384  
FXUS02 KWBC 040553  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
152 AM EDT SAT NOV 04 2017  
 
VALID 12Z TUE NOV 07 2017 - 12Z SAT NOV 11 2017  
   
..PATTERN OVERVIEW/GUIDANCE/UNCERTAINTY ASSESSMENT/PREFERENCES
 
 
INITIALLY...THE UPPER PATTERN WILL BE DOMINATED BY BROAD RIDGING  
EXTENDING FROM MEXICO EASTWARD THROUGH THE GULF OF MEXICO INTO  
FLORIDA. MEANWHILE...AN AMPLIFIED PROGRESSIVE FLOW REGIME WILL BE  
THE NORM TO THE NORTH. THE KEY FEATURES OF INTEREST ARE A MODEST  
PERTURBATION IN THE FLOW MOVING FROM THE CENTRAL GREAT BASIN  
TOWARD THE SOUTHERN/CENTRAL APPALACHIANS BETWEEN TUESDAY AND  
THURSDAY. THESE HEIGHT FALLS SHOULD EVENTUALLY NUDGE A  
QUASI-STATIONARY BOUNDARY OVER THE MID-SOUTH/CAROLINAS BACK TOWARD  
THE GULF OF MEXICO. WITHIN THE ACTIVE NORTHERN STREAM...A LOBE OF  
LONGWAVE ENERGY SHOULD PEEL AWAY FROM A STRONG/EXPANSIVE NEGATIVE  
HEIGHT ANOMALY OVER THE HIGHER LATITUDES OF CENTRAL/EASTERN  
CANADA. THE ATTENDANT LOWERING OF HEIGHTS WILL SWEEP THROUGH THE  
GREAT LAKES/NORTHEASTERN U.S. DURING THE DAY 5/6...NOVEMBER 9/10  
TIMEFRAME. COASTAL CYCLOGENESIS IS LIKELY TO TAKE PLACE AS THE  
SYSTEM MIGRATES TOWARD NOVA SCOTIA. LOOKING BACK TOWARD THE  
WESTERN U.S...A POTENT UPPER LOW EMERGING OUT OF THE GULF OF  
ALASKA WILL EVENTUALLY TRACK INTO THE PACIFIC NORTHWEST BY  
SOMETIME THURSDAY. MODELS SHOW RATHER IMPRESSIVE SURFACE PRESSURES  
OFF THE COAST BEFORE LANDFALL WITH THE 12Z GFS SHOWING A 974-MB  
LOW A FEW HUNDRED MILES WEST OF WASHINGTON/OREGON ON 09/1200Z.  
EVENTUALLY THIS SYSTEM SHOULD SHEAR WHILE MOVING INLAND AS THE  
SYNOPTIC-SCALE FLOW TURNS MORE QUASI-ZONAL IN NATURE TOWARD THE  
DAY 6/7...NOVEMBER 10/11 PERIOD.  
 
GENERALLY SPEAKING...MODELS ARE IN FAIRLY GOOD AGREEMENT WITH THE  
INITIAL SHORTWAVES MOVING OUT OF THE INTERMOUNTAIN WEST ON  
TUESDAY. THE GFS HAS SHOWN A SLOWING TREND BRINGING IT CLOSER TO  
WHAT THE ECMWF HAS BEEN SHOWING THE LAST SEVERAL RUNS. RELATIVE TO  
EARLIER CYCLES...MODELS HAVE BEEN MORE DAMPENED WITH REGARD TO  
AMPLITUDE. WITH RESPECT TO THE NORTHERN STREAM  
AMPLIFICATION...THERE HAS BEEN A MOVEMENT TOWARD GREATER DIGGING  
OF THIS TROUGH WHICH BRINGS MUCH COOLER AIR TOWARD THE CONTINENTAL  
U.S. THE 12Z UKMET WAS THE MOST AGGRESSIVE WITH THIS SCENARIO  
ALTHOUGH THE MORE RECENT 00Z CYCLE HAS BACKED OFF SOME. GIVEN THE  
VAST SPREAD NOTED IN THE ENSEMBLE SPAGHETTI PLOTS...THERE MAY BE  
ADDITIONAL CHANGES IN FUTURE GUIDANCE. AN EVENTUAL NEGATIVE TILT  
TO THE TROUGH BY LATE THURSDAY/EARLY FRIDAY WILL FAVOR SURFACE  
INTENSIFICATION TOWARD NOVA SCOTIA. ENSEMBLE LOW PLOTS DO SHOW  
LOWS SCATTERED ALL OVER THE WESTERN ATLANTIC SUGGESTING MUCH  
UNCERTAINTY. OUT WEST...IT APPEARS MODELS HAVE BEEN MOVING MORE  
TOWARD THE WESTWARD DISPLACED 12Z ECMWF/ECMWF ENSEMBLE MEAN.  
ENSEMBLE SPAGHETTI PLOTS FROM THE 12Z CYCLE AS WELL AS THE  
PREVIOUS 00Z SHOW A MARKED WESTWARD SHIFT IN THE  
GUIDANCE...PARTICULARLY THE GEFS/CMC MEMBERS. GRADUALLY THIS  
CLOSED LOW WILL BARREL EASTWARD TOWARD THE PACIFIC NORTHWEST WITH  
500-MB HEIGHT DEPARTURES ON THE ORDER OF 2 TO 2.5 STANDARD  
DEVIATIONS BELOW AVERAGE BASED ON THE 00Z GFS. A POTENTIALLY  
ROBUST SURFACE CYCLONE MAY LURK OFF THE COAST WITH SOME FAIRLY  
INTENSE PRESSURE GRADIENTS. THE UPPER TROUGH SHOULD MOVE TOWARD  
THE PACIFIC NORTHWEST ON THURSDAY WITH A GRADUAL DE-AMPLIFICATION  
OF THE LONGWAVE PATTERN MOVING TOWARD THE END OF THE FORECAST.  
 
AS MODELS CAME INTO BETTER AGREEMENT DURING THIS CYCLE...WAS ABLE  
TO MAINTAIN SOME APPRECIABLE AMOUNT OF OPERATIONAL MODELS IN THE  
MIX THROUGHOUT. UTILIZED A COMBINATION OF THE 18Z GFS AND 12Z  
ECMWF/UKMET THROUGH DAY 4 WITH MORE CONTRIBUTIONS TOWARD THE 18Z  
GFS/12Z ECMWF. THEREAFTER...STARTED TO INCREASE THE INFLUENCES OF  
THE TRIO OF ENSEMBLE MEANS WHILE REMOVING THE 12Z UKMET GIVEN THE  
STRONGER TROUGH OVER THE GREAT LAKES BY DAY 5/THURSDAY. STILL KEPT  
AROUND 20 TO 30 PERCENT OF THE OPERATIONAL GUIDANCE IN THE  
FORECAST FOR DAYS 6/7...NOVEMBER 10/11 WITH SLIGHTLY MORE  
WEIGHTING ON THE 12Z ECMWF/ECMWF ENSEMBLE MEAN GIVEN BETTER  
CONSISTENCY WITH THE EASTERN PACIFIC UPPER LOW.  
 
   
..SENSIBLE WEATHER/THREAT HIGHLIGHTS
 
 
FOR MUCH OF THE PERIOD...BELOW AVERAGE TEMPERATURES WILL BE THE  
NORM AS AMPLIFIED NORTHERN STREAM FLOW DRAWS IN A SHOT OF ARCTIC  
AIR. A FAIRLY SHARP FRONTAL ZONE WILL SET UP OVER THE  
CENTRAL/EASTERN U.S. ON TUESDAY WITH OVERNIGHT LOWS IN THE LOW/MID  
60S ACROSS THE MID-SOUTH TOWARD THE CENTRAL GULF COAST WHICH MAY  
BREAK A FEW DAILY RECORDS FOR WARM NIGHTTIME TEMPERATURES. WITHIN  
THE DOME OF ARCTIC AIR...LOWS WILL BE AROUND 0 DEGREES OR SLIGHTLY  
BELOW ACROSS THE NORTHERN ROCKIES INTO NORTH DAKOTA. THE LATTER  
AIR MASS SHOULD CONTINUE PUSHING SOUTH AND EAST WITH ANOMALIES ON  
THE ORDER OF 15 TO 20 DEGREES BELOW AVERAGE. BY FRIDAY...HIGHS  
WILL ONLY BE IN THE 40S FROM THE MISSOURI VALLEY EASTWARD THROUGH  
THE OHIO VALLEY AND INTO THE MID-ATLANTIC. A GRADUAL WARM UP OVER  
THE NATION IS LIKELY BY NEXT WEEKEND AS THE FLOW BECOMES MORE  
ZONAL IN NATURE.  
 
REGARDING PRECIPITATION THREATS...THE STALLED FRONTAL ZONE OVER  
THE TENNESSEE VALLEY/MID-SOUTH/CAROLINAS WILL KEEP RAIN IN THE  
FORECAST EARLY ON. THIS SYSTEM WILL EVENTUALLY EXIT AS THE COLD  
FRONT SWINGS SOUTHWARD LATE IN THE WORK WEEK. THE AMPLIFICATION IN  
THE NORTHERN STREAM COULD BRING SOME SNOW TO THE INTERIOR  
NORTHEAST ALTHOUGH MODELS DO NOT SHOW APPRECIABLE MOISTURE WITH  
THIS TROUGH. THE BIGGEST WEATHER MAKER WILL BE ACROSS THE WEST  
COAST GIVEN THE STRONG UPPER LOW MOVING INLAND. HEAVY RAINFALL IS  
EXPECTED FROM NORTHERN CALIFORNIA UP ALONG THE COAST TOWARD THE  
OLYMPICS. THIS WILL BE ACCOMPANIED BY SOME GUSTY WINDS DEPENDING  
ON HOW DEEP THE OFFSHORE LOW BECOMES. STRONG FORCING COUPLED WITH  
AMPLE COLD AIR ALOFT SHOULD BRING SNOW TO THE  
CASCADES...BITTERROOTS...SAWTOOTH...AND INTO THE TETONS.  
 
RUBIN-OSTER  
 

 
 
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