526  
FXUS02 KWBC 031558  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1058 AM EST SAT FEB 3 2018  
 
VALID 12Z TUE FEB 6 2018 - 12Z SAT FEB 10 2018  
 
16Z UPDATE:  
 
THE LARGE SCALE SYNOPTIC PATTERN CONTINUES TO SHOW AN EASTERN  
PACIFIC RIDGE AND A CENTRAL-EASTERN U.S. UPPER LEVEL TROUGH WITH  
PERIODIC COLD FRONTAL PASSAGES AND A STATIONARY FRONT NEAR THE  
CONTINENTAL DIVIDE. THE MAJORITY OF THE MODEL SOLUTIONS ARE ON  
BOARD WITH THIS GENERAL IDEA, EXCEPT THAT THE 00Z CMC IS FAVORING  
AN UPPER LEVEL LOW SINKING SOUTHWARD ACROSS CALIFORNIA BY THE END  
OF THE FORECAST PERIOD AS PART OF A REX BLOCK PATTERN OVER THE  
EASTERN PACIFIC. THE DETERMINISTIC GFS AND ECMWF, ALONG WITH  
THEIR RESPECTIVE ENSEMBLE MEANS, KEEP THIS SHORTWAVE ENERGY MUCH  
FARTHER TO THE EAST ACROSS THE ROCKIES AND INTERMOUNTAIN WEST, AND  
THESE SOLUTIONS ARE FAVORED MORE FOR THE MEDIUM RANGE FORECAST  
PERIOD. SOME OF THE UKMET WAS ALSO INCORPORATED THROUGH DAY 4  
BEFORE GREATER DIFFERENCE BECOME APPARENT OVER THE WESTERN U.S.  
WITH THE UKMET BY DAY 5. THE PREVIOUS DISCUSSION FROM OVERNIGHT  
FOLLOWS BELOW.  
 
D. HAMRICK  
   
..PATTERN OVERVIEW
 
 
LATEST GUIDANCE MAINTAINS MEAN TROUGHING ALOFT OVER A LARGE PART  
OF NORTH AMERICA DOWNSTREAM FROM A PERSISTENT AND STRONG EAST  
PACIFIC RIDGE. WITHIN THIS PATTERN THERE IS DECENT AGREEMENT AT  
LEAST IN PRINCIPLE ON A SIGNIFICANT SYSTEM TRACKING NORTHEAST FROM  
THE SOUTHERN PLAINS AROUND TUE-WED. SOME IMPORTANT DETAILS WILL  
TAKE MORE TIME TO BE RESOLVED THOUGH. ON THE OTHER HAND MODELS  
AND ENSEMBLE MEMBERS HAVE BEEN MORE VARIABLE AND LESS AGREEABLE  
WITH LARGER SCALE FLOW DETAILS BY THE LATTER HALF OF THE PERIOD,  
ASSOCIATED WITH DIFFERENCES INVOLVING ENERGY PASSING  
AROUND/THROUGH THE EAST PACIFIC RIDGE AND TO A LESSER EXTENT  
CANADIAN FLOW.  
 
   
..MODEL GUIDANCE/UNCERTAINTY ASSESSMENT/PREFERENCES
 
 
FOR THE SYSTEM TRACKING NORTHEAST FROM THE SOUTHERN PLAINS DURING  
THE FIRST HALF OF THE PERIOD, THERE IS STILL GOOD CONSENSUS FOR  
ITS EXISTENCE BUT ONGOING DIFFERENCES FOR DETAILS THAT WILL PLAY A  
SIGNIFICANT ROLE IN DETERMINING SENSIBLE WEATHER EFFECTS AT SOME  
LOCATIONS. THROUGH THE 12Z/18Z CYCLES THE GFS WAS ON THE FASTER  
SIDE OF THE OVERALL SPREAD, WITH GREATER EASTWARD PROGRESSION OF  
HEIGHT FALLS ALOFT PARTICULARLY NOTICEABLE OVER THE EASTERN U.S.  
BY DAY 4 WED. THE GEFS MEAN WAS AT LEAST SOMEWHAT CLOSER TO  
REMAINING GUIDANCE ALOFT SO ITS SURFACE WAVE WAS NOT AS FAST AS  
THE GFS. MEANWHILE GENERAL TRENDS HAVE BEEN TOWARD A SOMEWHAT  
MORE SHEARED DEPICTION OF THE UPPER SHORTWAVE, LEADING TO A WEAKER  
SURFACE LOW TREND IN THE ECMWF IN PARTICULAR. THIS TREND ALOFT  
HAS ALSO SLOWED THE TRAILING FRONT SOMEWHAT. ECMWF MEANS HAVE  
BEEN STABLE IN THE MIDDLE OF THE SPREAD FOR TIMING WITH  
OPERATIONAL ECMWF RUNS FAIRLY CLOSE, WHILE CMC RUNS HAVE TENDED TO  
BE ON THE SLOWER SIDE. UKMET RUNS HAVE VARIED A BIT BUT THE 12Z  
VERSION WAS CLOSE TO THE ECMWF/ECMWF MEAN. COMPARISONS AT THE  
SURFACE AND ALOFT LED TO REDUCING GFS WEIGHT IN THE FORECAST  
COMPARED TO WHAT WOULD TYPICALLY BE THE CASE FOR THE EARLY PART OF  
THE EXTENDED PERIOD, WITH PREFERENCES LEANING MORE TOWARD THE  
ECMWF/ECMWF MEAN SCENARIO. WITH THE NEW 00Z GFS/UKMET/CMC IN SO  
FAR THE PREVAILING TREND IS SLOWER. THE GFS IS STILL ON THE FAST  
SIDE BUT NOT TO THE EXTENT OF THE 12Z/18Z RUNS, WHILE THE  
UKMET/CMC ARE BOTH SLOWER THAN THE 12Z ECMWF/ECMWF MEAN.  
 
SOLUTIONS INCREASINGLY DIVERGE AS EASTERN PACIFIC ENERGY REACHES  
WESTERN NORTH AMERICA AND CONTINUES INLAND. RECENT GFS AND ECMWF  
RUNS PROVIDE A SIMPLE CONTRAST WITH THE FORMER SHOWING  
CONSIDERABLE WESTERN U.S. TROUGH AMPLIFICATION AND THE LATTER  
KEEPING VERY HIGH HEIGHTS OVER THE WEST. BOTH OF THESE IDEAS FIT  
WITHIN THE FULL ENSEMBLE SPREAD. AT THE VERY LEAST, THE MOST  
AMPLIFIED SIDE OF THE SPREAD LOOKS LESS LIKELY AS THE ENERGY IS IN  
THE PROCESS OF ROUNDING THE UPPER RIDGE. CONFIDENCE IS LOWER IN  
WHAT HAPPENS AFTER THAT POINT. LOOKING AT THE CORE OF POSITIVE  
HEIGHT ANOMALIES FORECAST TO BE OFF THE PACIFIC NORTHWEST COAST IN  
D+8 MULTI-DAY MEANS, TELECONNECTIONS OFFER POTENTIAL FOR SOME  
TROUGHING TO EXTEND INTO THE SOUTHWEST U.S. BUT PERHAPS NOT TO THE  
EXTENT SEEN IN SOME GFS RUNS, THE 00Z UKMET AT THE END OF ITS RUN,  
OR THE 00Z CMC (CLOSED LOW NEAR THE CA COAST). LATEST ENSEMBLE  
MEANS OFFERED THE CLOSEST COMPARISON TO THE TELECONNECTION-FAVORED  
PATTERN, LEADING TO THEIR PREFERENCE LATER IN THE PERIOD.  
 
BASED ON THE FAVORED EVOLUTION OF THE EASTERN SYSTEM, DAYS 3-5  
TUE-THU STARTED WITH THE 12Z ECMWF HAVING THE HIGHEST WEIGHT AMONG  
OPERATIONAL SOLUTIONS. LESSER WEIGHT OF THE GFS/UKMET AND 18Z  
GEFS/12Z ECMWF MEANS KEPT THE FORECAST SIMILAR TO CONTINUITY AND  
THE MIDDLE OF THE SPREAD OVER THE EAST. THIS BLEND YIELDED A  
CONSERVATIVE INTERMEDIATE FORECAST FOR ENERGY APPROACHING THE  
NORTHWEST BY WED. BASED ON THE TELECONNECTION-FAVORED FLOW  
THEREAFTER THE FORECAST QUICKLY ADJUSTED TO THE 18Z GEFS/12Z ECMWF  
MEANS EXCLUSIVELY FOR DAYS 6-7 FRI-SAT.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE SYSTEM TRACKING NORTHEAST FROM THE SOUTHERN PLAINS TUE-WED  
WILL BRING VARIOUS TYPES OF SIGNIFICANT WEATHER TO THE EASTERN  
HALF OF THE COUNTRY. IN THE WARM SECTOR THERE IS STILL A FAIRLY  
CONSISTENT SIGNAL FOR GREATEST HEAVY RAINFALL POTENTIAL FROM THE  
SOUTHEAST PLAINS/LOWER MS VALLEY INTO THE TN VALLEY/SOUTHERN  
APPALACHIANS. AT THE SAME TIME EXPECT A BAND OF MEANINGFUL SNOW  
ACCUMULATIONS TO THE NORTHWEST OF THE SURFACE LOW TRACK. GUIDANCE  
SPREAD FOR WHERE THE AXIS OF HIGHEST SNOWFALL WILL BE HAS NARROWED  
SOMEWHAT OVER THE PAST DAY, CENTERING APPROXIMATELY FROM THE MID  
MS VALLEY THROUGH THE LOWER GREAT LAKES INTO NEW ENGLAND. LATER  
IN THE PERIOD PRECIP COVERAGE/INTENSITY OVER THE EASTERN HALF OF  
THE LOWER 48 BECOMES UNCERTAIN CORRESPONDING TO GUIDANCE  
DIFFERENCES FOR FLOW ALOFT. TELECONNECTION-FAVORED FLOW OF THE  
ENSEMBLE MEANS WOULD LEAD TO AN EXPANDING AREA OF PRECIP ACROSS  
THE SOUTHERN TIER TOWARD FRI-SAT. OVER THE WEST, THE PATTERN WILL  
FAVOR PERIODS OF SNOW OVER THE NORTHERN ROCKIES AND POTENTIALLY  
INTO THE CENTRAL ROCKIES BY THE END OF THE WEEK. SOME RAIN/HIGHER  
ELEVATION SNOW IS POSSIBLE OVER THE NORTHERN PACIFIC NORTHWEST.  
SOME OF THIS WESTERN U.S. MOISTURE MAY STREAM ACROSS PARTS OF THE  
NORTHERN TIER IN THE FORM OF SNOW.  
 
MUCH OF THE WEST SHOULD SEE ABOVE NORMAL TEMPS THROUGH THE PERIOD  
BUT WITH THE POTENTIAL FOR A COOLING TREND BY FRI-SAT DEPENDING ON  
HOW MUCH UPPER TROUGHING DEVELOPS OVER THE REGION. SOME LOCATIONS  
WILL LIKELY SEE ONE OR MORE DAYS WITH PLUS 10-20F ANOMALIES. THE  
PLAINS SHOULD SEE A COUPLE COLD SURGES, ONE IN PROGRESS AT THE  
START OF THE PERIOD TUE AND ANOTHER THU ONWARD, WITH TEMPS AT  
LEAST 10-20F BELOW NORMAL AT SOME LOCATIONS. THE SOUTH AND EAST  
WILL BE QUITE WARM AHEAD OF THE SOUTHERN PLAINS-NEW ENGLAND SYSTEM  
AND TRAILING COLD FRONT, FOLLOWED BY A COOLING TREND TO NEAR OR  
SLIGHTLY BELOW NORMAL LEVELS.  
 
RAUSCH  
 

 
 
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