908  
FXUS02 KWBC 131600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1159 AM 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, THERE MAY BE ANOTHER WEAK SHORTWAVE ACROSS THE PACIFIC  
NORTHWEST, WITH A BUILDING RIDGE JUST TO THE WEST ACROSS THE  
EASTERN PACIFIC INTO WESTERN CANADA. WHILE MOST PIECES OF GUIDANCE  
DO SHOW THIS ANOMALOUSLY HIGH HEIGHTS WITH THE RIDGE, MODELS  
CONTINUE TO SHOW DISAGREEMENT WITH EXACTLY HOW AMPLIFIED THIS  
RIDGE MAY BE.  
 
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.  
 
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  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page