742  
FXUS02 KWBC 231905  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
305 PM EDT TUE APR 23 2019  
 
VALID 12Z FRI APR 26 2019 - 12Z TUE APR 30 2019  
   
..16Z UPDATE
 
 
NO MAJOR CHANGES WERE NEEDED TO THE OVERNIGHT MEDIUM RANGE  
FORECAST PACKAGE. STILL EXPECT AN UPPER LOW TO MOVE THROUGH THE  
EASTERN PACIFIC THIS WEEKEND, INTO CALIFORNIA ON MON, AND INTO THE  
FOUR CORNERS OR CENTRAL PLAINS BY TUE. THIS FEATURE SEEMED TO BE  
HANDLED REASONABLY WELL BY MOST DETERMINISTIC AND OPERATIONAL  
MODELS, THOUGH BY SUN THE 00Z CMC PROPAGATES THAT LOW FASTER THAN  
CONSENSUS, SO CHOSE TO EXCLUDE THAT FROM THE FORECAST PACKAGE,  
GOING WITH A BLEND OF THE 00Z AND 06Z GFS, 00Z GEFS, AND THE 00Z  
ECMWF AND ITS MEAN. THE 06Z GFS DOES STRENGTHEN A SURFACE LOW IN  
THE PLAINS MORE THAN WE WERE PREPARED TO PUT IN OUR FORECAST YET.  
 
IN THE NORTHERN STREAM, SHORTWAVES ARE EXPECTED TO TRAVERSE THE  
NORTHERN TIER OF THE U.S. THROUGHOUT THE PERIOD, WITH SOME MODEL  
DIFFERENCES IN STRENGTH AND TIMING. AS DISCUSSED OVERNIGHT, A  
SHORTWAVE ON SAT-SUN HAS COME INTO BETTER AGREEMENT, BUT A  
DIFFERENT SHORTWAVE MOVING THROUGH THE GREAT LAKES REGION ON SUN  
IS STRONGER IN THE GFS/GEFS THAN IN THE EC SUITE, WHICH LEADS TO  
SLOWER MOVEMENT AND THE GFS SUITE KEEPING THE FEATURE BACK ON MON,  
WHILE THE EC PROGRESSES THE ENERGY MORE. A COMPROMISE SEEMS LIKE  
THE BEST SOLUTION AT THIS POINT.  
 
FOR MORE DETAILS, SEE THE PREVIOUS DISCUSSION BELOW.  
 
TATE  
   
..PREVIOUS DISCUSSION
 
 
   
..OVERVIEW AND GUIDANCE PREFERENCES
 
 
UPPER-LEVEL FLOW ACROSS THE CONUS IS NOT EXPECTED TO CHANGE  
SIGNIFICANTLY DURING THE EXTENDED FORECAST PERIOD. A PROGRESSIVE  
NORTHERN STREAM AND LESS PROGRESSIVE SOUTHERN STREAM, SIMILAR TO  
THE PATTERN SEEN IN RECENT WEEKS, IS EXPECTED TO PERSIST.  
MODELS/ENSEMBLES DO HINT AT POTENTIAL NORTHERN STREAM FLOW  
AMPLIFICATION FROM THE NORTH PACIFIC INTO CANADA BY NEXT WEEKEND,  
HOWEVER. DEVELOPMENT OF A BROAD REX BLOCK EXTENDING FROM HAWAII TO  
ALASKA IS EXPECTED, WHICH WILL REINFORCE UPPER TROUGHING CENTERED  
WEST OF HUDSON BAY. CONCEPTUALLY, THIS SETUP SEEMS TO FAVOR  
POTENTIAL SOUTHWARD TRANSPORT OF POLAR AIR INTO CENTRAL CANADA AND  
THE NORTH CENTRAL U.S., AND THIS BEARS OUT WHEN TELECONNECTIONS  
ASSOCIATED WITH THE PACIFIC REX BLOCK ARE EXAMINED.  
 
BEGAN THE FORECAST DURING DAYS 3-5 (FRI-SUN) WITH A MULTI-MODEL  
DETERMINISTIC BLEND (12Z ECMWF/UKMET/CMC/18Z GFS). MODELS HAVE  
TRENDED SLIGHTLY MORE AMPLIFIED WITH NORTHERN STREAM SHORTWAVE  
ENERGY CROSSING THE GREAT LAKES ON FRI, RESULTING IN A QUICKER  
PROGRESSION OF ENERGY INITIALLY ACROSS THE SOUTHEAST, AND MORE  
PHASING BETWEEN THE TWO STREAMS. THE RESULTING PHASED LOW PRESSURE  
SYSTEM SHOULD DEEPEN AS IT QUICKLY LIFTS INTO THE CANADIAN  
MARITIMES BY SAT. CONSENSUS HAS IMPROVED AROUND AN ENERGETIC  
SHORTWAVE EXPECTED TO DIVE SOUTHEASTWARD INTO THE PACIFIC  
NORTHWEST AND NORTHERN ROCKIES SAT-SUN, AS ANOTHER CUTOFF SOUTHERN  
STREAM UPPER LOW APPROACHES CALIFORNIA.  
 
BEYOND SUN, THERE REMAINS A SIGNIFICANT QUESTION AS TO HOW MUCH  
HEIGHTS FALL FROM THE NORTHERN ROCKIES TO THE NORTHERN HIGH  
PLAINS. UPSTREAM RIDGE AMPLIFICATION ACROSS THE NORTH PACIFIC  
SHOULD FAVOR THE TENDENCY FOR CYCLONIC UPPER-LEVEL FLOW AND  
POTENTIAL TROUGH AMPLIFICATION ACROSS THE NORTHWEST, AND THIS IS  
THE DIRECTION MODELS SEEM TO BE TRENDING AT THE MOMENT. CURRENT  
MODEL TRENDS SUGGEST THAT THE AMPLIFYING TROUGH ACROSS THE  
NORTHWEST BY EARLY NEXT WEEK SHOULD BEGIN TO ABSORB THE INITIALLY  
CUTOFF SOUTHERN STREAM FEATURE AS IT MOVES INTO THE FOUR CORNERS  
REGION, MORE RAPIDLY KICKING IT EASTWARD INTO THE SOUTHERN/CENTRAL  
PLAINS BY TUE. ENSEMBLE SPAGHETTI PLOTS REVEAL A FAIR AMOUNT OF  
SPREAD INVOLVING THE TROUGH AMPLIFICATION AS WELL AS THE EVOLUTION  
OF THE FLOW ACROSS CANADA (WITH IMPLICATIONS FOR WHETHER COLDER  
POLAR AIR IS ABLE TO SPREAD SOUTH INTO THE NORTH CENTRAL U.S.).  
THUS, CONFIDENCE IS BELOW AVERAGE BY EARLY NEXT WEEK, AND A TREND  
TOWARD HEAVIER WEIGHT OF ENSEMBLE MEANS WAS FAVORED IN THE  
FORECAST. REGARDLESS OF THE SPECIFICS OF THE WESTERN TROUGH,  
ENSEMBLES SHOW BROAD AGREEMENT THAT SUBTROPICAL RIDGING SHOULD  
GRADUALLY BUILD INTO THE SOUTHEAST AND SOUTHERN MID-ATLANTIC  
REGIONS BY EARLY NEXT WEEK.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE SHORTWAVE AND SURFACE LOW PRESSURE SYSTEM TRAVERSING THE  
SOUTHEAST FRI-SAT SHOULD BE ACCOMPANIED BY SCATTERED TO NUMEROUS  
SHOWERS AND THUNDERSTORMS, WITH THE POTENTIAL FOR AT LEAST LOCALLY  
HEAVY RAINFALL. AS THE PHASED LOW PRESSURE SYSTEM DEVELOPS ACROSS  
NEW ENGLAND LATE FRI INTO SAT, WIDESPREAD RAIN IS EXPECTED,  
PERHAPS EVEN MIXING WITH SNOW ACROSS INTERIOR PORTIONS OF FAR  
NORTHERN NEW ENGLAND. FARTHER WEST, A LOW PRESSURE SYSTEM MOVING  
FROM THE NORTHERN/CENTRAL PLAINS TO THE GREAT LAKES SAT-SUN SHOULD  
BRING RAIN TO MUCH OF THE MIDWEST, WITH THE POTENTIAL FOR SOME  
AREAS OF SNOWFALL ON THE NORTHERN EDGE OF THE SYSTEM, FROM  
NORTHERN MINNESOTA EAST INTO NORTHERN WISCONSIN AND MICHIGAN. A  
STALLED FRONTAL BOUNDARY ACROSS THE CENTRAL U.S. IN THE WAKE OF  
THIS SYSTEM WILL CONTINUE TO GENERATE AREAS OF SHOWERS AND  
THUNDERSTORMS INTO EARLY NEXT WEEK FROM THE CENTRAL PLAINS TO THE  
MIDWEST, WITH FURTHER ENHANCEMENT POSSIBLE BY TUE AS THE NEXT  
SHORTWAVE APPROACHES. RAIN AND MOUNTAIN SNOW WILL BECOME MORE  
COMMON ACROSS MUCH OF THE INTERIOR WESTERN U.S. BY THE WEEKEND  
INTO EARLY NEXT WEEK AS WELL, AS UPPER TROUGHING AMPLIFIED  
OVERHEAD.  
 
ABOVE AVERAGE TEMPERATURES ARE EXPECTED FR-SUN FROM CALIFORNIA AND  
THE GREAT BASIN EAST TO THE HIGH PLAINS. HIGH TEMPERATURES OF 10  
TO 15 DEG F ABOVE AVERAGE ARE EXPECTED TO BE COMMON. EARLY NEXT  
WEEK, WARM TEMPERATURES ARE EXPECTED TO SPREAD EASTWARD TO THE  
SOUTHEAST/SOUTHERN MID-ATLANTIC AS THE SUBTROPICAL RIDGE BUILDS.  
AT THE SAME TIME, FALLING HEIGHTS ACROSS THE NORTHERN TIER SHOULD  
BRING BELOW AVERAGE TEMPERATURES FROM THE NORTHERN ROCKIES TO THE  
UPPER MIDWEST FROM SUN ONWARD.  
 
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  
 
HAZARDS:  
- HEAVY RAIN ACROSS PORTIONS OF THE CENTRAL PLAINS AND THE MIDDLE  
MISSISSIPPI VALLEY, TUE, APR 30.  
- HEAVY RAIN ACROSS PORTIONS OF THE NORTHEAST, SAT, APR 27.  
- FLOODING POSSIBLE ACROSS PORTIONS OF THE SOUTHERN ROCKIES, THE  
GREAT LAKES, THE NORTHEAST, THE SOUTHERN PLAINS, AND THE UPPER  
MISSISSIPPI VALLEY.  
- FLOODING OCCURRING OR IMMINENT ACROSS PORTIONS OF THE LOWER  
MISSISSIPPI VALLEY, THE NORTHERN PLAINS, THE TENNESSEE VALLEY, THE  
NORTHERN GREAT BASIN, THE MIDDLE MISSISSIPPI VALLEY, THE  
NORTHEAST, THE UPPER MISSISSIPPI VALLEY, THE GREAT LAKES, AND THE  
OHIO VALLEY.  
 
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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