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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0612Z Apr 17, 2015)
 
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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
212 AM EDT FRI APR 17 2015

VALID 12Z MON APR 20 2015 - 12Z FRI APR 24 2015

...UNCERTAINTY ASSESSMENT/MODEL PREFERENCES...
BLENDED THE 16/12Z ECENS...NAEFS AND GEFS AT A 50/30/20 RATIO---TO
HANDLE THE 'VAGARIES' OF THE FLOW PATTERN---ALONG THE WEST COAST
OF NORTH AMERICA. FOR THE EAST AND CENTRAL UNITED STATES---THIS
BLEND REASONABLY HANDLES THE EAST-NORTHEASTWARD MIGRATION OF A
CUTOFF 500MB LOW FROM THE WEST CENTRAL HIGH PLAINS TO THE CENTRAL
GREAT LAKES AT DAY 4. AND ITS GRADUAL 'DECAY' AND SHEARED
APPEARANCE ACROSS THE ST LAWRENCE VALLEY AND NORTHERN NEW
ENGLAND---DAY 5 THROUGH DAY 7---WILL BE A 'PLACE-HOLDER' FOR
NOW---BUT EXPECT SOME ALTERATIONS FOR THE NEW ENGLAND AND CANADIAN
MARITIME LOW TRACK FORECASTS IN THE FUTURE. 

IN THE WEST...MUCH OF THE MODEL SPREAD AND CONTENTION ORIGINATES
WEST OF HAIDA GWAII AROUND DAY 5---AND CONCERNS THE LONGITUDE
WHERE---AND STRUCTURE OF---ORGANIZED ENERGY AND ITS TRANSFER
DOWNSTREAM FROM THE GULF OF ALASKA TO THE LOWER 48'S WEST COAST.
WHAT ENERGY THAT DOES MIGRATE EASTWARD ALONG 50N---WILL ENCOUNTER 
A BROAD UPPER-LEVEL RIDGE---ANCHORED ALONG THE CANADIAN ROCKIES
AND QUICKLY DIG SOUTHWARD ALONG 125W-130W. AT THIS POINT...CANNOT
TELL IF MID-LEVEL ENERGY PINCHES OFF INTO A CLOSED LOW...SINGLE
OPEN WAVE...OR A SERIES OF SHORTWAVES. THE GFS...ECMWF AND
CANADIAN SEEM TO BE IN GENERAL AGREEMENT AT THE DAY3-DAY 4 TIME
FRAME---WITH A LEAD IMPULSE DIGGING SOUTH-SOUTHEASTWARD ALONG THE
CENTRAL AND SOUTHERN COAST LINE OF CALIFORNIA. BUT THAT IS AS FAR
AS THE DETERMINISTIC GUIDANCE COULD REASONABLY BE COUNTED UPON (TO
DETERMINE COLD ADVECTION)---AND DEFERRED TO THE BLENDED MEANS AS
BROAD TROUGHING BEGINS TO TAKE SHAPE BETWEEN 120W-125W FOR DAYS
6-7.

IN THE EAST...THE 16/12Z DETERMINISTIC GFS/ECMWF WERE 'USE-ABLE'
INTO DAY 4---BEFORE THE RATE AT WHICH NORTHERN STREAM ENERGY
ORIGINATING IN CENTRAL CANADA---BEGINS TO ALTER THE SPECIFIC
TIMING OF NOTABLE SHORTWAVES---AND PRECIPITATION GENERATION
(P-TYPE). THE CYCLONIC FLOW DOES OFFER SOME COLD ADVECTION NO
DOUBT---BUT ALSO RAISES QUESTIONS. FIRST QUESTION---CAN THE
CYCLONIC FLOW THAT APPEARS TO BE SHEARING THROUGH TIME---GENERATE
AND SUSTAIN A SERIES DEEPENING SURFACE WAVES ALONG ITS BASE? AND
QUESTION TWO...CAN SOUTHERN STREAM ENERGY 'PHASE' THE PATTERN
ALONG THE EAST COAST (BETWEEN CAPE HATTERAS AND THE GULF OF MAINE)
BEYOND DAY 5---AS THE 16/12Z ECMWF AND CANADIAN CURRENTLY SUGGEST?
BY NEXT WEDNESDAY---THIS PATTERN AND SCENARIO (FOR MORE WINTER
WEATHER?) SHOULD BECOME MUCH CLEARER THAN IT SITS RIGHT NOW.

...SENSIBLE WEATHER HIGHLIGHTS...
THE LOWER 48 WILL GRADUALLY GET AWAY FROM THE SPLIT-FLOW REGIME
CURRENTLY IN PLACE AS THE CUTOFF LOW OVER EASTERN COLORADO SLOWLY
DRIFTS OFF INTO THE GREAT LAKES.

EXPECT A WET PATTERN FOR DAYS 3-4 FOR THE MIDWEST---THE LOWER HALF
OF THE MISSOURI VALLEY...THE ENTIRE MISSISSIPPI VALLEY AND WESTERN
OHIO VALLEY. BY DAY 5...AS THE CUTOFF SYSTEM BEGINS TO STALL INVOF
WESTERN ONTARIO---THE PRECIPITATION BEGINS TO SHIFT
NORTHEASTWARD---IN EARNEST---ACROSS THE APPALACHIANS AND THE EAST
COAST.

TEMPERATURES BENEATH THE SLOW-MOVING UPPER-LEVEL LOW SHOULD BE
10-15F BELOW NORMAL FOR THE LAST TEN DAYS OF APRIL (IE GREAT LAKES
AND MIDWEST IN GENERAL).

IN THE WEST---ISOLATED TO SCATTERED PRECIPITATION APPEARS TO BE
MORE OF A PRE-FRONTAL CHALLENGE AND CONVECTIVE IN NATURE FROM
NORTHERN CALIFORNIA TO EASTERN WASHINGTON---WITH A SOUTHERLY
MID-LEVEL FLOW PRECEDING A SLOW-MOVING BUT 'SHALLOW' COLD
FRONT---DAYS 4-7. THE 'SHALLOW' FRONT---THE RESULT OF A DIGGING
TROUGH AXIS ALONG THE WEST COAST---PULLING MARITIME PACIFIC AIR
SOUTHWARD RATHER THAN EASTWARD---PREVENTING A DEEPENING OF THE
COLD ADVECTION INTO THE NORTHERN GREAT BASIN AND EAST OF THE
CASCADES.

VOJTESAK