458  
FXUS02 KWBC 151600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1059 AM EST THU FEB 15 2018  
 
VALID 12Z SUN FEB 18 2018 - 12Z THU FEB 22 2018  
 
...COOLING/UNSETTLED TROUGHING FOR THE WRN US AND N-CENTRAL  
STATES...  
...LOWER TO MID-MISSISSIPPI/OHIO VALLEY AND MID-SOUTH HEAVY  
RAINS...  
 
GENERAL FLOW PATTERN/MODEL PREFERENCES  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
A STRONG GUIDANCE SIGNAL CONTINUES TO POINT TO AN INCREASINGLY  
AMPLIFIED PATTERN NEXT WEEK WITH A DEEP AND POSITIVELY-TILTED  
WESTERN US TROUGH HELD IN PLACE BY PERSISTENT/SIGNIFICANT  
MID-LEVEL POSITIVE HEIGHT ANOMALIES SOUTH OF ALASKA AND OVER THE  
MID-ATLANTIC/SERN US.  
 
FORECAST SPREAD/UNCERTAINTY SEEMS TO BE MOST ASSOCIATED WITH THE  
EVOLUTION OF THE EMBEDDED SHORTWAVES IN THE WESTERN US TROUGH AND  
THE SPEED/SOUTHEASTWARD PROGRESSION OF THE COLD FRONT IN THE EAST  
LATER NEXT WEEK. UPSTREAM SPREAD IN THE NORTH PACIFIC SHOULD  
CONTINUE TO LEAD TO RUN-RUN AND MODEL-MODEL VARIANCE WITH TROUGH  
SPECIFICS IN THE WEST...BUT RECENT TRENDS ARE TOWARD MORE  
AMPLIFIED FLOW. LEAD FRONTAL TIMING/PROGRESSION REMAINS LESS  
CLEAR...BUT RECENT GFS/GEFS RUNS HAVE BEEN POSITIONED ON THE  
WARMER SIDE OF THE GUIDANCE ENVELOPE POST-FRONTAL ACROSS THE WRN  
AND CENTRAL STATES. THE PATTERN/TELECONNECTIONS SEEMS COLDER WITH  
SEVERAL SURFACE HIGH PRESSURE SURGES...SO THE WPC MEDIUM RANGE  
GUIDANCE BLEND WAS WEIGHTED MORE TOWARD CONTINUITY AND 00 UTC  
NAEFS/ECMWF ENSEMBLE MEANS THAN THE 06 UTC GEFS MEAN OR WITH  
INCLUSION OF MORE VARIED INDIVIDUAL MODEL RUNS.  
 
WEATHER IMPACTS  
~~~~~~~~~~~~~~~  
LOWERING HEIGHTS WILL LOWER SNOW LEVELS LEADING TO MODERATELY COLD  
TEMP ANOMALIES ACROSS THE WESTERN US. TEMP ANOMALIES ARE EXPECTED  
TO BE MORE SEVERE OVER THE NRN ROCKIES/PLAINS AND WILL GRADUALLY  
MODIFY AS POST-FRONTAL HIGH PRESSURE DIGS THROUGH A TRANSITIONAL  
SOUTH-CENTRAL US. LOCALLY TERRAIN ENHANCED SNOWS THIS WEEKEND  
SPREAD SOUTHWARD THROUGH THE GREAT BASIN AND INTERIOR  
WEST/SOUTHWEST NEXT WEEK. LIGHTER SNOWS SLIP EASTWARD ON THE  
NORTHERN SIDE OF THE PRECIPITATION SHIELD FARTHER EAST FOR THE NRN  
PLAINS AND UPPER MIDWEST.  
 
TEMPERATURES WILL RISE IN THE EAST AS AN APPROACHING COLD FRONT  
AND RETURNING WARM FRONT BECOME THE FOCI/OVERRUNNING SURFACE FOR  
SOME LOCALLY MODEST TO HEAVY RAIN TUE-THU. ENSEMBLE MEAN AMOUNTS  
WERE ALREADY OVER 2-3 INCHES WHICH SUGGESTS LOCAL AMOUNTS OF TWICE  
THAT OVER A SEVERAL DAY PERIOD. 12Z GEFS M-CLIMATE QPF VALUES SAT  
ABOVE THE 99.5 PERCENTILE FROM NORTHEAST TX TO SOUTHERN INDIANA  
AND THE ECMWF EXTREME FORECAST INDEX SHOWS VALUES ABOVE THE 90TH  
PERCENTILE. THE DEGREE OF WEDGING/COLD AIR DAMMING (WHICH WOULD BE  
REINFORCED BY ANTICIPATED RAINFALL EAST OF THE APPALACHIANS)  
REMAINS A QUESTION MARK, BUT WARM SECTOR TEMPS HAVE HIGH UPSIDE  
POTENTIAL NEAR RECORD HIGHS. EVEN SO...LINGERING TRAPPED COLD AIR  
WILL NOT MODIFY ENOUGH TO PREVENT WINTERY PRECIPITATION FOR THE  
FAR NRN TIER OF THE NERN US.  
 
SCHICHTEL  
 

 
 
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