344  
FXUS02 KWBC 070650  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
150 AM EST WED FEB 07 2018  
 
VALID 12Z SAT FEB 10 2018 - 12Z WED FEB 14 2018  
   
..OVERVIEW AND GUIDANCE/UNCERTAINTY ASSESSMENT
 
 
MOST GUIDANCE IS FAIRLY CONSISTENT AND AGREEABLE IN DEPICTING A  
POSITIVELY TILTED MEAN TROUGH ALOFT OVER THE LOWER 48, BETWEEN A  
PERSISTENT EAST PACIFIC RIDGE AND WESTERN ATLANTIC RIDGE THAT  
SHOULD GRADUALLY BUILD ACROSS THE BAHAMAS/CUBA. PRIMARY FORECAST  
CHALLENGES CONTINUE TO INVOLVE INDIVIDUAL SHORTWAVES ROUNDING THE  
PACIFIC RIDGE AND PROGRESSING INTO THE MEAN TROUGH, WHILE DETAILS  
OF CANADIAN/NORTHERN TIER U.S. FLOW WILL ADD SOME UNCERTAINTY TO  
THE FORECAST AS WELL.  
 
AFTER MULTIPLE DAYS OF WIDELY VARYING SOLUTIONS AND SIGNIFICANT  
RUN TO RUN VARIABILITY, IT APPEARS GUIDANCE IS STARTING TO LATCH  
ONTO A MORE AGREEABLE SOLUTION FOR LEADING SHORTWAVE ENERGY OVER  
THE WEST AS OF THE START OF THE PERIOD EARLY SAT. MOST SOLUTIONS  
FROM THE 12Z-18Z CYCLE ADJUSTED TOWARD A MORE OPEN/PROGRESSIVE  
DEPICTION VERSUS YESTERDAY'S 00Z RUNS, AND NEW 00Z RUNS IN SO FAR  
APPEAR TO PROVIDE REASONABLE CONTINUITY. WITHIN THE FULL RANGE OF  
EARLIER GUIDANCE THIS EVOLUTION IS A LITTLE CLOSER TO RECENT ECMWF  
RUNS THAT WERE QUITE PROGRESSIVE VERSUS A FEW GFS AND OTHER MODEL  
RUNS THAT HAD A CLOSED LOW CROSSING PORTIONS OF THE SOUTHWEST/CA.  
EASTWARD PROGRESSION OF THIS ENERGY COMBINED WITH NORTHERN TIER  
DYNAMICS WILL SUPPORT GRADUALLY INCREASED DEFINITION OF EASTERN  
U.S. FRONTAL WAVINESS THAT SHOULD REACH THE CANADIAN MARITIMES BY  
AROUND SUN NIGHT/EARLY MON. THE 12Z ECMWF WAS A BIT ON THE SLOWER  
SIDE OF THE ENVELOPE FOR THIS SYSTEM.  
 
THE NEXT SHORTWAVE IN THE SERIES SHOULD MOVE INTO THE WEST BY  
SUN-MON WITH A PORTION OF THE ENERGY CONTINUING ACROSS THE  
NORTHERN TIER AND THE REST DIGGING SOUTHWARD THROUGH THE WEST INTO  
A SHARPENING TROUGH. AT LEAST OVER THE PAST COUPLE DAYS THERE HAS  
BEEN BETTER THAN AVERAGE CLUSTERING AMONG MOST MODELS/MEANS FOR  
THE EVOLUTION OF THIS ENERGY OVER THE WEST INTO TUE-WED. THEN  
THERE IS A REASONABLE MODEL/ENSEMBLE SIGNAL FOR ANOTHER SHORTWAVE  
NEARING/REACHING THE NORTHWEST BY NEXT WED. REGARDLESS OF CURRENT  
CLUSTERING OF FEATURES MID-LATE PERIOD, RECENT MODEL BEHAVIOR  
SUGGESTS THAT AT LEAST A MEDIUM AMOUNT OF UNCERTAINTY REMAINS.  
 
THE EARLY PART OF THE FORECAST EMPHASIZED AN OPERATIONAL MODEL  
BLEND (18Z GFS AND 12Z ECMWF/UKMET/CMC) TO REFLECT LATEST  
CONSENSUS IDEAS--INCLUDING A NORTHEASTERN U.S. WAVE SOMEWHAT  
FASTER THAN THE 12Z ECMWF. THE MID-LATE PART OF THE PERIOD  
TRANSITIONED TO A BLEND OF 18Z GFS/12Z ECMWF AND THEIR RESPECTIVE  
ENSEMBLE MEANS AS DETAIL QUESTIONS INCREASE BUT WITH THE  
OPERATIONAL RUNS REMAINING SIMILAR ENOUGH IN PRINCIPLE TO MAINTAIN  
IN THE BLEND.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
ACROSS THE WEST EXPECT AN AREA OF SNOW TO DESCEND ALONG THE  
NORTHERN-CENTRAL ROCKIES EARLY IN THE WEEKEND. TRAILING PACIFIC  
SHORTWAVES ALOFT SHOULD BRING A COUPLE PERIODS OF PRECIP FROM THE  
PACIFIC NORTHWEST EASTWARD/SOUTHEASTWARD THROUGH THE ROCKIES. A  
MAJORITY OF THIS ACTIVITY SHOULD BE OF LIGHT-MODERATE INTENSITY  
AND IN THE FORM OF SNOW ASIDE FROM COASTAL/LOW ELEVATION RAIN IN  
THE NORTHWEST. EXPECT TEMPS TO BE NEAR TO ABOVE NORMAL AT MOST  
LOCATIONS WITH WARMEST ANOMALIES POSSIBLE NEXT WED AS HEIGHTS  
ALOFT BRIEFLY RISE BETWEEN SHORTWAVES.  
 
FARTHER EAST THE OVERALL PATTERN WILL PROMOTE A WET PERIOD FOR THE  
SOUTHEASTERN THIRD TO HALF OF THE LOWER 48. THE GREATEST COVERAGE  
OF MODERATE TO HEAVY RAINFALL SHOULD BE DURING THE WEEKEND AS  
GULF/ATLANTIC INFLOW INTERACTS WITH ONE OR TWO WAVY FRONTS OVER  
THE EASTERN STATES. HIGHEST TOTALS ARE MOST LIKELY TO BE WITHIN  
AN AREA FROM THE GULF COAST INTO PARTS OF THE MID ATLANTIC. THERE  
MAY BE A BRIEF DRYING TREND AFTER THE WEEKEND EVENT BUT  
PERSISTENCE OF SOUTHWESTERLY FLOW ALOFT THROUGH THE MIDDLE OF NEXT  
WEEK AND POSSIBLE APPROACH OF ANOTHER FRONT WILL LIKELY PROMOTE  
ADDITIONAL WAVES OF MOISTURE/RAINFALL OF VARYING INTENSITY WITHIN  
THE SAME GENERAL AREA. TO THE NORTHWEST OF THE RAINFALL AREA, THE  
BEST POTENTIAL FOR ONE OR MORE PERIODS OF SNOW EXTENDS FROM THE  
CENTRAL PLAINS THROUGH THE GREAT LAKES AND NORTHERN NEW ENGLAND.  
COLD TEMPS OVER THE PLAINS DURING THE WEEKEND, INCLUDING SOME  
HIGHS 20-30F BELOW NORMAL ON SAT, WILL MODERATE CONSIDERABLY  
THEREAFTER. THE WET PATTERN OVER THE SOUTH AND EAST SHOULD LEAD  
TO ABOVE AVERAGE TEMPS. EXPECT MINS TO BE PARTICULARLY WARM WITH  
ANOMALIES OF PLUS 10-20F MOST DAYS AND POSSIBLY AS HIGH AS PLUS  
20-30F AT SOME LOCATIONS ON SUN.  
 
RAUSCH  
 

 
 
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