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000
FXUS02 KWNH 180448
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1247 AM EDT TUE JUN 18 2013

VALID 12Z FRI JUN 21 2013 - 12Z TUE JUN 25 2013


SO FAR, THE SUMMER HAS BEEN MARKED BY MEAN TROUGHS CENTERED OVER
THE PACIFIC NORTHWEST AND ALONG THE EAST COAST, AND A MEAN RIDGE
CENTERED OVER THE HIGH PLAINS. AT TIMES--SUCH AS THE CURRENT
MEDIUM RANGE--ENERGY EJECTS FROM THE NORTHWEST TROUGH, FLATTENING
THE HIGH PLAINS RIDGE AND FILLING THE DOWNSTREAM EAST COAST
TROUGH. ONCE THE SHORTWAVE ENERGY PROGRESSES FARTHER, IT BEGINS TO
DROP SOUTHEASTWARD INTO THE WEAKNESS NEAR THE CANADIAN MARITIME
PROVINCES, WITH THE UPSTREAM FLOW PATTERN BOUNCING BACK AS WELL.
THE LAST TWO ECENS MEANS HAVE INDICATED THAT THIS SHUFFLING SONG
AND DANCE AT THE MEDIUM RANGE WILL BE FOLLOWED BY A RETROGRESSION
OF THE SUBTROPICAL RIDGE INTO THE FOUR CORNERS BY WEEK TWO. THE
GEFS MEANS HAVE SIMPLY RETURNED THE PATTERN TO THE STATUS QUO.
CLIMATOLOGY SUPPORTS A SHIFT OF THE RIDGE TO THE FOUR CORNERS BY
MIDSUMMER, WITH ITS ATTENDANT MONSOON INTO THE DESERT SOUTHWEST.
WITH A NOD TOWARD THE STRONG SIGNAL OF CLIMATOLOGY, STUCK CLOSE TO
THE SYNOPTIC DETAILS OF THE MOST RECENT EUROPEAN CENTRE ENSEMBLE
MEAN FOR THIS FORECAST.

WITH THE POLAR JET MEANDERING ACROSS THE NORTHERN TIER OF STATES
THIS PERIOD, EXPECT TEMPERATURES TO REMAIN NEAR OR BELOW NORMAL
THERE, WITH PERIODS OF SHOWERS AS NEW BATCHES OF SHORTWAVE ENERGY
PASS. THE ESSENTIALLY WHOLESALE EJECTION OF THE NORTHWEST TROUGH
ACROSS THE NORTHERN PLAINS AND UPPER MIDWEST SHOULD BE ACCOMPANIED
BY A VIGOROUS BOUT OF CONVECTION. BY THE END OF THE PERIOD, NEW
SHORTWAVE ENERGY SHOULD BE APPROACHING THE NORTHWEST COAST, WITH A
FRESH ROUND OF SHOWERS THERE. THE SOUTHEAST WILL BE SHOWERY ON THE
EQUATORIAL SIDE OF THE ZONAL SUBTROPICAL RIDGE AXIS THERE, AIDED
EARLY IN THE FORECAST BY THE LOW-LEVEL CONVERGENCE ASSOCIATED WITH
FRONTOLYSIS OVER THE REGION. THE FAR SOUTHWEST MAY SEE SOME
HIGH-BASED, DRY CONVECTION--ESPECIALLY DURING THE FIRST HALF OF
THE PERIOD WHILE HEIGHTS ARE STILL RELATIVELY LOW IN THE WAKE OF
THE DEPARTING TROUGH TO THE NORTH.


CISCO

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