982  
FXUS02 KWBC 131619  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1218 PM EDT MON AUG 13 2018  
 
VALID 12Z THU AUG 16 2018 - 12Z MON AUG 20 2018  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE BEGINNING OF THE PERIOD (THURSDAY MORNING) SHOWS SOME  
COMBINATION OF SHORTWAVE ENERGY LIFTING OUT OF THE NORTHERN PLAINS  
INTO A SECOND FEATURE/SHORTWAVE ELONGATING ACROSS THE UPPER  
MIDWEST. A SURFACE WAVE AHEAD OF THIS SHOULD GRADUALLY DEEPEN AS  
IT PROGRESSES FROM THE OHIO VALLEY INTO NEW ENGLAND THURSDAY AND  
FRIDAY. OVERALL, THE GUIDANCE IS IN RELATIVELY GOOD AGREEMENT WITH  
THIS FEATURE. IT IS INTERESTING TO NOTE THE ECMWF AND ITS ENSEMBLE  
MEAN WERE NOTICEABLY SLOWER WITH THE SHORTWAVE PLACEMENT ON DAY 3,  
BUT BY DAY 4 AND BEYOND, SEEMS TO COME INTO BETTER AGREEMENT WITH  
THE REST OF THE MODEL SUITE.  
 
OUT WEST, MODELS CONTINUE TO SHOW WEAK HEIGHT FALLS SPILLING INTO  
THE PACIFIC NORTHWEST ON DAY 3, INTERACTING WITH AMPLIFICATION IN  
THE NORTHERN STREAM FLOW. THIS SHOULD ULTIMATELY LEAD TO SOME  
LEVEL OF AMPLIFIED FLOW ACROSS THE MIDDLE OF THE COUNTRY BY THIS  
WEEKEND. THE LATEST RUN OF THE GFS (06Z ON AUG 13) HAS BACKED OFF  
ON THE MORE AMPLIFIED TROUGH NOTED PREVIOUSLY, AND NOW LOOKS  
CLOSER IN LINE WITH THE REST OF THE GUIDANCE. WHATEVER FEATURE  
DOES EMERGE WITHIN THE FLOW REGIME WILL ADVANCE DOWNSTREAM AND  
SHOULD BE AT A POSITION NEAR THE MISSISSIPPI RIVER BY DAY 7/AUGUST  
20 (MONDAY MORNING), AS SUGGESTED BY THE ENSEMBLE MEANS. IN ITS  
WAKE, A POSSIBLY ANOMALOUS RIDGE BUILDS JUST TO THE WEST ACROSS  
THE EASTERN PACIFIC INTO WESTERN CANADA. WHILE MOST PIECES OF  
GUIDANCE DO SHOW SOME DEGREE OF ANOMALOUS HEIGHTS, MODELS CONTINUE  
TO SHOW DISAGREEMENT WITH HOW STRONG THESE MAY BE. THE LATEST RUNS  
OF THE GFS CONTINUE TO BE THE MOST AMPLIFIED, BUT THIS MAY NOT BE  
A BAD THING AS A MORE AMPLIFIED PATTERN UPSTREAM (WITH WHATEVER  
REMAINS OF T.S. HECTOR PHASING INTO A LARGER UPPER WEST OF ALASKA)  
WOULD SUPPORT A MORE AMPLIFIED RIDGE DOWNSTREAM.  
 
AS FOR MODEL CHOICES WITH THIS CYCLE OF THE MEDIUM RANGE PROGS,  
THROUGH DAY 5, THERE WAS ENOUGH OPERATIONAL SUPPORT AND AGREEMENT  
TO CONSTITUTE A MAJORITY OPERATIONAL 06Z GFS/00Z ECMWF BLEND.  
BEYOND, NORMAL TIMING AND INTENSITY DIFFERENCES ARISE WITHIN THE  
SMALLER SCALE FEATURES SO A LARGER INFLUENCE OF THE ENSEMBLE MEANS  
(06Z GEFS/00Z ECENS) WAS USED. DID PUT SLIGHTLY MORE WEIGHT ON THE  
LATEST GFS/GEFS MEAN THAN THE ECMWF/ECENS BY DAY 7 TO SHOW THAT  
SLIGHTLY MORE AMPLIFIED RIDGING ACROSS WESTERN CANADA. THE OVERALL  
BLEND FOR THIS CYCLE FIT WELL WITH PREVIOUS SHIFT CONTINUITY SO  
ONLY MINOR CHANGES WERE NEEDED TO THE SURFACE FRONTS AND PRESSURES  
AND 500 PROGS.  
 
   
..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 TO NEAR 80 ACROSS THE LOWER ELEVATIONS, SUCH  
READINGS WOULD BE AROUND 10 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, 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.  
 
SANTORELLI/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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