333  
FXUS02 KWBC 210554  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1254 AM EST SUN JAN 21 2018  
 
VALID 12Z WED JAN 24 2018 - 12Z SUN JAN 28 2018  
   
..OVERVIEW
 
 
WAVY AND PROGRESSIVE UPPER FLOW IS EXPECTED TO PERSIST ACROSS THE  
CONUS THROUGH THE MEDIUM RANGE. A BLOCKING UPPER RIDGE EXPECTED TO  
DEVELOP FROM THE NORTH CENTRAL PACIFIC OCEAN TO THE BERING SEA  
WILL PROMOTE CONTINUED NEGATIVE HEIGHT ANOMALIES ACROSS THE GULF  
OF ALASKA. AN ENERGETIC SHORTWAVE IS FORECAST BY ALL GUIDANCE TO  
EJECT FROM THIS AREA OF PERSISTENT TROUGHING AND MOVE EASTWARD  
INTO THE CONUS. THE WAVE WILL AMPLIFY QUICKLY AS IT CROSSES THE  
ROCKIES AND MOVES INTO THE CENTRAL U.S., AS IT ENCOUNTERS  
INCREASING LOW-LEVEL BAROCLINICITY. THUS, EXPECT ANOTHER CENTRAL  
U.S. LOW PRESSURE SYSTEM BY LATE THIS WEEK INTO THE WEEKEND,  
CHARACTERIZED BY AN INTENSE THERMAL GRADIENT AND A WIDE ARRAY OF  
SENSIBLE WEATHER IMPACTS. MEANWHILE, SUBTROPICAL RIDGING IS  
EXPECTED TO BUILD NORTH ALONG THE EASTERN SEABOARD THROUGH THE  
WEEK, AND ALONG THE WEST COAST BY LATE IN THE WEEK.  
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT AND PREFERENCES
 
 
MODELS/ENSEMBLES SHOWED BETTER THAN AVERAGE LARGE SCALE AGREEMENT,  
AND THIS ALLOWED FOR SIGNIFICANT USE OF DETERMINISTIC SOLUTIONS  
THROUGH MOST OF THE FORECAST PERIOD. A BLEND OF THE 18Z GFS/12Z  
ECMWF SERVED AS A BASIS FOR THE FORECAST THROUGH DAY 5 (WITH THE  
ADDITION OF THE 12Z UKMET ON DAY 3). THESE SOLUTIONS REPRESENTED  
CONSENSUS WILL FOR THE DEEP CYCLONE AFFECTING NEW ENGLAND EARLY  
WED. SPREAD WAS SOMEWHAT HIGHER WITH RESPECT TO THE SURFACE LOW  
OFF THE PAC NW LATE WED-EARLY THU, BUT THE DESCRIBED BLEND ALSO  
REPRESENTED THE GUIDANCE CONSENSUS WELL.  
 
THE GFS/ECMWF AND THEIR RESPECTIVE ENSEMBLE MEANS CONTINUE TO SHOW  
RELATIVELY GOOD AGREEMENT AS THE UPPER TROUGH AMPLIFIES ACROSS THE  
FOUR CORNERS/ROCKIES THROUGH FRI, AND WITH THE DEVELOPMENT OF A  
SURFACE CYCLONE ACROSS THE HIGH PLAINS FRI. THE ECMWF WAS SLIGHTLY  
FASTER THAN THE GFS WITH MOVING THE SURFACE LOW NORTHEASTWARD INTO  
WESTERN ONTARIO. BUT THE ECENS/GEFS MEANS REMAINED VERY CLOSE.  
THUS, BY DAY 6 (SAT) A 50/50 BLEND OF DETERMINISTIC/ENSEMBLE MEANS  
WAS USED, TIPPING TO A SLIGHT MAJORITY ECENS/GEFS WEIGHTING BY DAY  
7 (SUN). ONE POINT OF CONTENTION WAS HOW STRONG THE EVENTUAL  
RIDGING ALONG THE WEST COAST WILL BE, AND HOW FAR NORTH THE AXIS  
EXTENDS. AT THIS TIME, A BLENDED/COMPROMISE SOLUTION APPEARS BEST.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
THE APPROACH OF A COUPLE FRONTAL SYSTEMS ALONG WITH CONTINUED  
ONSHORE FLOW WILL KEEP THE PACIFIC NORTHWEST AS WELL AS THE GREAT  
BASIN AND PORTIONS OF THE NORTHERN ROCKIES WET THROUGH THE PERIOD.  
HEAVY RAIN AND MOUNTAIN SNOW WILL BE POSSIBLE. FARTHER EAST, A  
COLD FRONT WILL BRING SHOWERS AND THUNDERSTORMS TO SOUTH FLORIDA  
WED-THU. AS THE UPPER TROUGH EXITS THE ROCKIES AND A NEW SURFACE  
CYCLONE TAKES SHAPE ACROSS THE CENTRAL U.S. LATE IN THE WEEK,  
EXPECT INCREASING GULF OF MEXICO MOISTURE TO EVENTUALLY PRODUCE  
WIDESPREAD PRECIPITATION FROM THE GULF COAST AND SOUTHEAST TO THE  
GREAT LAKES BY LATE FRI INTO THE WEEKEND. MODELS SUGGEST  
SUFFICIENT MOISTURE RETURN TO POTENTIALLY SUPPORT THUNDERSTORMS  
AND HEAVY RAINFALL FROM PORTIONS OF THE TN/OH VALLEYS TO THE GULF  
COAST AND SOUTHEAST. ON THE NORTHERN SIDE OF THE SYSTEM, ANOTHER  
ROUND OF SNOW IS POSSIBLE ACROSS THE MIDWEST AND UPPER GREAT LAKES.  
 
THE BUILDING SUBTROPICAL RIDGE ALONG THE EAST COAST ALONG WITH  
WARM AIR ADVECTION BY MID/LATE WEEK AHEAD OF THE DEVELOPING  
CENTRAL U.S. LOW PRESSURE WILL RESULT IN ABOVE AVERAGE  
TEMPERATURES ACROSS MUCH OF THE CENTRAL/EASTERN U.S., CONTINUING  
TO RISE THROUGH LATE IN THE WEEK. HIGH TEMPS OF 10-20 DEG F WILL  
BE POSSIBLE BY MID/LATE WEEK ACROSS A LARGE AREA. MEANWHILE, BELOW  
AVERAGE TEMPS (5-10 DEG F) WILL ACCOMPANY THE UPPER TROUGH ACROSS  
THE WESTERN U.S. THU-FRI.  
 
RYAN  
 

 
 
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