Prognostic Meteorological Discussion
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FXUS02 KWBC 050538
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
138 AM EDT SAT SEP 05 2015

VALID 12Z TUE SEP 08 2015 - 12Z SAT SEP 12 2015

AN ACTIVE HIGH-LATITUDE NORTHERN JET STREAM IS EVIDENT DURING THE
MEDIUM RANGE FORECAST PERIOD---CARTING REMNANTS OF `IGNACIO`
EASTWARD FROM THE ALASKA PANHANDLE AND NORTH CENTRAL BC AROUND DAY
3---BRUSHING THE UPPER GREAT LAKES AND WESTERN ONTARIO AROUND DAY
5. THIS MIGRATION COVERS A VAST AMOUNT OF DISTANCE IN A RATHER
SHORT PERIOD OF TIME. AND AT 700 MBS---LIKED THE WAY THE
DETERMINISTIC 4/12Z ECMWF HANDLED THE NATURE OF THE
MOISTURE/TEMPERATURE ADVECTION PATTERN IN RESPONSE TO THE
INCORPORATION OF THIS SUBTROPICAL DISTURBANCE (NORTH OF 50 NORTH
LATITUDE) AND INTO THE NORTHERN STREAM BETWEEN 145W-130W...AND
THEN OVER CANADA---EASTWARD TO 100W.

THE 4/12Z DETERMINISTIC GFS IS NOT `UN-USABLE` WHATSOEVER HEADING
INTO THE DAY 5 TIME FRAME. IT WAS THE WEAKER SOLUTION OVER THE
SOUTHERN PLAINS AND SOUTHEAST WITH THE COLD FRONT YESTERDAY---AND
LIKE YESTERDAY...WOULD BELIEVE THE MOMENTUM OF A
TROPICALLY-INFUSED SYSTEM CAUGHT IN A NEARLY WEST-TO-EAST NORTHERN
STREAM JET OVER CANADA WOULD BE `WEAKER` WITH THE INTRODUCTION OF
APPRECIABLE COLD ADVECTION INTO THE LOWER 48...EAST OF THE
DIVIDE---AND MORE IMPORTANTLY---SOUTH OF 35N LATITUDE.

THE SUBTROPICAL FLOW AND ITS MID-LEVEL WARM ADVECTION DOES SPREAD
QUICKLY ACROSS CANADA AND THOUGHT THE 4/12Z ECENS/GEFS MEANS WOULD
BE A VERY GOOD SOLUTION---IN LIEU OF---A PURE DETERMINISTIC
SOLUTION AND REMOTE CHANCE OF A `PERFECT PROG` BEYOND 120
HOURS---USING ONE OR THE OTHER (4/12Z DETERMINISTIC ECMWF/GFS). IN
THIS CASE---TO COMPILE A REALISTIC DEPICTION OF THE DAY4-5 AND
DAY6-7 48-HOUR QPFS---I AM NOT EMOTIONALLY `TIED` OR FAVORING ONE
MODEL OVER ANOTHER.

BOTH DETERMINISTIC PIECES OF GUIDANCE (ECMWF/GFS) HAVE THE LOWER
THIRD OF THE MISSOURI VALLEY...AND LOWER ARKANSAS RIVER BASIN WET
EARLY IN THE PERIOD---THEN GRADUALLY TRANSLATE THE PRECIPITATION
DOWNSTREAM ALONG THE FRONT. A MORE PROGRESSIVE FRONTAL PUSH IS THE
`STYLE` OF THE ECMWF---CREATING AN EXTREMELY WET WESTERN GULF
COAST AND NEW ENGLAND FOR DAY 6-7. WHILE THE GFS---A WEAKER
`STYLE` FRONTAL PROGRESSION ALONG THE NORTHERN FLANKS OF THE
SOUTHEAST RIDGE---CREATES A `WETTER` OHIO VALLEY.

IN EITHER CASE---THE RATHER PROGRESSIVE SERIES OF WAVES GENERATED
BY THE INFLUX OF SUBTROPICAL PACIFIC MOISTURE WILL KEEP MUCH OF
THE COLDER NORTHERN STREAM HIGH-LATITUDE AIRMASS LOCKED UP OVER
NORTHERN CANADA AND ALLOW AN AUTUMN-LIKE EAST PAC SURFACE RIDGE TO
MIGRATE INTO THE WEST COAST (AND OFFSHORE FLOW EVENT) THIS MEDIUM
RANGE PERIOD.

VOJTESAK



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