647  
FXUS02 KWBC 100610  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
209 AM EDT TUE OCT 10 2017  
 
VALID 12Z FRI OCT 13 2017 - 12Z TUE OCT 17 2017  
   
..OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES
 
 
WHILE MODELS REMAIN IN GENERAL AGREEMENT ON THE LARGE SCALE  
PATTERN ACROSS THE CONUS DURING THE MEDIUM RANGE, SOME INCREASE IN  
SPREAD DURING THE MID TO LATE FORECAST PERIOD WAS NOTED COMPARED  
TO THIS TIME LAST NIGHT. THE RESULTANT UNCERTAINTY SUPPORTS HEAVY  
ENSEMBLE MEAN USE BY THE LATTER HALF OF THE FORECAST PERIOD. AN  
ANOMALOUS UPPER RIDGE INITIALLY CENTERED ACROSS THE LOWER MS  
VALLEY ON DAY 3 (FRI) IS FORECAST BY ALL GUIDANCE TO FLATTEN BY  
SUN-MON AS AMPLIFIED SHORTWAVE ENERGY MOVES FROM THE ROCKIES INTO  
THE CENTRAL U.S. GUIDANCE CONTINUES TO DIFFER ON JUST HOW QUICKLY  
THIS UPPER ENERGY MOVES EAST ACROSS THE PLAINS, AS WELL AS THE  
STRUCTURE OF THE WAVE. THE PAST FEW RUNS OF THE GFS HAVE MOVED THE  
WAVE EAST ACROSS THE PLAINS MUCH MORE QUICKLY THAN THE  
ECMWF/UKMET/CMC, WHICH REPRESENTS A REVERSAL OF WHAT THE GFS  
SHOWED THIS TIME LAST NIGHT WHEN IT WAS ON THE SLOWER SIDE OF THE  
SPREAD. MEANWHILE, THE ECMWF HAS CONTINUED TO BOUNCE AROUND WITH  
RESPECT TO TIMING, BUT HAS GENERALLY TRENDED A BIT SLOWER AS OF  
THE 12Z MON RUN. ENSEMBLE SPAGHETTI PLOTS DO LITTLE TO ADD CLARITY  
TO THE SITUATION, WITH A VISIBLE INCREASE IN SPREAD AMONG ECMWF  
AND GEFS ENSEMBLE MEMBERS OVER THE PAST COUPLE RUNS. DESPITE THE  
SPREAD, THE EC ENSEMBLE MEAN HAS REMAINED RELATIVELY STEADY, WITH  
A TIMING THAT LOOKS VERY MUCH LIKE THE 12Z MON UKMET. AT THE  
SURFACE, SOLUTIONS AT 12Z SUN RANGE FROM A SURFACE FRONTAL WAVE  
OVER NERN KS ON THE 12Z ECMWF, TO LAKE ONTARIO ON THE 18Z GFS AND  
MICHIGAN ON THE 00Z GFS. THE WPC FORECAST LEANED HEAVILY TOWARD A  
SOLUTION RESEMBLING THE 12Z ECMWF ENSEMBLE MEAN AFTER DAY 5 (SUN)  
GIVEN ITS RELATIVE CONSISTENCY COMPARED TO OTHER GUIDANCE. THIS  
PLACES A TROUGH AXIS ACROSS THE GREAT LAKES ON DAY 6 (MON) MOVING  
TOWARD THE NORTHEAST BY DAY 7 (MON).  
 
NEXT SIGNIFICANT SHORTWAVE LOOKS TO APPROACH THE NORTHWEST BY DAY  
7 (TUE). WHILE ALL MODEL SOLUTIONS AGREE ON THE EXISTENCE AND  
GENERAL TIMING OF THIS FEATURE, THEY DIFFER SUBSTANTIALLY AS TO  
THE CHARACTER OF THE WAVE, WITH THE ECMWF CONSISTENTLY SHOWING A  
FLATTER LESS AMPLIFIED WAVE AND THE GFS SHOWING A SHARPER TROUGH.  
ENSEMBLE SPAGHETTI PLOTS SHOW A SLIGHT REDUCTION IN SPREAD  
SURROUNDING THIS FEATURE COMPARED TO LAST NIGHT, WITH THE ECMWF  
ENSEMBLE MEAN GENERALLY IN THE MIDDLE OF THE SPREAD. THUS, THE  
ECMWF ENSEMBLE MEAN IS ALSO THE PREFERRED SOLUTION FOR THIS  
FEATURE.  
 
GIVEN THE ABOVE CONSIDERATIONS, THE WPC FORECAST WAS INITIALLY  
BASED ON A MULTI-MODEL BLEND OF DETERMINISTIC GUIDANCE INCLUDING  
THE 12Z ECMWF/UKMET AND 18Z GFS FOR DAYS 3-5. FOR DAYS 6-7,  
WEIGHTING OF THE 12Z ECMWF ENSEMBLE MEAN WAS INCREASED  
SUBSTANTIALLY, WITH SMALLER COMPONENTS OF THE 12Z NAEFS/18Z GEFS  
ALSO INCORPORATED.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
MODELS/ENSEMBLE CONTINUE TO HINT AT THE POTENTIAL FOR A ROUND OF  
HEAVY CONVECTION ACROSS THE MIDWEST FRI-SUN AS A AN UPPER  
SHORTWAVE APPROACHES AND THE ASSOCIATED SURFACE FRONT MOVES INTO  
THE REGION AND THEN STALLS WHILE ADDITIONAL WAVES DEVELOP ALONG  
IT. DEVELOPMENT OF A HEALTHY LOW-LEVEL JET SHOULD TRANSPORT  
SUFFICIENT MOISTURE/INSTABILITY NORTHWARD INTO THE FRONT TO  
SUPPORT CONVECTION WITH POTENTIALLY HEAVY RAINFALL. DESPITE THE  
UNCERTAINTY AMONG DETERMINISTIC GUIDANCE SURROUNDING THE MASS  
FIELDS, THE ECMWF/GEFS ENSEMBLES HAVE BEEN STEADFAST AND  
REMARKABLY SIMILAR IN THEIR DEPICTION OF A BAND OF HEAVY RAINFALL  
ALONG/JUST NORTH OF THE FRONT. FARTHER SOUTH, THE FLORIDA  
PENINSULA WILL REMAIN WET THROUGH THE PERIOD. THE TAIL END OF A  
WEAKENING FRONTAL BOUNDARY ALONG WITH AN ELONGATED TROUGH/EASTERLY  
WAVE MOVING FROM THE BAHAMAS ACROSS THE FL PENINSULA WILL SUPPORT  
SCATTERED TO NUMEROUS SHOWERS AND STORMS FRI-SUN. BY EARLY NEXT  
WEEK ANOTHER SURFACE FRONT WILL STALL ACROSS THE PENINSULA  
BRINGING A CONTINUATION OF WET CONDITIONS. THE PACIFIC NORTHWEST  
WILL SEE THE POTENTIAL FOR PRECIPITATION SAT-SUN AS A WEAKENING  
SURFACE FRONT MOVES INLAND. DRY CONDITIONS WILL PREVAIL FOR MOST  
OF THE NORTHWEST ON MON BEFORE ANOTHER FRONT APPROACHES BY TUE.  
 
UPPER RIDGING AND WARM SOUTHERLY FLOW AHEAD OF THE FRONTAL SYSTEM  
CROSSING THE CENTRAL/EASTERN U.S. WILL SUPPORT WELL ABOVE AVERAGE  
TEMPERATURES THROUGH MUCH OF THE MEDIUM RANGE. HIGHS RANGING FROM  
5 TO 15 DEG F ABOVE AVERAGE ARE EXPECTED ACROSS MUCH OF THE  
CENTRAL/EASTERN U.S. FRI-SUN. THE ONE EXCEPTION WILL BE PORTIONS  
OF THE MID-ATLANTIC EAST OF THE APPALACHIANS ON FRIDAY WHERE  
EASTERLY FLOW OFF THE ATLANTIC WILL SUPPORT CLOUDS AND PERHAPS A  
FEW SHOWERS RESULTING IN COOLER TEMPERATURES, NEAR SEASONAL NORMAL  
VALUES. AS THE COLD FRONT SWEEPS EAST BY EARLY NEXT WEEK EXPECT  
DROPPING TEMPERATURES IN ITS WAKE, ALTHOUGH HIGHS WILL REMAIN NEAR  
OR JUST ABOVE NORMAL VALUES. ACROSS THE WEST TROUGHING AND  
PRECIPITATION WILL SUPPORT BELOW AVERAGE TEMPERATURES FRI-SUN,  
WITH HIGHS 5 TO 15 DEG F BELOW NORMAL. TEMPERATURES WILL MODERATE  
SOME BY EARLY NEXT WEEK AS CONDITIONS BRIEFLY DRY OUT AND WEAK  
UPPER RIDGING BUILDS IN.  
 
RYAN  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page