Prognostic Meteorological Discussion
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FXUS02 KWBC 270642
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
241 AM EDT WED MAY 27 2015

VALID 12Z SAT MAY 30 2015 - 12Z WED JUN 03 2015

PATTERN SHIFT ACROSS ARCTIC CANADA AND THE TEMPORARY DISLODGING OF
A BROAD UPPER-LEVEL RIDGE IN ALASKA OFFERS A SLIGHTLY MORE
PROGRESSIVE MID-LATITUDE FLOW PATTERN FOR THE LOWER 48. FROM WEST
TO EAST---A SERIES OF PACIFIC TROUGHS WILL MIGRATE ACROSS THE
NORTHERN HALF OF THE COUNTRY---WITH A SHORTWAVE RIDGE ATTEMPTING
TO SETTLE --- STABLE AND DRY CONDITIONS --- OVER THE DESERT
SOUTHWEST AND UPPER RIO GRANDE VALLEY THIS MEDIUM RANGE PERIOD. IN
THE SOUTHEAST AND GULF COAST...MORE OF A SUMMER PATTERN EMERGES
--- WITH A PRONOUNCED EASTERLY FLOW FROM THE BAHAMAS TO THE LOWER
RIO GRANDE.

THE 26/12Z ECENS/GEFS ENSEMBLE MEANS AND THEIR DETERMINISTIC RUNS
HAVE A REASONABLE HANDLE ON THE DETAILS OF A MID-LEVEL
CIRCULATION`S MIGRATION THROUGH THE CENTRAL AND NORTHEAST PORTION
OF THE COUNTRY EARLY IN THE FORECAST PERIOD. FROM ITS CURRENT
LOCATION IN THE PACIFIC NORTHWEST--- IT WILL BE MOVING FAIRLY
RAPIDLY THROUGH THE ST LAWRENCE VALLEY --- SWEPT INTO ATLANTIC
CANADA BY A STRONG SHORTWAVE TROUGH THAT MAINTAINS A TRACK ACROSS
EASTERN CANADA.

THERE SHOULD BE MULTIPLE AREAS OF ACTIVE WEATHER MIGRATING
EAST-NORTHEASTWARD ACROSS THE LOWER 48 --- AND THE DETAILS VARY
SLIGHTLY.

ACROSS THE EASTERN GREAT LAKES...OHIO VALLEY AND NEW ENGLAND---
THE SEQUENCE IS STRAIGHT-FORWARD IN NATURE---WITH MID-LEVEL WARM
ADVECTION AND LIFT FOLLOWED BY AN APPRECIABLE LOW-LEVEL COLD
FRONTAL PASSAGE (FOR SUNDAY AND MONDAY). THE DEPTH OF THE CANADIAN
AIRMASS IS SHALLOW --- AND IS MODIFIED CONSIDERABLY EAST OF THE
APPALACHIANS --- AND SOUTH OF THE MASON-DIXON.

THE DETERMINISTIC GUIDANCE --- WITH REASONABLE ACCURACY --- HAS
KEYED-IN ON THE MID-LEVEL WAVE RACING EASTWARD `IN POST-FRONTAL
FASHION` THROUGH THE LOWER MISSOURI VALLEY AND WESTERN OHIO VALLEY
FOR SUNDAY AND MONDAY. THE UNCERTAINTY LIES IN THE FORWARD
MOMENTUM OF THIS SYSTEM ---WHICH IS COMPLETELY DETACHED FROM THE
TROUGH SHEARING OUT ACROSS THE CANADIAN MARITIMES. ITS NOT A
STRETCH TO HAVE THIS WAVE MIGRATE EASTWARD ALONG THE PRE-EXISTING
FRONTAL BOUNDARY AND EXIT THE DELMARVA MONDAY NIGHT AND TUESDAY
MORNING. THE LATITUDE OF THIS SYSTEM ---AS IT EXITS THE EASTERN
SEABOARD --- IS `NOT SET IN STONE`. AND WILL A THIRD SHORTWAVE ?!
--- EMERGE FROM THE WESTERN PERIPHERY OF THE SURFACE RIDGE AND
LOW-LEVEL MOISTURE PLUME SETTING UP ALONG THE WESTERN GULF COAST.
RATHER THAN RELY ON ONE SPECIFIC DETERMINISTIC RUN OUT AT DAY 6
--- WILL FAVOR THE ECENS/NAEFS MEANS.

IN THE WEST AND NORTHERN ROCKIES/PLAINS --- A BLEND OF THE
NAEFS/ECENS/GEFS WILL BRING THE BASE OF THE MID-LEVEL TROUGH INTO
NORTHWEST CALIFORNIA AT/AROUND 1/12Z. AND FOR A LATE MAY SYSTEM
--- IT`S REASONABLE TO SUGGEST THIS SYSTEM WILL BE CONVECTIVE IN
NATURE. TO VARYING DEGREES --- THE DETERMINISTIC GUIDANCE SPREADS
THE SYSTEM (IN A SOUTHWEST FLOW ALOFT) ACROSS THE CASCADES AND
NORTHERN DIVIDE --- BUT THE ECMWF IS THE AGGRESSIVE SOLUTION BY
2/12Z ---WITH ITS COMPACT CLOSED LOW ALONG THE LOWER COLUMBIA
RIVER BASIN. THE PATTERN OVER WESTERN CANADA ---BEYOND DAY 5---
WOULD NOT SUGGEST THE SYSTEM WILL DIG SOUTHEASTWARD --- LIKE THE
26/12Z DETERMINISTIC ECMWF/CANADIAN SUGGEST.

SO A BLEND OF THE ECENS/NAEFS MEANS WOULD BE THE PREFERENCE BEYOND
DAY 5 --- AND LET THE SYSTEM MIGRATE EASTWARD INTO ROCKIES...GREAT
BASIN AND NORTHERN HIGH PLAINS---WITH THE BULK OF THE COLD
ADVECTION AND SENSIBLE WEATHER TARGETING AREAS NORTH OF I-80 FROM
RENO TO CHEYENNE.

VOJTESAK

$$





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