041  
FXUS02 KWBC 280653  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
252 AM EDT SAT JUL 28 2018  
 
VALID 12Z TUE JUL 31 2018 - 12Z SAT AUG 04 2018  
   
..HOT TEMPERATURES OVER THE WEST MODERATING
 
   
..HEAVY RAINFALL THREAT OVER PORTIONS OF THE EAST
 
 
   
..PATTERN OVERVIEW AND WEATHER HIGHLIGHTS/THREATS
 
 
THROUGH A COMBINATION OF SOME ENERGY EJECTING NORTHEASTWARD AND  
THE REMAINDER WEAKENING IN PLACE, EXPECT THE AMPLIFIED TROUGH  
ALOFT OVER OR JUST EAST OF THE MS VALLEY AS OF TUE-WED TO BECOME  
MORE DIFFUSE THEREAFTER. THE EVENTUAL WEAKENING TREND FOR BOTH  
THIS TROUGH AND THE INITIAL WESTERN U.S. RIDGE WILL LEAD TO A FLOW  
EVOLUTION THAT MORE CLOSELY RESEMBLES A MID-LATE SUMMER PATTERN BY  
FRI-SAT WITH LOW AMPLITUDE WESTERLIES ACROSS THE NORTHERN TIER.  
STRONG RIDGING OVER THE WESTERN ATLANTIC WILL BE A MORE PERSISTENT  
FEATURE.  
 
OVER THE EAST, THE FIRST HALF OF THE PERIOD WILL FEATURE MS VALLEY  
LOW PRESSURE TRACKING NORTHEASTWARD AND ASSOCIATED LEADING  
WARM/TRAILING COLD FRONTS PROVIDING WELL-DEFINED FOCUS FOR  
CONVECTION AND HEAVY RAINFALL POTENTIAL. THE COLD FRONT WILL HAVE  
DIFFICULTY REACHING ANY FARTHER THAN NEAR THE EAST COAST DUE TO  
THE STRONG DEEP-LAYER RIDGE OVER THE WESTERN ATLANTIC. MOIST  
GULF/ATLANTIC FLOW AROUND THE BERMUDA HIGH WILL LIKELY INTERACT  
WITH THE STALLING FRONT AND MAINTAIN THE POSSIBILITY OF  
SIGNIFICANT RAINFALL OVER PARTS OF THE  
APPALACHIANS/SOUTHEAST/MID-ATLANTIC. THE FRONT WILL LIKELY  
DISSIPATE BY FRI-SAT BUT SOME DIURNALLY FAVORED CONVECTION SHOULD  
CONTINUE WITH MORE OF A TERRAIN/LOCALIZED BOUNDARY FOCUS. THE  
BEST SIGNAL FOR HIGHEST 5-DAY TOTALS EXISTS OVER THE SOUTHERN  
APPALACHIANS/SOUTHEAST, WHILE OTHER RELATIVE MAXIMA ARE POSSIBLE  
OVER THE OHIO VALLEY/EASTERN GREAT LAKES AND PARTS OF THE  
MID-ATLANTIC.  
 
FLATTENING OF THE WESTERN U.S. RIDGE WILL BE AIDED BY THE EASTWARD  
PROGRESSION OF ENERGY WITHIN/AROUND AN UPPER LOW STILL FORECAST TO  
BE NEAR NORTHERN VANCOUVER ISLAND AS OF EARLY WED. THIS UPPER  
SUPPORT WILL BRING A GENERAL AREA OF LOW PRESSURE WITH ONE OR MORE  
CENTERS INTO THE NORTHERN PLAINS/SOUTHERN CANADA BY THE LATTER  
HALF OF THE PERIOD--LED BY A WARM FRONT REPRESENTING THE NORTHWARD  
RETURN OF THE COLD FRONT THAT INITIALLY DROPS INTO THE NORTHERN  
TIER TUE-WED. THE INITIAL COLD FRONT WILL BRING SOME  
PRECIPITATION TO NORTHERN AREAS AND THEN THE LATE-PERIOD SYSTEM  
SHOULD LEAD TO ONE OR MORE AREAS OF CONVECTION--SOME POSSIBLY  
HEAVY--WITHIN AN AREA BETWEEN THE NORTHERN PLAINS AND WESTERN  
GREAT LAKES.  
 
MEANWHILE EXPECT DIURNALLY FAVORED CONVECTION OF VARYING INTENSITY  
TO PERSIST OVER THE FOUR CORNERS STATES, THOUGH WITH UTAH TENDING  
TO SEE LIGHTER/MORE SCATTERED ACTIVITY THAN THE REST OF THE REGION.  
 
THE PATTERN CHANGE ALOFT OVER THE NORTHWEST WILL BRING A  
PRONOUNCED COOLING TREND TO THAT REGION. SOME LOCATIONS IN THE  
INTERIOR MAY SEE HIGHS 10-15F ABOVE NORMAL TUE BUT THEN EXPECT  
MODESTLY BELOW NORMAL HIGHS BY THU-SAT. FROM THE GREAT BASIN  
SOUTHWARD MIN TEMPS WILL LIKELY REMAIN WELL ABOVE NORMAL THROUGH  
THE PERIOD WHILE ANOMALIES FOR HIGHS SHOULD BE LESS EXTREME. MOST  
OF THE CENTRAL U.S. WILL SEE WELL BELOW NORMAL READINGS ESPECIALLY  
TUE-WED WITH LESSER COOL ANOMALIES FOR HIGHS EXTENDING INTO THE  
SOUTHEAST FOR MOST OF THE PERIOD GIVEN PERSISTENT CLOUDS/RAINFALL.  
ON THE OTHER HAND MOIST FLOW WILL KEEP MIN TEMPS WELL ABOVE  
NORMAL OVER THE MID-ATLANTIC/NORTHEAST. AFTER THE COOL START TO  
THE PERIOD, THE NORTHERN TIER WILL SEE TEMPERATURES REBOUND LATE  
IN THE WEEK AS SOUTHERLY FLOW DEVELOPS AHEAD OF NORTHERN PLAINS  
LOW PRESSURE.  
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT
 
 
THE 18Z GFS/12Z ECMWF AND THEIR ENSEMBLE MEANS (PLUS THE 12Z UKMET  
DAYS 3-5 TUE-THU) AGREED FAIRLY WELL IN PRINCIPLE THROUGH THE  
PERIOD. THE OPERATIONAL MODEL CLUSTER REPRESENTED CONSENSUS  
THROUGH MID-PERIOD, WITH ENSEMBLE GUIDANCE REACHING HALF OF THE  
TOTAL FORECAST WEIGHT BY DAY 7 SAT. CMC/CMC MEAN RUNS CONTINUE TO  
BE WEAKER WITH THE WESTERN ATLANTIC RIDGE, ALLOWING EASTERN U.S.  
HEIGHT FALLS ALOFT/SURFACE LOW PRESSURE TO EXTEND FARTHER EAST  
THAN CONSENSUS. INTERACTION BETWEEN AMPLIFYING SOUTHERN CANADA  
ENERGY AND THE MS VALLEY TROUGH AROUND MIDWEEK STILL LEADS TO SOME  
DETAIL UNCERTAINTY IN THE FORECAST OVER EASTERN NORTH AMERICA.  
THERE ARE STILL SIGNIFICANT QUESTION MARKS OVER SPECIFICS OF  
EJECTING UPPER LOW ENERGY THAT IS NEAR VANCOUVER ISLAND AS OF WED.  
HOWEVER AS WAS THE CASE YESTERDAY THERE IS STILL DECENT  
MODEL/ENSEMBLE AGREEMENT FOR THE GENERAL AREA OF SURFACE LOW  
PRESSURE EMERGING OVER THE NORTHERN PLAINS/SOUTHERN CANADA. THE  
PROGRESSIVE NATURE OF MEAN FLOW LATE IN THE PERIOD MAY MAKE IT  
DIFFICULT TO RESOLVE FINER DETAILS UNTIL THE SHORT RANGE TIME  
FRAME.  
 
ANOTHER INFLUENCE ON THE FORECAST THAT MAY COME INTO PLAY LATE IN  
THE PERIOD IS THE AMPLIFICATION EXPECTED ACROSS THE PACIFIC. THUS  
FAR THERE HAVE BEEN SOME SIGNIFICANT DIFFERENCES ON THE  
PLACEMENT/AMPLITUDE OF STRONG NORTH PACIFIC/ALEUTIANS RIDGING AND  
RESULTING NORTHEASTERN PACIFIC TROUGH, WITH GFS RUNS TENDING TO BE  
SOMEWHAT AMPLIFIED/SHIFTED A LITTLE WEST VERSUS CONSENSUS FOR THE  
OVERALL PATTERN. TELECONNECTIONS RELATIVE TO THE ALEUTIANS  
POSITIVE HEIGHT ANOMALY CENTER DEPICTED IN THE D+8 MULTI-DAY MEANS  
SEEM TO FAVOR LEANING CLOSER TO THE ECMWF/ECMWF MEAN OVER THE  
NORTHEASTERN PACIFIC AT THAT TIME.  
 
RAUSCH  
 
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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