385  
FXUS02 KWBC 080651  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
150 AM EST THU FEB 08 2018  
 
VALID 12Z SUN FEB 11 2018 - 12Z THU FEB 15 2018  
   
..OVERVIEW AND GUIDANCE/UNCERTAINTY ASSESSMENT
 
 
TODAY'S GUIDANCE MAINTAINS THE SIMILAR THEMES FROM RECENT DAYS  
WITH RESPECT TO THE OVERALL PATTERN AND UNCERTAINTIES. MODELS AND  
MEANS AGREE UPON A PERSISTENT EASTERN PACIFIC RIDGE AND A  
POSITIVELY TILTED MEAN TROUGH COVERING A MAJORITY OF NORTH  
AMERICA. TO THE SOUTHEAST OF THE MEAN TROUGH, CONSENSUS STILL  
SHOWS AN INITIAL WESTERN ATLANTIC RIDGE RETROGRADING FROM EAST OF  
THE BAHAMAS TOWARD THE GULF OF MEXICO. FORECAST UNCERTAINTIES  
INVOLVE THE EVOLUTION OF INDIVIDUAL SHORTWAVES PROGRESSING AROUND  
THE EAST PACIFIC RIDGE AND FEEDING INTO THE MEAN TROUGH, AS WELL  
AS DETAILS OF CANADIAN/NORTHERN TIER U.S. FLOW AROUND THE  
PERIPHERY OF A BROAD UPPER LOW ANCHORED OVER NORTHEASTERN CANADA.  
 
THE UPDATED FORECAST MAINTAINED FAIRLY GOOD CONTINUITY BY WAY OF A  
70/30 MODEL/ENSEMBLE MEAN BLEND (12Z ECMWF/UKMET/CMC AND 18Z  
GEFS/12Z ECMWF MEANS) EARLY IN THE PERIOD FOLLOWED BY A RAPID  
TRANSITION TO ENSEMBLE MEANS EXCLUSIVELY BY DAYS 6-7 WED-THU. FOR  
THAT TIME FRAME THE FORECAST INCORPORATED HALF ECMWF MEAN AND ONE  
QUARTER EACH 18Z GEFS/12Z NAEFS MEANS.  
 
THERE WERE TWO AREAS WHERE 12Z AND/OR 18Z GFS RUNS DIFFERED TOO  
MUCH FROM REMAINING GUIDANCE TO INCLUDE IN THE DETERMINISTIC  
FORECAST. WITHIN THE CANADIAN/NORTHERN TIER STREAM, THE 18Z GFS  
BECAME SLOWER THAN CONSENSUS WITH A LEADING SHORTWAVE WITH  
CORRESPONDINGLY SLOW PROGRESSION OF EASTERN U.S. LOW  
PRESSURE/FRONTAL EVOLUTION. THEN IT BECAME QUITE AMPLIFIED WITH A  
TRAILING SHORTWAVE, BY EARLY DAY 5 TUE LEADING TO A PRONOUNCED  
SHORTWAVE WHERE NEARLY ALL OTHER GUIDANCE INCLUDING THE GEFS MEAN  
SHOWED SOUTHWESTERLY FLOW ALOFT. MEANWHILE BOTH GFS RUNS DEPICTED  
A COMPACT CLOSED LOW REACHING THE WEST COAST BY SUN NIGHT-EARLY  
MON WHILE OTHER SOLUTIONS DEPICTED SOME VARIATION OF BROADER  
TROUGHING. BASED ON NEW 00Z GUIDANCE IN THUS FAR, THE 00Z GFS  
SHOWS SOME FAVORABLE TRENDS ESPECIALLY IN THE NORTHERN STREAM.  
WEST COAST DETAILS ARE STILL A QUESTION MARK BUT AT LEAST IT HAS  
OPENED UP ITS WEST COAST TROUGH AFTER EARLY SUN.  
 
AS WESTERN TROUGH ENERGY SETTLES OVER/NEAR THE SOUTHWEST, RECENT  
TRENDS SHOWN BY MODELS AND HINTED AT BY THE ENSEMBLE MEANS HAVE  
BEEN TOWARD GREATER POTENTIAL FOR AN EMBEDDED LOW TO CLOSE OFF BY  
NEXT TUE-THU. THIS TREND CORRESPONDS TO A FLATTER/NORTHWARD  
ADJUSTMENT FOR THE NEXT UPSTREAM BUNDLE OF SHORTWAVE ENERGY  
ROUNDING THE EAST PACIFIC RIDGE. GIVEN THAT THIS CLOSED LOW  
SCENARIO IS RELATIVELY NEW, PLUS THE HIGH SPREAD/VARIABILITY THAT  
MODELS/INDIVIDUAL ENSEMBLE MEMBERS FROM RECENT DAYS HAVE HAD FOR  
INDIVIDUAL FEATURES WITHIN THIS PATTERN, THE ENSEMBLE MEANS SEEM  
TO PROVIDE THE BEST STARTING POINT AT THIS TIME. THE DISPARITY  
BETWEEN RECENT GFS RUNS (OFFSHORE CLOSED LOW) AND 00Z CMC THAT  
CHANGED TO A PROGRESSIVE/OPEN SHORTWAVE HIGHLIGHTS THE UNCERTAINTY  
IN THIS ASPECT OF THE FORECAST. CONSENSUS EVOLUTION OF ENERGY  
MOVING INTO THE WEST WOULD SUPPORT A COLD FRONT CROSSING THE  
CENTRAL-EASTERN STATES BY EARLY TO MIDDLE PART OF NEXT WEEK.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
SHORTWAVE ENERGY MOVING INTO AND EVOLVING OVER THE WEST WILL  
SUPPORT AREAS OF MOSTLY LIGHT-MODERATE PRECIP, FIRST OVER THE  
PACIFIC NORTHWEST AND NORTHERN ROCKIES THEN PUSHING SOUTHWARD WITH  
TIME. INCREASING GUIDANCE SPREAD FOR SPECIFICS OF FLOW ALOFT  
LEADS TO DECLINING CONFIDENCE IN PRECISE COVERAGE/INTENSITY OF  
PRECIP ESPECIALLY OVER THE CENTRAL/SOUTHERN PARTS OF THE WEST.  
EXPECT RAIN OVER COASTAL/LOW ELEVATION AREAS AND SNOW AT HIGHER  
ELEVATIONS. ANOTHER AREA OF LIGHT PRECIP MAY BRUSH NORTHERN AREAS  
AROUND MIDWEEK. SLIGHTLY BELOW TO MODERATELY ABOVE NORMAL TEMPS  
WILL LIKELY TREND WARMER BY WED-THU AS HEIGHTS ALOFT RISE.  
 
GUIDANCE HAS BEEN CONSISTENT IN SUGGESTING THAT THE MEAN PATTERN  
WILL SUPPORT A RELATIVELY WET PERIOD OVER THE SOUTH AND EAST WITH  
AREAS FROM THE SOUTHEASTERN PLAINS THROUGH SOUTHEAST/MID ATLANTIC  
HAVING THE HIGHEST POTENTIAL FOR ONE OR MORE PERIODS OF MODERATE  
TO HEAVY RAINFALL. ONE AREA OF ORGANIZED RAIN WITH A WAVY FRONT  
SHOULD TAPER OFF AFTER SUN. FURTHER ACTIVITY IS LIKELY TO DEVELOP  
BY TUE-THU WITH THE INITIAL FRONT LINGERING NEAR GULF/SOUTHEAST  
COASTS AND THE NEXT FRONT APPROACHING FROM THE PLAINS. EXPECT  
MOST SNOW THAT OCCURS OVER THE EAST TO BE CONFINED TO PORTIONS OF  
THE GREAT LAKES AND NEW ENGLAND. THE SOUTHEAST WILL SEE THE MOST  
PERSISTENT WARMTH DURING THE PERIOD WITH CLOUDS/MOISTURE ENHANCING  
ANOMALIES FOR MORNING LOWS IN PARTICULAR. THIS INCLUDES SOME  
READINGS 20-30F ABOVE NORMAL ON SUN. THE PLAINS AND MID-UPPER MS  
VALLEY WILL START THE PERIOD ON THE COLD SIDE WITH TEMPS 10-20F  
BELOW NORMAL. THE COLD SHOULD PERSIST OVER THE NORTHERN PLAINS  
INTO WED WITH MOST OF THE CENTRAL U.S. WITHIN A FEW DEGREES ON  
EITHER SIDE OF NORMAL BY THU.  
 
RAUSCH  
 

 
 
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