045  
FXUS02 KWBC 050650  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
149 AM EST MON FEB 05 2018  
 
VALID 12Z THU FEB 08 2018 - 12Z MON FEB 12 2018  
   
..OVERVIEW
 
 
INDIVIDUAL MODELS AND ENSEMBLE MEMBERS CONTINUE TO SHOW  
INCREASINGLY DRAMATIC SPREAD WITH TIME REGARDING SPECIFICS OF WHAT  
SHOULD BE GENERALLY CYCLONIC FLOW OVER MOST OF NORTH AMERICA,  
WHILE RIDGES PERSIST OFF THE WEST COAST AND OVER OR JUST EAST OF  
THE BAHAMAS. THIS SPREAD KEEPS CONFIDENCE LOWER THAN DESIRED FOR  
SOME ASPECTS OF THE FORECAST THOUGH TRENDS AND TELECONNECTIONS  
PROVIDE SOME DIRECTION FOR LEANING AWAY FROM THE MORE EXTREME  
EVOLUTIONS. THE MEAN PATTERN ALOFT SHOULD FAVOR HIGHEST PRECIP  
TOTALS DURING THE PERIOD FROM THE SOUTHERN TIER/GULF COAST STATES  
NORTHEASTWARD.  
 
   
..MODEL GUIDANCE/UNCERTAINTY ASSESSMENT/PREFERENCES
 
 
THE GREATEST FORECAST PROBLEM OVER RECENT DAYS HAS BEEN THE DEGREE  
OF WESTERN TROUGHING THAT EVOLVES FROM SHORTWAVE ENERGY INITIALLY  
FORECAST TO BE OVER THE ALASKA PANHANDLE/BC AS OF EARLY DAY 3 THU.  
UPSTREAM NORTH PACIFIC SHORTWAVE ENERGY PLAYS A ROLE AS WELL.  
AMONG GUIDANCE THROUGH THE 12Z/18Z CYCLES, THE UKMET/CMC APPEARED  
TO BE A LITTLE FAST WITH THIS LEADING ENERGY FROM THE START OF THE  
PERIOD BASED ON GFS/GEFS CONTINUITY AND SLOWER ECMWF/ECMWF MEAN  
TRENDS. NEW 00Z RUNS SEEM TO FIT THIS SLOWER IDEA. UPSTREAM, THE  
PAST COUPLE ECMWF RUNS--ESPECIALLY THE 12Z/04 VERSION--HAVE BEEN  
QUITE EAGER TO PUSH A SIGNIFICANT BUNDLE OF SHORTWAVE ENERGY  
THROUGH THE EAST PACIFIC RIDGE BY FRI, LEADING TO A MUCH FLATTER  
PATTERN OVER THE LOWER 48 THAN REMAINING GUIDANCE. THE 12Z ECMWF  
WAS NOT AN OUTLIER VERSUS THE FULL ENSEMBLE SPREAD BUT DEFINITELY  
IN THE MINORITY. THE OLD 00Z/04 RUN WAS CLOSER TO THE MAJORITY  
SCENARIO. ON THE OTHER SIDE OF THE SPECTRUM, THE CMC HAS GREATLY  
MODERATED ITS EARLIER IDEA OF A DEEP WESTERN TROUGH WITH EMBEDDED  
CLOSED LOW NEAR THE COAST. SOME 12Z CMC ENSEMBLES STILL SUGGEST  
THAT IDEA THOUGH. TELECONNECTIONS RELATIVE TO THE EAST PACIFIC  
RIDGE'S CORE OF POSITIVE HEIGHT ANOMALIES CONTINUE TO FAVOR AT  
LEAST MODERATE WESTERN TROUGHING WITH SOME SENSITIVITY TO EXACT  
POSITION OF THE RIDGE. AMONG NON-ECMWF SOLUTIONS THERE IS SOME  
LOOSE AGREEMENT ON THE NEXT MEANINGFUL SHORTWAVE REACHING THE WEST  
BY THE DAYS 6-7 SUN-MON TIME FRAME. 12Z/18Z GFS RUNS HAD LOW  
CONFIDENCE EVOLUTIONS INVOLVING A CLOSED LOW OFF THE WEST COAST  
WHILE THE NEW 00Z VERSION AT LEAST SEEMS TO FIT THE MEAN PATTERN  
BETTER BY BRINGING THE ENERGY INTO THE NORTHWEST. SUMMING UP  
THESE CONSIDERATIONS LED TO EMPHASIZING THE 18Z GEFS/12Z ECMWF  
MEANS AND GRADUALLY DECREASING INPUT OF THE 18Z GFS WITH TIME FOR  
THE LAST THREE DAYS OF THE FORECAST. THE FORECAST INCLUDED SOME  
12Z ECMWF WEIGHT THU-FRI BUT DECREASING ON FRI DUE TO ITS LESS  
LIKELY EAST PACIFIC SOLUTION BY THAT TIME.  
 
TO THE EAST OF THE ROCKIES, NORTHERN STREAM FLOW WILL PROVIDE THE  
SUPPORT FOR A WAVY COLD FRONT PUSHING TO THE SOUTH AND EAST OF THE  
NORTHERN U.S. SPECIFICS OF AN INDIVIDUAL WELL DEFINED WAVE IN THE  
LATE WEEK TIME FRAME HAVE BEEN FADING IN AND OUT OVER CONSECUTIVE  
RUNS/DAYS. BY THE LATTER HALF OF THE PERIOD WAVE DETAILS WILL BE  
INFLUENCED NOT ONLY BY THE NORTHERN TIER FLOW ALOFT BUT ALSO  
IF/HOW ENERGY EJECTS FROM THE WEST. AS A RESULT ENSEMBLES ARE  
QUITE DISPERSED FOR INDIVIDUAL SURFACE LOW CENTERS OVER THE EAST.  
CORRESPONDING TO PREFERENCES UPSTREAM, PREFERRED AN 18Z GFS/GEFS  
AND 12Z ECMWF/ECMWF MEAN CONSENSUS EARLY TRENDING TO INCREASING  
ENSEMBLE MEAN EMPHASIS WITH MINORITY INPUT FROM THE 18Z GFS.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
EXPECT THE HIGHEST PRECIP TOTALS DURING THE PERIOD TO BE FROM THE  
WESTERN GULF COAST/LOWER MS VALLEY NORTHEASTWARD FROM AROUND FRI  
NIGHT ONWARD. THE COMBINATION OF A PERIOD OF AT LEAST MODERATE  
LOW LEVEL GULF INFLOW AHEAD OF THE FRONT PUSHING SOUTHEAST FROM  
THE PLAINS, AND SOUTHWESTERLY FLOW ALOFT WITH ONE OR MORE EMBEDDED  
SHORTWAVES, SHOULD HELP TO ENHANCE RAINFALL OVER SOME OF THIS  
AREA. AMOUNTS WILL BE LIGHTER WITH ONE OR MORE PERIODS OF SNOW  
EXTENDING FROM THE NORTHERN AND POSSIBLY CENTRAL ROCKIES INTO THE  
NORTH-CENTRAL PLAINS AND PARTS OF THE GREAT LAKES. THE  
VARIABILITY AND SPREAD OF GUIDANCE FOR DETAILS OF FLOW ALOFT  
CONTINUE TO TEMPER CONFIDENCE IN PRECISE  
LOCATION/INTENSITY/DURATION OF SNOWFALL. THE DEGREE OF  
INTERACTION BETWEEN THE SOUTHERN TIER MOISTURE SHIELD AND COLDER  
AIR TO THE NORTH WILL NEED TO BE MONITORED BY SAT-MON. MOST  
LOCATIONS WEST OF THE ROCKIES SHOULD GENERALLY BE DRY DURING THE  
PERIOD BUT SOMEWHAT MORE PRECIP COVERAGE WILL BE POSSIBLE IF FLOW  
ALOFT TRENDS TOWARD THE DEEPER TROUGH SIDE OF THE SPECTRUM.  
 
THE MOST EXTREME ANOMALIES FOR TEMPS SHOULD EXIST DURING THU-SAT.  
A DECENT PERCENTAGE OF THE WEST WILL SEE MAX AND/OR MIN READINGS  
OF 10-20F ABOVE NORMAL WHILE THE NORTHERN INTO CENTRAL PLAINS WILL  
LIKELY BE BELOW NORMAL BY A SIMILAR MAGNITUDE WITH SOME NORTHERN  
PLAINS LOCATIONS POSSIBLY 20-30F BELOW NORMAL FOR AT LEAST ONE  
DAY. SCATTERED DAILY RECORD HIGHS/WARM LOWS ARE POSSIBLE OVER THE  
WEST EARLY IN THE PERIOD. BY SUN-MON THE COLD OVER THE CENTRAL  
U.S. SHOULD MODERATE WHILE THE WEST SHOULD STAY ABOVE NORMAL BUT  
WITH MODERATE CONFIDENCE AT BEST AS TO WHAT EXTENT.  
 
RAUSCH  
 

 
 
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