531  
FXUS02 KWBC 200610  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
210 AM EDT WED MAR 20 2019  
 
VALID 12Z SAT MAR 23 2019 - 12Z WED MAR 27 2019  
   
..OVERVIEW AND GUIDANCE ASSESSMENT/PREFERENCES
 
 
A SPLIT FLOW PATTERN IS EXPECTED TO PERSIST THROUGH THE MEDIUM  
RANGE. ANOMALOUS UPPER RIDGE AXIS EXTENDING FROM ALASKA AND  
NORTHWEST CANADA INTO THE NORTH CENTRAL U.S. IS EXPECTED TO  
PERSIST, WITH DOWNSTREAM MEAN TROUGHING CENTERED EAST OF HUDSON  
BAY - MAKING FOR HIGHLY AMPLIFIED, IF NOT BLOCKED NORTHERN STREAM  
FLOW. MEANWHILE, THE STREAM WILL BE FAIRLY PROGRESSIVE ACROSS THE  
CONUS, WITH SHORTWAVES SPACED A COUPLE DAYS APART AS ENERGY EJECTS  
FROM AN UPSTREAM MEAN TROUGH ACROSS THE NORTH CENTRAL PACIFIC.  
 
DAYS 3-5  
 
IN GENERAL, A MULTI-MODEL BLEND OF THE LATEST DETERMINISTIC  
GUIDANCE SERVED AS A GOOD FORECAST STARTING POINT DURING DAYS 3-5  
(SAT-MON). MODEL SOLUTIONS STILL DIFFER TO SOME DEGREE ON THE  
POSITION OF A DEEP SURFACE LOW NEAR/EAST OF MAINE SAT MORNING, BUT  
SOLUTIONS ARE GENERALLY WELL-CLUSTERED AROUND THE ENSEMBLE MEANS,  
INDICATING THAT A CONSENSUS APPROACH IS LIKELY BEST. AS THE SAT  
SYSTEM QUICKLY PULLS AWAY, ANOTHER SHORTWAVE SHOULD TRAVERSE THE  
BROADER MEAN TROUGH, DIVING INTO THE GREAT LAKES/NORTHEAST  
SUN-MON. SOLUTIONS HAVE TRENDED MORE AMPLIFIED WITH THIS WAVE OVER  
THE PAST SEVERAL MODEL CYCLES, AND A CONSENSUS APPROACH IS ALSO  
FAVORED HERE AT THIS TIME. FARTHER WEST, MODELS SHOW GOOD  
AGREEMENT THAT A PACIFIC SHORTWAVE SHOULD REACH THE WEST COAST  
EARLY SAT, THEN CROSS THE FOUR CORNERS SUN, REACHING THE  
SOUTHERN/CENTRAL PLAINS BY MON. AT LEAST THROUGH SUN, MODELS  
HANDLE THIS FEATURE SIMILARLY, WITH DIFFERENCES BEGINNING TO  
EMERGE BY MON AS THE SYSTEM MOVES INTO THE PLAINS. NONETHELESS,  
THERE IS AT LEAST MODEST CONSENSUS THAT A SURFACE LOW SHOULD  
DEVELOP ACROSS THE SOUTHERN PLAINS SUN NIGHT-MON.  
 
DAYS 6-7  
 
THE FORECAST GETS QUITE TRICKY BY NEXT TUE-WED, PARTICULARLY WITH  
RESPECT TO THE SOUTHERN STREAM SYSTEM MOVING FROM THE PLAINS  
TOWARD THE EAST COAST. THE GFS AND ECMWF HAVE BOTH SHOWN  
SIGNIFICANT DIFFICULTY HANDLING THIS SYSTEM. THE ECMWF HAS BEE  
HIGHLY VARIABLE FROM RUN-TO-RUN, WHILE THE GFS HAS CONSISTENTLY  
BEEN A FAST OUTLIER, ONLY SEEMING TO GET FASTER WITH EACH RUN. A  
SIZABLE NUMBER OF ECENS AND CMCE MEMBERS HAVE SHOWN GOOD  
CLUSTERING FOR A FEW RUNS NOW AROUND A SURFACE LOW TRACKING FROM  
THE SOUTHERN PLAINS INTO THE TENNESSEE VALLEY MON NIGHT-TUE (A  
SOLUTION STARKLY DIFFERENT FROM THE 12Z ECMWF WHICH SEEMED TO BE  
AN OUTLIER IN THE CONTEXT OF ITS ENSEMBLE). THE 12Z CMC/GEM GLOBAL  
AND NAVGEM WERE THE ONLY DETERMINISTIC SOLUTIONS REPRESENTING THIS  
SIGNIFICANT CLUSTER, WHICH IS ALSO REFLECTED IN THE ECENS MEAN. BY  
TUE NIGHT-WED, ENERGY APPEARS LIKELY TO TRANSFER TO A NEW SURFACE  
LOW ALONG THE SOUTHEAST U.S. COAST, IN A "MILLER-B" SCENARIO. THIS  
PROCESS SEEMS TO BE BEST REPRESENTED BY THE ECENS AND CMCE MEANS,  
WITH THE 12Z ECMWF SOLUTION ALSO IN THE BALL PARK (IF YOU IGNORE  
ITS OUTLIER SOLUTION FOR THE GULF COAST LOW). FINALLY, FARTHER  
WEST, CONSENSUS REMAINS RELATIVELY GOOD THAT STRONG HEIGHT FALLS  
SHOULD REACH THE WEST COAST TUE-WED AS A SIGNIFICANT TROUGH MOVES  
ONSHORE. WHILE TIMING AMONG MODELS IS SIMILAR, THERE ARE  
STRUCTURAL DIFFERENCES THAT REMAIN, LEADING TO THE CONCLUSION THAT  
AN ENSEMBLE APPROACH FOR THAT SYSTEM IS LIKELY BEST. GIVEN THESE  
CONSIDERATIONS, THE FORECAST FOR DAYS 6-7 WAS BASED HEAVILY ON THE  
ECENS AND NAEFS ENSEMBLE MEANS, WITH COMPONENTS OF THE CMC/GEM  
GLOBAL AND NAVGEM ALSO INCLUDED, GIVEN THEIR AFOREMENTIONED  
HANDLING OF THE CENTRAL/EASTERN U.S. SYSTEM.  
 
   
..WEATHER HIGHLIGHTS AND HAZARDS
 
 
SHOWERS AND THUNDERSTORMS ARE EXPECTED TO DEVELOP ACROSS THE  
SOUTHERN AND CENTRAL PLAINS SAT-SUN. A LEADING UPPER SHORTWAVE SAT  
AND A SURFACE WARM FRONT/DRY LINE WILL INITIALLY FOCUS CONVECTION  
ACROSS THESE AREAS. THE POTENTIAL EXISTS FOR SOME SEVERE WEATHER  
AS WELL AS LOCALLY HEAVY RAINFALL THREATS. PLEASE REFER TO  
PRODUCTS ISSUED BY THE SPC FOR FURTHER INFORMATION ON THE SEVERE  
WEATHER THREAT. BY MON-TUE, SHOWERS AND THUNDERSTORMS SHOULD  
SPREAD EAST INTO THE LOWER/MID-MISSISSIPPI VALLEY AS WELL AS THE  
OHIO/TENNESSEE VALLEYS AND THE SOUTHEAST. DEVELOPMENT OF THE  
SURFACE LOW ALONG THE SOUTHEAST U.S. COAST TUE NIGHT-WED WOULD  
ALSO LIKELY BRING AN ENHANCEMENT OF RAINFALL TO PORTIONS OF THE  
CAROLINAS AND SOUTHERN MID-ATLANTIC. ENSEMBLE PROBABILITIES  
SUGGEST SOME POTENTIAL FOR WINTRY WEATHER ON THE NORTHERN  
PERIPHERY OF THIS SYSTEM TUE-WED, FROM PORTIONS OF THE UPPER OHIO  
VALLEY TO THE MID-ATLANTIC AND NORTHEAST. CONFIDENCE IN THE EXTENT  
OF COLD AIR IN PLACE BY THAT TIME IS VERY LOW, AND ONLY LOW-END  
PROBABILITIES WERE SHOWN AT THIS TIME ON THE DAY 7 WINTER WEATHER  
OUTLOOK. FARTHER WEST, RAIN AND MOUNTAIN SNOWS ARE EXPECTED ACROSS  
THE GREAT BASIN/ROCKIES WITH THE PASSAGE OF THE SHORTWAVE SAT-SUN.  
A MORE SIGNIFICANT AREA OF PRECIPITATION COULD SPREAD INTO  
NORTHERN CALIFORNIA BY NEXT MON-TUE AS THE STRONGER UPPER TROUGH  
APPROACHES, WITH SOME SIGNAL FOR A HEAVY PRECIPITATION EVENT IN  
THE MODEL GUIDANCE.  
 
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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