998  
FXUS02 KWBC 051557  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1157 AM EDT SUN MAY 05 2019  
 
VALID 12Z WED MAY 08 2019 - 12Z SUN MAY 12 2019  
   
..BUSY SPRINGTIME PATTERN FOR MUCH OF THE LOWER 48
 
 
   
..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. THE STORM PREDICTION  
CENTER HAS OUTLINED AN AREA OVER THE SOUTHERN PLAINS TO  
MISSOURI/ARKANSAS ON WEDNESDAY AND THEN FROM THE ARKLATEX  
NORTHEASTWARD TO THE MIDWEST ON THURSDAY FOR THE POTENTIAL FOR  
SEVERE WEATHER. 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. MARGINALLY COLD AIR AT HIGHER ELEVATIONS  
SUPPORTS THE CHANCE FOR SNOW WED-FRI AS THE LEAD UPPER TROUGH  
MOVES THROUGH.  
 
   
..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/NORTHWESTERN MEXICO AS OF EARLY DAY 7 SUN--SLOWER THAN THE  
00Z GFS BUT FASTER THAN THE 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. SPREAD FAVORS A BLENDED APPROACH WITH INCREASING  
ENSEMBLE MEAN EMPHASIS AT LATER TIMES.  
 
A CONSENSUS APPROACH HAS HELD UP FAIRLY WELL FOR THE STORM  
TRACKING FROM THE PLAINS INTO MIDWEST WED-THU, BUT THE LATEST  
00Z/06Z GFS RUNS WERE DISPLACED A BIT SOUTH/EAST OF THE  
ECMWF/UKMET/CANADIAN CONSENSUS EARLY THU INTO FRI. PREFERRED THIS  
NON-GFS BLEND GIVEN THE BETTER ENSEMBLE CLUSTERING TO ITS NORTH  
BUT THE GFS RUNS DO COME BACK INTO AGREEMENT AS THE LOW MOVES INTO  
CANADA. THE FORECAST AFTER EARLY THU BECOMES DEPENDENT ON MORE  
VARIABLE CENTRAL CANADA INTO NORTHERN U.S. UPPER TROUGH DETAILS  
WHERE THE GFS WAS QUICKER THAN THE REST WITH LEAD HEIGHT FALLS  
PRECEDING THE SYSTEM, WHICH MAY TARNISH ITS FORECAST THEREAFTER.  
OVERALL TREND STILL POINTS TO A STRONGER/FARTHER NORTHWARD LOW  
PRESSURE REACHING INTO CANADA WHILE A TRIPLE POINT WAVE MAY BREAK  
OFF JUST EAST OF NEW ENGLAND.  
 
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, BUT THERE  
REMAINS MODEST SPREAD IN TRACK/INTENSITY OF THIS FRONT.  
 
THE FIRST HALF OF THE FORECAST REFLECTED THE MOST COMMON IDEAS (OR  
AVERAGE DEPENDING ON THE SYSTEM) AMONG THE 00Z  
ECMWF/UKMET/CANADIAN. THE RAPIDLY INCREASING DETAIL UNCERTAINTY  
OVER SOME AREAS LED TO INCREASING EMPHASIS ON THE 06Z GEFS/00Z  
ECMWF MEANS BY NEXT WEEKEND ALONG WITH THE PREVIOUS FORECAST.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE STORM TRACKING NORTHEAST FROM THE CENTRAL PLAINS WED ONWARD  
WILL PRODUCE VARIOUS HAZARDS. THESE INCLUDE:  
 
1) HEAVY RAINFALL POTENTIAL OVER THE MIDWEST INTO THE GREAT LAKES  
WITH MEANINGFUL RAIN LIKELY EXTENDING INTO PARTS OF THE NORTHEAST  
2) THUNDERSTORMS WITH AN EMBEDDED SEVERE THREAT FROM THE SOUTHERN  
PLAINS INTO LOWER MISSISSIPPI VALLEY THAT EXTENDS INTO THE OHIO  
VALLEY/LOWER GREAT LAKES AS THE SYSTEM MOVES NORTHEASTWARD.  
CONSULT STORM PREDICTION CENTER PRODUCTS FOR LATEST INFO ON THE  
SEVERE WEATHER FORECAST.  
3) MOUNTAIN SNOW OVER COLORADO WED/THU WITH SEVERAL INCHES LIKELY  
AT HIGHER ELEVATIONS ABOVE 7000-8000FT BUT EVEN SOME SNOW TO  
6000FT POSSIBLE. MORE LOCALIZED POCKETS OF SNOW IN THE NORTHERN  
FRINGE OF THE PRECIP SHIELD ARE POSSIBLE WITH BEST PROBABILITIES  
(ALBEIT LOW IN ABSOLUTE TERMS) OVER THE WESTERN UP OF MICHIGAN AND  
NORTHEASTERN WISCONSIN BUT ALSO OVER WESTERN NEBRASKA EARLY IN THE  
PERIOD.  
 
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 (PLEASE CONSULT THE LATEST DAY 4-7 WINTER  
WEATHER OUTLOOK PROBABILITIES).  
 
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 ESPECIALLY FRIDAY FROM OLYMPIA SOUTHWARD  
TO PORTLAND AND SALEM (MID TO UPPER 80S).  
 
FRACASSO/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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