939  
FXUS02 KWBC 131601  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1100 AM EST MON NOV 13 2017  
 
VALID 12Z THU NOV 16 2017 - 12Z MON NOV 20 2017  
 
...PATTERN OVERVIEW/GUIDANCE/UNCERTAINTY  
ASSESSMENT/PREFERENCES......  
 
THE LARGE-SCALE PATTERN DURING THE PERIOD WILL FAVOR A HIGHLY  
AMPLIFIED AND PROGRESSIVE SET-UP WITH SOME FAIRLY IMPRESSIVE  
HEIGHT ANOMALY CENTERS. AS ONE SHORTWAVE EXITS TOWARD NOVA SCOTIA  
EARLY FRIDAY...STRONG HEIGHT FALLS EMERGING OUT OF THE  
NORTHWESTERN U.S. WILL SET THE STAGES FOR A POWERFUL CYCLONE  
LIKELY TO EMERGE OUT OF THE MIDDLE MS VALLEY BY FRIDAY AFTERNOON.  
RAPID INTENSIFICATION IS EXPECTED WHILE MOVING INTO THE GREAT  
LAKES THIS WEEKEND WITH THE POTENTIAL FOR SECONDARY WAVE  
DEVELOPMENT OFF THE MID-ATLANTIC/NEW ENGLAND COAST BY LATE  
SATURDAY. THE RESULTANT CLOSED LOW ANCHORING THE EASTERN SECTOR OF  
NORTH AMERICA WILL BE ACCOMPANIED BY ROBUST 500-MB HEIGHT  
ANOMALIES WHICH MAY BE ON THE ORDER OF 3 STANDARD DEVIATIONS BELOW  
AVERAGE. WHILE THIS REMAINS A FIXTURE IN THE FORECAST FROM DAY  
5-7...NOVEMBER 18-20...THE PATTERN OUT WEST IS A BIT MORE NEBULOUS  
WHICH MAY RANGE FROM QUASI-ZONAL TO GENERAL RIDGING. COMPLEXITIES  
OVER THE NORTHEASTERN PACIFIC WILL HELP DECIDE THE EVENTUAL FATE  
OF THE DOWNSTREAM PATTERN BUT IT DOES APPEAR THE PRIMARY STORM  
TRACK SHOULD LOOM CLOSE TO THE INTERNATIONAL BORDER WITH CANADA.  
 
WHILE SOME TIMING ISSUES EXIT WITH THE DEPARTING CYCLONE OVER THE  
WESTERN ATLANTIC ON FRIDAY...SUCH DIFFERENCES APPEAR CONFINED TO  
THE OFFSHORE WATERS. IT IS WORTH NOTING THE 00Z UKMET IS ON THE  
SLOWER SIDE OF THE SOLUTION ENVELOPE. FARTHER  
UPSTREAM...UNCERTAINTIES WITH THE WESTERN U.S. TROUGH APPEAR EARLY  
ON. THE 06Z/00Z GFS ARE APPROXIMATELY 10 MILLIBARS DEEPER WITH THE  
PARENT SURFACE LOW MOVING THROUGH SOUTHERN ALBERTA RELATIVE TO THE  
MORE SUBDUED 00Z ECMWF. THE 00Z CMC/UKMET SEEM TO FALL SOMEWHERE  
IN THE MIDDLE COMPARED TO THESE PAIR OF EXTREME. IT APPEARS THE  
06Z/00Z GFS AND 00Z CMC/UKMET MORE STRONGLY SUPPORT THIS LOW AS IT  
TRAVERSES THROUGH SOUTH-CENTRAL CANADA WHILE THE PREVIOUS TWO  
ECMWF RUNS DEPICT A WEAK AREA OF LOW PRESSURE. FARTHER  
SOUTH...STRONG FORCING WITHIN THE TROUGH WILL SUPPORT AN  
ADDITIONAL AREA OF CYCLOGENESIS ACROSS THE NORTHERN/CENTRAL PLAINS  
LATE THURSDAY/EARLY FRIDAY. TIMING DIFFERENCES PLAGUE THE  
PROGRESSION OF THIS SYSTEM AS IT MOVES TOWARD THE GREAT LAKES/OH  
VALLEY WITH PAST RUNS OF THE GFS BEING AHEAD OF THE PRIMARY  
CONSENSUS. MULTI-DAY ENSEMBLE SPAGHETTI COMPARISONS HAS SHOWN A  
SLOWING TREND WITH THE SMALLEST RUN-TO-RUN ADJUSTMENTS NOTED IN  
THE ECMWF GUIDANCE. ALL GLOBAL MODELS AGREE ON RAPID DEEPENING OF  
THIS SYSTEM WHILE IT TRACKS TOWARD THE ONTARIO/QUEBEC BORDER ON  
SATURDAY WITH THE PAST FOUR RUNS OF THE GFS AS WELL AS THE 00Z CMC  
SHOWING COASTAL CYCLOGENESIS OFF THE MID-ATLANTIC COAST ON  
SATURDAY EVENING. WHILE THIS NEGATIVE HEIGHT ANOMALY ENCOMPASSES  
MUCH OF THE NORTHEASTERN U.S. OVER THE WEEKEND...A MORE UNCERTAIN  
PATTERN LOOMS ACROSS THE WESTERN STATES. LONGWAVE TROUGHING OVER  
THE EASTERN PACIFIC IS AGREED UPON BUT THE DOWNSTREAM EFFECTS  
APPEAR MORE FUZZY IN NATURE. THE PREVIOUS THREE RUNS OF THE ECMWF  
SHOW ENOUGH SHORTWAVE ENERGY PUSHING EAST TO ALLOW A MORE ZONAL  
PATTERN TO EMERGE RELATIVE TO THE PRIOR FOUR RUNS OF THE GFS AS  
WELL AS THE 00Z CMC DEPICTING A MEAN RIDGE RATHER. THE LAST ECMWF  
RUN TO FAVOR A RIDGE OUT WEST WAS THE 11/1200Z CYCLE. THESE PHASE  
DIFFERENCES OF COURSE CREATE A MORE CHALLENGING FORECAST MOVING  
INTO EARLY NEXT WEEK.  
 
WITH THE TOUGH FORECAST AHEAD...PRIMARILY LEANED ON AN ENSEMBLE  
MEANS APPROACH THROUGHOUT. KEPT AN EQUAL BLEND OF THE 06Z GFS/00Z  
ECMWF FOR DAY 3/THURSDAY BUT QUICKLY WENT TOWARD A 50/50  
COMBINATION OF THE 06Z GEFS/00Z ECMWF ENSEMBLE MEANS THEREAFTER.  
IDEALLY WOULD HAVE LIKED TO HAVE SLOWED THE EASTERN TROUGH EVEN  
FURTHER AND CARVE OUT A MORE PRONOUNCED RIDGE ACROSS THE WEST.  
HOWEVER...THIS APPROACH OFFERS A COMPROMISE TO THESE IDEAS HERE.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
INITIALLY A MILD PATTERN WILL BE IN PLACE ACROSS THE CENTER OF THE  
COUNTRY IN ADVANCE OF THE MEAN WESTERN U.S. TROUGH. EXPECT HIGHS  
TO BE AROUND 1O TO 20 DEGREES ABOVE CLIMATOLOGY ON THURSDAY OVER  
THE ROCKIES INTO THE GREAT PLAINS WITH MID 60S EXPANDING NORTHWARD  
INTO THE BLACK HILLS OF SD. THE ADVANCING COLD FRONT SHOULD BRING  
A QUICK END TO THESE MILD CONDITIONS AS READINGS DROP BY OVER 20  
DEGREES BY THE FOLLOWING DAY. ONE MORE WARM DAY WILL BE IN STORE  
FOR THE SOUTHERN PLAINS ON FRIDAY AS HIGHS SOAR INTO THE LOW 80S  
BEFORE TEMPERATURES COME CRASHING DOWN FOR THE WEEKEND. THE DAY  
6/7...NOVEMBER 19/20 TIMEFRAME SHOULD BE QUITE CHILLY ACROSS THE  
EASTERN STATES AS LOWER 1000-500 MB THICKNESSES ADVECT INTO THE  
REGION. HIGHS ON SUNDAY SHOULD STRUGGLE TO ESCAPE THE 30S OVER THE  
OH VALLEY/LOWER GREAT LAKES/INTERIOR NEW ENGLAND. IN ADDITION TO  
THE COLDER TEMPERATURES...WIND CHILLS SHOULD COME INTO PLAY AS A  
STRONG PRESSURE GRADIENT ACCOMPANIES THE DEEP CYCLONE.  
 
CONDITIONS SHOULD GRADUALLY IMPROVE OVER UPPER NEW ENGLAND AS THE  
SURFACE LOW EXITS INTO THE ATLANTIC. FARTHER UPSTREAM...WIDESPREAD  
PRECIPITATION WILL BE COMMONPLACE OVER THE WESTERN U.S. WITH SNOW  
EXPECTED ACROSS THE HIGHER ELEVATIONS. MANY SOLUTIONS FAVOR A  
LOCAL MAXIMA OVER THE CO ROCKIES INTO FRIDAY. LOOKING  
EAST...SHOWERS AND THUNDERSTORMS SHOULD EXPAND IN COVERAGE ALONG  
THE ADVANCING FRONT WITH A SEVERE WEATHER RISK EXPECTED ON FRIDAY  
FROM THE ARKLATEX UP INTO IN/IL PER THE OVERNIGHT OUTLOOK FROM THE  
STORM PREDICTION CENTER. THE HEAVIEST RAINFALL SHOULD GENERALLY  
STAY NORTH OF 40N LATITUDE WHERE THE MORE ROBUST FORCING WILL  
RESIDE. SOME WINTRY PRECIPITATION WILL BE POSSIBLE ANYWHERE FROM  
THE UPPER MIDWEST THROUGH THE GREAT LAKES AND INTO INTERIOR NEW  
ENGLAND. THE POTENTIAL OF LOW PRESSURE FORMING OFF THE  
MID-ATLANTIC COAST MAY INCREASE AMOUNTS CLOSER TO THE LAND/SEA  
INTERFACE BUT MODELS FAVOR CARRYING THE WAVE OUT TO SEA.  
EVENTUALLY PRECIPITATION WILL RETURN TO THE PACIFIC NORTHWEST LATE  
IN THE PERIOD AS THE OFFSHORE TROUGH EDGES CLOSER TO THE COAST.  
 
RUBIN-OSTER  
 

 
 
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