426  
FXUS02 KWBC 231531  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1131 AM EDT THU AUG 23 2018  
 
VALID 12Z SUN AUG 26 2018 - 12Z THU AUG 30 2018  
   
..PATTERN OVERVIEW
 
 
A STRONG UPPER HIGH SOUTH OF THE GULF OF ALASKA ON SUNDAY WILL  
DRIFT SLOWLY WESTWARD NEXT WEEK. TO ITS EAST, TROUGHING WILL  
LINGER AND BE REINFORCED OVER THE PACIFIC NORTHWEST WHILE AN UPPER  
HIGH IN THE SOUTHEAST DRIFTS EASTWARD. THIS WILL ACT TO KEEP ANY  
FRONTAL BOUNDARIES NEAR THE US/CANADIAN BORDER (GREAT LAKES TO THE  
NORTHEAST) AS SURFACE LOW PRESSURE LIFTS THROUGH THE UPPER MIDWEST  
MON/TUE. TAIL END OF THAT SYSTEM WILL MEANDER ACROSS THE CENTRAL  
PLAINS TUE-THU.  
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
00Z/06Z MODELS/ENSEMBLES SHOWED MORE SPREAD THAN NORMAL IN THE  
NORTHWEST SUN-MON WITH THE LEAD TROUGH BUT BETTER THAN AVERAGE  
AGREEMENT IN THE EAST WITH THE RIDGING. THE ECMWF AND ITS  
ENSEMBLES WERE QUICKER TO LIFT THE TROUGH EASTWARD COMPARED TO THE  
OTHER 00Z GUIDANCE AND ITS PREVIOUS (12Z/22) RUNS IN RESPONSE  
WEAKER MID-LEVEL VORTICITY ON THE EAST SIDE OF THE UPPER HIGH.  
THOUGH ITS PREVIOUS RUNS SEEMED TOO AGGRESSIVE/OVERDONE, THE 00Z  
RUNS OF THE ECMWF/EC-ENS MAY BE TOO WEAK/QUICK AS THE ABSOLUTE  
EAST-WEST ENSEMBLE SPREAD HAS NOT CHANGED MUCH BETWEEN THE PAST  
TWO CYCLES. 00Z CANADIAN WAS THE SLOWEST/DEEPEST (WITH A CLOSED  
LOW) AND WAS NOT PREFERRED PAST MONDAY. PREFERRED TO YIELD ABOUT  
25% TOWARD THE ECMWF-LED QUICKER SOLUTIONS AND 75% WITH  
CONTINUITY/GFS/GEFS GIVEN UNCERTAINTY WAY UPSTREAM WITH TROPICAL  
STORMS CIMARON/SOULIK IN THE W PAC AS THEY LIFT INTO THE  
WESTERLIES.  
 
FOR NEXT WED-THU, POSITIVELY TILTED TROUGH IN THE WEST WILL MOSTLY  
LINGER AS THE UPPER HIGH IN THE EAST TRIES TO PUSH TOWARD NEW  
ENGLAND (AT LEAST MAINTAINING HIGHER THAN AVERAGE HEIGHTS).  
ANOTHER TROUGH MAY TRY TO PUSH INTO THE PAC NW AGAIN NEXT THURSDAY  
THROUGH THERE HAS BEEN A LARGE AMOUNT OF E-W SPREAD IN ITS AXIS,  
AGAIN RELATED TO UPSTREAM W PAC FLOW ATOP THE UPPER HIGH.  
INCREASED THE ENSEMBLE WEIGHTING IN TURN TO ACCOUNT FOR THE  
UNCERTAINTY.  
 
FRACASSO  
 
   
..WEATHER HIGHLIGHTS AND HAZARDS
 
 
THE MOST ACTIVE WEATHER IN TERMS OF PRECIPITATION WILL BE ACROSS  
THE NORTHERN TIER DURING THE MEDIUM RANGE GIVEN THE  
TROUGH/SHORTWAVE ENERGY AND FRONTAL BOUNDARY DESCRIBED ABOVE.  
MODELS SHOW THE POTENTIAL FOR MULTIPLE AREAS OF POTENTIALLY HEAVY  
CONVECTION ACROSS MULTIPLE DAYS FROM THE NORTHERN ROCKIES TO THE  
GREAT LAKES. BY NEXT TUE-WED, SOME CONVECTION MAY SPREAD AS FAR  
EAST AS THE NORTHEAST. THE CONSENSUS OF MODEL/ENSEMBLE SOLUTIONS  
SUGGEST THAT THE GREATEST POTENTIAL FOR HEAVY RAINFALL WILL EXIST  
ACROSS PORTIONS OF THE UPPER MIDWEST AND GREAT LAKES, AS THE  
FRONTAL BOUNDARY WILL BE IN THE VICINITY THROUGH MOST OF THE  
FORECAST PERIOD. FARTHER SOUTH, SCATTERED AFTERNOON/EVENING  
SHOWERS AND THUNDERSTORMS WILL PERSIST ACROSS THE SOUTHEAST AS A  
STATIONARY SURFACE FRONTAL BOUNDARY SLOWLY DECAYS. MONSOONAL  
CONVECTION IN THE FOUR CORNERS REGION SHOULD GRADUALLY REDUCE IN  
COVERAGE/INTENSITY THROUGH THE MEDIUM RANGE AS THE LOW/MID-LEVEL  
FLOW BECOMES LESS FAVORABLE FOR TRANSPORT OF DEEP MOISTURE INTO  
THE REGION.  
 
EXPECT BELOW AVERAGE TEMPERATURES ACROSS MUCH OF THE NORTHWESTERN  
U.S. GIVEN THE PERSISTENT UPPER TROUGH OVERHEAD. HIGH TEMPERATURES  
OF 5 TO 15 DEG F BELOW AVERAGE ARE EXPECTED INTO EARLY-MID NEXT  
WEEK. FARTHER EAST, THE EXPANDING UPPER RIDGE WILL SUPPORT  
INCREASING TEMPERATURES, WITH HIGHS 5 TO 10 DEG F ABOVE AVERAGE  
ACROSS MUCH OF THE CENTRAL AND EASTERN U.S. THROUGH THE PERIOD.  
 
RYAN  
 
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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