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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0555Z Nov 20, 2014)
 
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1255 AM EST THU NOV 20 2014

VALID 12Z SUN NOV 23 2014 - 12Z THU NOV 27 2014

...PATTERN OVERVIEW...

AT THE START OF THE PERIOD...23/1200Z...A SPLIT FLOW REGIME WILL
BE IN PLACE WITH A SOUTHERN STREAM SHORTWAVE ATTAINING A NEGATIVE
TILT WHILE LIFTING FROM THE OZARKS UP TOWARD THE UPPER GREAT
LAKES. IT APPEARS THE NORTHERN BRANCH OF THE JET WILL BECOME
ACTIVE AS MID-LEVEL VORTICITY TRACKS FROM THE CENTRAL ROCKIES INTO
THE SOUTHERN PLAINS ON SUNDAY. THE COMBINATION OF THESE TWO
FEATURES WILL HELP STRENGTHEN A LONGWAVE TROF ACROSS THE CENTER OF
THE COUNTRY. ADDITIONALLY...THE INITIAL CYCLONE MOVING UP THE MS
RIVER IS EXPECTED TO RAPIDLY STRENGTHEN POSSIBLY LEADING TO A
SUB-975 MB PRESSURE IN THE VICINITY OF THE UPPER GREAT LAKES BY
24/1200Z. GIVEN A STRONG DOWNSTREAM RIDGE ANCHORING THE
ATLANTIC...MODELS SHOW THIS SYSTEM GAINING ADDITIONAL LATITUDE
WITH THE STRONGEST HEIGHT FALLS LIFTING INTO THE NORTHEASTERN U.S.
THE LOW-LEVEL BAROCLINIC ZONE SHOULD EXIT THE EASTERN SEABOARD
SOMETIME TUESDAY WITH ITS FORWARD PROGRESSION TEMPERED BY THE
RIDGE TO ITS EAST. WHILE THE PATTERN IS MUCH MORE CLEAR-CUT ACROSS
THE EASTERN HALF OF THE U.S...MODELS VARY WITH THE HANDLING OF THE
EASTERN PACIFIC/WESTERN STATES. DEPENDING ON THE PIECE OF
GUIDANCE...EITHER AN UPPER RIDGE WILL BE MAINTAINED THROUGHOUT THE
REMAINDER OF THE MEDIUM RANGE OR ENOUGH SHORTWAVE ENERGY WILL
SLIDE ONSHORE TO ALLOW A MEAN TROF TO BE FEATURED ACROSS THE WEST
BY MID-WEEK. MORE DETAILS FOLLOW IN THE NEXT SECTION.


...MODEL EVALUATION/WPC PREFERENCES...

WITH THE EXCEPTION OF THE 12Z UKMET WHICH IS DISPLACED TO THE
SOUTH...ALL MODELS SEEM TO HANDLE THE SYSTEM TRACKING THROUGH THE
OZARKS COMPARABLY EARLY SUNDAY. THE GUIDANCE VARY WITH HOW DEEP
THE CYCLONE BECOMES OVER THE UPPER GREAT LAKES WITH THE 12Z CMC
BEING THE WEAKEST AMONG THE DETERMINISTIC MODELS. DID FEEL THE 18Z
GFS WAS A BIT SLOW WITH THE EASTWARD PROGRESSION OF THE COLD FRONT
WHILE THE 18Z GFS PARALLEL/12Z ECMWF APPEARED MORE COMPARABLE TO
THE ENSEMBLE MEANS. WPC FAVORED A COMBINATION OF THE 18Z GFS
PARALLEL/12Z ECMWF FOR DAYS 3 AND 4...NOV 23/24. AS MENTIONED IN
THE PREVIOUS SECTION...THE MODELS AS A WHOLE SEEM TO HANDLE THE
CENTRAL TO EASTERN U.S. TROF WELL BUT VARY GREATLY ACROSS THE
WEST. LOOKING FARTHER UPSTREAM FOR THE ORIGIN OF THIS SYSTEM...IT
APPEARS TO EMERGE FROM A BLOCK SET UP ACROSS THE BERING SEA. THUS
FAR IT SEEMS LIKE THE GUIDANCE DOES NOT HAVE A GOOD HANDLE ON HOW
SHORTWAVES EJECTING OUT OF THE CLOSED LOW ACROSS THE ALEUTIANS
WILL NAVIGATE DOWNSTREAM. THE PAST THREE CYCLES OF THE ECMWF
CONTINUE TO SHOW A RATHER POTENT SHORTWAVE DIGGING INTO THE
WESTERN U.S. DURING THE DAY 6/7 TIME FRAME. SUCH A SOLUTION IS
SUPPORTED BY THE 12Z CMC AS WELL...ALBEIT WITH TIMING DIFFERENCES.
THE PAST SEVERAL CYCLES OF THE GFS SEEM TO BE MOST ADAMANT ABOUT
MAINTAINING A MEAN RIDGE ACROSS THE WEST. THE RESPECTIVE ENSEMBLE
MEANS REFLECT THESE VARYING OPINIONS. THE 12Z CMC/NAEFS ENSEMBLE
MEANS DO NOT SHOW THE STOUT RIDGE OF THE 18Z GEFS MEAN BUT ALSO DO
NOT SHOW AS MUCH AMPLIFICATION AS THE 12Z ECMWF ENSEMBLE MEAN.
WITH RATHER LOW CONFIDENCE...DECIDED TO DISCOUNT THE SOLUTION OF
THE GFS/GEFS MEAN SUITE AND SPLIT THE DIFFERENCE BETWEEN THE 12Z
ECMWF/NAEFS ENSEMBLE MEANS FOR DAYS 5-7.


...SENSIBLE WEATHER HIGHLIGHTS...

ABOVE NORMAL TEMPERATURES WILL DOMINATE FOR MUCH OF THE BEGINNING
OF THE FORECAST GIVEN STRONG WARM ADVECTION AHEAD OF THE DEEP
SURFACE LOW. ABUNDANT CLOUD COVER/WARM ADVECTION/WIDESPREAD
PRECIPITATION ARE ALL FACTORS IN KEEPING MILD OVERNIGHT
TEMPERATURES IN PLACE. THE CURRENT WPC FORECAST SUGGESTS LOW
TEMPERATURE ANOMALIES APPROACHING 20 DEGREES ABOVE CLIMATOLOGY
FROM THE MS RIVER VALLEY TO THE EAST COAST FOR DAYS 3-5. THE
DEPARTURES FROM NORMAL WILL NOT BE QUITE AS SIGNIFICANT REGARDING
THE HIGH TEMPERATURES BUT IT WILL STILL BE RATHER MILD FOR LATE
NOVEMBER. THE POST COLD FRONTAL AIR MASS IS EXPECTED TO BE CHILLY
BUT NOTHING LIKE THE PREVIOUS BOUTS OF COLD WEATHER. THERE ARE
SUGGESTIONS OF ANOMALIES APPROACHING THE 10 TO 15 DEGREE BELOW
NORMAL RANGE WHICH WOULD CARRY HIGHS INTO THE TEENS AND 20S ACROSS
THE NORTHERN TIER OF THE COUNTRY.

REGARDING PRECIPITATION...A WET PERIOD IS IN STORE ALONG AND AHEAD
OF THE ADVANCING COLD FRONT. THE GUIDANCE SHOWS TWO BATCHES OF
PRECIPITATION EAST OF THE ROCKIES. ONE IS EXPECTED TO FOLLOW THE
SURGE OF HIGH PWAT AIR ACROSS THE SOUTHERN PLAINS/LOWER MS VALLEY
EASTWARD ALONG THE GULF COAST REGION. THE OTHER SWATH OF
PRECIPITATION SHOULD BE A MIXED PHASE EVENT ACROSS THE GREAT LAKES
IN ASSOCIATION WITH THE DEEPENING SURFACE LOW. IT IS DIFFICULT TO
PINPOINT PRECIPITATION TYPE DETAILS...BUT THE STRENGTH OF THE
SURFACE CYCLONE SHOULD ALLOW SUFFICIENT COLD AIR TO WRAP IN AROUND
THE BACKSIDE TO SUPPORT COMMA-HEAD SNOWS. ACROSS THE WESTERN
U.S...IT SHOULD BE RATHER ACTIVE AS WELL GIVEN SUFFICIENT ONSHORE
FLOW COMBINING WITH MID-LEVEL IMPULSES EMBEDDED WITHIN THE
NORTHWESTERLY FLOW ALOFT. THE ASSOCIATED PACIFIC MOISTURE SHOULD
MIGRATE INLAND SPREADING PRECIPITATION FROM THE OLYMPICS/CASCADES
INTO THE INTERMOUNTAIN WEST AND ROCKIES. THE NEBULOUS FORECAST FOR
DAYS 6/7 DEFINITELY MAKES IT DIFFICULT TO SUGGEST HIGHLIGHTS AT
THIS TIME OVER THE WEST.


RUBIN-OSTER