659  
FXUS02 KWBC 201554  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1154 AM EDT WED JUN 20 2018  
 
VALID 12Z SAT JUN 23 2018 - 12Z WED JUN 27 2018  
 
16Z UPDATE:  
 
THE WPC FORECAST DURING THE MEDIUM RANGE PERIOD FOR THIS WEEKEND  
THROUGH THE MIDDLE OF NEXT WEEK REMAINS FAIRLY CONSISTENT FROM THE  
PREVIOUS FORECAST ISSUANCE, WITH ABOVE AVERAGE FORECAST CONFIDENCE  
THROUGH MONDAY GIVEN OVERALL SYNOPTIC SCALE AGREEMENT WITH  
DETERMINISTIC AND ENSEMBLE MEAN GUIDANCE. IN TERMS OF  
DIFFERENCES, THE 00Z UKMET APPEARED SLOWER WITH THE NORTHEAST LOW  
AND STRONGER WITH THE LOW DEVELOPING OVER THE CENTRAL PLAINS  
SUNDAY AND INTO MONDAY. THE CMC BECOMES MORE AMPLIFIED WITH THE  
UPPER TROUGH CROSSING THE GREAT LAKES AND THEN NEW ENGLAND BY  
MONDAY COMPARED TO THE MODEL CONSENSUS, BUT IT HAS SOME SUPPORT  
FROM SOME OF THE GEFS MEMBERS. THE EC ENSEMBLE MEMBERS ARE  
HOLDING ONTO THE CLOSED UPPER LOW OVER THAT IS INITIALLY OVER THE  
WESTERN HIGH PLAINS LONGER THAN THE GFS AND CMC ENSEMBLE MEMBERS,  
AND OVERALL THE EC MEAN IS MORE IN LINE WITH THE DETERMINISTIC GFS  
AND ECMWF COMPARED TO THE 6Z GEFS MEAN. THE UPDATED WPC FORECAST  
WAS DERIVED PRIMARILY FROM DETERMINISTIC GFS/ECMWF/CMC THROUGH  
THIS WEEKEND WITH GREATEST WEIGHTING TO THE ECMWF GIVEN LESS MODEL  
RUN-TO-RUN VARIABILITY DURING THAT TIME, AND GRADUALLY INCREASING  
CONTRIBUTIONS FROM THE EC MEAN AND SOME OF THE GEFS MEAN BY THE  
END OF THE FORECAST PERIOD. THE PREVIOUS DISCUSSION FROM  
OVERNIGHT IS APPENDED BELOW FOR REFERENCE.  
 
D. HAMRICK  
   
..GUIDANCE AND PREDICTABILITY ASSESSMENT
 
 
GUIDANCE OVERALL SHOWS ABOVE NORMAL AGREEMENT WITH THE FLOW  
PATTERN EVOLUTION FOR THE MEDIUM RANGE, BOLSTERING FORECAST  
CONFIDENCE. NRN STREAM FLOW FEATURES A PERSISTENT CENTRAL CANADA  
UPPER RIDGE BOOK-ENDED BY AMPLIFIED WRN CANADA/NW U.S. AND ERN  
CANADA/NE U.S. COOLING TROUGHS. A MODESTLY PROGRESSIVE PATTERN  
UNDERNEATH OVER THE CONUS HAS SOME TENDENCY FOR SYSTEM  
AMPLIFICATION. ENERGY CROSSING FROM THE ROCKIES TO THE CENTRAL  
U.S. SHOWS POTENTIAL FOR DEEPENING AND/OR CLOSING OFF, MOVING  
THROUGH A REGION OF RELATIVELY WEAK UPPER FLOW BENEATH THE  
CANADIAN RIDGE.  
SOME RECENT GUIDANCE INCLUDING THE 18/00 UTC GFS RUNS AND  
YESTERDAYS 00 UTC CANADIAN SHOW POTENTIAL FOR CLOSING OFF A  
NORTHEAST TROUGH MON/TUE AS PER NRN STREAM ENERGY SPLIT. HOWEVER,  
THE LAST TWO CANADIAN RUNS BACKED OFF FROM FLOW  
SEPARATION/CYCLOGENESIS AND THE LAST FEW ECMWF/UKMET RUNS AND  
ENSEMBLES SUPPORT MORE MODEST AMPLITUDE. THE AMPLIFIED CENTRAL  
CANADIAN RIDGE POSITION OFFERS SOME 18/00 UTC GFS SUPPORT, BUT  
THEY ARE LESS LIKELY SCENARIOS. THE 12 UTC ECMWF ON THE OTHER HAND  
MAY BE TOO PROGRESSIVE LATE IN THE PERIOD GIVEN UPSTREAM AMPLITUDE.  
 
ACCORDINGLY, THE WPC MEDIUM RANGE PRODUCT SUITE WAS PRIMARILY  
DERIVED FROM A COMPOSITE BLEND OF REASONABLY CLUSTERED GUIDANCE  
FROM 12 UTC GFS/ECMWF AND GEFS/ECMWF ENSEMBLE MEANS. THIS  
MAINTAINS EXCELLENT WPC CONTINUITY.  
   
..WEATHER HIGHLIGHTS AND THREATS
 
 
LEAD OH VALLEY TROUGH/LOW ENERGY SATURDAY SHEARS NORTHEASTWARD  
OVER THE NORTHEAST SUNDAY. POST-FRONTAL HIGH PRESSURE WILL NUDGE A  
MODERATING TRAILING FRONT FAR DOWN OVER THE ERN U.S. NEXT WEEK.  
UPSTREAM, TWO MAIN AMPLIFYING SHORTWAVE TROUGHS DIG FROM THE NW  
U.S. SOUTHEASTWARD OVER THE ROCKIES TO THE NORTH-CENTRAL STATES.  
THESE SYSTEMS PRODUCE ORGANIZED SURFACE LOWS ALONG A MAIN FRONTAL  
BOUNDARY FROM THE PLAINS TO THE MS/OH/TN VALLEYS AND THE EAST.  
CONVECTIVE COMPLEXES/HEAVY RAIN FOCUS ALONG THE FRONT AND INTO THE  
WARM SECTOR FUELED BY POOLED MOISTURE. A LATE SEASON COOLING  
FRONT/MODEST PCPN SHOWS INCREASED PROMISE TO WORK MORE EARNESTLY  
OVER THE NW U.S. NEXT WEEK WITH SUPPORTING MID-UPPER LEVEL TROUGH  
AMPLIFICATION.  
 
SCHICHTEL  
 
WPC MEDIUM RANGE FORECASTS OF 500 MB HEIGHTS, SURFACE SYSTEMS,  
WEATHER GRIDS, QUANTITATIVE PRECIPITATION, AND WINTER WEATHER  
OUTLOOK PROBABILITIES ARE FOUND AT:  
 
HTTP://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTP://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTP://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTP://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTP://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
 

 
 
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