885  
FXUS02 KWBC 200650  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
249 AM EDT FRI OCT 20 2017  
 
VALID 12Z MON OCT 23 2017 - 12Z FRI OCT 27 2017  
   
..OVERVIEW
 
 
GUIDANCE AGREES WELL ON A MEAN PATTERN FEATURING AN AMPLIFIED  
EAST-CENTRAL U.S. TROUGH BETWEEN STRONG RIDGES OVER EASTERN  
PACIFIC/WEST COAST AS WELL AS WEST-CENTRAL ATLANTIC. HOWEVER  
THERE ARE MEANINGFUL DIFFERENCES FOR MULTIPLE EMBEDDED FEATURES  
WITHIN THIS FLOW. FROM THE SENSIBLE WEATHER PERSPECTIVE THE  
PRIMARY THEMES WILL BE VERY WARM TEMPS OVER THE WEST AND THE  
SOUTHERN HALF OF THE REGION IN PARTICULAR, AND FARTHER EAST A  
PERIOD OF HEAVY RAIN POTENTIAL FOLLOWED BY A PRONOUNCED COOLING  
TREND.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
AT THE START OF THE PERIOD THERE IS ALREADY SIGNIFICANT SPREAD  
WITH MS VALLEY TROUGH ENERGY THAT MAY CLOSE OFF AN UPPER LOW OVER  
THE SOUTHERN HALF OF THE REGION. ASIDE FROM THE 00Z CMC THAT HAS  
A PROGRESSIVE TROUGH, THERE IS AT LEAST CONSENSUS ON FLOW  
SEPARATION AS OF EARLY DAY 3 MON BUT THEN WIDENING SPREAD FOR HOW  
QUICKLY THIS FEATURE GETS ABSORBED BY RAPIDLY AMPLIFYING NORTHERN  
STREAM FLOW. WHAT MODEL TRENDS EXIST SEEM TO BE LEANING FASTER,  
ESPECIALLY IN THE CASE OF THE 12Z ECMWF WHICH OFFERS A FAST  
EXTREME. EVEN IF NOT FROM THE SOUTHERN U.S. WAVE AS IN THE ECMWF  
EVOLUTION, THERE IS A GOOD ENSEMBLE MEAN AGREEMENT TOWARD DEEPEST  
LOW PRESSURE EVOLVING OVER AND JUST NORTHEAST OF THE GREAT LAKES  
BY DAY 4 TUE IN RESPONSE TO THE NORTHERN STREAM DYNAMICS. THE 00Z  
GFS HAS TRENDED A BIT FASTER FROM ITS PREVIOUS COUPLE RUNS WHILE  
THE UKMET CONTINUES TO BE ONE OF THE SLOWER SOLUTIONS. OVERALL  
THE GUIDANCE HAS BEEN SUFFICIENTLY INCONSISTENT OVER RECENT DAYS  
TO HAVE FAIRLY LOW CONFIDENCE IN ANY PARTICULAR SCENARIO THUS FAR,  
FAVORING A MODEL/ENSEMBLE MEAN COMPROMISE FOR THIS ASPECT OF THE  
FORECAST. AS THE LARGER SCALE TROUGH REACHES THE EAST COAST THE  
12Z-18Z GEFS/12Z ECMWF MEANS ARE SIMILAR IN SUGGESTING A SURFACE  
WAVE TRACKING FROM THE WESTERN ATLANTIC INTO NEW ENGLAND LATE IN  
THE WEEK WHILE INDIVIDUAL MODEL RUNS VARY CONSIDERABLY FOR  
STRENGTH/TIMING. THE MEANS PROVIDE THE MOST APPEALING  
LATER-PERIOD OPTION WHILE WAITING FOR BETTER AGREEMENT IN THE  
MODELS.  
 
THE NEXT AREA OF UNCERTAINTY INVOLVES FLOW SEPARATION LIKELY TO  
OCCUR OVER THE PACIFIC SOUTH OF ALASKA AROUND LATE MON-TUE WITH  
THE NORTHERN PORTION OF THE TROUGH ENERGY QUICKLY HEADING INTO  
NORTH AMERICA TO REINFORCE THE MEAN TROUGH. THE 00Z ECMWF IS  
SOMEWHAT TOWARD THE AGGRESSIVE SIDE OF THE ENSEMBLE SPREAD IN HOW  
QUICKLY IT BRINGS IN THE HEIGHT FALLS. HOWEVER MULTIPLE  
ECMWF/ECMWF MEAN RUNS HAVE BEEN DEPICTING THIS FEATURE AND THE  
GFS/GEFS MEAN HAVE MADE A NOTICEABLE TREND IN THAT DIRECTION OVER  
THE PAST 12 HOURS OF RUNS, SO CONFIDENCE IS NOW INCREASING IN THE  
IDEA OF A STRONGER SHORTWAVE BY WED ONWARD.  
 
THE NEXT FEATURE UPSTREAM THAT REQUIRES MONITORING IS THE  
COMBINATION OF TYPHOON LAN AND NORTHERN STREAM ENERGY LEADING TO A  
VERY DEEP SYSTEM REACHING THE BERING SEA BY AROUND MIDWEEK.  
GUIDANCE APPEARS TO SHOW LESS SPREAD THAN HAD BEEN THE CASE OVER  
RECENT DAYS BUT TYPICAL DIFFICULTY THAT MODELS CAN HAVE IN  
PROPERLY DEPICTING TROPICAL-EXTRATROPICAL TRANSITIONS OFFERS  
POTENTIAL FOR DOWNSTREAM ADJUSTMENTS NOT YET APPARENT.  
 
THE UPDATED FORECAST INCORPORATES VARIOUS WEIGHTS OF THE 12Z-18Z  
GFS/12Z ECMWF AND 18Z GEFS/12Z ECMWF MEANS WITH SLIGHTLY MORE  
WEIGHT GIVEN TO THE ECMWF/ECMWF MEAN VERSUS GFS/GEFS MEAN,  
ESPECIALLY WITH CONTINUITY/TRENDS FOR THE NORTHERN PACIFIC  
SHORTWAVE MID-LATE PERIOD. THE BLEND INCREASES EMPHASIS OF THE  
ENSEMBLE MEANS LATE IN THE PERIOD GIVEN THEIR RELATIVE AGREEMENT  
VERSUS THE SPREAD OF OPERATIONAL RUNS.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
MULTIPLE AREAS OF HEAVY RAINFALL ARE POSSIBLE TO THE EAST OF THE  
MS RIVER AS A WAVY FRONT GRADUALLY PROGRESSES ACROSS THE EASTERN  
STATES. FLOW AHEAD OF AN INITIAL MS VALLEY UPPER TROUGH/LOW  
SHOULD SUPPORT AN AXIS OF HEAVIEST ACTIVITY FROM THE EAST-CENTRAL  
GULF COAST NORTHWARD EARLY IN THE WEEK WITH ATLANTIC INFLOW  
EVENTUALLY SUPPORTING ENHANCED AMOUNTS OVER THE NORTHEAST MID-LATE  
WEEK AS THE FRONT CONTINUES ONWARD. DETAILS/TIMING OF INDIVIDUAL  
FEATURES ALOFT ARE STILL UNRESOLVED, TEMPERING CONFIDENCE IN  
SPECIFICS OF EXACTLY WHERE/WHEN HEAVIEST RAINFALL WILL OCCUR.  
TOWARD FRI ANOTHER AREA OF PRECIP MAY DEVELOP WITH ANOTHER FRONT  
REACHING THE EAST-CENTRAL U.S.. MEANWHILE AN AREA OF MOSTLY LIGHT  
PRECIP SHOULD EXTEND FROM THE NORTHWEST INTO THE CENTRAL  
ROCKIES/HIGH PLAINS AROUND WED-THU.  
 
THE AMPLIFIED PATTERN ALOFT WILL LEAD TO SOME SIGNIFICANT  
TEMPERATURE ANOMALIES. LOCATIONS OVER CALIFORNIA AND THE  
SOUTHWEST/GREAT BASIN MAY SEE ONE OR MORE DAYS WITH HIGHS OF  
10-20F ABOVE NORMAL WITH A FEW DAILY RECORDS BEING CHALLENGED.  
THIS HEAT ALONG WITH DRY/GUSTY WINDS ASSOCIATED WITH OFFSHORE  
GRADIENTS WILL LIKELY ENHANCE WILDFIRE DANGER ONCE AGAIN. A VERY  
BRIEF EPISODE OF WARMTH MAY OCCUR OVER THE NORTHERN-CENTRAL PLAINS  
AROUND MIDWEEK. THE EASTERN U.S. WILL ALSO BE QUITE WARM AT THE  
START OF THE WEEK BEFORE FRONTAL PASSAGE, BUT THEN TREND BELOW  
NORMAL WITH A DAY OR TWO OF MINUS 5-15F ANOMALIES BY WED-THU.  
 
RAUSCH  
 

 
 
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