629  
FXUS02 KWBC 251526  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1026 AM EST THU JAN 25 2018  
 
VALID 12Z SUN JAN 28 2018 - 12Z THU FEB 01 2018  
 
16Z UPDATE...  
 
NO BIG CHANGES FROM PREVIOUS SHIFT, AND A DETERMINISTIC BLEND  
(GFS/ECMWF/UKMET/CANADIAN) SUFFICED FOR THE FIRST COUPLE DAYS OF  
THE FORECAST PERIOD (SUN/MON) BEFORE A TREND TOWARD THE ENSEMBLE  
MEANS (06Z GEFS AND 00Z ECMWF ENSEMBLE MEAN) WAS USED. THIS  
REMAINED CLOSE TO CONTINUITY. 06Z/00Z GFS AND 00Z ECMWF WERE CLOSE  
ENOUGH TO THE ENSEMBLE CONSENSUS TO USE INTO NEXT THURSDAY AS A  
PACIFIC SYSTEM MOVES OUT OF THE ROCKIES AND THROUGH THE PLAINS.  
THIS WILL BRING IN MUCH COLDER AIR TO THE NORTHERN TIER WITH NEAR  
ZERO HIGHS POSSIBLE FOR NORTHERN ND/MN NEXT THURSDAY.  
 
FRACASSO  
 
PREVIOUS DISCUSSION ISSUED AT 0638Z...  
 
   
..PATTERN OVERVIEW
 
 
PROGRESSIVE FLOW PATTERN EXPECTED TO REMAIN IN PLACE ACROSS THE  
CONUS, WITH TWO FAVORED REGIONS FOR SURFACE CYCLOGENESIS: ACROSS  
THE CENTRAL U.S. AND OFF THE EASTERN SEABOARD. BLOCKING RIDGE  
DEVELOPING ACROSS THE BERING SEA/EASTERN RUSSIA EARLY NEXT WEEK  
WILL SUPPORT POOLING OF FRIGID ARCTIC AIR ACROSS NORTH CENTRAL  
CANADA BETWEEN THE RIDGE AND THE HUDSON BAY UPPER LOW. BY THE  
MIDDLE OF NEXT WEEK, AS UPPER RIDGING ONCE AGAIN RETURNS OFF THE  
WEST COAST, THIS PATTERN WILL FAVOR SOUTHWARD TRANSPORT OF ARCTIC  
AIR INTO THE NORTH CENTRAL U.S.  
 
   
..MODEL GUIDANCE/UNCERTAINTY ASSESSMENT/PREFERENCES
 
 
SOLUTIONS HAVE CONTINUED TO CONVERGE WITH RESPECT TO THE UPPER  
TROUGH CROSSING THE GREAT LAKES/APPALACHIANS ON SUN, AND HAVE ALSO  
COME TO A CONSENSUS ON A MORE SUPPRESSED SOLUTIONS FOR THE WAVE OF  
LOW PRESSURE EXPECTED TO DEVELOP ALONG THE FRONT SUN NIGHT/MON,  
WITH MOST SOLUTIONS KEEPING THE LOW FARTHER OFF THE EASTERN  
SEABOARD RELATIVE TO THIS TIME LAST NIGHT. THE ONE EXCEPTION WAS  
THE 12Z UKMET (ALONG WITH A HANDFUL OF ECENS/CMCE MEMBERS) THAT  
PERSIST IN SHOWING A SURFACE LOW TRACKING FROM THE NORTHERN  
MID-ATLANTIC ACROSS SOUTHERN NEW ENGLAND. GIVEN THE SHIFT, AND THE  
DECREASING NUMBER OF ENSEMBLE MEMBERS SUPPORTING A MORE NORTHERLY  
LOW TRACK, THE FORECAST WAS ADJUSTED ACCORDINGLY.  
 
MODELS ALSO CONTINUE TO SHOW DECENT AGREEMENT WITH RESPECT TO  
SHORTWAVE/FRONT REACHING THE PAC NW BY MON, WITH RELATIVELY SUBTLE  
DIFFERENCES MAINLY WITH RESPECT TO TIMING. THESE TIMING  
DIFFERENCES INCREASE A BIT BY TUE-WED AS THE TROUGH CROSSES THE  
ROCKIES AND AMPLIFIES. A SLIGHT TREND TOWARD LESS AMPLIFICATION  
WAS NOTED COMPARED TO LAST NIGHT, ALTHOUGH IN GENERAL MODEL  
SOLUTIONS HAVE REMAINED FAIRLY CONSISTENT. THE GFS AND ECMWF  
SHOWED VERY SIMILAR SOLUTIONS EVEN BY LATE TUE AS THE SURFACE LOW  
EMERGES FROM THE CANADIAN ROCKIES AND SKIRTS ALONG THE U.S.  
CANADIAN BORDER, WITH THE TRAILING COLD FRONT AND PERHAPS ANOTHER  
TRANSIENT WAVE OF LOW PRESSURE ALONG IT CROSSING THE PLAINS.  
 
GIVEN IMPROVED MODEL SPREAD, THE WPC FORECAST WAS INITIALLY BASED  
ON A MULTI-MODEL DETERMINISTIC BLEND INCLUDING THE 12Z ECMWF/UKMET  
AND 18Z GFS DURING DAYS 3-5 (SUN-TUE). FROM DAY 6 ONWARD, A TREND  
WAS SHOWN TOWARD MAJORITY ECENS/GEFS MEAN WEIGHTING, ALTHOUGH  
SUFFICIENT CONSENSUS WAS SHOWN TO KEEP A SUBSTANTIAL PORTION OF  
ECMWF/GFS IN THE FORECAST THROUGH DAY 7 (THU).  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
HEAVY RAIN IS POSSIBLE ON SUN ACROSS PORTIONS OF THE SOUTHEASTERN  
U.S., AS GULF OF MEXICO MOISTURE SURGES NORTHWARD AHEAD OF THE  
COLD FRONT AND DEVELOPING WAVE OF LOW PRESSURE. MULTI-INCH  
RAINFALL TOTALS ARE POSSIBLE. FARTHER NORTH, RELATIVELY LIGHTER  
PRECIP TOTALS ARE EXPECTED WITH THE FRONTAL PASSAGE FROM THE  
MID-ATLANTIC TO THE NORTHEAST. SUFFICIENT COLD AIR WILL BE IN  
PLACE FROM PORTIONS OF THE LOWER GREAT LAKES TO NORTHERN NEW  
ENGLAND TO SUPPORT SNOW, OR A RAIN SNOW MIX. HIGH TEMPS WILL BE  
10-20 DEG F ABOVE AVERAGE ON SUN ACROSS MUCH OF THE EASTERN U.S.,  
AHEAD OF THE COLD FRONT, WITH COOLER TEMPERATURES (NEAR SEASONAL  
AVERAGES) ARRIVING BY MONDAY IN THE WAKE OF THE COLD FRONT.  
WARMING TEMPERATURES AR EXPECTED FOR MUCH OF THE EASTERN U.S.  
AGAIN BY MID TO LATE NEXT WEEK, AS SUBTROPICAL RIDGING BUILDS  
OVERHEAD AND WARM ADVECTION COMMENCES AHEAD OF THE NEXT CENTRAL  
U.S. FRONTAL SYSTEM.  
 
THE PACIFIC NORTHWEST WILL SEE HEAVY PRECIPITATION, ESPECIALLY  
SUN-TUE, AS AN AMPLIFYING UPPER TROUGH AND ASSOCIATED SURFACE COLD  
FRONT MOVE ASHORE. HEAVY RAIN WILL FALL NEAR THE COAST AND AND  
LOWER ELEVATIONS, WHILE SNOWFALL TOTALS MAY BE MEASURED IN FEET AT  
THE HIGHER ELEVATIONS OF THE CASCADES (PARTICULARLY IN WA).  
ONSHORE FLOW WILL PERSIST IN THE WAKE OF THIS SYSTEM, WITH THE  
POTENTIAL FOR HEAVY PRECIP PERSISTING, ESPECIALLY ACROSS THE  
CASCADES.  
 
RYAN  
 

 
 
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