Preliminary Forecasts
Issued by NWS

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403
FXUS02 KWNH 200513
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
112 AM EDT SUN AUG 20 2017

VALID 12Z WED AUG 23 2017 - 12Z SUN AUG 27 2017


...GUIDANCE EVALUATION/PREFERENCES...

MODELS/ENSEMBLES SHOW GENERALLY GOOD AGREEMENT WITH RESPECT TO THE
LARGE-SCALE PATTERN, WITH THE LATE WEEK PATTERN CONSISTING OF AN
EASTER PACIFIC TROUGH...DOWNSTREAM HIGH PLAINS RIDGE...AND GREAT
LAKES TO OHIO VALLEY TROUGH.
NEXT WEEKEND...THE EASTERN PACIFIC TROUGH MOVES ONSHORE AND
DEAMPLIFIES...WITH THE DOWNSTREAM RIDGE MOVING EAST AND
DEAMPLIFYING AS WELL.
THE NEXT PACIFIC TROUGH COMING INTO THE EASTERN PACIFIC RESULTS IN
BUILDING HEIGHTS IN THE PACIFIC NORTHWEST NEXT WEEKEND...WITH THE
DOWNSTREAM TROUGH REMAINING IN PLACE OVER THE NORTHEAST US.

TYPICAL SPREAD EXIST WITH EACH OF THE SYSTEMS WITH GROWTH IN THE
SPREAD NEXT WEEKEND.  DIFFERENCES IN THE TIMING OF THE COLD FRONT
IN THE SOUTHEAST ARE RELATIVELY SMALL...WITH DIFFERENCES REGARDING
MESOSCALE RIPPLES OF LOW PRESSURE ALONG THE FRONT.

THE MODELS CONTINUE TO SHOW LOW PRESSURE OFF THE BRITISH COLUMBIA
COAST WED MORNING THAT WEAKENS AS IT COMES ONSHORE THU...WITH LEE
LOW DEVELOPMENT IN THE CANADIAN PRAIRIES AND NORTHERN PLAINS THAT
MOVES SLOWLY EAST ACROSS MT/WY THU NIGHT AND ACROSS THE DAKOTAS
AND CENTRAL HIGH PLAINS FRI-EARLY SAT...CONTINUING A SLOW EAST
DRIFT INTO SUN AS A CLOSED HIGH OVER THE LAKES DRIFTS INTO NEW
YORK/NEW ENGLAND.

THE WPC FORECAST BLENDED THE DETERMINISTIC 12Z ECMWF AND THE
GEFS/ECMWF ENSEMBLE MEAN FORECASTS THROUGH NEXT WEEKEND.

MANY OF THE MODELS AND ENSEMBLE MEMBERS INDICATE HARVEY WILL CROSS
THE YUCATAN PENINSULA AND ACROSS THE BAY OF CAMPECHE INTO THE EAST
COAST OF MEXICO.


...WEATHER HIGHLIGHTS...

SOUTH OF THE COLD FRONT...SHOWERS AND STORMS FOCUS WED AND THU
ACROSS THE SOUTHERN MID ATLANTIC TO THE PIEDMONT OF THE CAROLINAS.
FLORIDA SHOULD ALSO SEE AN INCREASE IN RAINFALL AS MOISTURE POOLS
AHEAD OF THE STALLED COLD FRONT IN THE NORTHERN PENINSULA.

THE COMBINATION OF THE WEAKENING UPPER LOW/TROUGH PUSHING INTO
SOUTHERN CALIFORNIA AND THE UPPER RIDGE FOCUSING OVER TEXAS SHOULD
SUPPORT INCREASED SHOWERS/CONVECTION OVER THE SOUTHERN ROCKIES
LATE IN THE WEEK AND THEN AS THE RETURN SOUTHERLY FLOW DEVELOPS
SHOWERS AND STORMS DEVELOP IN TX AND INTO THE SOUTHERN AND CENTRAL
PLAINS.

WITH THE UPPER TROUGH ACROSS THE LAKES AND OH
VALLEY...TEMPERATURES WILL BE SEVERAL DEGREES COOLER THAN NORMAL
LATER THIS WEEK INTO NEXT WEEKEND.  THE COOLER AIR SPREADS ACROSS
NEW YORK AND NEW ENGLAND.  THE RETURN OF ABOVE NORMAL HEIGHTS
LEADING TO WARMER TEMPERATURES NEXT WEEKEND IN THE NORTHERN HIGH
PLAINS TO THE GREAT BASIN AND DESERT SOUTHWEST.

PETERSEN


$$





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