691  
FXUS02 KWBC 210558  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
157 AM EDT SUN APR 21 2019  
 
VALID 12Z WED APR 24 2019 - 12Z SUN APR 28 2019  
   
..OVERVIEW AND GUIDANCE PREFERENCES
 
 
MODELS CONTINUE TO STRUGGLE WITH INTERACTION BETWEEN A MORE  
PROGRESSIVE NORTHERN STREAM AND LESS PROGRESSIVE SOUTHERN STREAM  
THROUGH MUCH OF THE EXTENDED PERIOD. TIMING AND AMPLITUDE  
DIFFERENCES OF FEATURES WITHIN EACH STREAM ARE COMPOUNDED BY  
DIFFERENCES IN THE POTENTIAL INTERACTION (OR LACK THEREOF) BETWEEN  
STREAMS. AS A RESULT, FORECAST CONFIDENCE/PREDICTABILITY CONTINUES  
TO QUICKLY DECREASE BY THE LATTER PORTION OF THE FORECAST PERIOD.  
 
SOUTHERN STREAM SHORTWAVE ENERGY IS FORECAST BY ALL GUIDANCE TO  
CROSS THE SOUTHERN PLAINS WED/WED NIGHT AND MOVE INTO THE LOWER  
MISSISSIPPI VALLEY ON THU, WITH PERHAPS A CLOSED CENTER DEVELOPING  
AT 500 HPA (ALTHOUGH THIS CONTINUES TO VARY BY MODEL AND FROM RUN  
TO RUN). IN GENERAL, TIMING OF THE SYSTEM HAS CONTINUED TO DRIFT  
TOWARD A MIDDLE GROUND, AND A CONSENSUS BLEND OF DETERMINISTIC  
GUIDANCE SHOULD NOW REPRESENT THE SYSTEM REASONABLY WELL AT LEAST  
THROUGH DAY 4 (THU). TIMING DIFFERENCES CONTINUE TO GROW FROM FRI  
ONWARD, GENERALLY RELATED TO INTERACTION WITH NORTHERN STREAM  
ENERGY CROSSING THE GREAT LAKES. PREFERRED TO GRADUALLY LEAN MORE  
TOWARD ENSEMBLE TIMING OF THIS SYSTEM STARTING ON FRI AND THROUGH  
NEXT WEEKEND, AS MOST SOLUTIONS SHOW A LARGER DEGREE OF PHASING  
OCCURRING AS THE SYSTEM PASSES NEW ENGLAND.  
 
FARTHER WEST, MODELS SHOW GENERAL AGREEMENT THAT A RATHER  
AMPLIFIED NORTHERN STREAM SHORTWAVE SHOULD APPROACH THE PACIFIC  
NORTHWEST ON FRI, MOVING INLAND FRI NIGHT-SAT, LOWERING HEIGHTS  
AND PUSHING A COLD FRONT INLAND. SOLUTIONS VARY WIDELY AS TO WHAT  
WILL OCCUR WITH THE SOUTHERN STREAM, HOWEVER. MODELS HAVE  
OSCILLATED BETWEEN A CUTOFF UPPER LOW APPROACHING CALIFORNIA LATE  
SAT INTO SUN, OR A MORE ELONGATED AND PROGRESSIVE FEATURE THAT  
QUICKLY MOVES INLAND. AT THIS TIME, PREFER TO LEAN HEAVILY TOWARD  
ENSEMBLE MEANS GIVEN THE SPREAD AND INCONSISTENCY OF SOLUTIONS  
WITH THE SOUTHERN STREAM FEATURE.  
 
GIVEN THESE CONSIDERATIONS, THE FORECAST DURING DAYS 3-4 (WED-THU)  
WAS BASED HEAVILY ON A DETERMINISTIC MULTI-MODEL BLEND, INCLUDING  
THE 12Z ECMWF/UKMET AND 18Z GFS. BY DAYS 5-7 (FRI-SUN), WEIGHTING  
OF ECENS AND NAEFS ENSEMBLE MEANS WAS SIGNIFICANTLY INCREASED TO  
ACCOUNT FOR INCREASING SPREAD/UNCERTAINTY.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE SYSTEM CROSSING THE SOUTHERN PLAINS AND LOWER MISSISSIPPI  
VALLEY WED INTO THU IS EXPECTED TO PRODUCE WIDESPREAD SHOWERS AND  
THUNDERSTORMS, WITH MODELS SHOWING A STRONG SIGNAL FOR HEAVY  
RAINFALL FROM PORTIONS OF CENTRAL TEXAS EAST TOWARD THE  
MISSISSIPPI RIVER. SEVERE THUNDERSTORMS ARE ALSO POSSIBLE. PLEASE  
SEE CONVECTIVE OUTLOOKS ISSUED BY THE STORM PREDICTION CENTER FOR  
FURTHER DETAILS ON THE SEVERE WEATHER THREAT. SCATTERED TO  
NUMEROUS SHOWERS AND THUNDERSTORMS ARE EXPECTED TO ACCOMPANY THE  
SYSTEM EASTWARD INTO THE SOUTHEAST AS WELL AS THE TENNESSEE/OHIO  
VALLEYS BY LATE THU INTO FRI, BUT ANY SIGNAL FOR HEAVY RAINFALL IS  
MUCH LESS CLEAR IN THE GUIDANCE GIVEN INCREASING FORWARD SPEED OF  
THE SYSTEM AND INABILITY OF MODELS TO RESOLVE CONVECTIVE DETAILS  
AT THAT TIME SCALE. NONETHELESS, WOULD EXPECT CONTINUED POTENTIAL  
FOR AT LEAST LOCALLY HEAVY RAINFALL. FARTHER WEST, THE ENERGETIC  
UPPER-LEVEL TROUGH ENTERING THE NORTHWESTERN U.S. LATE IN THE WEEK  
IS EXPECTED TO PRODUCE RAIN AND MOUNTAIN SNOW FROM THE NORTHWEST  
TO THE NORTHERN ROCKIES. AS THE ENERGY MOVES INTO THE CENTRAL U.S.  
BY NEXT WEEKEND, RAIN AND POSSIBLY EVEN SNOW MAY SPREAD INTO AREAS  
FROM THE NORTHERN PLAINS TO THE UPPER MIDWEST/GREAT LAKES.  
 
ABOVE AVERAGE TEMPERATURES ARE EXPECTED THROUGH MUCH OF THE PERIOD  
ACROSS PORTIONS OF THE WESTERN U.S. HIGH TEMPERATURES ARE FORECAST  
TO BE 10 TO 20 DEG F ABOVE AVERAGE FROM CALIFORNIA TO THE GREAT  
BASIN WED-THU, WITH THESE TEMPERATURE ANOMALIES SPREAD INTO THE  
ROCKIES AND PORTIONS OF THE HIGH PLAINS BY FRI-SAT AS COOLER  
TEMPERATURES OVERSPREAD THE NORTHWEST/NORTHERN ROCKIES IN  
ASSOCIATION WITH THE ARRIVING UPPER TROUGH/COLD FRONT.  
 
RYAN  
 
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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