483  
FXUS02 KWBC 050604  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
103 AM EST SUN NOV 05 2017  
 
VALID 12Z WED NOV 08 2017 - 12Z SUN NOV 12 2017  
   
..PATTERN OVERVIEW/GUIDANCE/UNCERTAINTY ASSESSMENT/PREFERENCES
 
 
A SPLIT FLOW REGIME IS NOTED DURING THE PERIOD WITH A FEW PRIMARY  
FEATURES TO NOTE. ON WEDNESDAY MORNING...A STRETCHED OUT STRIP OF  
VORTICITY SLIDING THROUGH THE CENTER OF THE COUNTRY WILL CONTINUE  
TO ACCELERATE EASTWARD REACHING THE MID-SOUTH BY THE FOLLOWING  
MORNING. DEPENDING ON THE DEPTH OF THIS SHORTWAVE...SOME WAVES OF  
LOW PRESSURE SHOULD DEVELOP ALONG THE EXITING WAVY FRONTAL  
BOUNDARY. THE PRIMARY SURFACE CYCLONE MAY STRENGTHEN OVER THE  
WESTERN ATLANTIC BUT SHOULD REMAIN FAR ENOUGH OFF THE EASTERN  
SEABOARD. LOOKING BACK TOWARD THE WEST COAST...A POWERFUL UPPER  
LOW WILL DROP DOWN FROM THE GULF OF ALASKA WITH FORECAST 500-MB  
HEIGHT ANOMALIES ON THE ORDER OF 2.5 TO POSSIBLY 3 STANDARD  
DEVIATIONS BELOW AVERAGE VIA THE 00Z GFS. THIS CLOSED CIRCULATION  
SHOULD REACH THE NORTHERN CA/PACIFIC NORTHWEST COAST BY LATE  
WEDNESDAY WHILE GRADUALLY EVOLVING INTO MORE OF AN OPEN WAVE  
THEREAFTER. A LIKELY PROGRESSIVE STACK OF SHORTWAVES SHOULD  
ACCELERATE TOWARD THE GREAT PLAINS/MISSISSIPPI RIVER VALLEY BY  
NEXT WEEKEND. LASTLY...AMPLIFICATION IN THE NORTHERN STREAM WILL  
POTENTIALLY DRAW A SIGNIFICANT SHOT OF ARCTIC AIR TO SECTIONS OF  
THE CENTRAL/EASTERN U.S. FROM THURSDAY ONWARD. MODELS EXHIBIT  
EXTREME DIFFERENCES WITH THIS FEATURE WHICH WILL BE NOTED IN THE  
FOLLOWING SECTION. LASTLY...AMPLIFIED FLOW RETURNS TO THE EASTERN  
PACIFIC TOWARD THE LATTER HALF OF NEXT WEEKEND.  
 
REGARDING THE INITIAL STRUNG OUT SHORTWAVES MOVING TOWARD THE  
LOWER TENNESSEE VALLEY...MODELS HAVE BEEN TRENDING TOWARD A  
SLIGHTLY HIGHER AMPLITUDE WAVE. THIS WOULD INDUCE A MODEST REGION  
OF LOW PRESSURE ALONG THE FRONT WHICH COULD DEEPEN WHILE MOVING  
OUT OVER THE WESTERN ATLANTIC. SHIFTING FOCUS TO THE WEST  
COAST...MULTI-MODEL CYCLE TRENDS SHOW AN EASTWARD JOG WITH THE  
POSITION OF THE EASTERN PACIFIC UPPER LOW. IT APPEARS THAT THE  
LARGEST DIFFERENCES ARE WITH REGARD TO THE STRENGTH OF THE  
DOWNSTREAM RIDGE WITH MANY OF THE 12Z ECMWF ENSEMBLE MEMBERS  
FAVORING A HIGHER DEGREE OF AMPLITUDE. AS THE PROGRESSIVE  
SHORTWAVE CONTINUES ITS MARCH TOWARD THE CENTER OF THE  
COUNTRY...THE SURFACE PATTERN BECOMES COMPLICATED BY EFFECTS FROM  
THE AMPLIFICATION IN THE NORTHERN STREAM. WITH THE PAST FEW RUNS  
OF THE ECMWF SHOWING MUCH GREATER SOUTHWARD ELONGATION OF THIS  
TROUGH...A ROBUST SURFACE ANTICYCLONE SETS UP OVER THE UPPER  
MIDWEST BY FRIDAY MORNING. MEAN SEA LEVEL PRESSURES WOULD BE  
AROUND 1036-MB ON 10/1200Z BASED ON THESE ECMWF RUNS.  
MEANWHILE...THE 00Z/18Z/12Z GFS WOULD SUPPORT A SURFACE CYCLONE IN  
THIS EXACT POSITION WHICH LEADS TO QUITE A FORECAST CHALLENGE. THE  
00Z GFS FROM YESTERDAY WAS THE LAST RUN IN ITS SUITE WHICH  
DEPICTED THE DEGREE OF AMPLIFICATION SHOWN BY THE ECMWF. UNTIL THE  
00Z CMC ARRIVED LOOKING MORE LIKE THE 12Z ECMWF...THE CMC/UKMET  
HAD NOT OFFERED MUCH RECENT SUPPORT FOR THIS AMPLIFIED SCENARIO.  
COMPARISON OF MULTI-DAY ENSEMBLE SPAGHETTI PLOTS...WITH THE  
EXCEPTION OF THE ECMWF ENSEMBLE MEMBERS...THE CMC/GEFS HAVE BEEN  
WAFFLING AROUND ALL OVER THE PLACE. SUFFICE TO SAY...A LOW  
CONFIDENCE FORECAST IS AT HAND HERE. FINALLY...ALL GLOBAL  
ENSEMBLES AGREE ON A RETURN OF LONGWAVE TROUGHING TO THE EASTERN  
PACIFIC BY NEXT WEEKEND....A GENERAL REPEAT OF WHAT IS IN PLACE ON  
DAY 3/WEDNESDAY.  
 
GIVEN THE RUN-TO-RUN CONSISTENCY EXHIBITED BY THE ECMWF AND ITS  
ENSEMBLE MEAN...DECIDED TO LEAN MORE TOWARD ITS SOLUTION  
THROUGHOUT THE PERIOD. AMONG THE GFS CYCLES...FAVORED THE PREVIOUS  
00Z RUN AS IT HAD THE MOST IN COMMON WITH THE ECMWF WHILE ALSO  
STAYING SLIGHTLY CLOSER TO CONTINUITY. GENERALLY TOOK AN EVEN  
SPLIT OF THESE TWO SOLUTIONS THROUGHOUT THE PERIOD WHILE QUICKLY  
INCORPORATING A MORE ENSEMBLE BASED APPROACH AFTER FRIDAY. DID  
THROW A TOKEN AMOUNT OF THE 18Z GFS IN THE MIX TO ACCOUNT FOR ANY  
SHIFT TOWARD A MORE DE-AMPLIFIED SCENARIO.  
 
   
..SENSIBLE WEATHER/THREATS
 
 
EXCEPT FOR THE SOUTHEASTERN U.S. AS WELL AS THE DESERT  
SOUTHWEST/FOUR CORNERS REGION...MUCH OF THE COUNTRY WILL BE  
ENTRENCHED IN BELOW AVERAGE TEMPERATURES THROUGHOUT THE FORECAST.  
OF COURSE THIS IS HIGHLY DEPENDENT ON THE MODEL CHOICE WITH THE  
GFS/ECMWF DIVERGING BY ALMOST 30 DEGREES BY DAY 5/FRIDAY. GIVEN A  
COLD SOLUTION WAS FAVORED...EXPECTED DEPARTURES FROM NORMAL WILL  
BE AROUND 10 TO 20 DEGREES BELOW AVERAGE AS THE MODIFIED ARCTIC  
AIR WORKS ITS WAY INTO THE CENTRAL/EASTERN U.S. THE COLD SPOT  
SHOULD BE FROM NORTHEASTERN MONTANA THROUGH NORTH DAKOTA AND INTO  
NORTHERN MINNESOTA WHERE LOWS WILL STRUGGLE TO ESCAPE THE SINGLE  
DIGITS UNTIL FRIDAY. AT THIS TIME...IT DOES NOT APPEAR DAILY  
TEMPERATURE RECORDS WILL BE BROKEN.  
 
THE SLOW-MOVING FRONT STRETCHING FROM THE ARKLATEX INTO THE  
CAROLINAS WILL BE AN INITIAL FOCUS FOR WIDESPREAD RAINFALL. ALL  
THIS ACTIVITY WILL SHIFT SOUTHWARD WITH CONDITIONS GRADUALLY  
IMPROVED TOWARD THE END OF THE WORK WEEK. ACROSS THE WEST  
COAST...HEAVY PRECIPITATION ACCOMPANIED THE APPROACHING CLOSED LOW  
WILL IMPACT NORTHERN CA UP ALONG THE PACIFIC NORTHWEST COAST. THE  
ACTIVITY IS FORECAST TO SHIFT TOWARD THE UPPER INTERMOUNTAIN WEST  
WITH ENOUGH COLD AIR TO ALLOW SNOWFALL OVER THE HIGHER TERRAIN  
INCLUDING THE CASCADES...BITTERROOTS...SAWTOOTH...AND TETONS.  
FARTHER DOWNSTREAM...SOME WARM ADVECTION SHOWERS ARE POSSIBLE OVER  
THE CENTRAL U.S. ALTHOUGH IT IS UNKNOWN HOW MUCH MOISTURE RETURN  
THERE WILL BE. AND ACROSS INTERIOR NEW ENGLAND...WHILE MOISTURE  
WILL LIKELY BE MEAGER...SOME ACCUMULATING SNOWS ARE POSSIBLE  
ACCOMPANYING THE POTENTIAL ARCTIC INTRUSION.  
 
RUBIN-OSTER  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page