY  
IN THE 970-985 MB RANGE (THE 00Z ECMWF WAS THE DEEPEST SHOWING A  
962 MB LOW AT 12Z MON). IF THE SYSTEM IS ABLE TO MAINTAIN AT LEAST  
A PARTIAL WARM CORE OR WARM SECLUSION (DUE TO ITS TROPICAL ORIGIN)  
THIS WOULD LIKELY TIP THE PEAK INTENSITY TOWARD A DEEPER SOLUTION  
AS SHOWN BY THE 00Z ECMWF. IMPRESSIVE UPPER DYNAMICS ASSOCIATED  
WITH THE DEEPENING CYCLONE AND ANOMALOUS TROPICAL MOISTURE  
TRANSPORTED NORTHWARD INTO THE SYSTEM ARE EXPECTED TO PRODUCE A  
HEAVY RAINFALL AND GUSTY WIND EVENT SUNDAY AND MONDAY FROM  
PORTIONS OF THE MID-ATLANTIC TO THE NORTHEAST, BEFORE THE SYSTEM  
LIFTS QUICKLY NORTH AND OCCLUDES EAST OF HUDSON BAY BY DAY 5  
(TUE). HEAVY RAIN MAY FALL OVER SOME AREAS RECEIVING ADDITIONAL  
HEAVY RAINS DURING THE SHORT RANGE, INCREASING THE POTENTIAL FOR  
FLOODING IN SOME AREAS. SUFFICIENT AGREEMENT WAS NOTED AMONG  
DETERMINISTIC GUIDANCE THAT A MULTI-MODEL BLEND INCLUDING THE 00Z  
ECMWF/CMC AND 06Z GFS FORMED THE BASIS FOR THE WPC FORECAST DURING  
DAYS 3-5.  
 
IN THE WAKE OF THIS SYSTEM, ADDITIONAL SHORTWAVES WILL REINFORCE  
THE TROUGH ACROSS THE CENTRAL/EASTERN U.S, KEEPING BELOW AVERAGE  
TEMPERATURES IN PLACE. AT THE SAME TIME, THE BUILDING RIDGE ACROSS  
THE HIGH LATITUDES OF THE PACIFIC WILL SUPPORT SOME DEGREE OF  
ARCTIC AIR TRANSPORT SOUTHWARD ACROSS CENTRAL CANADA AND INTO THE  
NORTH CENTRAL U.S. A VIGOROUS SHORTWAVE IS SHOWN BY THE MAJORITY  
OF GUIDANCE REACHING THE NORTHERN PLAINS BY WED-THU, PUSHING A  
SURFACE COLD FRONT INTO THE CENTRAL U.S. A COMPLEX FLOW PATTERN  
EVOLUTION INVOLVING THE EXACT ORIENTATION OF THE BLOCKING RIDGE  
AND THE DOWNSTREAM SHORTWAVE ENERGY ACROSS CANADA RENDERS THE  
DEGREE OF COLD AIR TRANSPORT SOUTHWARD INTO THE NORTHERN CONUS  
UNCERTAIN AT THIS TIME, BUT THERE REMAINS SOME POTENTIAL FOR  
ARCTIC HIGH PRESSURE TO WORK ITS WAY SOUTHWARD INTO THE NORTHERN  
PLAINS/MIDWEST BY THE MIDDLE OF NEXT WEEK. GIVEN GENERAL LARGE  
SCALE AGREEMENT BUT INCREASING DETERMINISTIC SPREAD ON THE  
SPECIFICS, THE WPC FORECAST WAS TRENDED HEAVILY TOWARD ENSEMBLE  
MEANS (00Z ECENS/NAEFS/06Z GEFS) DURING DAYS 6-7 (WED-THU).  
 
RYAN  
 

 
 
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