Preliminary Forecasts
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974
FXUS02 KWNH 220654
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
253 AM EDT FRI MAY 22 2015

VALID 12Z MON MAY 25 2015 - 12Z FRI MAY 29 2015

...OVERVIEW...
THE LOWER 48 WEATHER PATTERN WILL BE IN A TRANSITION PERIOD
BETWEEN DAYS 4-6 --- AS THE UPPER-LEVEL RIDGE OVER THE NORTHWEST
PORTION OF NORTH AMERICA AND DOWNSTREAM TROUGH OVER EASTERN CANADA
ARE TEMPORARILY ALTERED AND INFLUENCED BY A SERIES OF UNRELATED
SYNOPTIC-SCALE FEATURES. IN THE WEST---THE RIDGE IS INFLUENCED BY
A DEEP TROUGH MIGRATING FROM THE ALEUTIANS TO THE NORTHWEST GULF
OF ALASKA. AND IN THE EAST--- A BROAD SUBTROPICAL RIDGE ---
ORIGINATING IN THE WESTERN ATLANTIC--- MIGRATES INTO SOUTHERN
APPALACHIA.

...MODEL PREFERENCES...
THE 21/12Z GFS/ECMWF MAINTAIN A BROAD CYCLONIC CIRCULATION THROUGH
THE MEDIUM RANGE PERIOD ALONG AND WEST OF THE CONTINENTAL
DIVIDE---AND BOTH SUPPORT THE GRADUAL AMPLIFICATION OF THE
DOWNSTREAM PATTERN FROM THE PLAINS STATES TO THE APPALACHIANS.
LIKEWISE---THEY ARE IN VERY GOOD AGREEMENT ACROSS THE SOUTHEAST
AND MID-ATLANTIC REGION WITH A BUILDING UPPER-LEVEL RIDGE AXIS
FROM FLORIDA INTO SOUTHERN NEW ENGLAND. THE PAST TWO OR THREE WPC
FORECAST CYCLES HAVE GENERALLY FOLLOWED AN ECENS/GEFS BLEND AND
COULD NOT FIND FAULT IN CONTINUING THIS TRAIN OF THOUGHT THIS
MORNING.

...UNCERTAINTY ASSESSMENT...
BETWEEN DAYS 4-6 --- THE PATTERN IS IN A BRIEF TRANSITION AND THE
UNCERTAINTY LIES ALONG THE WESTERN PERIPHERY OF THE SOUTHEAST
RIDGE...ACROSS BRITISH COLUMBIA --- UNDERNEATH THE UPPER-LEVEL
RIDGE... AND EMBEDDED WITHIN AN UNUSUALLY-ACTIVE EASTERN PACIFIC
SUBTROPICAL JET STREAM AXIS BETWEEN 20N-30N.

IN THE MIDWEST AND EAST --- THE CHALLENGE IS WITH TRAJECTORY AND
DEPTH OF SHORTWAVE ENERGY MIGRATION (DAYS 4-6) FROM THE PLAINS TO
THE MISSISSIPPI RIVER THEN CONSOLIDATION OF A COLD FRONT AND
SURFACE CYCLONE MIGRATING EASTWARD ACROSS THE GREAT LAKES-OHIO
VALLEY AND ITS INTERACTION WITH THE CORE OF THE UPPER-LEVEL RIDGE.
THE TEMPERATURE GUIDANCE SHOWS THE GREATER UNCERTAINTY EAST OF THE
APPALACHIANS AND THE FRONT`S ABILITY TO TEMPER THE HEAT/HUMIDITY
UNDERNEATH THE BUILDING RIDGE. ECMWF GUIDANCE APPEARED TO BE A
GOOD DEAL WARMER THAN THE GFS --- AND LOCAL/REGIONAL FORECASTER
PREFERENCES MAY ADD VALUE HERE.

IN THE WEST---THE UNCERTAINTY LIES IN TIMING THE ARRIVAL OF
SHORTWAVE ENERGY THAT SHEARS OUT ACROSS THE GULF OF ALASKA AND
CONSOLIDATES ALONG A STATIONARY BOUNDARY DRAPED OVER THE PANHANDLE
AND COASTAL BRITISH COLUMBIA. THIS IMPACTS THE PACIFIC NORTHWEST
AND NORTHERN GREAT BASIN. AND TO COMPLICATE MATTERS --- THE
PERSISTENCE OF A SPLIT-FLOW PATTERN IN THE EAST PAC --- ALONG 140W
BETWEEN 20N AND 40N WILL BE EMBEDDED WITH CUTOFF MID-LEVEL
ENERGY---ALLOWING A SECOND ENTRY POINT FOR MOISTURE TO MIGRATE
ACROSS CALIFORNIA AND THE SOUTHERN GREAT BASIN.

...SENSIBLE WEATHER HIGHLIGHTS...
A SUMMER-LIKE AIRMASS AND `FEEL` ARE EXPECTED BENEATH THE
AMPLIFYING RIDGE ACROSS THE EAST.

FOR THE PLAINS AND IMMEDIATELY DOWNWIND OF THE CONTINENTAL DIVIDE
--- AN ACTIVE PERIOD OF SHOWERS AND THUNDERSTORMS AS MID-LEVEL
ENERGY EJECTS NORTHEASTWARD FROM THE BROAD TROUGH IN THE GREAT
BASIN. A CONSOLIDATED SYNOPTIC-SCALE FRONT IS EXPECTED TO MIGRATE
INTO THE MIDWEST AND GREAT LAKES TUESDAY AND AND WEAKEN AS IT
REACHES THE APPALACHIANS AND LOWER MS VALLEY ON WEDNESDAY.

UNSETTLED IN THE WEST...WITH A COOLER...ONSHORE FLOW COMPONENT
EXPECTED TO  DIRECT UPSLOPE CONDITIONS TO WEST-FACING AND
SOUTHWEST-FACING SLOPES.

VOJTESAK

$$





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