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FXUS02 KWNH 230553
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
152 AM EDT THU MAY 23 2013

VALID 12Z SUN MAY 26 2013 - 12Z THU MAY 30 2013


THE MODELS CONTINUE TO FCST 500 MB TROUGHS ALONG BOTH WEST/EAST
COASTS TO DRIFT SLOWLY EAST EARLY NEXT WEEK.  A DEEP LAYER RIDGE
RIDGE DRIFTS EAST OVER THE CENTRAL STATES AND INTO THE MS VALLEY
MID-LATE NEXT WEEK.  700 MB TEMP ANOMALIES AND 500 MB HEIGHT
ANOMALIES RUN 1-2 STD DEVIATIONS ABOVE NORMAL FROM THE ECMWF
ENSEMBLE MEAN AND GEFS MEAN ACROSS THE PLAINS IN THE EARLY PART OF
THE WEEK...EXTENDING EAST INTO THE MS VALLEY VALLEY MID WEEK AND
INTO THE LAKES AND OHIO VALLEY LATE IN THE WEEK. TEMPS IN THESE
AREAS ARE FORECAST TO RUN SEVERAL DEGREES ABOVE NORMAL.

SOUTHERLY FLOW AHEAD OF THE TROUGH AND SFC LOW/WARM FRONT SHOULD
FOCUS PRECIP OVER THE CENTRAL PLAINS LATE IN THE WEEKEND AND INTO
THE NORTHERN PLAINS/MID-UPPER MS VALLEY/GREAT LAKES EARLY TO MID
NEXT WEEK WHERE CONVECTION IS LIKELY AS INDICATED IN THE
MODELS/MULTIPLE GEFS/ECMWF ENSEMBLE MEMBERS WITHIN PERIODS OF
UPPER DIVERGENCE MAXIMA/LOW LEVEL CONVERGENCE COUPLETS.

ADDITIONAL SHORTWAVE ENERGY REINFORCES THE WEST COAST TROUGH EARLY
NEXT WEEK.  AS THE PACIFIC CYCLONE/ANTICYCLONE COUPLET MOVE EAST
NEXT WEEK...THE WEST COAST TROUGH MOVES SLOWLY INLAND IN THE
MIDDLE OF THE WEEK.  HEIGHT REMAINS BELOW NORMAL IN THE PACIFIC
NORTHWEST...WITH WET CONDITIONS EXPECTED THERE INTO THE NORTHERN
ROCKIES UNDER DIFLUENT FLOW ALOFT AND PERSISTENT 700 MB RH ABV 70
PERCENT AND SPORADIC VERTICAL VELOCITY MAXIMA.

MANUAL PROGS USED A BLEND OF THE 12Z ECMWF ENSEMBLE MEAN/18Z GEFS
MEAN WITH LESS WEIGHTING ON THE 12Z ECMWF AND 12Z NORTH AMERICAN
ENSEMBLE MEAN HEIGHTS/SEA LEVEL PRESSURES...AS WELL AS THE
TEMPERATURES/PROBABILITY OF PRECIP FCSTS.  THE 12-00Z GFS SHOW A
FASTER INLAND DEPARTURE OF THE WEST TROUGH ACROSS THE GREAT
BASIN/ROCKIES ONTO THE PLAINS...WITH THE 18Z GEFS MEAN/12Z ECMWF
MEAN SHOWING BETTER AGREEMENT ON A SLOWER MOTION.  GIVEN THE LONG
TERM GFS BIAS OF MOVING HIGH AMPLITUDE TROUGHS EAST TOWARDS HIGH
AMPLITUDE RIDGES...A BLEND OF THE MEANS WAS USED.

IN THE EAST... THE MODELS/ENSEMBLES SHOW AN UPPER TROUGH WITH AN
EMBEDDED CLOSED LOW ACROSS NEW ENGLAND OVER THE WEEKEND....THEN
DEPARTING EARLY NEXT WEEK AS UPSTREAM RIDGING IN
THE NORTHERN PLAINS BUILDS INTO THE UPPER MS VALLEY MON AND THEN
THE UPPER GREAT LAKES ON TUE/WED.  THIS SETS THE STAGE FOR A COLD
WEEKEND/MON OVER THE NORTHEAST AND THEN A EARLY-MID WEEK WARMING
TREND FOR THE OH VALLEY/TN VALLEY/UPPER LAKES...AND LATE WEEK
WARMING TREND ACROSS THE NORTHEAST.

PETERSEN




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National Weather Service Weather Forecast Office
