803  
FXUS02 KWBC 190534  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
133 AM EDT FRI APR 19 2019  
 
VALID 12Z MON APR 22 2019 - 12Z FRI APR 26 2019  
   
..OVERVIEW AND GUIDANCE PREFERENCES
 
 
UPPER-LEVEL FLOW ACROSS NORTH AMERICA DURING THE EXTENDED PERIOD  
WILL BE LARGELY SEPARATED INTO A PROGRESSIVE NORTHERN STREAM  
ACROSS CANADA AND THE U.S. NORTHERN TIER, AND A MUCH LESS  
PROGRESSIVE SOUTHERN STREAM, WITH THE POTENTIAL/TENDENCY FOR  
SYSTEMS TO CLOSE OFF ALOFT. OVERALL, MODELS HAVE STRUGGLED WITH A  
SIMILAR FLOW PATTERN FOR THE PAST WEEK OR TWO, AND THIS CERTAINLY  
SEEMS TO BE THE CASE GOING FORWARD AS WELL. PREDICTABILITY  
REDUCTIONS RESULTING FROM TIMING/AMPLITUDE DIFFERENCES WITHIN EACH  
STREAM ARE FURTHER AMPLIFIED BY POTENTIAL INTERACTION (OR LACK  
THEREOF) BETWEEN THE STREAMS. AS A RESULT,  
CONFIDENCE/PREDICTABILITY AT THE BEGINNING OF THE EXTENDED  
FORECAST PERIOD IS AVERAGE AT BEST, AND QUICKLY DECREASES TO WELL  
BELOW AVERAGE BY LATER IN THE PERIOD.  
 
A MULTI-MODEL DETERMINISTIC BLEND (INCLUDING THE 12Z  
ECMWF/UKMET/CMC AND 18Z GFS) WAS FAVORED DURING DAYS 3-4  
(MON-TUE). DETERMINISTIC SOLUTIONS AND ENSEMBLE MEMBERS VARY WITH  
RESPECT TO HOW QUICKLY A COMPACT UPPER LOW INITIALLY ACROSS THE  
MID-ATLANTIC REGION ON MON WILL LIFT OUT, BUT A CONSENSUS  
SOLUTIONS CLOSE TO THE ENSEMBLE MEANS WAS PREFERRED. A TREND  
TOWARD A SOMEWHAT SLOWER PROGRESSION OF THIS FEATURE MON-TUE WAS  
NOTED AMONG MUCH OF THE GUIDANCE AND ENSEMBLE MEMBERS OVER THE  
PAST COUPLE RUNS. THIS PROCESS IS RELATED TO POTENTIAL INTERACTION  
WITH PROGRESSIVE NORTHERN STREAM ENERGY CROSSING NORTH OF THE  
GREAT LAKES ON TUE, WHICH ALSO SHOWS TIMING/AMPLITUDE DIFFERENCES  
AMONG THE GUIDANCE. FARTHER WEST, MODELS ARE IN RELATIVELY GOOD  
AGREEMENT THAT ANOTHER SLOW-MOVING SOUTHERN STREAM SYSTEM SHOULD  
CROSS THE FOUR CORNERS REGION MON-TUE, WITH DIFFERENCES (AT LEAST  
EARLY ON) CONFINED TO HOW AMPLIFIED THE FEATURE BECOMES.  
 
FROM DAY 5 (WED) ONWARD, MODEL SPREAD QUICKLY GROWS WITH RESPECT  
TO THE SOUTHERN STREAM FEATURE APPROACHING THE SOUTHERN/CENTRAL  
PLAINS BY THAT TIME. THE PRECISE TIMING/AMPLITUDE OF THE FEATURE  
(WHICH REMAINS UNCERTAIN) WILL PLAY A SIGNIFICANT ROLE IN WHETHER  
OR NOT ANY INTERACTION OCCURS WITH PROGRESSIVE NORTHERN STREAM  
FLOW AND ULTIMATELY HOW QUICKLY THE FEATURE MOVES EASTWARD. A  
SIGNIFICANT TREND WAS NOTED IN THE ENSEMBLE GUIDANCE TOWARD A  
SLOWER/DEEPER SOUTHERN STREAM SYSTEM WITH LESS NORTHERN STREAM  
INTERACTION. THIS HAS BEEN A TREND AMONG THE GUIDANCE WITH A  
COUPLE RECENT SIMILAR SYSTEMS, SO GIVEN THE TREND WAS INCLINED TO  
LEAN MORE HEAVILY TOWARD THAT IDEA, AS SHOWN BY THE 12Z ECMWF AND  
ECENS MEAN. ADDITIONALLY, MODELS GENERALLY AGREE ON ANOTHER ROUND  
OF PACIFIC SHORTWAVE ENERGY APPROACHING THE WEST COAST BY THU OR  
FRI, ALTHOUGH GIVEN THE TIME RANGE, SOLUTIONS VARY WIDELY ON THE  
NATURE AND TIMING OF THE FEATURE. WHAT DOES SEEM AT LEAST A BIT  
MORE CLEAR IS THAT A STRONG UPPER-LEVEL JET SHOULD BE ACCOMPANIED  
BY HEIGHT FALL AND A SURFACE COLD FRONT PUSHING INTO THE  
NORTHWESTERN U.S. BY THU-FRI. GIVEN THESE CONSIDERATIONS, THE  
FORECAST DURING DAYS 5-7 WAS TRENDED HEAVILY TOWARD ENSEMBLE MEANS  
(THE ECENS MEAN IN PARTICULAR), WITH SOME CONTINUED WEIGHT FROM  
THE DETERMINISTIC ECMWF AS WELL.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE SOUTHERN STREAM SYSTEM SLOWLY MOVING FROM THE SOUTHWEST INTO  
THE CENTRAL U.S. IS EXPECTED TO PRODUCE WIDESPREAD SHOWERS AND  
THUNDERSTORMS WITH POTENTIALLY HEAVY RAINFALL. THE MOST  
WIDESPREAD/HEAVIEST RAINS SHOULD BE ACROSS THE SOUTHERN/CENTRAL  
PLAINS BY MON NIGHT/TUE, AND SPREADING EAST TOWARD THE GULF COAST  
AND LOWER MISSISSIPPI VALLEY BY WED-THU. MEANWHILE, WELL ABOVE  
AVERAGE TEMPERATURES ARE EXPECTED ACROSS PORTIONS OF THE WESTERN  
U.S. FROM TUE ONWARD, WITH HIGH TEMPERATURES EXPECTED TO BE 10-20  
DEG F ABOVE AVERAGE FOR SOME AREAS. THE WARM TEMPERATURES SHOULD  
GRADUALLY SPREAD EAST TOWARD THE CENTRAL U.S. BY LATER NEXT WEEK,  
WITH A COLD FRONT BRINGING A RETURN TO NEAR NORMAL TEMPERATURES  
FOR THE PACIFIC NORTHWEST.  
 
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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