Prognostic Meteorological Discussion
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FXUS02 KWBC 030612
PMDEPD

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





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