713  
FXUS02 KWBC 271526  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1124 AM EDT FRI OCT 27 2017  
 
VALID 12Z MON OCT 30 2017 - 12Z FRI NOV 03 2017  
   
..POWERFUL STORM TO EXIT THE NORTHEAST MONDAY
 
 
   
..OVERVIEW
 
 
A STRONG SURFACE LOW WILL LIFT OUT OF THE NORTHEAST ON MONDAY  
WHILE A COUPLE WEAKER SYSTEMS FOLLOW IN ITS WAKE ACROSS THE  
NORTHERN TIER. FARTHER UPSTREAM, IMPRESSIVELY STRONG UPPER RIDGING  
OVER THE GULF OF ALASKA WILL MIGRATE NORTHWARD INTO WESTERN ALASKA  
MID WEEK WHICH WILL FAVOR TROUGHING INTO THE PACIFIC NORTHWEST.  
BROAD CYCLONIC FLOW AROUND AN UPPER LOW IN NUNAVUT WILL BE IN  
PLACE ACROSS THE CENTRAL/EASTERN CONUS FOR MUCH OF THE WEEK.  
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT AND PREFERENCES
 
 
MODELS/ENSEMBLE SHOW RELATIVELY LOW SPREAD INITIALLY ACROSS THE  
CONUS. POWERFUL STORM SYSTEM IS FORECAST TO BE ACROSS NORTHERN NEW  
ENGLAND AT 12Z ON DAY 3 (MON). MODELS STILL SHOW SOME SPREAD WITH  
RESPECT TO TIMING/INTENSITY OF THIS SURFACE LOW, BUT THE GUIDANCE  
SEEMED MOSTLY WELL CLUSTERED AROUND THE 00Z ECENS MEAN. THE 00Z  
ECMWF/CMC AND 06Z GFS WERE THE CLOSEST DETERMINISTIC SOLUTIONS TO  
THE CONSENSUS, SO A BLEND OF THESE FORMED THE BASIS FOR THE WPC  
FORECAST FOR DAYS 3-4.  
 
FORECAST CONFIDENCE DECREASES A BIT FROM DAY 5 ONWARD, WITH SOME  
FAIRLY SIGNIFICANT CHANGES MADE BY THE 00Z ECMWF. WITH SHORTWAVE  
ENERGY SKIRTING THE NORTHERN PLAINS/GREAT LAKES DAYS 5-6  
(WED-THU), THE 00Z ECMWF IS MUCH FASTER/WEAKER THAN ALL OTHER  
GUIDANCE, INCLUDING THE 00Z ECENS MEAN. THIS RESULTS IN VERY WEAK  
OR EVEN NON-EXISTENT SURFACE LOW ASSOCIATED WITH THE CLIPPER-TYPE  
SYSTEM CROSSING THE NORTHERN TIER WED-THU. PRIOR RUNS OF THE ECMWF  
WERE MORE IN LINE WITH THE CONSENSUS HERE, SO GIVEN THE LACK OF  
ENSEMBLE SUPPORT AND THE RUN-TO-RUN INCONSISTENCY, WAS INCLUDED TO  
REDUCE ECMWF WEIGHTING IN THE FORECAST SIGNIFICANTLY FROM DAY 5  
ONWARD. THE SAME HOLDS TRUE FOR HEIGHT FALLS REACHING THE  
NORTHWESTERN U.S. DAYS 6-7 (THU-FRI), WITH THE 00Z ECMWF MOVING  
THE ENERGY INTO THE NORTHWEST MUCH MORE QUICKLY THAN CONSENSUS,  
INCLUDING THE 00Z ECENS MEAN. IN BOTH OF THESE AREAS THE 06Z GFS  
WAS MUCH CLOSER TO THE MODEL/ENSEMBLE CONSENSUS THAN THE 00Z  
ECMWF. GIVEN THESE CONSIDERATIONS, THE FORECAST WAS TRENDED TOWARD  
HEAVIER ENSEMBLE (00Z ECENS/NAEFS/06Z GEFS) WEIGHTING FROM DAY 5  
ONWARD, WITH A LARGER COMPONENT OF THE 06Z GFS CARRIED LATER IN  
THE PERIOD RELATIVE TO THE ECMWF.  
 
   
..WEATHER THREATS/HIGHLIGHTS
 
 
MULTI-HAZARD THREAT WITH THE CYCLONE WILL SLOWLY WIND DOWN IN THE  
NORTHEAST ON MONDAY AS IT CONTINUES TO DEEPEN INTO CANADA. NEXT  
SYSTEM ON ITS HEELS WILL BRING MUCH COLDER AIR TO THE ROCKIES AND  
SOUTHERN PLAINS AS THE COLD FRONT SPILLS OVER THE DIVIDE. SNOW IS  
LIKELY OVER SOUTHERN COLORADO MON-TUE THANKS TO UPSLOPE FLOW AS  
THE SFC HIGH SLIDES OUT OF MONTANA INTO NE/KS. FINAL SYSTEM DURING  
THIS BUSY WEEK WILL DROP SOUTHEASTWARD OUT OF CANADA INTO THE  
UPPER MIDWEST WED/THU, APPROACHING THE NORTHEASTERN U.S. BY  
FRIDAY. TAIL END OF THIS FRONT WILL HAVE A HARDER TIME CRESTING  
THE DIVIDE FROM ID/MT INTO WY AS THE HEIGHT FALLS WILL BE OFFSHORE  
THE WEST COAST. THIS SHOULD INCREASE THE RAIN/SNOW CHANCES FOR  
WASHINGTON AND NW MONTANA WITH MUCH COLDER TEMPERATURES LIKELY FOR  
NORTHERN MONTANA (10-20F BELOW AVERAGE) BY NEXT FRIDAY.  
 
RYAN/FRACASSO  
 

 
 
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