284  
FXUS02 KWBC 170546  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
145 AM EDT SAT APR 17 2021  
 
VALID 12Z TUE APR 20 2021 - 12Z SAT APR 24 2021  
   
..HEAVY RAINFALL FOR PARTS OF FLORIDA TUESDAY
 
 
   
..OVERVIEW
 
 
STRONG UPPER RIDGING IS FORECAST TO DRIFT WEST-NORTHWESTWARD OUT  
OF NORTHWESTERN CANADA AND THROUGH ALASKA AS AN UPPER LOW IN THE  
NORTHEASTERN PACIFIC WOBBLES ITS WAY TOWARD THE PACIFIC NORTHWEST.  
THIS FAVORS TROUGHING OVER MOST OF THE CONUS NEXT WEEK AND COOLER  
THAN NORMAL TEMPERATURES TO ALL BUT THE WEST. A WAVY FRONTAL  
BOUNDARY WILL BRING A CHANCE OF HEAVY RAIN OVER FLORIDA FROM THE  
SHORT RANGE INTO THE MEDIUM RANGE EARLY IN THE WEEK BEFORE  
DISSIPATING. A LEAD BOUNDARY OVER THE PLAINS TO MIDWEST ON TUESDAY  
WILL BRING LIGHT RAIN AND SNOW INTO THE GREAT LAKES AS IT WINDS UP  
OVER SOUTHEASTERN CANADA. A WESTERN SYSTEM MAY TAP THE GULF FOR  
MOISTURE LATER NEXT WEEK AND BRING WIDESPREAD RAINFALL TO THE  
LOWER MISSISSIPPI VALLEY BY NEXT WEEKEND.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT AND PREFERENCES
 
 
THE GFS/GEFS HAVE STRUGGLED OVER THE PAST WEEK OR SO WITH THE  
AMPLIFIED/CUTOFF FLOW FROM THE PACIFIC ACROSS NORTH AMERICA,  
THOUGH OVER MANY AREAS THEY SEEMED FAIRLY ADEQUATE. HOWEVER,  
OVERALL PREFERRED THE ECMWF-LED CONSENSUS WITH THE 12Z CANADIAN  
AND THE ECMWF ENSEMBLES WHICH WERE GENERALLY MORE AMPLIFIED THAN  
THE GFS/GEFS. THERE WAS A NOTABLE TREND TOWARD A SLOWER  
PROGRESSION ACROSS CANADA AND THE NORTHEAST NEXT THURSDAY WITH A  
FARTHER WEST UPPER LOW NEAR THE CANADIAN ARCHIPELAGO, ALLOWING THE  
NORTHEAST SYSTEM TO CLOSE OFF AND WRAP UP OVER MAINE. WITH THE  
UPSTREAM UPPER LOW STILL FORECAST TO DROP DOWN INTO SOUTHERN  
CANADA FRIDAY, THIS SHOULD BE ENOUGH OF A KICKER OVERALL TO NUDGE  
THE MAINE SYSTEM FARTHER TO THE NORTHEAST. BY LATER IN THE WEEK,  
THE GFS MAY BE TOO SLOW OVER TEXAS/MEXICO WITH AN UPPER/SFC SYSTEM  
(RELATED TO A SLOWER PROGRESSION OF THE INCOMING PACIFIC SYSTEM  
TOWARD WA/OR) BUT THE BREAKDOWN/RELAXATION OF THE AMPLIFIED  
PATTERN SUGGESTS LOW CONFIDENCE IN EVOLUTION. OPTED TO REMAIN  
CLOSER TO THE ECMWF ENSEMBLE MEAN WHICH TYPICALLY HANDLES  
AMPLIFIED FLOW THE BEST, BUT EVEN IT CAN BE TOO SLOW ONCE THE  
PATTERN OPENS UP (WHICH CAN FAVOR THE GFS/GEFS).  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
NORTHERN AND CENTRAL FLORIDA MAY SEE A CONTINUATION OUT OF THE  
SHORT RANGE (MON) INTO THE MEDIUM RANGE (TUE) WITH RAINFALL ALONG  
A FRONT THAT WILL SLOWLY PUSH SOUTHWARD. SEVERAL INCHES ARE  
POSSIBLE IN SOME AREAS DURING THE PERIOD. GENERALLY LIGHT  
PRECIPITATION IS EXPECTED NEAR A FRONTAL SYSTEM MOVING INTO THE  
GREAT LAKES AND NORTHEAST TUE-WED WITH SOME SNOW ON ITS NORTHWEST  
SIDE. AS THE SYSTEM DEEPENS THROUGH THE NORTHEAST, RAIN/SNOW COULD  
INCREASE OVER NEW ENGLAND WHERE TEMPERATURES MAY BE COLD ENOUGH  
FOR SNOW ESPECIALLY AT HIGHER ELEVATIONS AND OVERNIGHT. THE NEXT  
SYSTEM OF INTEREST WOULD BE OUT OF THE SOUTHERN ROCKIES AROUND FRI  
AND THROUGH THE PLAINS. A MODEST SIGNAL EXISTS FOR AT LEAST SOME  
HEAVY RAINFALL OVER THE LOWER MISSISSIPPI VALLEY NEXT FRI-SAT.  
 
WARMER THAN NORMAL TEMPERATURES ARE EXPECTED FOR THE WEST COAST  
CLOSER TO OR UNDERNEATH UPPER RIDGING, WITH MUCH MILDER AIR JUST  
AHEAD OF THE PACIFIC SYSTEM LATE NEXT WEEK. THE CENTRAL STATES  
WILL SEE THE COLDEST READINGS RELATIVE TO NORMAL FOR LATE APRIL,  
PERHAPS 10-25 DEGREES BELOW NORMAL ON TUESDAY EAST OF THE ROCKIES  
BEHIND THE FRONTAL BOUNDARY. THAT COLD AIR MASS WILL PUSH EASTWARD  
BUT MODERATE THOUGH PARTS OF THE EAST MAY SEE AT LEAST ONE DAY  
WITH TEMPERATURES 5-15 DEGREES BELOW NORMAL (THU ALONG THE I-81  
CORRIDOR).  
 
FRACASSO  
 
ADDITIONAL 3-7 DAY HAZARD INFORMATION CAN BE FOUND ON THE WPC  
MEDIUM RANGE HAZARDS OUTLOOK 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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