681  
FXUS02 KWBC 101544  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1143 AM EDT TUE OCT 10 2017  
 
VALID 12Z FRI OCT 13 2017 - 12Z TUE OCT 17 2017  
   
..OVERVIEW
 
 
SOME ATMOSPHERIC SHUFFLING IS FORECAST TO OCCUR OVER THE NEXT  
WEEK, RESULTING IN A FLATTENING OF THE UPPER PATTERN AFTER AN  
INITIAL SYSTEM PUSHES OUT OF THE CENTRAL STATES AND OFF THE EAST  
COAST BY EARLY NEXT WEEK. THIS WILL LEAD TO A COOLER PERIOD BY  
NEXT WEEK IN THE EAST, BUT ONLY BACK TO MORE TYPICAL VALUES FOR  
MID-OCTOBER. THE NORTHWESTERN PORTION OF THE LOWER 48 INTO THE  
PLAINS WILL SEE SEVERAL DAYS OF NEAR TO BELOW AVERAGE  
TEMPERATURES.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
WHILE A GENERAL MODEL BLEND SHOULD SUFFICE TO START THE PERIOD,  
DIFFERENCES APPEAR BY THE WEEKEND REGARDING THE UPPER TROUGH  
EXITING THE CENTRAL ROCKIES. THE 00Z ECMWF AND MANY OF THE ECMWF  
ENSEMBLE MEMBERS WERE SLOWER THAN THE GFS/UKMET/CANADIAN AND MOST  
OF THE GEFS/CMC ENSEMBLE MEMBERS. TREND IN THE ENSEMBLES HAS BEEN  
SOMEWHAT QUICKER BUT NOT ENTIRELY CONSISTENT. ECMWF MAY BE  
EXHIBITING ITS SLOW BIAS WITH DIGGING FEATURES SO A BLENDED  
POSITION IN BETWEEN THE QUICKER/SLOWER CAMPS SEEMED REASONABLE BY  
SATURDAY. AS THE SYSTEM CONTINUES TO PUSH EASTWARD, 00Z ECMWF  
CONTINUED TO LAG BEHIND THE OTHER MODELS AND MOST ENSEMBLES AS IT  
WRAPPED UP THE SYSTEM THROUGH WI AND THE U.P. OF MI WHILE THE  
ENSEMBLE CONSENSUS WAS FARTHER EAST, BUT NOT AS FAR EAST/QUICK AS  
THE 00Z/06Z GFS. THE 00Z UKMET/CANADIAN FELL IN BETWEEN AND NEAR  
THE 06Z GEFS/00Z ECMWF ENSEMBLE MEAN OVERLAP, WHICH WAS A GOOD  
MIDDLE GROUND POSITION AND NOT TOO FAR FROM CONTINUITY. BY NEXT  
MON/TUE, ENSEMBLES GENERALLY FAVORED A POSITION IN BETWEEN THE 00Z  
ECMWF AND 06Z GFS AS THE SFC LOW LIFTS THROUGH ONTARIO/QUEBEC AND  
PUSHES THE WARM FRONT BACK THROUGH MAINE. FRONT SHOULD BE OFFSHORE  
MOST OF THE EAST COAST BY NEXT TUESDAY EXCEPT FOR FLORIDA.  
 
FARTHER UPSTREAM STRONG JET BETWEEN ALASKA AND HAWAI'I BY NEXT  
WEEK SUGGESTS A LOT OF UNCERTAINTY IN SYSTEM TIMING INTO BRITISH  
COLUMBIA AND ALSO WASHINGTON. DETERMINISTIC MODELS ARE OF LITTLE  
USE FOR SPECIFICS BUT HAVE BEEN MOSTLY CONSISTENT IN BRINGING A  
WAVE INTO BC NEXT WEEK WHILE KEEPING THE SFC BOUNDARY NEAR THE  
US/CANADIAN BORDER.  
 
FINALLY, AN UPPER LOW INITIALLY OVER THE BAHAMAS WILL SLOWLY  
WEAKEN AS THE UPPER RIDGE TO ITS NORTH GETS SQUEEZED IN A NE-SW  
DIRECTION. A TROPICAL WAVE IS FORECAST TO MOVE THROUGH THE  
SUNSHINE STATE AND BRING AN INCREASED CHANCE OF SHOWERS TO MOST OF  
THE STATE, ESPECIALLY SOUTH OF THE PANHANDLE.  
 
FRACASSO  
 
   
..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 PUSH INTO THE PENINSULA BRINGING A  
CONTINUATION OF WET CONDITIONS. THE PACIFIC NORTHWEST WILL SEE THE  
POTENTIAL FOR PRECIPITATION SAT-SUN AS A SURFACE FRONT LINGERS TO  
THE NORTH. 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 US FRI-SUN. RECORD WARM MINIMUM TEMPERATURES ARE  
EXPECTED AHEAD OF THE FRONT THIS WEEKEND INTO MONDAY. 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 AVERAGE. 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  
 

 
 
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