EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
234 PM EDT MON AUG 24 2009
 
VALID 12Z THU AUG 27 2009 - 12Z MON AUG 31 2009
 
MODELS AND THEIR ENSEMBLES HAVE MAINTAINED GOOD CONTINUITY WITH
PRIOR RUNS IN ALLOWING A SERIES OF POWERFUL UPPER LOWS TO TRAVERSE
THE NORTHEAST PACIFIC AND GULF OF ALASKA DURING THE MEDIUM RANGE
PERIOD...WHICH HELPS TO SUSTAIN A NEGATIVE ANOMALY IN THE
NORTHEAST PACIFIC.  THIS ANOMALY FAVORS RIDGING ACROSS THE WEST
AND LEADS TO AMPLIFICATION OF A DOWNSTREAM UPPER TROUGH OVER
SOUTHEAST CANADA/NORTHEAST/GREAT LAKES.  OVERALL...THE GUIDANCE IS
RELATIVELY CLOSE THROUGH THE PERIOD...THOUGH LATER ON THE 00Z
CANADIAN SOLUTION IS CLOSEST TO THE ENSEMBLE MEAN PROGRESSION OF
THE TROUGH IN THE NORTHEAST THIRD OF THE COUNTRY.  USED A 00Z
CANADIAN/00Z ECMWF COMPROMISE IN MOST SPOTS...EXCEPT WHERE 00Z
MULTI-MODEL ENSEMBLE LOW CLUSTERING INDICATED OTHERWISE.  THIS
SOLUTION FAVORS A WET PATTERN FROM THE CENTRAL AND SOUTHERN
PLAINS/NORTHEAST GULF COAST NORTH AND EASTWARD IN ADVANCE OF THE
MAIN UPPER TROUGH AND POSSIBLE TROPICAL CYCLONE EXPECTED TO MOVE
CLOSE TO THE EASTERN SEABOARD.

POSSIBLE TROPICAL SYSTEM MOVING UP THE EAST COAST...
THE PROGRESS OF AN AREA OF DISTURBED WEATHER NEAR 18N/57W
CONTINUES TO BE MONITORED AND THE BIGGEST FORECAST PROBLEM OF THE
DAY...WHICH IS FORECAST BY ALL OF THE GUIDANCE TO STRENGTHEN AS IT
MOVES EAST OF THE BAHAMAS BEFORE LIFTING NORTHWARD AND JUST
OFFSHORE THE EAST COAST/ATLANTIC CANADA FROM THIS WEEKEND ONWARD
AHEAD OF THE NORTHEAST TROUGHING.  THE 00Z CANADIAN...AS
USUAL...WAS THE MOST BULLISH WITH ITS STRENGTH.  THE GFS WAS THE
WEAKEST.  THE ECMWF/CANADIAN MODELS HAVE GENERALLY TRENDED TOWARDS
A STRONGER/QUICKER SOLUTION OVER THEIR PAST COUPLE DAYS OF
RUNS...WITH THEIR SOLUTIONS CONVERGING WITH THE SYSTEMS POSSIBLE
TRACK OVER THAT TIME FRAME.  THIS COMBINED WITH THE 00Z
MULTI-MODEL ENSEMBLE CLUSTERING DECREASES THE UNCERTAINTY
ASSOCIATED WITH THIS EVENT.  

ALONG THE EAST COAST...THE FORECAST TRACK OF THE LOW IS CLOSE TO
THE SLOW 24/12Z ECMWF THROUGH FRIDAY NIGHT AFTER COORDINATION WITH
TPC AT 16Z...WHEN HPC SPLIT THE DIFFERENCE BETWEEN HPC CONTINUITY
AND NHCS SIGNIFICANTLY SLOWER PROPOSED TRACK TO COME UP WITH THE
CURRENT SOLUTION.  THEREAFTER...THERE IS NOT A SHRED OF MODEL
GUIDANCE AS SLOW AS THE MEDIUM RANGE PROGS...WHICH NOW LIE ALONG
THE SLOW/EXTREME SOUTHWEST FRINGE OF THE 90 MEMBER 00Z MULTI-MODEL
ENSEMBLE SPREAD.

ROTH/JAMES