155  
FXUS02 KWBC 210650  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
249 AM EDT SAT OCT 21 2017  
 
VALID 12Z TUE OCT 24 2017 - 12Z SAT OCT 28 2017  
   
..OVERVIEW
 
 
GUIDANCE AGREES IN PRINCIPLE ON AMPLIFIED TROUGHING DOWNSTREAM  
FROM A STRONG WEST COAST/EASTERN PACIFIC RIDGE. EVOLUTION OF  
INDIVIDUAL FEATURES LEADS TO SOME RETROGRESSION OF THE MEAN  
PATTERN, AS AN INITIAL WESTERN RIDGE COLLAPSES WHILE NORTHEAST  
PACIFIC ENERGY PROGRESSES INLAND AND AMPLIFIES OVER THE  
ROCKIES/PLAINS WITH A NEW RIDGE QUICKLY BUILDING OFFSHORE. THE  
ROCKIES/PLAINS TROUGH AMPLIFICATION WILL HELP TO EJECT A LEADING  
TROUGH OVER THE MS VALLEY AS OF TUE.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
BASED ON DATA THROUGH THE 12Z/18Z CYCLES THERE WERE STILL  
IMPORTANT DETAIL DIFFERENCES OVER THE EASTERN STATES AT THE START  
OF THE FORECAST 12Z TUE. AT THAT TIME AMPLIFYING NORTHERN STREAM  
FLOW WILL BE ABSORBING A SOUTHERN TIER UPPER LOW CLOSING OFF IN  
THE SHORT RANGE TIME FRAME. OVER THE PAST DAY GFS/GEFS/UKMET RUNS  
HAVE CONSISTENTLY BEEN SLOWER WITH THE SOUTHERN TIER LOW, LEADING  
TO A MORE ELONGATED SURFACE PATTERN IN CONTRAST TO ECMWF/ECMWF  
MEAN RUNS WHOSE QUICKER ABSORPTION LEADS TO A DEEPER/MORE  
CONSOLIDATED GREAT LAKES SYSTEM. THE 12Z CMC ADJUSTED TOWARD THE  
ECMWF VERSUS AN OUTLIER OPEN/PROGRESSIVE SOLUTION IN ITS 00Z/20  
RUN. BASED ON SHORT RANGE PREFERENCES AND MAINTAINING A SOLUTION  
CLOSEST TO CONTINUITY, PREFERRED A FORECAST CLOSER TO THE ECMWF  
CLUSTER. NEW 00Z GUIDANCE SEEMS TO SUPPORT THIS IDEA WITH  
CONTINUITY FROM THE CMC AND A SIGNIFICANT ADJUSTMENT IN THE UKMET.  
AS THE TROUGH EVENTUALLY LIFTS NORTHEAST, MODEL RUNS ARE  
SUGGESTING POTENTIAL FOR AN EMBEDDED CLOSED LOW OR AT LEAST A  
SHARPER TROUGH THAT WOULD SUPPORT SOME DEGREE OF SURFACE WAVE  
DEVELOPMENT THAT WOULD AFFECT NEW ENGLAND BY MID-LATE WEEK. THERE  
IS STILL A LOT OF MODEL/ENSEMBLE SPREAD AND LOWER CONFIDENCE FOR  
THIS ASPECT OF THE FORECAST. THE 12Z GFS AND PAST COUPLE ECMWF  
RUNS APPEAR TO COMPARE CLOSEST TO THE 18Z GEFS/12Z ECMWF MEANS  
THOUGH FASTER 18Z/00Z GFS RUNS ARE IN THE REALM OF POSSIBILITY.  
 
FARTHER UPSTREAM THERE IS STEADILY IMPROVING CONSENSUS IN  
PRINCIPLE FOR NORTHEAST PACIFIC ENERGY AMPLIFYING INTO THE LOWER  
48 AFTER MIDWEEK BUT WITH A LOT OF DETAIL DIFFERENCES REGARDING  
THE LEADING SURFACE REFLECTION. BY THU-SAT THE MOST PRONOUNCED  
TREND IN THE GUIDANCE OVER THE PAST DAY OR SO IS TOWARD  
SHARPER/WESTWARD TROUGH AMPLIFICATION IN RESPONSE TO A MUCH  
SHARPER/NORTHWARD EXTENDING EASTERN PACIFIC RIDGE. THIS TREND IS  
EVIDENT IN BOTH OPERATIONAL MODELS AND THE ENSEMBLE MEANS, AND  
SEEMS PLAUSIBLE GIVEN THE STRONG UPSTREAM BERING SEA SYSTEM THAT  
REPRESENTS THE EXTRATROPICAL EVOLUTION OF TYPHOON LAN. THE  
AMPLIFYING PATTERN SEEMS TO FAVOR THE CLUSTER OF 12Z  
GFS-GEFS/ECMWF-ECMWF MEAN VERSUS THE 18Z GFS THAT BECOMES MORE  
PROGRESSIVE WITH THE CONUS TROUGH. NOTE THAT WHILE THERE IS  
DECENT MODEL/ENSEMBLE MEAN AGREEMENT AT THE MOMENT, ENSEMBLE  
SPAGHETTI PLOTS SHOW SOME VERY DIFFERENT SOLUTIONS FROM SOME  
MEMBERS SO THERE IS POTENTIAL FOR EVOLUTION TO STRAY SIGNIFICANTLY  
FROM CURRENT CONSENSUS.  
 
LATE IN THE PERIOD GUIDANCE BECOMES INCREASINGLY DIVERGENT  
REGARDING FLOW BETWEEN THE NORTHWEST CARIBBEAN AND FLORIDA. WHILE  
GFS RUNS DO NOT HAVE THE LOW PROBABILITY TROPICAL SYSTEMS SEEN IN  
THE 12Z/00Z CMC, THEY STILL BRING MUCH MORE MOISTURE INTO FL THAN  
THE ECMWF/ECMWF MEAN. THE SURFACE PATTERN OF GEFS MEANS IS CLOSER  
TO THE ECMWF CLUSTER WHILE SUGGESTING AN INTERMEDIATE  
TIMING/MAGNITUDE OF MOISTURE INCREASE. THIS RECOMMENDS A SOLUTION  
CLOSER TO THE ECMWF/ECMWF MEAN/GEFS MEAN VERSUS GFS RUNS.  
 
THE UPDATED FORECAST INCORPORATES MORE 12Z ECMWF/ECMWF MEAN  
GUIDANCE RELATIVE TO THE 12Z GFS/18Z GEFS MEAN BASED ON  
CONSIDERATIONS FOR THE ABOVE FEATURES AS WELL AS THE TENDENCY OVER  
THE PAST COUPLE DAYS FOR THE ECMWF/ECMWF MEAN TO BE LEADING THE  
TRENDS FOR INDIVIDUAL FEATURES. ENSEMBLE MEAN WEIGHT INCREASES  
LATER IN THE PERIOD AS OPERATIONAL MODEL DETAILS BECOME MORE  
DISSIMILAR.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
THE INITIAL SYSTEM FORECAST TO BE OVER/NEAR THE GREAT LAKES MAY  
PRODUCE SOME STRONG WINDS EARLY IN THE PERIOD WHILE PUSHING ALONG  
A WAVY FRONT ACCOMPANIED BY LOCALLY MODERATE-HEAVY RAINFALL OVER  
THE MID ATLANTIC/NORTHEAST. POTENTIAL WAVE DEVELOPMENT ALONG THE  
FRONT MAY FOCUS ADDED RAINFALL OVER NEW ENGLAND DURING THE LATTER  
HALF OF THE WEEK DEPENDING ON THE EXACT TRACK/TIMING OF THE WAVE.  
UPSTREAM TROUGH ENERGY ALOFT WILL BRING AN AREA OF RAIN/SNOW FROM  
THE PACIFIC NORTHWEST/NORTHERN ROCKIES SOUTHWARD THROUGH THE  
CENTRAL ROCKIES AROUND WED-THU. AS THE TROUGH AMPLIFIES, THE  
LEADING COLD FRONT SHOULD DECELERATE SOME AS IT HEADS INTO THE MS  
VALLEY, WITH GULF INFLOW HELPING TO CONTRIBUTE TO INCREASING  
RAINFALL OVER THE EAST-CENTRAL U.S. LATE IN THE PERIOD. ALSO LATE  
IN THE PERIOD GUIDANCE DIFFERS SIGNIFICANTLY REGARDING HOW MUCH  
MOISTURE/RAINFALL MAY SPREAD ACROSS FLORIDA. OVERALL EXPECT SOME  
INCREASE BUT CONFIDENCE IS LOW REGARDING TIMING/EXTENT.  
 
AT THE START OF THE PERIOD EXPECT VERY WARM TO HOT CONDITIONS OVER  
THE SOUTHWEST/CALIFORNIA WITH HIGHS AT LEAST 10-20F ABOVE NORMAL.  
SPC OUTLOOKS INDICATE POTENTIAL FOR FIRE WEATHER CONCERNS OVER  
PARTS OF SOUTHERN CALIFORNIA. THE ABOVE NORMAL TEMPS WILL SPREAD  
VERY BRIEFLY INTO THE ROCKIES/PLAINS ON WED BEFORE A SURGE OF VERY  
CHILLY AIR PUSHES SOUTHEAST FROM THE NORTHERN ROCKIES THU ONWARD,  
WITH FAIRLY BROAD COVERAGE OF HIGHS 10-20F BELOW NORMAL OVER THE  
CENTRAL U.S. BY FRI. THE UPPER TROUGH CROSSING THE EAST FROM THE  
START OF THE PERIOD WILL BRING SOME CHILLY READINGS TO THE EAST AS  
WELL WITH MINUS 10-15F ANOMALIES MOST LIKELY AROUND WED.  
 
RAUSCH  
 

 
 
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