465  
FXUS02 KWBC 120638  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
237 AM EDT THU JUL 12 2018  
 
VALID 12Z SUN JUL 15 2018 - 12Z THU JUL 19 2018  
   
..OVERVIEW
 
 
AS AN ELONGATED UPPER TROUGH SLOWLY DEPARTS OFF THE EASTERN  
SEABOARD, A SHARPENING TROUGH WILL ZIPPER ACROSS CENTRAL CANADA  
REACHING THE VICINITY OF HUDSON BAY BY MONDAY. SUCH HEIGHT FALLS  
SHOULD GRADUALLY TRANSLATE EASTWARD POSSIBLY EXITING THE EASTERN  
U.S. BY DAY 7/JULY 19. IN ITS WAKE, THE SYNOPTIC PATTERN REMAINS  
MORE NEBULOUS IN NATURE AS GUIDANCE VARY IN WHICH PHASE OF THE  
SINUSOIDAL FLOW WILL BE FAVORED. IT DOES APPEAR SOME FORM OF  
LONGWAVE TROUGH SHOULD BE POSITIONED OVER WESTERN NORTH AMERICA  
TOWARD THE END OF THE FORECAST. TO THE SOUTH OF THE ACTIVE  
MID-LATITUDE PATTERN FOCUSING ALONG THE INTERNATIONAL BORDER WITH  
CANADA, A PROMINENT 594-DM MID-LEVEL RIDGE SHOULD BECOME FIXED  
OVER THE SOUTHWESTERN U.S. THROUGHOUT THE PERIOD.  
 
   
..GUIDANCE EVALUATION AND PREFERENCES
 
 
THERE DO NOT APPEAR TO BE ANY SIGNIFICANT ISSUES WITH THE HANDLING  
OF THE SLOW MOVING TROUGH LIFTING AWAY FROM THE GULF STREAM. THIS  
REGION IS CONTINUING TO BE MONITORED BY THE NATIONAL HURRICANE  
CENTER FOR TROPICAL CYCLONE DEVELOPMENT WITH A 50 PERCENT CHANCE  
OF SUCH OCCURRENCE THE NEXT 5 DAYS. THE 12Z UKMET IS THE MOST  
BULLISH WITH CYCLOGENESIS DEPICTING A SUB-1000 MB WAVE LIFTING  
TOWARD THE EASTERN TIP OF NOVA SCOTIA BY MONDAY EVENING.  
MEANWHILE, THE OTHER REGION OF ONLY MODEST DIFFERENCES IS THE  
SOUTHWESTERN U.S. WITH A 2 SIGMA ABOVE CLIMATOLOGY MID-LEVEL RIDGE  
SETTING UP FOR THE FORESEEABLE FUTURE. THE PAST FEW RUNS OF THE  
GFS ARE A BIT MORE EXPANSIVE WITH THE 594-DM HEIGHT CONTOUR  
RELATIVE TO OTHER GUIDANCE.  
 
A PRONOUNCED UPPER TROUGH AND THE ATTENDANT HEIGHT FALLS WILL  
SLIDE EASTWARD FROM CENTRAL NORTH AMERICA TOWARD THE EASTERN  
SEABOARD DURING THE PERIOD. RELATIVE TO YESTERDAY, THE MODELS HAVE  
TRENDED A BIT SLOWER WITH FRONTAL PROGRESSION WITH A POSITION  
LIKELY OFFSHORE OF NEW ENGLAND BY DAY 7/JULY 19. ENSEMBLE  
SPAGHETTI PLOTS FROM THE 12Z/00Z CYCLES WOULD SUGGEST THE ECMWF  
MEMBERS ARE SLOWER AS A WHOLE ALTHOUGH ITS PATTERN HAS LOOKED A  
BIT WONKY UPSTREAM. SO HAVE NOT PUT TREMENDOUS STOCK IN ITS  
SOLUTION SUITE. PLENTY OF RUN-TO-RUN VARIABILITY IN THE GUIDANCE  
THOUGH AS THIS LARGE-SCALE UPPER TROUGH MARCHES TOWARD EASTERN  
NORTH AMERICA BY MID/LATE NEXT WEEK.  
 
THE MOST CONTENTIOUS SECTOR OF THE MAP IS THE NORTHEASTERN PACIFIC  
EASTWARD INTO WESTERN NORTH AMERICA WHERE THE GUIDANCE ARE AT ODDS  
WITH ONE ANOTHER. PHASE DIFFERENCES ARE IMMEDIATELY EVIDENT IN  
OPERATIONAL AND ENSEMBLE MODEL COMPARISONS. DURING THE LAST THREE  
RUNS, THE ECMWF ENSEMBLES CONTINUE TO AGGRESSIVELY SHOVE A TROUGH  
INTO BRITISH COLUMBIA. INVESTIGATING WHERE THIS SYSTEM COMES FROM,  
IT CURRENTLY LURKS NEAR THE FAR WESTERN ALEUTIANS WHERE THE ECMWF  
SUSTAINS IT AS AN OPEN WAVE INTO NORTHERN BRITISH COLUMBIA WHILE  
THE GFS SHEARS IT APART OVER THE YUKON TERRITORY. SUFFICE TO SAY,  
THIS DIFFERENCE LEADS TO SIGNIFICANT PHASE DIFFERENCES ON THE MAP.  
THE PREVIOUS GFS RUNS AND 12Z UKMET FAVOR TROUGHING INTO WESTERN  
CANADA ON TUESDAY WHILE THE PAST 3 ECMWF CYCLES BUILD HEIGHTS  
AHEAD OF ANOTHER COMPACT UPPER LOW. STILL DIFFICULT TO TRUST THE  
ECMWF, PARTICULARLY ITS OPERATIONAL RUN GIVEN HOW MUCH IT DEPARTS  
FROM THE ENSEMBLES BY THE DAY 4/5, JULY 16/17 TIMEFRAME.  
 
DURING THE FORECAST, KEPT A 30 PERCENT CONTRIBUTION OF SOME  
VARIATION OF THE 12Z ECMWF/ECMWF ENSEMBLE MEAN THROUGHOUT THE  
PERIOD. DID NOT WANT TO COMPLETELY DISCOUNT IT BUT THE FORECAST  
WAS MORE HEAVILY BASED ON THE 18Z/12Z GFS, 18Z GEFS/12Z NAEFS  
ENSEMBLE MEANS, WITH SOME EARLY MINIMAL CONTRIBUTIONS FROM THE 12Z  
CMC/UKMET. A COMPLETE ENSEMBLE APPROACH WAS DEEMED NECESSARY  
BEYOND WEDNESDAY GIVEN A BREAKDOWN OF THE PATTERN.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
WHILE UPCOMING TEMPERATURES SHOULD REMAIN RATHER CLOSE TO  
CLIMATOLOGY DURING THE FORECAST PERIOD, THERE ARE A FEW EXCEPTIONS  
TO THIS RULE. SUNDAY SHOULD PROVE TO BE A RATHER COOL SUMMER DAY  
ACROSS THE NORTHERN ROCKIES/GREAT PLAINS DOWN INTO WESTERN  
NEBRASKA WITH HIGHS 10 TO 15 DEGREES BELOW AVERAGE. THIS WOULD  
SUGGEST HIGHS IN THE 70S BEFORE REBOUNDING BY EARLY NEXT WEEK AS  
THE COLD ADVECTION CEASES. ELSEWHERE, WELL ABOVE AVERAGE READINGS  
SHOULD BE COMMONPLACE ACROSS THE PACIFIC NORTHWEST WITH A BUILDING  
UPPER RIDGE. HIGH TEMPERATURES WILL LIKELY BE 10 TO 15 DEGREES  
ABOVE CLIMATOLOGY FROM SUNDAY THROUGH TUESDAY AS READINGS SOAR  
INTO THE 90S INLAND WITH 70S CLOSER TO THE PACIFIC COAST.  
 
WET CONDITIONS SHOULD PREVAIL OVER THE FOUR CORNERS REGION UP INTO  
THE CENTRAL ROCKIES ALTHOUGH COVERAGE WILL BE DEPENDENT ON HOW  
EXPANSIVE THE 594-DM RIDGE BECOMES. WARM MID-LEVEL TEMPERATURES  
UNDERNEATH THIS RIDGE SHOULD STUNT MORE VERTICALLY DEVELOPED  
CONVECTION. FARTHER EAST, AN EXPANSIVE REGION OF SHOWERS AND  
THUNDERSTORMS IS EXPECTED IN ADVANCE OF THE CENTRAL NORTH AMERICAN  
TROUGH. WHILE THE BETTER FOCUS SHOULD OCCUR CLOSER TO THE HEIGHT  
FALLS, ABUNDANT INSTABILITY WOULD PROMOTE WIDESPREAD CONVECTION  
WITHIN THE VAST WARM SECTOR.  
 
RUBIN-OSTER  
 
WPC MEDIUM RANGE 500 MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDEXES 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  
HTTP://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
 

 
 
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