632  
FXUS02 KWBC 080526  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
126 AM EDT FRI JUN 08 2018  
 
VALID 12Z MON JUN 11 2018 - 12Z FRI JUN 15 2018  
   
..PATTERN OVERVIEW
 
 
UPPER FLOW ACROSS NORTH AMERICA DURING THE MEDIUM RANGE IS  
EXPECTED TO SHOW A LARGE DEGREE OF BLOCKING, PARTICULARLY AT THE  
HIGHER LATITUDES. AN ANOMALOUS UPPER RIDGE AXIS IS EXPECTED TO  
INITIALLY BE IN PLACE ON DAY 3 (MON) FROM NEAR THE GREAT LAKES  
NORTH ACROSS HUDSON BAY. OVER THE FOLLOWING DAYS, THIS FEATURES IS  
FORECAST BY MOST GUIDANCE TO TRANSITION TO A HIGHLY ANOMALOUS  
BLOCKING UPPER HIGH CENTERED NORTH OF HUDSON BAY. ENERGY  
TRAVERSING THE WESTERLIES WILL BE FORCED SOUTH OF THIS FEATURE,  
RESULTING IN A RELATIVELY ACTIVE STORM TRACK ACROSS THE U.S.  
NORTHERN TIER. ADDITIONALLY, THE UPPER RIDGE ALONG WITH NEGATIVE  
HEIGHT ANOMALIES LOCATED ACROSS EASTERN CANADA WILL BE FAVORABLE  
FOR THE TRANSPORT OF COOLER AIR FROM HIGH LATITUDES SOUTHWARD,  
ENHANCING THERMAL GRADIENTS ACROSS THE NORTHERN U.S. FARTHER  
SOUTH, UPPER RIDGING IS LIKELY TO REMAIN STRONG ACROSS THE  
SOUTHERN PLAINS AND MISSISSIPPI VALLEY. LASTLY, UPPER TROUGHING IS  
LIKELY TO AMPLIFY ALONG THE WEST COAST BY MID TO LATE NEXT WEEK,  
ALTHOUGH THE EXACT NATURE OF THE TROUGH/UPPER LOW REMAINS UNCLEAR  
AT THIS TIME.  
 
   
..MODEL GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
MODELS/ENSEMBLES HAVE PERFORMED VERY POORLY WITH RESPECT TO THE  
AFOREMENTIONED UPPER RIDGE/BLOCKING HIGH NOW FORECAST TO DEVELOP  
ACROSS CANADA DURING THE MEDIUM RANGE. TELECONNECTIONS ASSOCIATED  
WITH A FEW OF THE STRONGER HEIGHT ANOMALIES AROUND THE NORTHERN  
HEMISPHERE DO LITTLE TO AID IN THE FORECAST OF THE CANADIAN BLOCK.  
FURTHER, THE SIGNIFICANT RUN-TO-RUN VARIABILITY AMONG THE GUIDANCE  
SURROUNDING THIS FEATURE SUGGESTS A HIGHLY UNSTABLE PATTERN WITH  
LOW PREDICTABILITY. THE EFFECTS OF THIS REVERBERATE TO OTHER  
ASPECTS OF THE FORECAST, INCLUDING SIGNIFICANT VARIABILITY EVEN BY  
DAYS 3-4 WITH RESPECT TO SHORT WAVE/FRONTAL TIMING ACROSS THE  
MID-ATLANTIC REGION. A THOROUGH LOOK AT SEVERAL OF THE MOST RECENT  
MODEL CYCLES SEEMED TO PERHAPS REVEAL SLIGHTLY MORE RUN-TO-RUN  
CONSISTENCY IN THE GFS RELATIVE TO OTHER GUIDANCE, ESPECIALLY FROM  
DAY 4 ONWARD, WITH PERHAPS THE LEAST CONSISTENCY SHOWN BY THE  
ECMWF. THE POSITION AND STRENGTH OF THE BLOCKING CANADIAN RIDGE  
AFFECTS THE TIMING AND AMPLITUDE OF SHORTWAVE ENERGY CROSSING THE  
NORTHERN PLAINS/MIDWEST TUE-WED. GIVEN THESE CONSIDERATIONS, THE  
FORECAST FOR DAYS 3-4 WAS BASED ON A MULTI-MODEL DETERMINISTIC  
BLEND INCLUDING THE 12Z GFS/UKMET/ECMWF, WITH THE ECMWF WEIGHTED  
LEAST OF THE THREE - COMPONENTS OF THE 12Z ECENS/GEFS MEANS WERE  
ALSO INCLUDED.  
 
FROM DAY 5 (WED) ONWARD, THERE IS AT LEAST A GENERAL CONSENSUS  
AMONG THE GUIDANCE THAT A RELATIVELY VIGOROUS SHORTWAVE/UPPER LOW  
WILL APPROACH THE PACIFIC NORTHWEST, AND POTENTIALLY AMPLIFY AS IT  
REACHES THE COAST (ALTHOUGH THE 12Z CMC KEPT A WEAKER SYSTEM AND  
QUICKLY MOVED IT INLAND). WHILE THERE IS AGREEMENT ON THE  
EXISTENCE AND GENERAL TIMING OF THIS FEATURE, THERE IS ONCE AGAIN  
A HIGH DEGREE OF RUN-TO-RUN VARIABILITY AS TO THE EXACT CHARACTER  
OF THE FEATURE (OPEN TROUGH VS. CLOSED LOW). ADDITIONAL  
COMPLICATIONS ARISE FROM WED ONWARD WITH THE GFS PERSISTENTLY  
DEVELOPING AND AREA OF LOW PRESSURE IN THE GULF OF MEXICO WHICH IT  
THEN MOVES NORTHWARD TOWARD THE GULF COAST, WHILE THE ECMWF AND  
CMC SHOW NO SUCH SYSTEM. AS A RESULT, THE FORECAST WAS TRENDED  
TOWARD ENSEMBLE (12Z ECENS/GEFS) MEANS FROM DAY 5 ONWARD.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
A SURFACE FRONTAL BOUNDARY AND UPPER-LEVEL SHORTWAVE ARE LIKELY TO  
BRING SCATTERED TO NUMEROUS SHOWERS AND STORMS WITH AREAS OF HEAVY  
RAINFALL TO PORTIONS OF THE SOUTHEAST MON-TUE. LINGERING MOISTURE  
ACROSS THE SOUTHEAST THROUGH THE END OF NEXT WEEK SHOULD RESULT IN  
CONTINUED AFTERNOON/EVENING CONVECTION WITH LOCALLY HEAVY RAINS.  
THE UPPER TROUGH AND RELATIVELY STRONG SURFACE FRONTAL SYSTEM  
CROSSING THE NORTHERN PLAINS/MIDWEST MON-TUE IS EXPECTED TO RESULT  
IN AREAS OF LOCALLY HEAVY CONVECTION, WITH THIS ACTIVITY SPREADING  
EASTWARD TO THE NORTHEAST AS WELL AS THE OHIO/TENNESSEE VALLEYS BY  
WED-THU. MEANWHILE, MUCH OF THE WESTERN U.S. WILL BE DRY. THE  
ARRIVAL OF THE UPPER TROUGH AND ASSOCIATED SURFACE FRONT ACROSS  
THE NORTHWEST BY NEXT WED-THU SHOULD BRING AN INCREASE IN SHOWER  
ACTIVITY, REACHING THE NORTHERN ROCKIES BY THU-FRI AS THE SYSTEM  
MOVES INLAND.  
 
CONTINUED UPPER RIDGING ACROSS THE SOUTH CENTRAL U.S. WILL RESULT  
IN PERSISTENT WARM/HOT TEMPERATURES, WITH HIGHS EXPECTED TO RANGE  
FROM 6 TO 12 DEG F ABOVE AVERAGE THROUGH MUCH OF THE FORECAST  
PERIOD FROM PORTIONS OF THE SOUTHERN/CENTRAL PLAINS TO THE  
LOWER/MIDDLE MISSISSIPPI VALLEY.  
 
RYAN  
 
WPC MEDIUM RANGE FORECASTS OF 500 MB HEIGHTS, SURFACE SYSTEMS,  
WEATHER GRIDS, QUANTITATIVE PRECIPITATION, AND WINTER WEATHER  
OUTLOOK PROBABILITIES CAN BE 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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