278  
FXUS02 KWBC 150637  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
136 AM EST THU FEB 15 2018  
 
VALID 12Z SUN FEB 18 2018 - 12Z THU FEB 22 2018  
 
...HEAVY RAIN POSSIBLE ACROSS THE MID-SOUTH, LOWER TO  
MID-MISSISSIPPI/OHIO VALLEY EARLY TO MID NEXT WEEK...  
 
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  
POSITIVE HEIGHT ANOMALIES SOUTH OF ALASKA AND ACROSS THE  
MID-ATLANTIC STATES. A BLEND OF THE RECENT ECMWF/CANADIAN WITH THE  
GFS/UKMET RUNS OFFERED A GOOD STARTING POINT WITH A TRANSITION  
TOWARD THE ECMWF ENSEMBLE MEAN AND GEFS MEAN BY NEXT WED/THU.  
QUESTIONS IN THE FORECAST LIE WITH THE EVOLUTION OF THE EMBEDDED  
SHORTWAVES IN THE WESTERN TROUGH AND THE SPEED/SOUTHEASTWARD  
PROGRESSION OF THE COLD FRONT IN THE EAST LATER NEXT WEEK (ECMWF  
MUCH SLOWER THAN THE GFS/CANADIAN). GIVEN THE UPSTREAM SPREAD IN  
THE NORTH PACIFIC WHICH WILL MODULATE THE TROUGH IN THE WEST, DID  
NOT FAVOR ONE SOLUTION OVER THE OTHER IN THE EAST RESULTING IN A  
MIDWAY POSITION BETWEEN THE ECMWF/GFS VIA THE ENSEMBLE MEANS.  
 
WEATHER IMPACTS  
~~~~~~~~~~~~~~~  
ACROSS THE WEST, LOWERING HEIGHTS WILL TAKE THE SNOW LEVEL DOWN TO  
1000-2000 FT AND BRING IN MUCH COLDER AIR TO ESPECIALLY MONTANA  
(ABOUT 20-40 DEGREES BELOW AVERAGE) BUT ALSO THROUGH MOST OF THE  
PAC NW, CENTRAL/NORTHERN ROCKIES, AND INTO THE PLAINS. MODEST SNOW  
IS EXPECTED FOR MIDDLE AND HIGHER ELEVATIONS THIS WEEKEND THAT  
WILL MOVE SOUTHWARD THROUGH THE GREAT BASIN AND INTERIOR  
WEST/SOUTHWEST NEXT WEEK. SOME LIGHTER SNOW WILL SLIP EASTWARD ON  
THE NORTHERN SIDE OF THE PRECIPITATION SHIELD FARTHER EAST ACROSS  
THE HIGH 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 LIE  
ABOVE THE 99.5 PERCENTILE FROM NORTHEAST TX TO SOUTHERN INDIANA  
AND THE ECMWF EFI (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 TEMPERATURES HAVE A  
HIGH UPSIDE POTENTIAL NEAR RECORD HIGHS. COLD FRONT WILL BRING IN  
COOLER TEMPERATURES TOWARD THE EAST COAST BY NEXT THURSDAY OR SO,  
DEPENDING ON THE SPEED OF THE FRONT.  
 
FRACASSO/ROTH  
 

 
 
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