865  
FXUS02 KWBC 202042  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
442 PM EDT WED MAR 20 2019  
 
VALID 12Z SAT MAR 23 2019 - 12Z WED MAR 27 2019  
   
..16Z UPDATE
 
 
THE ARRAY OF 00Z/06Z GUIDANCE YIELDS A FORECAST RATIONALE SIMILAR  
TO PREVIOUS CYCLE--A CONSENSUS APPROACH FOR DAYS 3-4 SAT-SUN AND  
THEN EMPHASIS ON THE 00Z ECMWF/CMC AND NAEFS/ECMWF MEANS BY DAYS  
5-7 MON-WED. GFS/FV3 GFS RUNS AND GEFS MEANS ARE STILL ON THE  
FASTER SIDE OF THE SPREAD WITH THE SHORTWAVE CROSSING THE SOUTHERN  
HALF OF THE LOWER 48 (THOUGH THE UKMET HEDGES IN THAT DIRECTION BY  
MON). AT THE SAME TIME THE GFS/GEFS MEAN ARE MORE AMPLIFIED WITH  
FLOW DROPPING INTO THE GREAT LAKES BY MON, LEADING TO MORE PHASING  
BETWEEN STREAMS THAN THE MAJORITY SCENARIO. FROM LATE MON ONWARD  
THE 00Z ECMWF HAS TRENDED BACK TO THE NORTH WITH ITS EASTERN U.S.  
SURFACE LOW TO AGREE BETTER WITH ECMWF/CMC MEANS. THE AVERAGE OF  
MEMBERS WITHIN THE CMC/ECMWF CLUSTER HAS ADJUSTED A TAD FASTER  
FROM CONTINUITY AND THIS IS REFLECTED IN THE UPDATED FORECAST.  
OVER THE EASTERN PACIFIC/WEST COAST THERE IS REASONABLE CONSENSUS  
INTO DAY 5 MON. AFTER THAT TIME THE MAJORITY OF GUIDANCE  
GENERALLY SUGGESTS THE UPPER LOW OFF THE COAST WILL BE MORE  
CONSOLIDATED THAN DEPICTED BY THE 06Z GFS/FV3 GFS. THE 00Z GFS IS  
CLOSER TO CONSENSUS. INCLUDING THE NAEFS MEAN IN THE BLEND  
ACCOUNTS FOR THE POSSIBILITY THAT THE ECMWF/CMC CLUSTER COULD BE  
SLIGHTLY FAST TO BRING THE UPPER TROUGH INLAND NEXT WED, GIVEN THE  
OVERALL BLOCKY NATURE OF THE EVOLVING WESTERN NORTH AMERICAN  
PATTERN AT THAT TIME.  
 
MODELS AND ENSEMBLES ARE STABLE WITH THE OVERALL SIGNAL FOR ONE OR  
MORE EPISODES OF HEAVY PRECIPITATION FOCUSED OVER/NEAR NORTHERN  
CALIFORNIA AND SIERRA NEVADA DURING MON-WED. SOME OF THIS  
MOISTURE AND/OR THE LEADING SYSTEM CROSSING THE WEST DURING THE  
WEEKEND WILL SPREAD RAIN AND HIGHER ELEVATION SNOW OVER REMAINING  
PARTS OF THE NORTHERN HALF OR MORE OF THE WEST. SPREAD AND  
VARIABILITY IN MODEL GUIDANCE KEEP CONFIDENCE ON THE LOW END OF  
THE SPECTRUM FOR COVERAGE/INTENSITY/TIMING OF PRECIPITATION FROM  
THE PLAINS TO EAST COAST, AS WELL AS FOR DETAILS OF ANY SNOW THAT  
MAY FALL IN THE NORTHERN FRINGE OF THE MOISTURE SHIELD.  
CLOUDS/PRECIP WILL CONTRIBUTE TO ABOVE NORMAL MORNING LOWS (BY  
10-20F IN SOME CASES) FROM PARTS OF THE WEST INTO THE PLAINS  
DURING THE WEEKEND AND CENTRAL/SOUTHERN PLAINS INTO LOWER  
MISSISSIPPI VALLEY EARLY NEXT WEEK. HIGHS WILL TEND TO BE NEAR TO  
BELOW NORMAL OVER THE WEST DURING THE WEEKEND. THE INTERIOR WEST  
AND ROCKIES/PLAINS SHOULD WARM UP AGAIN BY NEXT TUE OR WED. ON  
THE OTHER HAND BELOW NORMAL READINGS WILL LIKELY PUSH  
SOUTHEASTWARD FROM THE NORTHERN PLAINS/GREAT LAKES DURING MON-WED.  
 
RAUSCH  
 
PREVIOUS DISCUSSION:  
 
   
..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 BEEN  
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  
 
HAZARDS:  
- HEAVY PRECIPITATION ACROSS PORTIONS OF CALIFORNIA, THE CENTRAL  
GREAT BASIN, AND THE PACIFIC NORTHWEST, MON-WED, MAR 25-MAR 27.  
- HEAVY RAIN ACROSS PORTIONS OF THE LOWER MISSISSIPPI VALLEY AND  
THE SOUTHERN PLAINS, SAT-SUN, MAR 23-MAR 24.  
- HEAVY RAIN ACROSS PORTIONS OF THE MIDDLE/LOWER MISSISSIPPI  
VALLEY, THE CENTRAL/SOUTHERN APPALACHIANS, THE TENNESSEE VALLEY,  
THE MID-ATLANTIC, THE SOUTHEAST, AND THE OHIO VALLEY, MON-TUE, MAR  
25-MAR 26.  
- HEAVY SNOW ACROSS PORTIONS OF THE MID-ATLANTIC AND THE CENTRAL  
APPALACHIANS, TUE-WED, MAR 26-MAR 27.  
- HEAVY SNOW ACROSS PORTIONS OF THE NORTHEAST AND THE GREAT LAKES,  
SAT, MAR 23.  
- SEVERE WEATHER ACROSS PORTIONS OF THE SOUTHERN PLAINS, SUN, MAR  
24.  
- FLOODING POSSIBLE ACROSS PORTIONS OF THE NORTHERN/CENTRAL  
PLAINS, THE UPPER/MIDDLE MISSISSIPPI VALLEY, AND THE GREAT LAKES.  
- FLOODING OCCURRING OR IMMINENT ACROSS PORTIONS OF THE  
NORTHERN/CENTRAL PLAINS, THE MISSISSIPPI VALLEY, THE TENNESSEE  
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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