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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1559Z Dec 18, 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
1059 AM EST THU DEC 18 2014

VALID 12Z SUN DEC 21 2014 - 12Z THU DEC 25 2014

...INTENSE LOW PRESSURE TO IMPACT THE EASTERN 1/3RD OF THE NATION
ON CHRISTMAS EVE AND CHRISTMAS DAY...

...OVERVIEW...
ACTIVE SPLIT-FLOW PATTERN FOR THE WEST---WILL TRANSITION TO A MORE
AMPLIFIED PATTERN ACROSS THE LOWER 48 THIS MEDIUM RANGE
PERIOD---WITH A SHORTWAVE RIDGE IN THE WEST AND A
NEGATIVELY-TILTED TROUGH IN THE GREAT LAKES AND
MID-ATLANTIC/NORTHEAST REGION.

...UNCERTAINTY ASSESSMENT...
IN THE EAST...IT WILL ULTIMATELY COME DOWN TO SURFACE CYCLONE
DEPTH...BUT FROM A SENSIBLE WEATHER PERSPECTIVE---THE EMBEDDED
ENERGY NOTED IN THE DETERMINISTIC GUIDANCE AS A TRIPLE-POINT
LOW---SHOULD BE THE FOCUS FOR AN INTENSE REGION OF LOW-LEVEL
CONVERGENCE AND STRONG WINDS. THE FORECAST UNCERTAINTY IS HOW THE
FEATURE MIGRATES FROM THE PIEDMONT NORTH-NORTHWESTWARD THROUGH
TIME.

IN THE WEST...THE UNCERTAINTY IS WHETHER OR NOT THE ~150+ METER
HEIGHT FALLS MOVING SOUTHWARD FROM THE BC COAST INTO NORTHERN
CALIFORNIA...WILL CLOSE OFF INTO A CLOSED 500MB CIRCULATION IN THE
GREAT BASIN. AND HOW FAR SOUTH WILL COLD/DRY CANADIAN SPILL INTO
THE PACIFIC NORTHWEST---WEST OF THE CONTINENTAL DIVIDE.

IN THE CENTRAL PORTION OF THE COUNTRY...THE UNCERTAINTY LIES IN
THE OUTCOME OF  THE VERY DEEP CIRCULATION IN SOUTHEAST CANADA AND
TRAJECTORIES (SOUTHWESTWARD PUSH) OF LOW-LEVEL COLD
ADVECTION---ORIGINATING IN THE CANADIAN PRAIRIE PROVINCES
(PRIMARILY SASKATCHEWAN/MANITOBA) IN THE SYSTEM'S
POST-STRENGTHENING PHASE.

...MODEL PREFERENCES...
THE 18/00Z DETERMINISTIC GFS/ECMWF WERE REASONABLE PIECES OF
FORECAST GUIDANCE THROUGH DAY 7. THE ECMWF IS A FASTER SOLUTION IN
THE NORTHEAST...WITH THE TRIPLE POINT (OCCLUSION-COLD-WARM FRONT)
AND A DEEPER SOLUTION AT THE SURFACE IN THE GREAT BASIN. THE
18/00Z ECENS/GEFS MEANS ARE REASONABLE. ALOFT...BOTH CARRY A
CLOSED SUB-522DM 500MB LOW CENTER MIGRATION ACROSS SOUTHEAST
CANADA...BUT THEY LOOK UNDERDONE WITH THE PRIMARY SURFACE CYCLONE
DEPTH INVOF LAKE HURON EASTWARD TO CENTRAL QUEBEC (984MB)---25/00Z
AND 25/12Z TIME FRAME.

THE SURFACE/500MB GRAPHICS USED THE AFOREMENTIONED DETERMINISTIC
GUIDANCE AT A 30/30 CLIP TO F144 (24/12Z) WITH A 20/20 RATIO FOR
EACH CORRESPONDING MEAN TO ACCOUNT FOR SPREAD. I TAPER AWAY FROM
THE 'WEAKER' DETERMINISTIC GFS---PRIMARILY TO HANDLE THE
MID/UPPER-LEVEL WAVE MIGRATION IN THE WEST AND GREAT BASIN.
FINALLY...MORE OF A 60/40 ECENS/GEFS BLEND AT F168 (25/12Z). GIVEN
THE DYNAMIC PATTERN ACROSS THE LOWER 48...HAVE ABOVE AVERAGE
CONFIDENCE IN THE 18/00Z CYCLE GUIDANCE AND BLEND CONFIGURATION
ALOFT. BELIEVE THE SURFACE GRAPHIC FOR DAY 7 WILL REQUIRE SOME
ALTERATION AS THE DETAILS BECOMES BETTER DEFINED WITH RESPECT TO
THE INTENSITY OF THE LOW-LEVEL WIND FIELDS.

...SENSIBLE WEATHER HIGHLIGHTS...
CHRISTMAS EVE/DAY CYCLONE---24/12Z TO 25/12Z TIME FRAME---WILL
HAVE REACHED FULL MATURITY---USHERING IN COLD ADVECTION ACROSS THE
NORTHEAST QUARTER OF THE NATION.
LOOKS TO BE A BROAD-SCALE AND RATHER INTENSE WIND/RAIN SCENARIO
EMBEDDED WITHIN THE WARM SECTOR...ALONG THE COLD FRONT AND INVOF
THE TRIPLE-POINT. IN THE SOUTHEAST...THE FRONTAL PASSAGE AND
WINDS/RAIN WILL BE MORE OF A CHRISTMAS EVE EVENT...WITH A DRY WEST
WIND ON CHRISTMAS DAY. AT A MINIMUM---SURFACE LOW AND ITS TROWAL
FEATURE SHOULD BE CONTINUING TO LIFT MOISTURE---IN THE FORM OF
SNOW---FROM THE CANADIAN MARITIMES---WESTWARD TO LAKE
HURON/SUPERIOR. EMBEDDED WITHIN THE COLD SECTOR...SCATTERED SNOW
SHOWERS---THE UPSLOPE FLOW MECHANISM GENERATING THE PRECIPITATION
IN THE APPALACHIANS/NEW ENGLAND AND SYNOPTIC-SCALE LAKE EFFECT FOR
THE GREAT LAKES. FOR THE NORTHERN/CENTRAL PLAINS AND MIDWEST...A
SHALLOW BUT STEEP MID-LEVEL LAPSE RATE ENVIRONMENT MAY PRODUCE
ISOLATED SNOW SHOWER ACTIVITY.

THE WPC TEMPERATURE FORECAST IS GOING TO TREND COOLER THAN
CONTINUITY IN THE PACIFIC NORTHWEST---TO ACCOUNT FOR A COOLER
CANADIAN AIRMASS WEST OF THE CONTINENTAL DIVIDE. THE QPF/POP
FORECAST LEANING FOR SOME ADDITIONAL AREAS OF PRECIPITATION IN THE
GREAT BASIN ALONG THE LEADING EDGE OF THE HEIGHT FALLS.

VOJTESAK