622  
FXUS02 KWBC 211443  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
942 AM EST WED FEB 21 2018  
 
VALID 12Z SAT FEB 24 2018 - 12Z WED FEB 28 2018  
 
DUE TO TECHNICAL DIFFICULTIES, SOME PRODUCTS MAY NOT BE AVAILABLE  
OR UPDATED. PLEASE CHECK ISSUANCE TIMES. WE APOLOGIZE FOR ANY  
INCONVENIENCE.  
 
   
..PATTERN OVERVIEW
 
 
MOST GUIDANCE MAINTAINS AN AMPLIFIED MEAN PATTERN CONSISTING OF  
SHORTWAVES PERIODICALLY DIGGING INTO A WEST COAST/INTERIOR WEST  
MEAN TROUGH AND THEN EJECTING AROUND A DOWNSTREAM RIDGE WHOSE  
SHAPE WILL EVOLVE WITH TIME. THE MOST COMMON SCENARIO AMONG  
LARGEST GUIDANCE IS FOR A LEADING SHORTWAVE EJECTING FROM THE  
ROCKIES/HIGH PLAINS AROUND THE START OF THE WEEKEND TO GENERATE A  
POTENTIALLY MAJOR STORM REACHING THE GREAT LAKES BY SAT  
NIGHT/EARLY SUN, FOLLOWED BY A FAIRLY WEAK/DIFFUSE AND PROGRESSIVE  
SHORTWAVE, AND FINALLY DEEPER WESTERN TROUGH AMPLIFICATION NEXT  
MON-WED. THIS EVOLUTION/PROGRESSION WILL MOST LIKELY LEAD TO  
STEADY EROSION OF INITIALLY STRONG RIDGING ALONG/OFFSHORE THE EAST  
COAST FOLLOWED BY A SHIFT OF RIDGE EMPHASIS INTO THE GULF OF  
MEXICO AND THEN SOME REBUILDING OF A RIDGE OVER THE EAST-CENTRAL  
U.S. BY MIDWEEK AS WESTERN TROUGHING AMPLIFIES.  
 
   
..MODEL GUIDANCE/UNCERTAINTY ASSESSMENT/PREFERENCES
 
 
ONE SIGNIFICANT FOCUS OF THE FORECAST WILL BE ON THE STORM SYSTEM  
ASSOCIATED WITH THE SHORTWAVE EMERGING FROM THE ROCKIES/HIGH  
PLAINS AT THE START OF THE PERIOD EARLY SAT. THERE IS STILL A  
MODERATE AMOUNT OF SPREAD FOR EXACTLY HOW THIS SYSTEM WILL EVOLVE  
BUT MOST MODEL/ENSEMBLE SOLUTIONS HAVE TRENDED NOTICEABLY DEEPER  
VERSUS 24 HOURS AGO, WHILE THE GFS CONTINUES TO BE THE DEEPEST  
MODEL. THIS DEEPER TREND WOULD MERIT SOME INCORPORATION OF THE  
00Z/06Z GFS SCENARIO INTO THE FORECAST AS PART OF A MODEL  
COMPROMISE BUT OCCASIONALLY DEEP BIASES STILL LEAVE SOME LINGERING  
DOUBT IN GOING AS DEEP AS THE GFS. THERE ARE STILL INDICATIONS  
THAT A WEAK WAVE WILL FORM OFF THE NORTHEAST COAST AS THE PARENT  
LOW TRACKS OVER SOUTHEAST CANADA. SPECIFICS OF THIS WAVE WILL  
PLAY AN IMPORTANT ROLE IN HOW LONG COOL AIR REMAINS OVER NEW  
ENGLAND.  
 
THERE IS DECENT CONTINUITY FROM YESTERDAY THAT A TRAILING  
SHORTWAVE REACHING THE NORTHWEST DURING THE WEEKEND SHOULD BE  
FAIRLY WEAK AND PROGRESSIVE, WITH AT MOST A MODEST SURFACE  
REFLECTION AS THE FEATURE CONTINUES ACROSS LOWER 48.  
 
THE OTHER MEANINGFUL UNCERTAINTY IN TODAY'S FORECAST INVOLVES  
EASTERN PACIFIC/WESTERN U.S. EVOLUTION TOWARD THE LATTER HALF OF  
THE PERIOD, WITH MODELS AND TO A GREATER EXTENT INDIVIDUAL  
ENSEMBLE MEMBERS DISPLAYING BROADENING SPREAD FOR COMBINED  
EVOLUTION OF TROUGH ENERGY DIGGING INTO THE WEST AND  
UPSTREAM/INFLUENCING ENERGY REACHING THE NORTHEAST PACIFIC. BEST  
MODEL/MEAN CONSENSUS (ECMWF/CMC AND GEFS/ECMWF/CMC MEANS) HAS A  
FAIRLY SHARP AND DEEP UPPER TROUGH REACHING JUST INLAND FROM THE  
WEST COAST BY DAYS 6-7 TUE-WED, WITH INDICATIONS THAT THERE COULD  
BE AN EMBEDDED CLOSED LOW. THIS MAJORITY SOLUTION IS A LITTLE  
FASTER THAN THE 00Z GFS BUT SLOWER THAN THE 06Z RUN.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE DEEPENING STORM SYSTEM TRACKING FROM NEAR THE CENTRAL PLAINS  
THROUGH THE GREAT LAKES INTO SOUTHEAST CANADA DURING THE WEEKEND  
WILL BRING A BROAD AREA OF ACTIVE WEATHER TO THE CENTRAL AND  
EASTERN U.S. AHEAD OF THE FRONT TRAILING FROM THE SURFACE LOW,  
EXPECT AREAS OF HEAVY RAINFALL INCLUDING OVER PORTIONS OF THE  
MS/OH VALLEY REGION THAT HAS ALREADY EXPERIENCED  
CONSIDERABLE/EXCESSIVE RAINFALL. THUS FLOODING CONCERNS WILL  
CONTINUE. SOME CONVECTION MAY ALSO BE STRONG TO SEVERE WITH SPC  
CURRENTLY HIGHLIGHTING SUCH POTENTIAL FROM EXTREME NORTHEAST TX  
INTO WESTERN TN AND VICINITY. CONSULT LATEST SPC OUTLOOKS FOR  
LATEST INFO. MEANWHILE BEST POTENTIAL FOR COLD SECTOR SNOW WILL  
EXTEND FROM THE NORTH-CENTRAL PLAINS THROUGH THE UPPER MS  
VALLEY/UPPER GREAT LAKES AND INTO NEW ENGLAND, WITH A TRANSITION  
ZONE/CHANGEOVER POSSIBLE BETWEEN THE SNOW AND RAIN AREAS. STRONG  
WINDS MAY ALSO BECOME A CONCERN DEPENDING ON HOW THE SYSTEM  
DEVELOPS. AFTER SYSTEM PASSAGE RAINFALL OF VARYING INTENSITY MAY  
LINGER FOR A TIME OVER THE SOUTHERN TIER/SOUTHERN MID ATLANTIC.  
 
OVER THE WEST, INITIALLY EXPECT SOME COASTAL/LOW ELEVATION RAIN  
AND OTHERWISE SNOW FROM THE PACIFIC NORTHWEST/NORTHERN ROCKIES  
INTO THE GREAT BASIN/SOUTH-CENTRAL ROCKIES FOLLOWED BY ANOTHER  
PERIOD OF PRECIP THAT SHOULD EVENTUALLY PUSH SOUTHWARD FROM  
NORTHERN AREAS INTO THE GREAT BASIN AND CA. GUIDANCE SPREAD FOR  
DETAILS ALOFT IS STILL SUFFICIENTLY HIGH TO TEMPER CONFIDENCE IN  
DETERMINING COVERAGE AND INTENSITY OF ACTIVITY OVER CA/GREAT  
BASIN. HOWEVER THERE IS ENOUGH AGREEMENT IN PRINCIPLE THAT UPPER  
TROUGH DEPTH MAY SUPPORT FAIRLY LOW SNOW LEVELS OVER SOME AREAS BY  
NEXT MON-WED.  
 
THE AMPLIFIED PATTERN WILL MAINTAIN A SHARP CONTRAST IN  
TEMPERATURE ANOMALIES OVER THE LOWER 48. DURING THE WEEKEND SOME  
AREAS IN THE EAST MAY SEE MINS OF 20-30F ABOVE NORMAL FOR  
POTENTIAL DAILY RECORDS AND DAYTIME HIGHS 15-20F ABOVE NORMAL.  
LESS EXTREME WARM ANOMALIES SHOULD PERSIST OVER THE EAST INTO NEXT  
WEEK. ON THE OTHER HAND MUCH OF THE WEST INTO THE NORTHERN PLAINS  
SHOULD REMAIN WELL BELOW NORMAL DURING THE FORECAST PERIOD. THE  
NORTHERN HIGH PLAINS SHOULD BE COLDEST WITH MAX TEMPS AVERAGING  
15-20F BELOW NORMAL BUT MANY OTHER LOCATIONS OVER THE  
WEST/NORTHERN PLAINS SHOULD SEE ONE OR MORE DAYS WITH MIN AND/OR  
MAX TEMPS AT LEAST 10F BELOW NORMAL AS WELL.  
 
RAUSCH  
 

 
 
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