453  
FXUS02 KWBC 050701  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
300 AM EDT SUN MAY 05 2019  
 
VALID 12Z WED MAY 08 2019 - 12Z SUN MAY 12 2019  
   
..OVERVIEW
 
 
EXPECT ACTIVE WEATHER FOR ONE OR MORE DAYS OF THE PERIOD FROM  
PORTIONS OF THE CENTRAL/SOUTHERN WEST THROUGH A SIGNIFICANT PART  
OF THE CENTRAL/EASTERN STATES. A STORM EMERGING OVER THE PLAINS  
ON WED AND TRACKING NORTHEASTWARD THEREAFTER WILL PRODUCE A WARM  
SECTOR SEVERE WEATHER THREAT AND ONE AREA OF POTENTIALLY HEAVY  
RAINFALL FROM THE MIDWEST INTO THE NORTHEAST. LOCATIONS FROM THE  
EASTERN HALF OF TEXAS THROUGH THE LOWER MISSISSIPPI VALLEY AND  
POSSIBLY INTO THE TENNESSEE VALLEY WILL BE OF GREATER CONCERN FOR  
A SIGNIFICANT MULTI-DAY HEAVY RAINFALL EVENT. THE FRONT TRAILING  
FROM PLAINS LOW PRESSURE WILL GENERATE INITIAL CONVECTION AND THEN  
LIKELY STALL NEAR AND NORTHEAST FROM THE WESTERN GULF COAST  
FRI-SUN, INTERACTING WITH IMPULSES EMBEDDED IN SOUTHWESTERLY FLOW  
ALOFT AND A STEADY FLOW OF GULF MOISTURE. AT THE SAME TIME UPPER  
TROUGHING OVER THE WEST WILL PROMOTE TERRAIN-ENHANCED  
PRECIPITATION FROM PARTS OF CALIFORNIA INTO THE ROCKIES.  
 
   
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
 
 
FROM THE EASTERN PACIFIC INTO WESTERN NORTH AMERICA, GUIDANCE IS  
STILL BEGINNING TO ESTABLISH WESTERN U.S. TROUGHING ALOFT BY  
MIDWEEK IN RESPONSE TO A STRONG EASTERN PACIFIC RIDGE. HOWEVER  
LATER IN THE WEEK MOST SOLUTIONS ARE SHOWING A TREND TOWARD THE  
RIDGE EXTENDING FARTHER INTO SOUTHWESTERN CANADA AND THE PACIFIC  
NORTHWEST. THIS LEADS TO A FARTHER SOUTHWEST TRACK OF A POTENTIAL  
EMBEDDED UPPER LOW (ADVERTISED BY SOME EARLIER GFS RUNS), WITH THE  
ADDED COMPLICATION THAT THIS ENERGY COULD INTERACT WITH A SEPARATE  
SOUTHERN STREAM UPPER LOW THAT STARTS THE PERIOD AROUND 130-140W  
LONGITUDE. THE BEST CONSENSUS/AVERAGE AMONG MODELS AND MEANS THUS  
FAR WOULD PLACE THE CENTER OF THE UPPER LOW NEAR SOUTHERN ARIZONA  
AS OF EARLY DAY 7 SUN--SLOWER THAN THE 18Z/NEW 00Z GFS RUNS BUT  
FASTER THAN THE NEW 00Z CMC. MEANWHILE AFTER EARLY FRI MODELS AND  
ENSEMBLE MEMBERS CONTINUE TO BECOME CHAOTIC IN DEPICTING EXACTLY  
HOW NORTHERN PACIFIC FLOW WILL ERODE THE EASTERN PACIFIC RIDGE.  
AT THE VERY LEAST THE 12Z ECMWF MEAN HAS MADE A FAVORABLE  
ADJUSTMENT TOWARD SLOWER WEAKENING OF THE RIDGE VERSUS ITS RUN  
FROM 24 HOURS AGO. THE WIDE SPREAD FAVORS A BLENDED APPROACH WITH  
INCREASING ENSEMBLE MEAN EMPHASIS.  
 
A CONSENSUS APPROACH HAS HELD UP FAIRLY WELL FOR THE STORM  
TRACKING FROM THE PLAINS INTO MIDWEST WED-THU. FASTER SOLUTIONS  
FROM 1-2 DAYS AGO (CMC/UKMET AND TO SOME DEGREE ECMWF) HAVE  
TRENDED SLOWER WHILE THE GFS HAS TRENDED A BIT FASTER FROM ITS  
PRIOR SLOW TIMING. THE FORECAST AFTER EARLY THU BECOMES DEPENDENT  
ON MORE VARIABLE CENTRAL CANADA INTO NORTHERN U.S. UPPER TROUGH  
DETAILS. TRENDS HAVE BEEN FASTER WITHIN THIS NORTHERN  
STREAM--LEADING TO A MORE PHASED EVOLUTION ALOFT AND  
STRONGER/FARTHER NORTHWARD LOW PRESSURE REACHING INTO CANADA WHILE  
A TRIPLE POINT WAVE MAY BREAK OFF EAST OF NEW ENGLAND. THIS  
EVOLUTION WAS BECOMING EVIDENT IN YESTERDAY'S 00Z GUIDANCE AND  
SUBSEQUENT RUNS HAVE BEEN INCREASING THE CONFIDENCE IN SUCH A  
SCENARIO.  
 
AFOREMENTIONED ISSUES WITH INCOMING PACIFIC FLOW AFTER FRI ALONG  
WITH MODEL/ENSEMBLE SPREAD AND RUN-TO-RUN VARIABILITY DO NOT LEND  
MUCH CONFIDENCE TO DETAILS OF TRAILING FLOW WITHIN AND AROUND THE  
CENTRAL CANADA/NORTHERN U.S. MEAN TROUGH ALOFT. BEST CLUSTERING  
WOULD HAVE A CANADIAN WAVE ANCHORING A FRONT THAT DROPS INTO THE  
NORTHERN U.S. DURING THE LATTER HALF OF THE PERIOD.  
 
THE FIRST HALF OF THE FORECAST REFLECTED THE MOST COMMON IDEAS (OR  
AVERAGE DEPENDING ON THE SYSTEM) AMONG THE 12-18Z GFS AND 12Z  
ECMWF/CMC. THE RAPIDLY INCREASING DETAIL UNCERTAINTY OVER SOME  
AREAS LED TO INCREASING EMPHASIS ON THE 18Z GEFS/12Z ECMWF MEANS  
BY NEXT WEEKEND. THE CMC AND 18Z GFS CONTRIBUTIONS WERE PHASED  
OUT BEFORE DAY 7 SUN.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE STORM TRACKING NORTHEAST FROM THE CENTRAL PLAINS WED ONWARD  
WILL PRODUCE VARIOUS HAZARDS. THESE INCLUDE HEAVY RAINFALL  
POTENTIAL OVER THE MIDWEST INTO THE GREAT LAKES WITH MEANINGFUL  
RAIN LIKELY EXTENDING INTO PARTS OF THE NORTHEAST, LOCALLY HEAVY  
CONVECTION FROM THE SOUTHERN PLAINS INTO LOWER MISSISSIPPI VALLEY,  
AND A THREAT OF SEVERE WEATHER THAT MAY EXTEND FROM THE SOUTHERN  
HALF OF THE PLAINS INTO THE OHIO VALLEY/LOWER GREAT LAKES.  
CONSULT STORM PREDICTION CENTER PRODUCTS FOR LATEST INFO ON THE  
SEVERE WEATHER FORECAST. LOCALIZED POCKETS OF SNOW ARE POSSIBLE  
IN THE NORTHERN FRINGE OF THE PRECIP SHIELD WITH BEST  
PROBABILITIES (ALBEIT LOW IN ABSOLUTE TERMS) OVER THE WESTERN UP  
OF MICHIGAN AND NORTHEASTERN WISCONSIN.  
 
AREAS FROM EASTERN TEXAS THROUGH THE LOWER MISSISSIPPI VALLEY AND  
POSSIBLY FARTHER NORTHEAST WILL SEE A CONTINUED THREAT OF PERIODS  
OF HEAVY RAIN INTO THE WEEKEND AS LOW LEVEL GULF MOISTURE FEEDS  
INTO THE TRAILING FRONT THAT LIKELY STALLS NEAR AND NORTHEAST FROM  
THE WESTERN GULF COAST AND SOUTHWESTERLY MEAN FLOW ALOFT CARRIES  
ALONG CONVECTION-ENHANCING SHORTWAVES. FOR THE FULL FIVE-DAY  
PERIOD MODEL/ENSEMBLE GUIDANCE SHOWS VERY HIGH PROBABILITIES OF 2"  
OR GREATER ACCUMULATIONS OVER THIS AREA AND MODELS ARE SUGGESTING  
SOME 5-12" TOTALS.  
 
OVER THE WESTERN U.S. SOME DETAILS WILL LIKELY KEEP CHANGING AS  
CONSENSUS FLOW ALOFT EVOLVES, BUT THERE IS A CONSISTENT THEME OF  
ENHANCED PRECIPITATION FROM CALIFORNIA THROUGH THE SOUTHERN HALF  
OF THE ROCKIES IN ASSOCIATION WITH THE MEAN TROUGH ALOFT.  
CURRENTLY EXPECT HIGHEST PRECIPITATION TOTALS (RAIN AND HIGHER  
ELEVATION SNOW) OVER THE COLORADO/NEW MEXICO PORTIONS OF THE  
ROCKIES. SIGNIFICANT TOTALS ARE ALSO POSSIBLE OVER UTAH/NORTHERN  
ARIZONA, WHILE INCREASED POTENTIAL FOR AN UPPER LOW TO TRACE A  
WIDER ARC COMPARED TO PREVIOUS DAYS HAS RAISED AMOUNTS TO SOME  
EXTENT OVER FAVORED TERRAIN IN CENTRAL/SOUTHERN CALIFORNIA. SOME  
OF THE ROCKIES SNOW COULD BRIEFLY EXTEND INTO THE HIGH PLAINS  
EARLY IN THE PERIOD.  
 
CHILLY AIR BEHIND THE MIDWEEK PLAINS STORM WILL BRING A BROAD AREA  
OF MINUS 10-25F ANOMALIES FOR HIGHS THROUGH MUCH OF THE CENTRAL  
U.S. THE MOST EXTREME TEMPERATURES WITHIN THAT RANGE SHOULD BE  
OVER THE HIGH PLAINS. ON THE OTHER HAND SOUTHERLY FLOW AHEAD OF  
THE STORM WILL BRING WARMTH INTO THE EAST, WITH GREATEST ANOMALIES  
FOR MORNING LOWS (PLUS 10-20F). THE PACIFIC RIDGE ALOFT BUILDING  
INTO THE PACIFIC NORTHWEST WILL LEAD TO MULTIPLE DAYS OF HIGHS  
10-20F ABOVE NORMAL OVER THAT REGION--POSSIBLY APPROACHING RECORD  
VALUES AT SOME LOCATIONS.  
 
RAUSCH  
 
ADDITIONAL 3-7 DAY HAZARDS INFORMATION CAN BE FOUND ON THE WPC  
MEDIUM RANGE HAZARDS CHART AT:  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP  
 
WPC MEDIUM RANGE 500MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDICES ARE 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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