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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1936Z Dec 17, 2014)
 
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


EXTENDED FORECAST DISCUSSION...CORRECTED PARAGRAPH 3
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1100 AM EST WED DEC 17 2014

VALID 12Z SAT DEC 20 2014 - 12Z WED DEC 24 2014

...SOAKING RAIN FOR PORTIONS OF WASHINGTON/OREGON STARTING LATE
THIS WEEKEND...

..OVERVIEW...
ACTIVE SPLIT-FLOW PATTERN OVER THE CONUS WILL TRANSITION TO A MORE
AMPLIFIED RIDGE/TROUGH WITH A NEGATIVELY-TILTED TROUGH SETTING UP
OVER THE GREAT LAKES AND MID-ATLANTIC/NORTHEAST REGION(S) AROUND
THE CHRISTMAS EVE TIME FRAME.

A 'BUCKLING' IN THE PERSISTENTLY-STRONG WESTERLY PACIFIC JET WILL
LIKELY OCCUR DURING WEEKEND---AND BECOME THE IMPETUS FOR
CHANGE---AMPLIFYING THE PATTERN DOWNSTREAM OVER THE LOWER 48. THE
17/00Z GEFS/ECENS MEANS HAVE WAVERED ON THE SMALLER-SCALE DETAILS
SURROUNDING THE PATTERN DOWNSTREAM OF THE ROCKIES...BUT GENERALLY
AGREE THAT A 500MB WAVE FORMS INVOF THE CENTRAL MIDWEST---WITH
PACIFIC JET SUPPORT CARVING OUT THE 200MB-500MB TROUGH TOWARDS THE
END OF THIS MEDIUM RANGE PERIOD.

...UNCERTAINTY ASSESSMENT...
AFOREMENTIONED 'BUCKLING' OF THE PACIFIC JET ---IN THE OVERVIEW---
ALONG THE WEST COAST ON SAT/SUN APPEARED TO BE THE LEADING EDGE OF
A BROAD WARM FRONT THAT SURGES QUICKLY THROUGH THE NORTHERN
CONTINENTAL DIVIDE AND NORTHERN HIGH PLAINS. ON MONDAY...THE
FAST-MOVING NORTHWEST FLOW---NOSE OF THIS PACIFIC JET
CONFIGURATION ALOFT--SUPPORTS THE DEEPENING OF LEE SIDE
CYCLOGENESIS OVER SOUTHEASTERN ALBERTA AND THE BIRTH OF A
WELL-DEFINED SURFACE CYCLONE---MIGRATING ACROSS THE UPPER HALF OF
THE MISSOURI VALLEY EARLY IN THE PERIOD. ITS 'MORPHOLOGY' AROUND
DAY 6...IN THE MIDWEST---APPEARS TO BE A FUNCTION OF ITS DEPTH AND
FORWARD MOTION IN RELATIONSHIP TO THE UPPER-LEVEL JET. ONE OF THE
SMALLER-SCALE DETAILS IS---CAN THE SURFACE CYCLONE DRAW UPON
ENOUGH COLDER...CANADIAN AIR AND PULL THAT COLDER AIRMASS
SOUTHEASTWARD FASTER/ENOUGH---TO INTENSIFY AND AMPLIFY---THE
ENTIRE PATTERN IN THE NORTHEAST QUARTER OF THE NATION---IN TIME
FOR THE PEAK CHRISTMAS TRAVEL 'WINDOW'? THE SHORT ANSWER IS---YES.
DO THE 17/00Z DETERMINISTIC SOLUTIONS HAVE ALL THE 'MORPHOLOGY'
DETAILS "DOWN PAT"---THE ANSWER IS NO. THE 'NO' PART OF THE ANSWER
IS ON THE WINTER WEATHER SIDE--AND WHO MAY/MAY NOT SEE A 'WHITE
CHRISTMAS' MORNING.

WHAT CAN BE GLEANED FROM THE 17/00Z GUIDANCE IS...THERE WILL BE A
DYNAMIC UPPER-LEVEL TROUGH EMERGING AND THE THREAT OF ORGANIZED
SYNOPTIC-SCALE PRECIPITATION ASSOCIATED WITH THE SURFACE
CYCLONE--WITH A DISTINCT WINTER SIDE---ALONG ITS NORTHERN
PERIPHERY---AND WET SIDE AHEAD OF THE DEEPER---COLD AIRMASS THAT
IS RAIN.

...MODEL PREFERENCES...
THE 17/00Z GEFS/ECENS AND THEIR DETERMINISTIC RUNS WERE IN DECENT
AGREEMENT THROUGH 22/12Z...AND WPC LIKES THE SLIGHTLY SLOWER ECENS
VERSUS THE GEFS AFTER DAY 5 FROM THE DATELINE TO THE EASTERN
SEABOARD.  VOJTESAK

...SENSIBLE WEATHER HIGHLIGHTS...

WITH AN INCREASING JET INTO THE PAC NW THIS WEEKEND INTO MONDAY...
150KTS PLUS... EXPECT A PRETTY WET PERIOD FOR ESPECIALLY COASTAL
WA/OR AND INTO THE CASCADES. NEAR RIGHT ANGLE FLOW INTO THE
TERRAIN COUPLED WITH PW ANOMALIES IN THE +2 TO +3 RANGE SHOULD
HELP MAXIMIZE PRECIP POTENTIAL... WITH SOME HIGHER RES GUIDANCE
SHOWING SEVERAL INCHES OF QPF SAT-MON -- UP TO ABOUT TEN INCHES AT
THE HIGH SIDE OF THE SPREAD. SNOW LEVELS WILL BE FAIRLY HIGH IN
THE PACIFIC AIRMASS BUT WILL ACCUMULATE AT THE HIGHEST PEAKS.

IN THE EAST... THE PESKY SYSTEM FORECAST TO SLIDE OFF THE COAST
WILL LIKELY DEVELOP FAR ENOUGH OFFSHORE TO HAVE LOW IMPACTS ALONG
THE COAST... BUT COULD THROW SOME LIGHT RAIN/SNOW BACK INTO THE
MID-ATLANTIC/NORTHEAST AS IT EXITS INTO THE ATLANTIC.

MID-PERIOD... QUESTIONS FROM THE LAST FEW NIGHTS ABOUT HOW MUCH
ENERGY WOULD DIG/SPLIT INTO N MEXICO AS THE RIDGE BUILDS OFF
CALIFORNIA AND THE TROUGHING DIGS INTO AZ/NM LATE THIS WEEKEND.
TREND HAS BEEN AWAY FROM THE DIGGY SOLUTIONS AND MORE TOWARD WHAT
THE GEFS MEMBERS HAD 48 HRS AGO -- BUT IT APPEARS THAT THE GEFS
AND ECMWF ENSEMBLES CONVERGED SOMEWHAT MIDWAY BETWEEN EACH OF
THEIR FORMER SOLUTIONS. CURRENT 12Z ECMWF ENSEMBLES STILL LAG JUST
BEHIND THE GEFS MEMBERS AND SHOW A BIT MORE AMPLITUDE BUT EVEN A
BLEND BETWEEN THE TWO SIGNALS A NEGATIVELY TILTED TROUGH
DEVELOPING BY NEXT WEDNESDAY... 24 DECEMBER. SLOWER SOLUTIONS
WOULD ALLOW MUCH MORE RAIN TO DEVELOP IN THE SOUTHEAST AHEAD OF
THE TROUGH AXIS. GIVEN ITS TIMING... IMPACTS WILL BE GREATER THAN
AVERAGE. STAY TUNED.

FRACASSO