329  
FXUS02 KWBC 171917  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
316 PM EDT WED APR 17 2019  
 
VALID 12Z SAT APR 20 2019 - 12Z WED APR 24 2019  
   
..OVERVIEW AND GUIDANCE PREFERENCES
 
 
GUIDANCE AGREES THAT MEAN FLOW ACROSS THE LOWER 48 WILL BECOME  
LESS AMPLIFIED IN THE WAKE OF THE DEEP SYSTEM LIFTING  
NORTHEASTWARD OVER THE EAST DURING THE WEEKEND. AS ILLUSTRATED BY  
ENSEMBLE SPAGHETTI PLOTS, SHORTWAVE DETAILS WILL BE RATHER  
UNCERTAIN FOR MOST OF THE PERIOD--STARTING WITH ENERGY COMING INTO  
THE WEST ON DAY 3 SAT AND THEN FOR DETAILS OF PACIFIC FLOW  
REACHING WESTERN NORTH AMERICA. THIS FAVORS MAINTAINING A BLEND  
APPROACH FOR THE FORECAST, MOSTLY USING OPERATIONAL GUIDANCE EARLY  
AND THEN TRENDING TOWARD MORE ENSEMBLE MEAN INPUT LATE.  
 
FOR THE DEPARTING EASTERN SYSTEM THE PRIMARY CONSIDERATION IS TO  
DOWNPLAY THE 00Z GFS WHICH BECOMES FASTER THAN CONSENSUS AFTER  
LATE SAT. THE PAST DAY OF ECMWF RUNS HAS BEEN FAIRLY STABLE WHILE  
GFS RUNS HAVE BEEN LESS CONSISTENT.  
 
REGARDING THE WEEKEND WESTERN U.S. ENERGY AND TRAILING PACIFIC  
SHORTWAVE(S) WHOSE COMBINATION BECOMES ONE OF THE PRIMARY FORECAST  
PROBLEMS THROUGH THE REST OF THE PERIOD, GFS/GEFS RUNS HAVE BEEN  
PERSISTENT IN SHOWING A DEEPER AND SLOWER OVERALL TROUGH THAN MOST  
OTHER GUIDANCE. HOWEVER THE PAST COUPLE ECMWF RUNS HAVE TRENDED  
TOWARD DIGGING SOMEWHAT MORE ENERGY INTO THE SOUTHWESTERN  
U.S./ROCKIES AND NORTHERN MEXICO VERSUS PREVIOUS RUNS.  
TELECONNECTIONS RELATIVE TO NEGATIVE HEIGHT ANOMALY CENTERS WELL  
NORTH OF HUDSON BAY AND THE GULF OF ALASKA (OR FARTHER  
WEST-NORTHWEST DEPENDING ON THE MODEL/ENSEMBLE MEAN) ALL GENERALLY  
SUPPORT MODERATELY ABOVE NORMAL HEIGHTS AND POSSIBLY MODEST MEAN  
RIDGING OVER THE WEST-CENTRAL U.S. THIS RELATIONSHIP WOULD  
SUGGEST THAT WHATEVER TROUGHING REACHES THE WEST SHOULD BE A  
LITTLE WEAKER AND/OR FASTER THAN DEPICTED IN THE GFS/GEFS. ECMWF  
TRENDS AND A GFS-LIKE SOLUTION IN THE 00Z UKMET FAVOR AN  
INTERMEDIATE BLEND APPROACH BUT TILTED JUST A BIT IN THE  
ECMWF/ECMWF MEAN DIRECTION IN LIGHT OF TELECONNECTIONS.  
 
DURING THE LATTER HALF OF THE PERIOD THE 00Z CMC/CMC MEAN ARE MOST  
AGGRESSIVE TO BRING HEIGHT FALLS ACROSS THE NORTHWESTERN U.S.  
AFOREMENTIONED TELECONNECTIONS RECOMMEND STAYING CLOSER TO THE  
GFS/ECMWF AND THEIR ENSEMBLE MEANS. THE FAST MEAN FLOW AND  
SENSITIVITY TO SMALLER-SCALE SHORTWAVE DETAILS LEAD TO LOW  
CONFIDENCE FOR SURFACE FRONT/WAVE SPECIFICS AS EACH CYCLE OF  
GUIDANCE HAS SHOWN MEANINGFUL CHANGES FOR EASTERN PACIFIC FRONTS  
THAT SHOULD TEND TO DIE OUT UPON ENTERING WESTERN NORTH AMERICA.  
FARTHER EAST, GUIDANCE CONSENSUS OVER THE PAST COUPLE DAYS HAS  
BEEN TRENDING STRONGER/EASTWARD WITH EASTERN CANADA HIGH PRESSURE  
THAT WILL LIKELY HELP TO PUSH A BACKDOOR FRONT SOUTHWARD ALONG THE  
EAST COAST NEXT TUE-WED.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE STRONG SYSTEM AFFECTING THE EAST TO START THE WEEKEND MAY  
PRODUCE A BAND OF ENHANCED COOL-SECTOR RAINFALL OVER PARTS OF THE  
OHIO VALLEY/EAST-CENTRAL GREAT LAKES AND A LEADING BAND OF LOCALLY  
MODERATE/HEAVY RAINFALL FOCUSED OVER PARTS OF NEW ENGLAND. WINDS  
MAY BE FAIRLY STRONG AT SOME LOCATIONS ON SAT AS WELL. THE  
OVERALL SHORTWAVE TROUGH ALOFT AFFECTING THE WESTERN/CENTRAL U.S.  
AND ASSOCIATED SURFACE FEATURES WILL SPREAD RAIN AND HIGH  
ELEVATION SNOW OVER CENTRAL LATITUDES OF THE WEST AND THEN  
RAINFALL OF VARYING INTENSITY OVER CENTRAL/SOUTHERN PARTS OF THE  
PLAINS AND POSSIBLY THE MISSISSIPPI VALLEY. CONFIDENCE IN  
SPECIFICS OF THIS PRECIPITATION AREA IS LOWER THAN DESIRED DUE TO  
THE GUIDANCE SPREAD FOR DETAILS ALOFT. THERE IS A SIGNAL FOR LOW  
LEVEL UPSLOPE FLOW TO PROVIDE SOME PRECIPITATION ENHANCEMENT ALONG  
THE EASTERN SLOPES OF THE CENTRAL ROCKIES AROUND SUN. THEREAFTER  
ENSEMBLE GUIDANCE SHOWS RELATIVELY HIGHER PROBABILITIES FOR  
LOCALLY HEAVY RAINFALL OVER THE SOUTHERN PLAINS WHICH SHOULD SEE A  
MULTI-DAY FEED OF MOISTURE FROM THE GULF. AFTER A RELATIVELY DRY  
WEEKEND THE PACIFIC NORTHWEST SHOULD SEE PERIODS OF PRECIPITATION  
FROM MON ONWARD AS MULTIPLE FRONTS APPROACH/CROSS THE REGION.  
MOST OF THIS ACTIVITY SHOULD BE IN THE LIGHT-MODERATE RANGE.  
 
THE STRONG EASTERN SYSTEM WILL BRING A BRIEF PERIOD OF CHILLY  
TEMPERATURES TO THE MISSISSIPPI VALLEY/EAST FROM SAT INTO SUN  
MORNING. HIGHS MAY BE AT LEAST 10-15F BELOW NORMAL ON SAT FROM  
PARTS OF THE OHIO VALLEY INTO THE SOUTHEAST. ASIDE FROM A POCKET  
OF BELOW NORMAL HIGHS DROPPING SOUTHWARD ALONG THE HIGH PLAINS,  
TEMPERATURES SHOULD OTHERWISE BE NEAR TO ABOVE NORMAL GIVEN THE  
FLATTER TREND OF FLOW ALOFT. EXPECT THE HIGHEST WARM ANOMALIES  
OVER THE WEST AND INTO THE NORTHERN HIGH PLAINS WITH ONE OR MORE  
DAYS OF MAX AND/OR MIN READINGS 10-20F ABOVE NORMAL.  
 
RAUSCH  
 
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 SOUTHERN PLAINS, TUE-WED, APR  
23-APR 24.  
- FLOODING POSSIBLE ACROSS PORTIONS OF THE MID-ATLANTIC, THE LOWER  
MISSISSIPPI VALLEY, THE CENTRAL/SOUTHERN APPALACHIANS, THE  
NORTHERN PLAINS, THE TENNESSEE VALLEY, THE GREAT LAKES, THE  
NORTHEAST, THE UPPER MISSISSIPPI VALLEY, THE SOUTHERN PLAINS, AND  
THE OHIO VALLEY.  
- FLOODING OCCURRING OR IMMINENT ACROSS PORTIONS OF THE NORTHEAST,  
THE NORTHERN PLAINS, THE MISSISSIPPI VALLEY, AND THE OHIO VALLEY.  
- FLOODING LIKELY ACROSS PORTIONS OF THE SOUTHEAST, THE UPPER  
MISSISSIPPI VALLEY, THE MID-ATLANTIC, THE NORTHERN PLAINS, AND THE  
NORTHERN GREAT BASIN.  
- HIGH WINDS ACROSS PORTIONS OF THE CENTRAL PLAINS AND THE  
SOUTHERN PLAINS, SUN-MON, APR 21-APR 22.  
 
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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