956  
FXUS02 KWBC 230620  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
120 AM EST FRI FEB 23 2018  
 
VALID 12Z MON FEB 26 2018 - 12Z FRI MAR 02 2018  
   
..PATTERN OVERVIEW
 
 
THE SUBTROPICAL RIDGE IS EXPECTED TO SHIFT A BIT WESTWARD DURING  
THE MEDIUM RANGE. THE STRONGEST POSITIVE HEIGHT ANOMALIES  
ASSOCIATED WITH THE FEATURE, INITIALLY CENTERED OFF THE EAST  
COAST, WILL SHIFT INTO THE GULF OF MEXICO BY EARLY NEXT WEEK, WITH  
A RIDGE AXIS ATTEMPTING TO EXERT INFLUENCE FARTHER NORTH ACROSS  
THE SOUTHERN U.S. AND PERHAPS THE TN/OH VALLEYS. MEANWHILE,  
PERSISTENT UPPER RIDGING ACROSS THE NORTH PACIFIC WILL CONTINUE TO  
FAVOR TROUGHING ACROSS THE WEST. THE COMBINATION OF CONTINUED  
WESTERN TROUGHING AND GULF OF MEXICO RIDGING WILL FAVOR A  
RELATIVELY ACTIVE WEATHER PATTERN ACROSS THE CENTRAL AND EASTERN  
U.S., WITH THE POTENTIAL FOR ADDITIONAL AREAS OF HEAVY RAINFALL.  
 
   
..MODEL GUIDANCE/UNCERTAINTY ASSESSMENT/PREFERENCES
 
 
IN GENERAL, THE MODELS/ENSEMBLES SHOWED DECENT AGREEMENT WITH  
RESPECT TO THE LARGE SCALE PATTERN THROUGHOUT THE PERIOD. FOR THE  
MOST PART, SIGNIFICANT DIFFERENCES AMONG THE GUIDANCE WERE MATTERS  
OF TIMING. FIRST SIGNIFICANT TIMING ISSUE ARISES BY TUE-WED, AND  
RESULTS FROM UNCERTAINTY SURROUNDING HOW QUICKLY THE TROUGH/UPPER  
LOW OVER THE SOUTHWEST OPENS UP AND MOVES EAST. THE ECMWF HAS  
SHOWN A FAIR DEGREE OF RUN-TO-RUN VARIABILITY WITH THE 00Z RUN  
LAST NIGHT SHOWING A CLOSED UPPER LOW LINGERING FOR LONGER ACROSS  
THE WEST, RESULTING IN MUCH SLOWER TIMING FROM TUE-WED ONWARD. THE  
12Z ECMWF, ON THE OTHER HAND, TRANSITIONS THE FEATURE INTO AN OPEN  
WAVE, AND MOVES IT EASTWARD MORE QUICKLY RELATIVE TO THE PREVIOUS  
RUN. THE GFS HAS BEEN A BIT MORE CONSISTENT WITH WEAKENING THE  
UPPER LOW/MOVING THE SYSTEM EASTWARD A BIT MORE QUICKLY. ENSEMBLE  
SPREAD IS RATHER HIGH, AND COVERS THE ENTIRE RANGE OF  
POSSIBILITIES. THESE DIFFERENCES TRANSLATE DOWNSTREAM TO THE  
MIDWEST/OH VALLEY BY THU, AND IMPACT THE DEGREE OF PHASING THAT  
BEGINS TO OCCUR WITH AN AREA OF RELATIVELY WEAK NORTHERN STREAM  
ENERGY. THE GFS HAS BEEN CONSISTENT IN DEVELOPING A RELATIVELY  
STRONG SURFACE LOW ACROSS THE GREAT LAKES/OH VALLEY BY WED  
NIGHT/THU, WHILE THE ECMWF HAS VARIED SIGNIFICANTLY FROM  
RUN-TO-RUN, WITH THE LATEST RUN DEVELOPING A MUCH WEAKER LOW  
RELATIVE TO THE GFS AND KEEPING A MORE OPEN FRONTAL WAVE.  
UNCERTAINTY INCREASES FURTHER BY FRI AS ADDITIONAL PHASING COULD  
OCCUR ACROSS THE NORTHEASTERN U.S., WITH SOME SOLUTIONS SUGGESTING  
SURFACE LOW DEVELOPMENT NEAR OR OFF THE EAST COAST. BY THIS TIME  
PERIOD BOTH THE ECMWF AND GFS HAVE VARIED SUBSTANTIALLY FROM  
RUN-TO-RUN, AND CONFIDENCE IN THE FORECAST DETAILS IS LOW. FARTHER  
WEST, MODELS ARE IN GENERAL AGREEMENT ON ANOTHER SHORTWAVE DIGGING  
SOUTHWARD ALONG THE WEST COAST AND AMPLIFYING INTO A LARGE TROUGH  
OR PERHAPS ANOTHER UPPER LOW.  
 
THE WPC FORECAST WAS INITIALLY BASED HEAVILY ON DETERMINISTIC  
GUIDANCE DURING DAYS 3-4, WITH THE 12Z ECMWF/GFS/CMC MAKING UP THE  
MAJORITY OF THE BLEND. AFTER THAT TIME, DUE TO INCREASING  
UNCERTAINTY, ENSEMBLE MEAN (12Z ECENS/NAEFS) WEIGHTING WAS  
GRADUALLY BOOSTED THROUGH TIME, WITH MAJORITY ENSEMBLE MEANS  
DURING DAYS 6-7.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
WIDESPREAD CONVECTION AND POTENTIALLY HEAVY RAINS WILL ONCE AGAIN  
POSE A THREAT ACROSS PORTIONS OF THE LOWER MS VALLEY, TN VALLEY,  
AND SOUTHEAST FROM LATE TUE THROUGH THU NIGHT AS LOW-LEVEL  
SOUTHERLY FLOW SPREADS AMPLE MOISTURE NORTHWARD, AND A WEAK  
REMNANT SURFACE BOUNDARY PROVIDES A FOCUS FOR CONVECTIVE  
DEVELOPMENT. FARTHER NORTH, ANY POTENTIAL LOW PRESSURE SYSTEMS  
TRACKING ACROSS THE CENTRAL PLAINS/MIDWEST/GREAT LAKES WOULD BRING  
CHANCES OF SNOWFALL TO PORTIONS OF THOSE REGIONS, ALTHOUGH  
CONFIDENCE IS LOW IN THE SPECIFICS AT THIS POINT GIVEN THE TIME  
FRAME AND THE DESCRIBED MODEL SPREAD. PERSISTENT TROUGHING ALONG  
THE WEST COAST WILL KEEP CONDITIONS UNSETTLED THERE, WITH THE  
THREAT FOR HEAVY PRECIPITATION INCREASING BY MID TO LATE NEXT WEEK  
AS THE NEXT TROUGH/UPPER LOW DIGS ALONG THE WEST COAST. HEAVY  
PRECIPITATION IS POSSIBLE FROM THE CASCADES SOUTHWARD THROUGH MUCH  
OF CALIFORNIA AS A COLD FRONT PUSHES INLAND. BROAD POSITIVE HEIGHT  
ANOMALIES WILL KEEP TEMPERATURES ABOVE AVERAGE FOR MUCH OF THE  
CENTRAL/EASTERN U.S., WITH HIGHS REACHING DOUBLE DIGITS ABOVE  
AVERAGE ACROSS PORTIONS OF THE MID-MS VALLEY/MIDWEST BY TUE.  
MEANWHILE, PERSISTENT TROUGHING AND PRECIPITATION WILL KEEP  
TEMPERATURES BELOW AVERAGE ACROSS MUCH OF THE WEST THROUGH NEXT  
WEEK.  
 
RYAN  
 

 
 
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