083  
FXUS02 KWBC 231559  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1159 AM EDT SUN SEP 23 2018  
 
VALID 12Z WED SEP 26 2018 - 12Z SUN SEP 30 2018  
   
..PATTERN OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES
 
 
EXPECT THE AMPLIFIED PATTERN FORECAST TO EVOLVE OVER NORTH AMERICA  
AND ADJACENT OCEANS DURING THE SHORT RANGE TIME FRAME TO CONTINUE  
AT LEAST INTO THE FIRST HALF OF THE MEDIUM RANGE, THROUGH LATE  
THIS WEEK. MOST GUIDANCE INDICATES THAT MEAN FLOW COULD FLATTEN  
SOMEWHAT AT MID-LATITUDES NEXT WEEKEND THOUGH. OVER THE PAST  
12-24 HOURS THERE ARE NOTICEABLE TRENDS SUGGESTING INCREASED  
POTENTIAL THAT THE STRONG NORTHEASTERN PACIFIC UPPER RIDGE  
BUILDING INTO ALASKA AND THE ARCTIC COULD CLOSE OFF A HIGH CENTER  
OVER OR NEAR THE ALASKA MAINLAND. RESULTING ELONGATION OF  
CYCLONIC FLOW OVER/OFFSHORE BRITISH COLUMBIA AS WELL AS THE  
APPROACH OF UNDERCUTTING EAST-CENTRAL PACIFIC TROUGH/CLOSED LOW  
ENERGY WOULD ENCOURAGE INLAND PROGRESSION AND RAPID WEAKENING OF A  
LEADING UPPER LOW DRIFTING TOWARD CALIFORNIA THROUGH FRI. AT THE  
SAME TIME ANOTHER STRONG UPPER RIDGE INITIALLY COVERING THE  
WESTERN ATLANTIC AND PARTS OF THE GULF OF MEXICO SHOULD GRADUALLY  
TRANSITION ITS EMPHASIS TO THE GULF AND SOUTHERN U.S. THIS  
PATTERN EVOLUTION SHOULD YIELD A STRENGTHENING TEMPERATURE  
CONTRAST BETWEEN VERY CHILLY AIR SETTLING OVER THE NORTHERN PLAINS  
REGION AND WARMTH PERSISTING OVER PORTIONS OF THE SOUTH AND WEST.  
 
FROM DAY 3 WED INTO DAY 5 FRI AN OPERATIONAL MODEL CONSENSUS  
CONTINUES TO PROVIDE A GOOD APPROACH (AND WITH MINIMAL CHANGE IN  
CONTINUITY) TO RESOLVE EXISTING DETAIL/TIMING DIFFERENCES FOR  
FEATURES OF INTEREST--NAMELY A LEADING FRONT HEADING INTO THE  
EAST/SOUTH AROUND MIDWEEK, CANADIAN LOW PRESSURE WHOSE TRAILING  
FRONT SHOULD REACH THE GREAT LAKES/PLAINS BY EARLY FRI, AND UPPER  
LOW APPROACHING CALIFORNIA. THEN BY DAYS 6-7 SAT-SUN FORECAST  
CONFIDENCE IN SPECIFICS RAPIDLY DECREASES AS INDIVIDUAL  
MODEL/ENSEMBLE SOLUTIONS DIVERGE FROM EACH OTHER AND EARLIER RUNS  
WITHIN THE COMPLEX EVOLUTION THAT MAY OCCUR OVER/NEAR THE EASTERN  
PACIFIC. THE ULTIMATE TIMING OF THE UPPER LOW NEARING THE WEST  
COAST AS OF EARLY SAT WILL BE VERY SENSITIVE TO DETAILS OF FLOW  
JUST TO THE NORTH, WHICH WILL BE DETERMINED BY EXACTLY HOW THE  
ALASKA RIDGE EVOLVES. DETAILS OF UPSTREAM PACIFIC UNDERCUTTING  
FLOW MAY PLAY A ROLE AS WELL TOWARD THE END OF THE PERIOD. WITH  
CURRENT SIGNALS LEANING TOWARD CLOSURE OF AN ALASKA UPPER HIGH, AT  
LEAST THERE IS BETTER AGREEMENT IN PRINCIPLE ON EJECTION OF THE  
UPPER LOW INTO THE WEST--IN CONTRAST TO SOME EARLIER GFS/GEFS RUNS  
THAT HAD LEFT IT STRANDED WELL OFFSHORE THROUGH THE WEEKEND.  
FASTER 06Z GEFS TREND TOWARD THE 00Z ECMWF MEAN REINFORCES THE  
CURRENT MAJORITY CLUSTER. ADJUSTING THE FORECAST TOWARD 40-60  
PERCENT ENSEMBLE INPUT BY NEXT WEEKEND WHILE KEEPING MODEST WEIGHT  
OF SOME OPERATIONAL GUIDANCE REFLECTS LATEST TRENDS FOR THE LARGE  
SCALE EVOLUTION WHILE DOWNPLAYING UNCERTAIN SPECIFICS. IN SPITE  
OF THE ONGOING SHIFTS IN SOME EASTERN PACIFIC DETAILS ALOFT, THE  
SURFACE FORECAST OVER MUCH OF THE CONTIGUOUS U.S. MAINTAINS DECENT  
CONSISTENCY.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS
 
 
THE BEST POTENTIAL FOR HIGHEST RAINFALL TOTALS DURING THE PERIOD  
EXTENDS FROM THE SOUTHERN TIER STATES AND PERHAPS INTO THE  
SOUTHERN MID-ATLANTIC, ASSOCIATED WITH A LEADING FRONT THAT WILL  
LIKELY STALL BEFORE DISSIPATING AND THEN ANOTHER FRONT REACHING  
THE AREA LATE IN THE WEEK. SOME CONVECTION ALONG THE  
CENTRAL-NORTHERN LATITUDES OF THE LEADING FRONT MAY BE STRONG TO  
SEVERE, SO CHECK SPC OUTLOOKS FOR LATEST DETAILS. THE  
CLIPPER-LIKE SYSTEM THAT MAY TRACK CLOSE TO THE CANADIAN BORDER  
FOR A TIME WILL BRING AN AREA OF GENERALLY LIGHT-MODERATE  
PRECIPITATION ACROSS EXTREME NORTHERN AREAS LATE IN THE WEEK.  
NEXT WEEKEND THE LARGE SCALE PATTERN EVOLUTION WILL LIKELY PROMOTE  
THE ESTABLISHMENT OF AN INCREASINGLY SHARP FRONTAL BOUNDARY FROM  
THE GREAT LAKES INTO CENTRAL PLAINS AND NORTHWEST. ALONG AND  
NORTH OF THIS FRONT THERE IS THE POSSIBILITY FOR ENHANCED  
PRECIPITATION BUT WITH FAIRLY LOW CONFIDENCE IN EXACT  
TIMING/AMOUNTS. LOW LEVEL UPSLOPE FLOW MAY AID ACTIVITY ALONG THE  
EASTERN SLOPES OF THE NORTHERN ROCKIES WITH SOME SNOW POSSIBLE  
OVER NORTHERN/WESTERN MONTANA. FARTHER EAST THERE ARE SOME  
SIGNALS FOR A PERIOD OF LOCALLY MODERATE-HEAVY RAINFALL OVER PARTS  
OF THE UPPER MISSISSIPPI VALLEY/MIDWEST/GREAT LAKES.  
 
EXPECT CHILLY AIR TO SETTLE OVER THE NORTHERN PLAINS AND VICINITY  
WITH HIGHS DROPPING TO 10-20F BELOW NORMAL BY THU AND 15-25 BELOW  
NORMAL FRI-SUN. ON THE OTHER HAND THE WARM SECTOR AHEAD OF THE  
INITIAL EASTERN COLD FRONT ON WED WILL CONTAIN VERY WARM MIN TEMPS  
IN PARTICULAR WITH PLUS 10-20F ANOMALIES. AFTER WED, MORNING LOWS  
OVER EASTERN/SOUTHERN AREAS SHOULD BE A LITTLE LESS EXTREME BUT  
STILL WELL ABOVE NORMAL. TEMPERATURES OVER THE SOUTHERN PLAINS  
WILL MODERATE AFTER A COOL DAY WED. A MAJORITY OF THE WEST WILL  
LIKELY SEE READINGS 5-10F OR SO ABOVE NORMAL THROUGH LATE WEEK,  
THOUGH WITH SOME COOLING EXPECTED NEXT WEEKEND AS THE EASTERN  
PACIFIC UPPER LOW EJECTS INLAND AND THE UPPER PATTERN FLATTENS.  
 
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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