438  
FXUS02 KWBC 110644  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
244 AM EDT WED OCT 11 2017  
 
VALID 12Z SAT OCT 14 2017 - 12Z WED OCT 18 2017  
   
..OVERVIEW
 
 
UPPER RIDGE INITIALLY CENTERED ACROSS THE LOWER MS VALLEY EARLY  
NEXT WEEKEND IS FORECAST BY ALL GUIDANCE TO WEAKEN THROUGH THE  
MEDIUM RANGE. AT THE SAME TIME, A PERSISTENT RIDGE ACROSS THE  
NORTH CENTRAL PACIFIC WILL ALLOW A RELATIVELY ACTIVE FLOW REGIME  
TO SET UP FROM THE HIGHER LATITUDES OF THE NORTH PACIFIC ACROSS  
NORTH AMERICA, BRINGING A PERIOD OF GENERALLY PROGRESSIVE FLOW  
ACROSS MUCH OF THE CONUS THROUGH THE PERIOD. TELECONNECTIONS  
ASSOCIATED WITH THIS NORTH CENTRAL PACIFIC RIDGE SUPPORT  
PERSISTENT NEGATIVE HEIGHT ANOMALIES ACROSS THE NORTHWESTERN U.S.  
WITH RIDGING ACROSS THE THE NORTHEAST. IN GENERAL, MODEL GUIDANCE  
FALLS IN LINE WITH THIS, WITH MULTIPLE SHORTWAVES/PACIFIC FRONTS  
AFFECTING THE NORTHWEST THROUGH THE PERIOD AND WEAK POSITIVE  
HEIGHT ANOMALIES PERSISTING ACROSS THE EAST (DESPITE THE WEAKENING  
OF THE HIGHLY ANOMALOUS RIDGE FARTHER SOUTH).  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
MODEL SPREAD IS SUFFICIENTLY LOW DURING DAYS 3-4 THAT A  
MULTI-MODEL BLEND INCLUDING THE 12Z ECMWF/GFS/CMC ALONG WITH  
LESSER COMPONENTS OF THE 12Z ECENS/18Z GEFS SERVED AS A BASIS FOR  
THE WPC FORECAST. FROM DAY 5 ONWARD, ENSEMBLE (ECENS/NAEFS/GEFS)  
WEIGHTING WAS INCREASED TO ACCOUNT FOR INCREASING FORECAST  
UNCERTAINTY, WITH THE 12Z EC ENSEMBLE MEAN WEIGHTED THE MOST  
HEAVILY BY DAYS 6-7.  
 
THE GFS CONTINUES TO FALL ON THE FAST SIDE OF THE GUIDANCE SPREAD  
WITH THE SHORTWAVE EXITING THE ROCKIES ON DAY 3 (SAT) AND REACHING  
THE MIDWEST ON DAY 4 (SUN). THIS PERSISTS ALL THE WAY THROUGH THE  
PROGRESSION OF THE SYSTEM EAST INTO THE OH VALLEY/NORTHEAST DAYS  
5-6 (MON-TUE). THE 12Z ECMWF REMAINS GENERALLY WELL CENTERED IN  
THE ENSEMBLE SPREAD, AND THUS WAS WEIGHTED A BIT HEAVIER RELATIVE  
TO THE GFS IN THE FORECAST. A SURFACE WAVE IS EXPECTED TO DEVELOP  
ALONG THE FRONT ACROSS THE CENTRAL PLAINS ON DAY 3 MOVING TO THE  
GREAT LAKES ON DAY 4 AND THEN NEARING THE NORTHEAST ON DAY 5.  
QUITE A BIT OF MODEL SPREAD EXISTS AS TO THE EXACT POSITION AND  
STRENGTH OF THIS LOW ESPECIALLY BY DAY 5, WITH THE 12Z ECMWF  
SHOWING A RELATIVELY STRONG 993 HPA LOW ACROSS SOUTHERN QUEBEC,  
AND THE GFS SHOWING A MUCH WEAKER AND FASTER 1000 HPA LOW ALREADY  
IN THE CANADIAN MARITIMES BY DAY 5. THE ECENS/NAEFS MEANS  
GENERALLY SUPPORT A MORE ECMWF-LIKE SOLUTION (BUT PERHAPS NOT AS  
DEEP GIVEN THE SPREAD), AND THIS IS THE SOLUTION SHOWN IN THE WPC  
FORECAST.  
 
AS MENTIONED IN THE OVERVIEW, A COUPLE SHORTWAVES OF NOTE WILL  
CROSS THE NORTH PACIFIC AND REACH THE NORTHWEST DURING THE MEDIUM  
RANGE, ESPECIALLY LATER IN THE PERIOD. MODEL CONSENSUS SUGGESTS  
ONE SHORTWAVE AND ASSOCIATED SURFACE FRONTAL SYSTEM SHOULD ARRIVE  
ON DAY 6. DETERMINISTIC SOLUTIONS DIFFER AS TO THE TIMING AND  
EXACT CHARACTER OF THE WAVE, WITH THE ECMWF PERSISTENT IN SHOWING  
A FLATTER WAVE AND THE GFS PERSISTENT IN SHOWING A MORE AMPLIFIED  
AND SOMEWHAT ELONGATED SHORTWAVE. THIS WAVE QUICKLY MOVES INLAND  
REACHING THE CANADIAN PRAIRIES BY DAY 7 AND YET ANOTHER STRONGER  
PACIFIC SHORTWAVE IS FORECAST TO QUICKLY FOLLOW IN ITS HEELS. THE  
GFS AND ECMWF ARE IN RELATIVELY GOOD AGREEMENT FOR DAY 7 IN  
SHOWING A QUICKLY AMPLIFYING SHORTWAVE DIVING SOUTHEASTWARD OFF  
THE COAST OF BRITISH COLUMBIA WITH PERHAPS THE FORMATION OF A  
CLOSED UPPER LOW. AT THE SURFACE, BOTH SOLUTIONS INDICATE A  
RELATIVELY DEEP (SUB-980 HPA) LOW OFF THE B.C. COAST AT 12Z WED.  
IN THIS CASE THE SEEMINGLY GOOD AGREEMENT BETWEEN THE ECMWF AND  
GFS COULD BE DECEIVING AS ENSEMBLE SCATTER PLOTS SHOW A LARGE  
SPREAD SURROUNDING THIS SYSTEM BY DAY 7. THUS, WAS NOT READY TO  
BUY FULLY INTO THE SPECIFIC DETAILS SHOWN BY THE 12Z ECMWF/00Z  
ECMWF BY DAY 7, AND THE WPC FORECAST GENERALLY FALLS IN LINE WITH  
A CONSENSUS OF ENSEMBLE MEANS.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
MODELS/ENSEMBLE CONTINUE TO HINT AT THE POTENTIAL FOR A ROUND OF  
HEAVY CONVECTION ACROSS THE MIDWEST/GREAT LAKES SAT-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 SOME  
UNCERTAINTY AMONG DETERMINISTIC GUIDANCE SURROUNDING THE MASS  
FIELDS, THE ECENS/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. 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 PUSH INTO  
THE PENINSULA BRINGING A CONTINUATION OF WET CONDITIONS. MOST OF  
THE WEST WILL INITIALLY BE DRY BEFORE RAPID SUCCESSION SHORTWAVES  
APPROACH THE REGION BY TUE-WED. THUS, THE NORTHWEST WILL GET  
INCREASINGLY WET BY EARLY TO MID NEXT WEEK, WITH HEAVIEST AMOUNTS  
FOCUSED ON FAVORED TERRAIN AREAS AND HIGH ELEVATION SNOWS  
POSSIBLE.  
 
TEMPERATURES ARE EXPECTED TO BE WELL ABOVE AVERAGE ACROSS MUCH OF  
THE CENTRAL AND EASTERN U.S. AHEAD OF THE COLD FRONT SAT-MON, WITH  
HIGH TEMPERATURES 5 TO 15 DEG F ABOVE AVERAGE COMMON. AS THE FRONT  
PASSES, TEMPERATURES WILL DROP BUT ONLY TO NEAR OR SLIGHTLY BELOW  
SEASONAL NORMS. IN THE WEST, TEMPERATURES WILL INITIALLY BE 5 TO  
15 DEG BELOW NORMAL ON SAT BEFORE UPPER TROUGHING MOVES EAST, BUT  
SHOULD RECOVER TO NEAR NORMAL LEVELS FROM SUN ONWARD AS HEIGHTS  
RISE AHEAD OF THE NEXT INCOMING SHORTWAVES.  
 
RYAN  
 

 
 
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