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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0621Z Apr 20, 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
211 AM EDT MON APR 20 2015

VALID 12Z THU APR 23 2015 - 12Z MON APR 27 2015

...OVERVIEW...
AN UPPER-LEVEL CIRCULATION ANCHORS ACROSS SOUTHEAST CANADA THIS
MEDIUM RANGE PERIOD---WITH A COOL APRIL WEATHER PATTERN FOR THE
GREAT LAKES AND NORTHEAST. A PERSISTENT WESTERLY JET STREAM
PRESENCE ACROSS THE SOUTHERN STATES---FROM COAST TO COAST--- WILL
PROMOTE MID-LEVEL ENERGY MIGRATION AND SCATTERED PRECIPITATION
EVENTS FROM CALIFORNIA TO THE CAROLINAS.

...MODEL PREFERENCES...
THOUGHT THE 19/12Z ECENS AND NAEFS MEANS (A 60/40 BLEND
RESPECTIVELY) ADEQUATELY HANDLE THE FLOW PATTERN---FROM WEST TO
EAST---THIS MEDIUM RANGE PERIOD. AND WILL COVER THE DIFFERENCES IN
THE SHORTWAVE DETAILS POSED BY THEIR INDIVIDUAL DETERMINISTIC
RUNS. THE 19/12Z DETERMINISTIC GFS/ECMWF WAS LESS THAN DESIRABLE
GUIDANCE AFTER THE MID-POINT OF DAY 4---FOR THE LOWER 48---AS A
WHOLE. VARIED RESULTS---AT REGIONAL SCALE ARE NOTED BELOW. 

...UNCERTAINTY ASSESSMENT...
CONCERNING THE UPPER-LEVEL CIRCULATION OVER QUEBEC---THE 19/12Z
DETERMINISTIC GFS/ECMWF AND CANADIAN APPEARED TO BE 'USE-ABLE'
PIECES OF GUIDANCE UNTIL LATE DAY 4---WHEN THE ECMWF APPEARED TO
BREAK CONTINUITY OVER SOUTHEAST CANADA AND BEGINS TO GENERATE A
SLIGHTLY MORE-AMPLIFIED TROUGH ALONG THE EASTERN SEABOARD.

CONCERNING THE ORGANIZED PACIFIC SHORTWAVE EXITING THE COLORADO
FRONT RANGE ON FRIDAY AFTERNOON---THE 19/12Z DETERMINISTIC
GFS/ECMWF AND CANADIAN WERE IN MUCH BETTER AGREEMENT---AND FRIDAY
NIGHT IS WHEN THE ECMWF BEGINS TO DEVELOP MORE SHORTWAVE RIDGING
OVER THE MISSISSIPPI VALLEY AND TAKES A MORE NORTHEASTWARD TRACK
VERSUS THE GFS AND CANADIAN.

CONCERNING THE EVOLVING UPPER-LEVEL TROUGH PATTERN IN THE WEST AND
PROJECTED DEEP CYCLONE MIGRATING THROUGH THE SOUTHEAST GULF OF
ALASKA TOWARDS THE PACIFIC NORTHWEST (DAYS 6-7)---

THE DETERMINISTIC GUIDANCE STILL LIKES THE IDEA OF A SECOND
MID-LEVEL WAVE TO MOVE INTO SOUTHERN CALIFORNIA SATURDAY
MORNING---BUT THERE REMAINS SOME QUESTION AS TO--IF AND
WHERE---THE WAVE TRIES TO PINCH OFF INTO A CLOSED 500MB
CIRCULATION. THE ECMWF/CANADIAN HAVE COMPLETELY BACKED OFF THE
IDEA OF A CLOSED CIRCULATION AND THE 12Z GFS CONTINUES TO FAVOR
THE FORMATION OF A CLOSED CIRCULATION BETWEEN POINT CONCEPTION AND
THE MEXICAN BORDER ON FRIDAY NIGHT/EARLY SATURDAY MORNING.

THE DEEP CYCLONE EDGING SOUTHEASTWARD TOWARDS THE OLYMPIC
PENINSULA DOES HAVE A GENERAL CONSENSUS WITH THE 'UPPER' FRONT
PROGRESSION ACROSS THE CASCADES---ITS STRENGTH---BEING THE
CHALLENGE RIGHT NOW. THE 19/12Z ECMWF/GFS TRENDED A TAD FASTER AND
THE CANADIAN---SLOWER WITH THE HEIGHT FALLS (THE 564 DM HEIGHT
CONTOUR) MOVING THROUGH NORTHERN CALIFORNIA AROUND THE 27/12Z
(MONDAY) TIME FRAME. HOWEVER---THE 19/12Z ECMWF NOTED A MUCH
STRONGER LEAD SHORTWAVE MOVING THROUGH THE PACIFIC NORTHWEST FOR
SATURDAY/SUNDAY---VERSUS THE GFS/CANADIAN.

...SENSIBLE WEATHER HIGHLIGHTS...
'CHILLY' APRIL WEATHER PATTERN FOR THE GREAT LAKES AND NORTHEAST
WITH BROAD CYCLONIC FLOW FROM EASTERN CANADA 'CYCLING'
COUNTERCLOCKWISE FROM NORTHWEST ONTARIO TO SOUTHERN NEW
ENGLAND...THE GULF OF MAINE AND NOVA SCOTIA. MORE OF A WESTERLY
LOW-LEVEL WIND COMPONENT FOR THE CENTRAL AND SOUTHERN APPALACHIANS
AND MID-ATLANTIC STATES---AFFORDING THE LOWER ELEVATIONS AND
COASTAL AREAS WITH SOME MODIFICATION OF THIS COLD ADVECTION
'CYCLING'.

IN THE WEST...AN ACTIVE SPRING WEATHER PATTERN COMES TO THE GREAT
BASIN... SIERRA... CENTRAL AND SOUTHERN CALIFORNIA... AND
DOWNSTREAM ACROSS THE COLORADO RIVER BASIN AND SOUTHERN ROCKIES
WITH A COUPLE OF ORGANIZED MID-LEVEL SYSTEMS MIGRATING INTO THE
SOUTHERN HIGH PLAINS DURING THE LATTER HALF OF THE PERIOD.

MEDIUM RANGE PRECIPITATION FOCUS---EAST OF THE CONTINENTAL DIVIDE
SHOULD BE ALONG/BENEATH THE WEST-SOUTHWESTERLY ORIENTED JET STREAM
AXIS FROM WEST TEXAS TO THE CAROLINAS. ACROSS THE SOUTH CENTRAL
PLAINS---WITH AN ORGANIZED SYSTEM FOR FRIDAY/SATURDAY APPEARS ON
TRACK.

VOJTESAK