Prognostic Meteorological Discussion
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FXUS02 KWBC 011450
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1050 AM EDT TUE SEP 01 2015

VALID 12Z FRI SEP 04 2015 - 12Z TUE SEP 08 2015


THE MEDIUM RANGE PERIOD WILL BE DOMINATED BY THE LIFTING OF A
STRONG TROUGH OVER THE NORTHWEST TO THE NORTHEAST...ALLOWING
COOLER AIR TO SPREAD SOUTH AND EASTWARD ACROSS THE ROCKIES INTO
THE NORTHERN AND CENTRAL PLAINS AND THEN THE UPPER MISSISSIPPI
VALLEY AT THE END OF THE PERIOD.  A VERY LARGE AND WARM UPPER
ANTICYCLONE WILL SLOWLY BE WEAKENED FROM THE NORTH AND WEST. HIGH
PRESSURE OVER SOUTHEAST CANADA WILL BUILD SLOWLY SOUTH AND
EASTWARD...BRINGING SOMEWHAT COOLER CONDITIONS TO THE NORTHEAST
AND EASTERLY TO NORTHEASTERLY FLOW ACROSS THE SOUTHEASTERN STATES
WHILE THE ENSEMBLE MEANS SHOW A BROAD AND RELATIVELY WEAK UPPER
LOW FORMING FROM DAY 5/SUNDAY ONWARD.

THE OVERNIGHT ENSEMBLE MEANS PRESENT A UNIFIED PICTURE OF THE
MOVEMENT OF THESE SYSTEMS WITH THE OVERNIGHT OPERATIONAL RUNS NOT
QUITE SHOWING THE KIND OF AGREEMENT EVEN EARLY IN THE FORECAST
OVER PORTIONS OF NORTH AMERICA...WITH THE GFS FASTER THAN THE
ECMWF...WHICH IS WHAT HAS BEEN SEEN DURING THE PAST SEVERAL DAYS.
HOWEVER...THE ECMWF APPEARS TO BE SOMEWHAT MORE OF THE OUTLIER
WITH A NUMBER OF SYSTEMS FROM DAY 4/SATURDAY ONWARD.
THEREFORE...A BLEND INVOLVING MORE OF THE MEANS THAN THE
OPERATIONAL RUNS STARTED RELATIVELY EARLY IN THE FORECAST...ON DAY
4/SATURDAY.

THE AREA OF MOST SIGNIFICANT QPF WILL OCCUR WITHIN THE LIFTING
TROUGH OVER THE WESTERN US EARLY ON...WITH THE POTENTIAL FOR
SIGNIFICANT QPF ACROSS THE WEST AND EVEN SOME EARLY SEASON SNOW IN
THE HIGHEST ELEVATIONS OF THE NORTHERN ROCKIES AND NORTHWEST.
SHOWERS AND THUNDERSTORMS ARE LIKELY TO FOCUS NEAR THE SLOW MOVING
FRONT WITH THE POTENTIAL FOR LOCALLY HEAVY RAINS AND EVEN SEVERE
WEATHER.  THE SLOW DEVELOPMENT OF THE UPPER LOW OVER THE SOUTHEAST
WITH THE EASTERLY FLOW DEVELOPING ALONG THE SOUTHEAST COAST WILL
ALSO GENERATE SOME SIGNIFICANT QPF ALONG THE GULF AND SOUTHEAST
COASTS.

AN INTERACTION BETWEEN HURRICANE IGNACIO AND THE WESTERLIES BEGINS
TO BE PICKED UP IN SOME OF THE MODELS AS EARLY AS DAY
5/SUNDAY...AND PROVIDES A SOURCE OF UNCERTAINTY IN HOW THE MODELS
RESOLVE THESE FEATURES BUT WILL LIKELY HAVE A MORE SIGNIFICANT
ROLE IN THE DAYS TO COME.

KOCIN




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