313  
FXUS02 KWBC 141424  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
923 AM EST WED FEB 14 2018  
 
VALID 12Z SAT FEB 17 2018 - 12Z WED FEB 21 2018  
 
...HEAVY RAIN POSSIBLE ACROSS THE CENTRAL APPALACHIANS, MID-SOUTH,  
AND THE LOWER TO MID MISSISSIPPI AND OHIO VALLEYS FROM SATURDAY  
INTO NEXT WEDNESDAY...  
 
GENERAL FLOW PATTERN/MODEL PREFERENCES  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
ENSEMBLE MEAN SOLUTIONS CONTINUE TO POINT TO AN INCREASINGLY  
AMPLIFIED PATTERN NEXT WEEK WITH A DEEP, POSITIVELY-TILTED WESTERN  
TROUGH BEING PINNED IN PLACE BY PERSISTENT/SIGNIFICANT MID-LEVEL  
HEIGHT ANOMALIES SOUTH OF ALASKA AND ACROSS THE MID-ATLANTIC  
STATES. FOR THE DAYS 3-4 (SATURDAY INTO SUNDAY) 500 HPA  
HEIGHTS/PRESSURES/WIND GRIDS, A COMPROMISE OF THE 00Z CANADIAN,  
00Z ECMWF, 06Z GFS, 00Z UKMET APPEARED REASONABLE, WITH SOME  
EMPHASIS ON THE 06Z GFS WITH THE WAVE OFFSHORE THE EAST COAST PER  
THE 00Z GLOBAL ENSEMBLE CLUSTERING. THEREAFTER, THE 00Z CANADIAN  
BECOMES MORE ZONAL WITH THE MID- TO UPPER-LEVEL FLOW WHICH NEITHER  
BEFIT TELECONNECTIONS WITH THE AFOREMENTIONED POSITIVE HEIGHT  
ANOMALIES NOR THE OTHER GUIDANCE SO IT WAS DISMISSED. USED A  
BLEND OF THE 00Z UKMET/06Z GFS/00Z ECMWF FOR MONDAY BEFORE  
INCLUDING INCREASING THE PERCENTAGES OF THE 00Z ECMWF/00Z NAEFS  
MEAN SOLUTIONS. THE REMAINING GRIDS (CLOUDS, DEW POINT,  
PRECIPITATION CHANCES, AND WEATHER) WERE MORE EVEN HANDED IN USE  
OF THE DETERMINISTIC ECMWF/GFS AND THE NAEFS/ECMWF ENSEMBLE MEAN  
SOLUTIONS. DAY SHIFT QPF HAS YET TO BE DETERMINED, BUT A  
COMPROMISE OF THE 06Z GFS/00Z ECMWF APPEARS TO BE THE LIKELY  
STARTING POINT.  
 
WEATHER IMPACTS  
~~~~~~~~~~~~~~~  
ACROSS THE WEST, LOWERING HEIGHTS WILL TAKE THE SNOW LEVEL DOWN TO  
1000-2000 FT AND BRING IN MUCH COLDER AIR TO MONTANA (ABOUT 20-30  
DEGREES BELOW AVERAGE). MODEST SNOW IS EXPECTED FOR MIDDLE AND  
HIGHER ELEVATIONS THIS WEEKEND IN WA/OR/ID/MT/WY THAT WILL MOVE  
SOUTHWARD THROUGH THE GREAT BASIN AND INTERIOR WEST INTO THE  
SOUTHWEST NEXT WEEK, WITH SOME LIGHTER SNOW SLIPPING EASTWARD ON  
THE NORTHERN SIDE OF THE PRECIPITATION SHIELD FARTHER EAST.  
 
AFTER A SNEAKY WINTER WEATHER THREAT FOR THE MID-ATLANTIC INTO THE  
NORTHEAST EARLY IN THE PERIOD (SAT NIGHT INTO EARLY SUNDAY),  
TEMPERATURES WILL MODERATE IN THE EAST AS AN APPROACHING COLD  
FRONT AND RETURNING WARM FRONT BECOME THE FOCI/OVERRUNNING SURFACE  
FOR SOME LOCALLY MODEST TO HEAVY RAIN TUESDAY INTO NEXT WEDNESDAY.  
THE DEGREE OF WEDGING/COLD AIR DAMMING WHICH WOULD BE REINFORCED  
BY ANTICIPATED RAINFALL EAST OF THE APPALACHIANS REMAINS A  
QUESTION MARK, SO REMAINED CONSERVATIVE ON THE NORTHWARD  
PROGRESSION OF THE WARM FRONT.  
 
ROTH/FRACASSO  

 
 
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