209  
FXUS02 KWBC 091558  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1057 AM EST FRI NOV 9 2018  
 
VALID 12Z MON NOV 12 2018 - 12Z FRI NOV 16 2018  
 
16Z UPDATE  
----------  
THE DETERMINISTIC MODELS AND THEIR RESPECTIVE MEANS ARE IN ABOVE  
AVERAGE AGREEMENT ON THE SYNOPTIC SCALE FOR THE BEGINNING OF THE  
FORECAST PERIOD ON MONDAY WITH A HIGH AMPLITUDE PATTERN IN PLACE,  
FEATURING AN UPPER RIDGE OFF THE WEST COAST, AN ELONGATED TROUGH  
OVER THE CENTRAL U.S., AND A BROAD SUBTROPICAL RIDGE OVER THE  
BAHAMAS AND FLORIDA. THE MAIN FORECAST CHALLENGE RELATES TO THE  
PROGRESSION OF THE SYNOPTIC SCALE TROUGH TOWARDS THE EAST COAST,  
AND THE AMOUNT OF EVENTUAL PHASING BETWEEN THE INITIAL NORTHERN  
STREAM AND SOUTHERN STREAM SHORTWAVE PERTURBATIONS WITHIN THE  
LARGE SCALE SYNOPTIC TROUGH AXIS. THE 00Z UKMET IS SLOWER AND  
MORE AMPLIFIED WITH THE UPPER LEVEL TROUGH AXIS THROUGH DAY 5 OF  
THE FORECAST, AND THE GFS IS MORE PROGRESSIVE BY THAT TIME AS THE  
TROUGH REACHES NEW ENGLAND. IN TERMS OF COLD FRONT TIMING, THE  
GEFS MEAN IS FASTER THAN THE EC MEAN BY 6-12 HOURS ONCE THE FRONT  
REACHES THE EAST COAST, AND THE UKMET INDICATING THE SLOWEST  
TIMING.  
 
ELSEWHERE ACROSS THE CONTINENTAL U.S., THE CMC BECOMES OUT OF  
PHASE WITH THE UPPER LEVEL PATTERN ACROSS THE NORTHERN PLAINS AND  
SOUTH-CENTRAL CANADA BY WEDNESDAY SINCE IT IS INDICATING A  
SHORTWAVE IMPULSE IN CONTRAST TO THE BROAD UPPER LEVEL RIDGING  
WITH THE OTHER DETERMINISTIC GUIDANCE. THE WPC FORECAST  
INCORPORATED MORE OF THE EC MEAN BY THURSDAY, AND THEN ALSO SOME  
OF THE GEFS MEAN BY THE END OF THE FORECAST PERIOD. THE PREVIOUS  
DISCUSSION FOLLOWS BELOW FOR REFERENCE.  
 
D. HAMRICK  
 
   
..OVERVIEW
 
 
A SHARP/AMPLIFIED FLOW PATTERN WILL PREVAIL AT THE START OF THE  
WEEK, FEATURING A WEST COAST RIDGE AND CENTRAL CANADA THROUGH  
SOUTHERN ROCKIES/HIGH PLAINS TROUGH. AS INCOMING PACIFIC ENERGY  
FILTERS THROUGH THE WESTERN RIDGE, THE DOWNSTREAM TROUGH SHOULD  
PROGRESS EASTWARD/NORTHEASTWARD AND ULTIMATELY SUPPORT A STORM  
SYSTEM THAT DEVELOPS NEAR THE EAST COAST AND LIKELY BECOMES QUITE  
STRONG BY THE TIME IT REACHES THE CANADIAN MARITIMES. BY LATE IN  
THE WEEK MOST GUIDANCE SHOWS THE RETURN OF A WESTERN RIDGE/EASTERN  
TROUGH CONFIGURATION BUT ONE THAT IS SOMEWHAT BROADER AND LESS  
AMPLIFIED THAN EXPECTED ON MON-TUE. THIS PATTERN WILL BRING MUCH  
BELOW NORMAL TEMPERATURES FROM THE CENTRAL INTO EASTERN U.S.,  
ALONG WITH SNOWFALL POTENTIAL TO SOME LOCATIONS DURING THE FIRST  
HALF OF THE WEEK TO THE WEST OF THE DEVELOPING EAST COAST SYSTEM.  
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT
 
 
FROM ABOUT TUE ONWARD MODELS AND ENSEMBLE MEMBERS DIVERGE  
REGARDING PROGRESSION OF THE SOUTHERN HALF OF THE OVERALL UPPER  
TROUGH THAT SUPPORTS EAST COAST/CANADIAN MARITIMES SYSTEM  
DEVELOPMENT. LOOKING AT GUIDANCE THROUGH THE 18Z CYCLE, THE 12Z  
ECMWF/UKMET/CMC TO VARYING DEGREES ALL SHOWED SOME SHORTWAVE  
ENERGY HANGING BACK FAR ENOUGH WESTWARD TO PRODUCE MORE  
MEANINGFUL/SLOWER WAVINESS ALONG THE FRONT THAT TRAILS THE PRIMARY  
EASTERN SYSTEM COMPARED TO GFS/GEFS MEAN RUNS. AMONG THE 12Z  
ECMWF ENSEMBLES THERE WAS ONLY 10-20 PERCENT SUPPORT FOR A  
SOLUTION AS SLOW AS THE OPERATIONAL 12Z ECMWF AND EVEN LESS FOR  
THE 12Z UKMET. THE COMBINATION OF TEPID ENSEMBLE SUPPORT FOR THE  
SLOW SCENARIO AND LACK OF CONTINUITY IN THE PAST FEW ECMWF RUNS  
FAVORED TRENDING THE FORECAST MORE TOWARD THE GFS AND GEFS/ECMWF  
MEANS BY MID-LATE WEEK. ARRIVAL OF THE 00Z UKMET/CMC PROVIDE SEEM  
TO REINFORCE THE GREATER PROBABILITY OF THE GFS AND GEFS/ECMWF  
MEAN SCENARIO.  
 
AS FOR THE PRIMARY EAST COAST STORM, THE PRIMARY CONSIDERATION  
AMONG 12Z/18Z SOLUTIONS WAS TO EXCLUDE THE UKMET AS IT SHOWED A  
TRACK WELL EAST OF OTHER SOLUTIONS OFF THE NEW ENGLAND COAST.  
OTHER MODELS DISPLAYED COASTAL VS INLAND TRACK DIFFERENCES THAT  
WOULD HAVE A SIGNIFICANT INFLUENCE ON PRECIPITATION TYPE AT SOME  
LOCATIONS, BUT OVERALL DIFFERENCES APPEARED TO BE WITHIN TYPICAL  
ERROR BOUNDS FOR FORECASTS 4-5 DAYS OUT IN TIME THUS FAVORING AN  
INTERMEDIATE TRACK AT THIS TIME.  
 
THE MOST NOTABLE DISCREPANCY UPSTREAM IS WITH LATEST CMC RUNS  
BEING MUCH FASTER THAN OTHER GUIDANCE WITH PUSHING EASTERN PACIFIC  
SHORTWAVE ENERGY THROUGH THE WESTERN RIDGE ALOFT. OVER THE PAST  
SEVERAL DAYS CMC AND/OR CMC ENSEMBLE MEAN RUNS WERE LAST TO CATCH  
ONTO THE SHARP/AMPLIFIED PATTERN EVOLVING IN THE SHORT RANGE AND  
TYPICAL BIASES ARGUE AGAINST THE CURRENT CMC IDEA.  
 
BY DAY 7 FRI THERE IS REASONABLE AGREEMENT AMONG MOST MODELS/MEANS  
THAT THE INCOMING PACIFIC ENERGY WILL AMPLIFY INTO MODERATE  
EAST-CENTRAL U.S. TROUGHING WHILE THE WESTERN RIDGE REBUILDS TO  
SOME DEGREE. THE TROUGH IS CONSISTENT WITH TELECONNECTIONS  
RELATIVE TO THE NEGATIVE HEIGHT ANOMALIES SEEN IN MOST D+8  
MULTI-DAY MEANS OVER THE NORTHERN ATLANTIC. THE EMBEDDED ENERGY  
MAY SUPPORT A MODEST CLIPPER TYPE SYSTEM LATE IN THE WEEK.  
 
BASED ON THE ABOVE GUIDANCE CONSIDERATIONS THE UPDATED FORECAST  
BLEND STARTED WITH MOSTLY 12Z/18Z GFS AND 12Z ECMWF RUNS ON MON  
BUT THEN STEADILY DECREASED ECMWF INPUT TO ZERO AFTER WED WHILE  
GRADUALLY INCREASING 18Z GEFS/12Z ECMWF MEAN WEIGHT. GFS WEIGHT  
ALSO DECLINED WITH TIME BUT MORE GRADUALLY, STILL AT 20 PERCENT BY  
DAY 7 FRI.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
AMPLIFIED UPPER TROUGHING THAT PROGRESSES FROM THE CENTRAL THROUGH  
NORTHEASTERN STATES MON-WED ALONG WITH THE CIRCULATION BEHIND THE  
LEADING EAST COAST STORM SYSTEM (LEADING TO A PERIOD OF BREEZY TO  
WINDY CONDITIONS) WILL BRING MUCH BELOW NORMAL TEMPERATURES FROM  
THE CENTRAL U.S. INTO THE EAST DURING THAT TIME FRAME. SOME HIGHS  
MAY BE AT LEAST 15-25F BELOW NORMAL WHILE ANOMALIES FOR MORNING  
LOWS MAY BE ONLY A FEW DEGREES LESS EXTREME. EASTERN U.S. STORM  
EVOLUTION THAT INCLUDES A WAVY WARM FRONT LIFTING NORTHEAST FROM  
THE GULF OF MEXICO WILL LIKELY BRING AREAS OF HEAVY RAINFALL FROM  
PARTS OF THE GULF COAST NORTHEASTWARD. EXTENT/AXIS OF COLD SECTOR  
SNOWFALL WILL BE SENSITIVE TO STORM TRACK. CURRENTLY EXPECT THE  
APPALACHIANS/WESTERN NEW ENGLAND AND POINTS WESTWARD TO SEE THE  
BEST POTENTIAL FOR ANY SNOW. COLD CYCLONIC LOW LEVEL FLOW WILL  
SUPPORT PERIODS OF LAKE EFFECT SNOW BEHIND THE SYSTEM. LATE IN  
THE WEEK A CLIPPER SYSTEM MAY BRING SOME MOISTURE TO PARTS OF THE  
GREAT LAKES REGION. AS THE UPPER PATTERN BROADENS SOMEWHAT THE  
COLD TEMPERATURES OVER THE EAST SHOULD MODERATE LATE IN THE WEEK  
BUT STILL REMAIN BELOW NORMAL.  
 
EXPECT THE WESTERN STATES TO SEE AN EXPANSION OF ABOVE NORMAL  
TEMPERATURES MON-WED WITH ABOVE NORMAL READINGS LIKELY TO EXTEND  
THROUGH LATE WEEK. PLUS 5-10F ANOMALIES SHOULD BE COMMON WITH  
LOCALIZED HIGHER VALUES POSSIBLE. AFTER SNOW DEPARTS FROM THE  
SOUTH-CENTRAL ROCKIES/HIGH PLAINS MON, PRECIPITATION WILL BE  
CONFINED TO THE PACIFIC NORTHWEST AND EXTREME NORTHERN ROCKIES.  
PRIMARY FOCUS FOR RAIN AND HIGHER ELEVATION SNOW WILL BE WITH  
SHORTWAVE ENERGY/ASSOCIATED SURFACE FRONT REACHING THE NORTHWEST  
AROUND LATE TUE-WED AND THEN WITH UPSTREAM TROUGHING ALOFT/FRONTAL  
SYSTEM THAT MAY APPROACH BY FRI.  
 
RAUSCH  
 
WPC MEDIUM RANGE 500 MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDEXES ARE FOUND AT:  
 
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
 

 
 
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