048  
FXUS02 KWBC 130548  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
148 AM EDT MON AUG 13 2018  
 
VALID 12Z THU AUG 16 2018 - 12Z MON AUG 20 2018  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE PREVAILING WESTERLIES SHOULD REMAIN CONFINED TO THE VICINITY  
OF THE INTERNATIONAL BORDER WITH CANADA. MULTIPLE FEATURES DIPPING  
SOUTHWARD WITHIN THIS ACTIVE FLOW WILL BE KEY FEATURES TO HONE IN  
ON DURING THE PERIOD. ON THURSDAY MORNING, THE GUIDANCE SHOW SOME  
COMBINATION OF SHORTWAVE ENERGY LIFTING OUT OF THE CENTRAL PLAINS  
AND A FEATURE ELONGATING OUT OF THE UPPER MIDWEST. A SURFACE WAVE  
ATTENDANT TO THE LEAD HEIGHT FALLS IS FORECAST TO GRADUALLY DEEPEN  
WHILE PROGRESSING FROM THE OHIO VALLEY INTO NEW ENGLAND. RECENT  
GFS RUNS REMAIN AT ODDS WITH OTHER MODELS AS THEY FAVOR ONE  
PRIMARY SHORTWAVE ACROSS THE UPPER GREAT LAKES ON THURSDAY.  
ADDITIONALLY, FORECAST SPREAD IS NOTED WITH SURFACE LOW  
PROGRESSION AS THE 18Z/12Z GFS ARE SLOWER THAN THE CONSENSUS WHILE  
THE 12Z ECMWF SITS WELL AHEAD OF ITS ENSEMBLE CLUSTER. REGARDING  
INTENSITY, THE 12Z UKMET IS EASILY 5 TO 10 MB DEEPER THAN MOST  
OTHER SOLUTIONS BY SATURDAY MORNING ALTHOUGH ITS MOST RECENT 00Z  
RUN DID SIGNIFICANTLY BACK OFF ON THIS.  
 
LOOKING FARTHER WEST, AS A GENERAL LONGWAVE TROUGH PATTERN SITS  
OVER THE NORTHEASTERN U.S. INTO THE WESTERN ATLANTIC, MODELS SHOW  
WEAK HEIGHT FALLS FROM THE PACIFIC NORTHWEST INTERACTING WITH  
AMPLIFICATION WITHIN THE NORTHERN STREAM. THIS SHOULD ULTIMATELY  
LEAD TO SOME LEVEL OF AMPLIFIED FLOW ACROSS THE MIDDLE OF THE  
COUNTRY DURING THE WEEKEND. ENSEMBLE SPAGHETTI PLOTS HAVE  
ROUTINELY FAVORED GENERAL WEST-NORTHWESTERLY FLOW ALOFT OVER THIS  
REGION WITH VARYING DEGREES OF AMPLIFICATION SUGGESTED IN SEVERAL  
MEMBERS. BY DAY 5/SATURDAY ONWARD, THE 12Z GFS DOES STAND OUT AS  
BEING THE MOST AMPLIFIED AMONG THE 90 ENSEMBLE MEMBER COMPARISON.  
WHAT EVER FEATURE EMERGES WITHIN THE FLOW REGIME SHOULD ADVANCE  
DOWNSTREAM WITH A POSITION NEAR OR EAST OF THE MISSISSIPPI RIVER  
BY DAY 7/AUGUST 20 AS SUGGESTED BY ENSEMBLE MEANS. IN ITS WAKE,  
HEIGHTS SIGNIFICANTLY BUILD ACROSS WESTERN NORTH AMERICA WITH THE  
00Z GFS 500-MB HEIGHT ANOMALIES AROUND 2 TO 2.5 SIGMA ABOVE  
CLIMATOLOGY ACROSS THE PACIFIC NORTHWEST INTO BRITISH COLUMBIA. IT  
IS ON THE STRONGER SIDE OF THE ENVELOPE WITH THE ECMWF ENSEMBLES  
DEPICTING A WEAKER RIDGE, GENERALLY AROUND A 1 SIGMA DEPARTURE.  
WHILE OUTLYING IN NATURE, IT IS STILL WORTH NOTING THE 00Z/12Z CMC  
SHOW A LONGWAVE TROUGH IN A SIMILAR LOCATION. THEIR SOLUTION HAS  
NEARLY ZERO ENSEMBLE SUPPORT SO IT CAN BE REJECTED HERE.  
 
THROUGH DAY 4/FRIDAY, THERE WAS ENOUGH OPERATIONAL MODEL SUPPORT  
TO USE AN ARRAY OF SOLUTIONS, PRIMARILY LED BY THE 18Z GFS/12Z  
ECMWF WITH SMALLER CONTRIBUTIONS FROM THE 12Z CMC/UKMET. BEYOND  
THIS PERIOD, THE 12Z UKMET GREW OUT OF TOLERANCE BEING DEEPER WITH  
THE NORTHEASTERN U.S. CYCLONE WHILE THE 00Z/12Z CMC DIVERGED  
SIGNIFICANTLY FROM ENSEMBLES BACK TOWARD THE NORTHEASTERN PACIFIC.  
THUS, LEANED MORE IN THE DIRECTION OF THE 18Z GFS/12Z ECMWF WITH  
AN INCREASING INFLUENCE OF THE TRIO OF ENSEMBLE MEANS, 18Z  
GEFS/12Z ECMWF/12Z NAEFS. CONFIDENCE THROUGHOUT THE PERIOD IS  
AROUND AVERAGE WITH GLOBAL SOLUTIONS AT LEAST IN AGREEMENT WITH  
THE GENERAL SYNOPTIC PATTERN.  
 
   
..WEATHER HIGHLIGHTS AND HAZARDS
 
 
WITH THE EXPECTATION OF A DEEP UPPER TROUGH PUSHING ACROSS THE  
NORTHERN ROCKIES INTO THE NORTHERN/CENTRAL GREAT PLAINS DURING THE  
DAY 5-7, AUGUST 18-20 TIME PERIOD, MUCH COOLER CONDITIONS CAN BE  
ANTICIPATED. WITH EXPECTED HIGHS ONLY IN THE 60S ACROSS THE HIGHER  
TERRAIN AND 70S ACROSS THE LOWER ELEVATIONS, SUCH READINGS WOULD  
BE AROUND 10 TO 15 DEGREES BELOW AVERAGE. BEFORE THIS TAKES PLACE,  
IT SHOULD BE QUITE WARM ACROSS THE NORTH-CENTRAL U.S. THROUGH  
FRIDAY AS HIGH TEMPERATURES SIT IN THE LOW/MID 90S. ELSEWHERE, A  
BUILDING UPPER RIDGE ALONG THE WEST COAST WILL ALLOW TEMPERATURES  
TO SOAR EACH DAY WITH MOST ANOMALOUS NUMBERS OVER THE PACIFIC  
NORTHWEST. BY DAY 6-7, AUGUST 19-20, PORTLAND AND SEATTLE MAY PUSH  
INTO THE 90S WHICH WOULD BE 10 TO 15 DEGREES ABOVE MID-AUGUST  
CLIMATOLOGY.  
 
MULTIPLE ROUNDS OF SHOWERS AND THUNDERSTORMS ARE LIKELY FROM THE  
MIDDLE OF THE COUNTRY EASTWARD INTO THE GREAT LAKES AND NEW  
ENGLAND. THE BEST FOCUS WILL INITIALLY BE NEAR THE LIFTING SURFACE  
WAVE WHILE VAST WARM SECTOR CONVECTION SHOULD PREVAIL TO THE  
SOUTH. WITH THE FLOW EXPECTED TO AMPLIFY THIS WEEKEND ACROSS THE  
ROCKIES, ROBUST QPF AMOUNTS ARE NOTED IN MANY MODEL SOLUTIONS WITH  
A PARTICULAR FOCUS ACROSS THE CENTRAL PLAINS INTO THE MIDDLE  
MISSISSIPPI VALLEY. ACROSS THE FOUR CORNERS REGION, ACTIVE  
MONSOONAL FLOW WILL AID IN DAILY THUNDERSTORM CHANCES WITH HEAVIER  
AMOUNTS POTENTIALLY OVER SOUTHERN ARIZONA WHERE HIGHER  
PRECIPITABLE WATER AIR IS TO RESIDE.  
 
RUBIN-OSTER  
 
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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