896  
FXUS02 KWBC 281553  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1152 AM EDT SUN APR 28 2019  
 
VALID 12Z WED MAY 01 2019 - 12Z SUN MAY 05 2019  
 
...STRONG TO SEVERE THUNDERSTORMS WITH HEAVY RAINS AND FLASH  
FLOODING/FLOODING POTENTIAL WILL THREATEN THE SOUTHERN PLAINS AND  
LOWER/MIDDLE MISSISSIPPI VALLEY WEDNESDAY INTO THURSDAY...  
   
..16Z UPDATE
 
 
THE OVERNIGHT MODEL GUIDANCE MAINTAINED ITS PATTERN OF SHOWING  
TROUGHING OVER THE WESTERN U.S. MIDWEEK WITH SHORTWAVES MOVING  
INTO THE CENTRAL U.S. ON DAY 4/THU, AND A GENERAL MODEL BLEND  
WORKED WELL THERE. BY FRI AND INTO THE WEEKEND, MODEL SPREAD HAS  
BEEN HIGH REGARDING AN UPPER LOW TRACKING ACROSS THE EASTERN  
PACIFIC TOWARD CALIFORNIA. FORTUNATELY, THE 00Z AND 06Z GFS/GEFS  
HAVE NOW COME MORE IN LINE WITH THE EC, NAMELY, THIS SOUTHERN  
STREAM LOW REMAINS SPLIT FROM THE NORTHERN STREAM FLOW THROUGHOUT  
THE PERIOD, WHEREAS OLDER GFS RUNS COMBINED THE STREAMS AND MOVED  
THE MERGED TROUGH QUICKLY ACROSS THE WEST. THE GFS SUITE REMAINS  
QUICKER THAN THE EC WITH THE UPPER LOW TRACK, BUT HAS BEEN  
TRENDING SLOWER, AND THERE IS MUCH BETTER AGREEMENT THAN  
PREVIOUSLY. HOWEVER, QUESTIONS REMAIN DOWNSTREAM ACROSS THE CONUS  
BY THE WEEKEND--THE 00Z EC/EC MEAN TRENDED TOWARD MORE ZONAL FLOW  
IN THE CENTRAL AND EASTERN CONUS, WHEREAS THE GFS/GEFS INDICATED A  
SOMEWHAT MORE AMPLIFIED PATTERN WITH RIDGING IN THE CENTRAL U.S.  
AND TROUGHING IN THE EAST. TRIED TO KEEP SOME AMPLIFICATION TO THE  
FLOW BY USING SOME WPC CONTINUITY FROM THE NIGHT SHIFT BY SAT/SUN.  
 
WIDESPREAD THUNDERSTORMS REMAIN LIKELY IN THE CENTRAL U.S.,  
CONTINUING FROM THE SHORT RANGE PERIOD INTO WED/THU. SEE THE  
PREVIOUS DISCUSSION FOR MORE DETAILS.  
 
TATE  
   
..PREVIOUS DISCUSSION
 
 
...OVERVIEW AND ASSESSMENT OF MODEL/ENSEMBLE GUIDANCE AND  
PREDICTABILITY...  
 
A LARGE CLOSED UPPER CIRCULATION WILL RETROGRESS AND SETTLE OVER  
WESTERN CANADA THIS WEEK. THIS OCCURS AS A SERIES OF NORTHERN  
STREAM TROUGHS, HEIGHT FALLS, AND PRECIPITATION SWATHS DIG INTO  
THE NORTHWEST THEN EASTWARD OVER THE U.S. NORTHERN TIER. THERE IS  
POTENTIAL FOR SOME DEGREE OF PHASING WITH A SOUTHERN STREAM FLOW  
OF IMPULSES/HEIGHT FALLS WORKING UNDERNEATH DURING THIS PERIOD  
FROM THE EASTERN PACIFIC INLAND OVER THE U.S. SOUTHERN TIER. THE  
GFS/GEFS AND TO A LESSER EXTENT THE FV3 HAVE TENDED TO BE ON THE  
PROGRESSIVE SIDE OF THE FULL ENVELOPE OF SOLUTIONS WITH RECENT  
ECMWF RUNS AND TO A LESSER EXTENT ECMWF ENSEMBLES AND THE  
UKMET/CANADIAN LIE ON THE SLOWER SIDE OF THE FORECAST ENVELOPE.  
THE WPC MEDIUM RANGE PRODUCT SUITE WAS PRIMARILY DERIVED FROM  
COMPOSITE SOLUTIONS INCLUDING THE NATIONAL BLEND OF MODELS  
INCREASINGLY TOWARD ECMWF ENSEMBLE MEAN GUIDANCE OVER TIME. THIS  
IS WITH CONSIDERATION OF THE OVERALL BLOCKY FLOW UPSTREAM OVER THE  
PACIFIC, UNCERTAINTY WITH EMBEDDED FEATURE TIMING AND PHASING IN  
TWO MAIN STREAM FLOWS, AND THE INHERENTLY SLOW NATURE OF CLOSED  
LOWS TO EJECT INTO THE FLOW.  
   
..WEATHER HIGHLIGHTS/THREATS
 
 
WITH THE TROUGHING IN THE WEST EARLY IN THE PERIOD, TEMPERATURES  
WILL BE COLD ENOUGH TO SUPPORT SNOW MAINLY IN THE NORTHERN/CENTRAL  
ROCKIES, THOUGH LATE SEASON LIGHT SNOWS ARE POSSIBLE IN THE  
NORTHERN/CENTRAL PLAINS. A RESULTANT LEAD AND WAVY FRONTAL SYSTEM  
WILL PROGRESS MID-LATE WEEK OVER THE CENTRAL THEN EASTERN U.S.  
DEEPENING INFLOW/MOISTURE FROM THE GULF OF MEXICO CONSISTENT WITH  
ULTIMATE FRONTAL WAVE DEVELOPMENTS WILL FEED INTO CONVECTIVELY  
ACTIVE WARM SECTOR MESO-BOUNDARIES AS WELL AS A STARK BAROCLINIC  
ZONE OVERTOP. STRONG TO SEVERE THUNDERSTORMS WITH HEAVY RAINS AND  
FLASH FLOODING/FLOODING POTENTIAL WILL FOCUS/THREATEN THE SOUTHERN  
PLAINS AND LOWER/MIDDLE MISSISSIPPI VALLEY WEDNESDAY INTO  
THURSDAY. FLOODING REMAINS POSSIBLE AS PARTICULARLY THE  
MISSISSIPPI VALLEY HAS HAD FLOODING CONCERNS FOR WEEKS, AND WITH  
THE CONVECTIVE NATURE OF THE PRECIPITATION, FLASH FLOODING IS  
POSSIBLE AS WELL. THE BULK OF ACTIVITY WILL PROGRESS ACROSS THE  
EAST AND SOUTH INTO FRIDAY BEFORE SHIFTING OFFSHORE BY NEXT  
WEEKEND AS NEW UPSTREAM NORTHERN AND SOUTHERN STREAM TROUGH  
ENERGIES/HEIGHT FALLS AND MODEST PRECIPITATION LIKELY WORKS BACK  
INTO THE WEST IN CONTINUED SPLIT FLOW.  
 
SCHICHTEL  
 
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  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page