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Extended Forecast Discussion
 
(Latest Discussion - Issued 0612Z Aug 03, 2015)
 
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Abbreviations and acronyms used in this product
 
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 AUG 03 2015

VALID 12Z THU AUG 06 2015 - 12Z MON AUG 10 2015

THE 2/12Z MODEL CYCLE AND GLOBAL GUIDANCE DID NOT SEEM TO OFFER
MANY SURPRISES---AND EASILY MAINTAINED CONTINUITY FROM THE
PREVIOUS SHIFT FOR THIS MEDIUM RANGE FORECAST PACKAGE. THE
PRESENCE OF TWO BROAD TROUGHS---ALONG BOTH COASTS OF NORTH
AMERICA---PROVIDE A RATHER UNIQUE AUGUST SENSIBLE WEATHER PATTERN
FOR THE LOWER 48. THE 2/12Z ECENS/NAEFS MEANS AND 2/12Z
DETERMINISTIC ECMWF/GFS WERE REASONABLE SOLUTIONS. THEIR
DIFFERENCES --- LIMITED USE OF A 'PURE' DETERMINISTIC APPROACH
AFTER DAY 5.  

IN VERY SIMPLISTIC TERMS--- THE MID-LEVEL FLOW PATTERN ALMOST
TAKES ON 'MID-WINTER' FORM. EAST OF THE ROCKIES--- WITH A BROAD
UPPER-LEVEL CYCLONE ANCHORED IN JAMES BAY AND SUFFICIENT AMOUNTS
OF PACIFIC ENERGY/MOISTURE STREAMING ACROSS THE NORTHERN
ROCKIES---TO ESTABLISH A LEE SIDE...MID-LEVEL CYCLONE TRACK THAT
ORIGINATES IN SOUTHEAST ALBERTA --- AND GENERALLY CARRIES A SERIES
OF SYSTEMS THAT FOLLOW THE ENTIRE COURSE OF THE MISSOURI RIVER
INTO KENTUCKY AND TENNESSEE. AND IN THE WEST---A DEEP MID-LEVEL
TROUGH ANCHORED ALONG 125W.

IN WINTER...WOULD BE DESCRIBING THE PATTERN EAST OF THE DIVIDE---
AS "A SERIES OF 'CLIPPERS'--- CARVING OUT THE BASE OF THE UPPER
TROUGH". FOR THIS  MEDIUM RANGE PERIOD AND SCENARIO...IT IS
UNUSUAL THAT THE NORTHEAST PACIFIC WOULD BE 'AS ACTIVE AS IT WILL
BE' ---TO BE MIGRATING A SERIES OF MOIST AND UNSTABLE PACIFIC
FRONTS ACROSS THE INTERMOUNTAIN WEST AND INTO THE HIGH PLAINS. 

AN OBVIOUS DIFFERENCE ---ASIDE FROM IT BEING THE 'WRONG SEASON'
--- TO BE DESCRIBING AND VISUALIZING THIS AS SUCH...

IS THE FACT THAT THE DOWNSTREAM SURFACE RIDGE OVER SOUTHEAST
CANADA/UPPER GREAT LAKES---DOES NOT CONTAIN THE REQUIRED DEPTH OF
COLD/DRY CANADIAN AIR...TO FORCE A 'SUPPRESSED' CYCLONE TRACK
ACROSS THE SOUTHEAST AND OFF THE EAST COAST INVOF THE OUTER BANKS.
EVEN THOUGH IT WILL BE SOMEWHAT 'COOLER'--- THE MID-LEVEL CYCLONE
TRACK FOLLOWS I-70 EARLY IN THE PERIOD (THU-FRI) AND I-90 LATE IN
THE PERIOD (SAT-SUN). THE LATTER OF THESE TWO
SYSTEMS---EFFECTIVELY SWEEPING THE BULK OF THE UPPER-LEVEL CYCLONE
AWAY FROM JAMES BAY AND INTO ATLANTIC CANADA. IF IT WERE MID
WINTER...THE STORM TRACK WOULD TAKE A BROAD...SUPPRESSED ARC
THROUGH THE SOUTHEAST.

AND ALTHOUGH...THE DETERMINISTIC RUNS ARE OCCASIONALLY 'SPINNING
UP' SURFACE CYCLONES EAST OF THE APPALACHIANS--A TYPICAL WINTER
SCENARIO WITH A STRONG SURFACE RIDGE OVER THE ST LAWRENCE
VALLEY/NORTHERN NEW ENGLAND---BELIEVE THIS WILL NOT BE THE CASE.
WHAT SURFACE REFLECTIONS DO EVOLVE IN THIS MID-LEVEL FLOW PATTERN
OFFSHORE---WILL BE THE ORGANIZED MID-LEVEL WAVE ENERGY THAT HOLDS
TOGETHER AS IT MIGRATES DOWNSTREAM OF THE MISSISSIPPI VALLEY AND
ACROSS THE SPINE OF THE APPALACHIANS. IF THE GULF STREAM DOES
PROVIDE ENHANCEMENT... BELIEVE A  DEEPENING SURFACE CYCLONE
SOLUTION IN THE WESTERN ATLANTIC...IS PART CONVECTIVE FEEDBACK AND
PART MCS DEVELOPMENT. THIS PARTICULAR MID-LEVEL FLOW
PATTERN---EAST OF THE MISSISSIPPI---IS WAY TOO PROGRESSIVE FOR A
'WINTER-STYLE' EAST COAST STORM PATTERN.

A SECOND DIFFERENCE IS...MOST WINTER 'CLIPPERS' HAVE VERY LITTLE
MOISTURE TO WORK WITH AND THEIR MID-LEVEL VORTICITY---'GENERALLY
SPEAKING'--- WORKS MOISTURE DOWN TO THE SURFACE FROM THE MID-LEVEL
DYNAMICS/SHEAR. THIS WILL NOT BE THE CASE. THIS BEING AUGUST AND
NOT JANUARY...THERE WILL BE ONE VERY IMPORTANT DIFFERENCE OBSERVED
IN THESE MID-LEVEL CYCLONES---THEY WILL CONTAIN GOOD INFLOW (EAST
TO SOUTHEAST WINDS) AND OCCASIONALLY BE ABLE TO ACCESS THE
LOW-LEVEL JET AXIS IN THE SOUTHERN AND SOUTH CENTRAL HIGH PLAINS
FOR FUEL. THIS HAS BEEN A BIG QPF FORECAST CHALLENGE---DETERMINING
THE AXIS OF HEAVY PRECIPITATION. NOT ONLY ALONG THE BASE OF THE
DOWNSTREAM UPPER-LEVEL TROUGH AND ALONG THE CANADIAN FRONT---BUT
ALSO...WHICH DETERMINISTIC MID-LEVEL STORM TRACK TO 'BELIEVE' FROM
RUN-TO-RUN.

WEST OF THE ROCKIES...
AN UNUSUALLY ACTIVE PATTERN ALONG 50 NORTH LATITUDE... FROM THE
EASTERN ALEUTIANS TO VICTORIA ISLAND BC---AND AN ESTABLISHED
MID-LEVEL TROUGH AXIS IN SOUTHWEST BC AND NORTHWEST WASHINGTON
THROUGH THE PERIOD. WHAT MAKES THE PATTERN 'DIFFERENT' WILL BE THE
RATHER MOIST ONSHORE FLOW. WHAT CURRENTLY PASSES AS 'MONSOONAL
MOISTURE' AND A 'MEANDERING' OFFSHORE CIRCULATION---ESTABLISHED 
ACROSS THE BASE OF THE TROUGH---WILL SHIFT TO MORE OF A WEST TO
NORTHWEST MID-LEVEL PATTERN DURING THE MEDIUM RANGE PERIOD. NOT
ONLY WILL PRECIPITATION BECOME ESTABLISHED ALONG THE LOW-LEVEL
FRONT...BUT THE LEADING EDGES OF THE MID-LEVEL FRONTS WILL
ENCOUNTER AND INTERACT WITH THE DOWNSTREAM UPPER-LEVEL RIDGE. AND
THERE WILL LIKELY BE SEVERAL CHANNELS OF MID-LEVEL MOISTURE TO
WORK WITH. NOT ONLY WILL THE ANTICYCLONIC FLOW ALOFT BE GENERATING
ISOLATED CONVECTION OVER THE TERRAIN INVOF THE 4 CORNERS...BUT
THERE ARE TWO ADDITIONAL SOURCES OF MOISTURE...PLUMES IN THE
SUBTROPICAL PACIFIC NEAR HAWAI'I---THAT WILL FACTOR IN ALONG THE
WEST COAST THIS MEDIUM RANGE PERIOD.

VOJTESAK