059  
FXUS02 KWBC 241554  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1053 AM EST FRI NOV 24 2017  
 
VALID 12Z MON NOV 27 2017 - 12Z FRI DEC 01 2017  
   
..PATTERN ASSESSMENT AND MODEL CHOICES
 
 
WE CONTINUE WITH RELATIVELY HIGH CONFIDENCE ON THE LARGEST  
SCALES...THAT DURING DAYS 3-7 A PROGRESSIVE WAVE TRAIN WILL SET UP  
ALL THE WAY FROM THE WESTERN PACIFIC ACROSS NORTH AMERICA...THEN  
SLOWING AND SPLITTING AROUND A PERSISTENT BLOCK OVER THE MIDDLE  
ATLANTIC OCEAN. GIVEN THE SEASON AND STRENGTH OF THE OVERALL POLE  
TO EQUATOR GRADIENTS THERE IS POTENTIAL FOR EMBEDDED NOTEWORTHY OR  
EVEN STRONG LOWS TO SPIN UP WITHIN THE PROGRESSIVE FLOW...ALTHOUGH  
THE DETERMINISTIC AND ENSEMBLE GUIDANCE SUGGESTS CYGLOGENESIS WILL  
FOR THE MOST PART REMAIN NORTH OF THE LOWER 48 STATES THROUGH NEXT  
FRIDAY. TO BEGIN THE MEDIUM RANGE PERIOD MONDAY MORNING...A TROUGH  
WILL BE EXITING THE EAST COAST OF NORTH AMERICA. THE D3-7 FORECAST  
THEN FEATURES A SUCCESSION OF THREE LARGE SCALE TROUGHS...THE  
FIRST OF WHICH WILL LIKELY INCLUDE A PRONOUNCED SOUTHERN STREAM  
COMPONENT...AND POTENTIALLY A CLOSED MID LEVEL CIRCULATION.  
 
TELECONNECTIONS BASED ON THE ATLANTIC BLOCK FAVOR AN EVENTUAL  
CONFLUENCE OF FLOW OVER THE EASTERN U.S. WHICH WOULD SHEAR AND  
WEAKEN THE SOUTHERN STREAM TROUGH...AND THIS DOES OCCUR IN THE  
GUIDANCE D5-6. MODELS STILL VARY GREATLY WITH THE DETAILS OF THE  
INITIAL STRENGTH OF THE SYSTEM OVER THE SOUTHWEST STATES...ITS  
FORWARD SPEED...AND RATE OF WEAKENING. THE ECMWF IS  
CHARACTERISTICALLY SLOWER/STRONGER...AND THE GFS IS  
CHARACTERISTICALLY FAST. ASIDE FROM THE VERY FLAT CANADIAN  
ENSEMBLE MEMBERS WHICH LEAN TOWARD A WEAKER REFLECTION...THERE IS  
A GROWING CONSENSUS OF OTHER GUIDANCE SOMEWHERE IN THE  
MIDDLE...AND SIMILAR TO THE 00Z UKMET THROUGH DAY 5.  
 
THIS FIRST WAVE IS FOLLOWED VERY QUICKLY BY A MORE CONSOLIDATED  
TROUGH THAT ENTERS THE NORTHWEST U.S. ON TUESDAY...FALLS INTO THE  
HEIGHT DEPRESSION LEFT BEHIND BY THE SHEARING WAVE IN THE PLAINS  
ON THURSDAY...AND SWEEPS TO THE EAST COAST NEXT FRIDAY.  
 
THE THIRD TROUGH OF INTEREST NEARS THE WEST COAST LATE NEXT WEEK.  
ECMWF/ECMWF ENSEMBLE MEAN RUNS HAVE GENERALLY BEEN STEADIEST OVER  
THE PAST COUPLE DAYS. GEFS MEANS HAVE MADE A NOTABLE TREND TOWARD  
THE ECMWF MEAN FOR THE DEPTH OF THE TROUGH...HELPING TO INCREASE  
CONFIDENCE SOMEWHAT. OCCASIONAL RUNS OF THE CMC ARE SIMILAR IN  
PRINCIPLE AS WELL.  
 
ONCE AGAIN...IN AN OVERALL SENSE...THE 00Z AND 06Z GFS WERE OUT OF  
PHASE WITH THE MEANS AT SEVERAL TIMES...ON SEVERAL PARTS OF THE  
MAP...WHICH DID NOT INSPIRE CONFIDENCE...BUT SEEING GOOD AGREEMENT  
BETWEEN THE GEFS MEAN AND ECMWF ENSEMBLE MEAN GIVES US A SIMPLE  
PATH FORWARD...AND ONE THAT STICKS CLOSE TO WPC CONTINUITY. OUR  
BLENDS FOR THE WEATHER ELEMENTS ACROSS THE CONUS INCLUDED 20 TO 30  
PERCENT EACH OF THE OPERATIONAL ECMWF AND UKMET THROUGH DAY  
5.5...THEN TRANSITIONING TO A 60/40 BLEND OF THE ECMWF ENSEMBLE  
MEAN/GEFS MEAN AS WAS DONE DURING THE PREVIOUS FORECAST CYCLE.  
 
   
..WEATHER HIGHLIGHTS
 
 
RECORD AND NEAR-RECORD WARMTH IS FORECAST ON DAY 3 FROM THE  
SOUTHWEST STATES INTO THE PLAINS...WITH HIGHS IN THE UPPER 60S TO  
MID 80S DEPENDING ON ELEVATION AND LATITUDE. TUESDAY WILL STILL  
SEE STRONGLY ANOMALOUS HIGHS AHEAD OF THE FRONT ENTERING THE  
SOUTHERN PLAINS...ALTHOUGH RECORDS THERE WILL BE LESS WITHIN  
REACH. THE SUCCESSION OF FULL LATITUDE TROUGHS THEN KNOCKS MUCH OF  
THE NATION BACK DOWN CLOSER TO AVERAGE...ALTHOUGH ANY ARCTIC AIR  
REMAINS TIED TO A BROAD AND UNCHANGING LOW ACROSS CENTRAL CANADA.  
 
ACROSS THE WEST...PROGRESSIVE EARLY-MID WEEK SYSTEMS MAY GENERATE  
A COUPLE PERIODS OF RAIN/HIGH ELEVATION SNOW. LATE PERIOD EASTERN  
PACIFIC TROUGHING ALOFT MAY BRING ANOTHER AREA OF MOISTURE TO THE  
NORTHWEST TOWARD THE END OF THE WEEK. THE WASHINGTON  
OLYMPICS/CASCADES HAVE THE GREATEST PROBABILITY OF SEEING THE  
HIGHEST 5-DAY PRECIP TOTALS BUT UNCERTAINTY IN SHORTWAVE DETAILS  
ALOFT ALLOW POTENTIAL FOR OTHER AREAS TO SEE FOCUSED ACTIVITY AS  
WELL. THE LEADING SOUTHERN STREAM SYSTEM EMERGING FROM THE WEST  
MAY GENERATE AN AXIS OF MEANINGFUL RAINFALL ACROSS THE  
CENTRAL/SOUTHERN PLAINS AND LOWER MISSOURI VALLEY AROUND MIDWEEK.  
AMOUNTS WILL DEPEND ON STRENGTH/TIMING OF THE UPPER SYSTEM. THE  
NORTHEAST QUADRANT OF THE LOWER 48 SHOULD SEE LIGHT TO PERHAPS  
LOCALLY MODERATE PRECIP IN ASSOCIATION WITH FRONTAL PASSAGES.  
EXPECT SNOW CHANCES AT LOWER ELEVATIONS TO BE CONFINED TO NEAR THE  
CANADIAN BORDER.  
 
BURKE/RAUSCH  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page