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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1553Z Sep 17, 2014)
 
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1153 AM EDT WED SEP 17 2014

VALID 12Z SAT SEP 20 2014 - 12Z WED SEP 24 2014

...HEAVY RAIN THREAT FOR THE SOUTHERN AND CENTRAL PLAINS...


...OVERVIEW...

AN AMPLIFIED TROUGH/RIDGE/TROUGH PATTERN IS EXPECTED TO SHIFT
EASTWARD THROUGH THE MEDIUM RANGE... CENTERED ALONG 140W/NORTHERN
ROCKIES/GREAT LAKES BY EARLY NEXT WEEK. A LEAD SYSTEM AND
SECONDARY COLD FRONT SHOULD PUSH EASTWARD THROUGH THE WEEKEND AND
OFF THE EAST COAST MON/D5 AND INTO THE GULF TUE/D6 WITH HIGH
PRESSURE IN ITS WAKE. A PACIFIC SYSTEM SHOULD ENTER THE PAC NW
LATE IN THE PERIOD. THE REMNANTS OF ODILE WILL GET ABSORBED INTO
THE LEAD FRONTAL BOUNDARY IN THE CENTRAL/SOUTHERN PLAINS WHICH
WILL ACT TO ENHANCE THE RAINFALL THERE.


...GUIDANCE/UNCERTAINTY ASSESSMENT AND PREFERENCES...

FOR THE FIRST PART OF THE FORECAST... THE MODELS/ENSEMBLE SHOW
GOOD AGREEMENT WITH THE TROUGH MOVING THROUGH THE LAKES WHILE AN
UPPER LOW INITIALLY NEAR CALIFORNIA DRIFTS NORTHEASTWARD INTO THE
ROCKIES AND THEN ONTO THE CENTRAL PLAINS BY NEXT WED/D7. A BLEND
AMONG THE 06Z GFS AND 00Z ECMWF/ECENS MEAN... ALONG WITH WPC
CONTINUITY... OFFERED A REASONABLE STARTING POINT.

FOR THE FORECAST NEXT WEEK... THE MODELS AND ENSEMBLES SHOW
CONSIDERABLE SPREAD IN THE CENTRAL/NW PACIFIC QUITE EARLY IN THE
FORECAST... WHICH EXPAND IN TIME. THE 00Z-06Z GEFS MEMBERS WERE
GENERALLY QUICKER WITH ENERGY WELL UPSTREAM THAT RESULTS IN A MORE
ZONAL FLOW INTO THE PAC NW BY WED/D7 COMPARED TO THE 00Z ECMWF
ENSEMBLES... WHICH MAINTAIN A SLOWER TROUGH MOVING TOWARD THE WEST
COAST. THE GFS HAS TRENDED AWAY FROM ITS INTENSE RIDGING THROUGH
MUCH OF THE WESTERN STATES TOWARD THIS ZONAL FLOW ATOP STRONG
RIDGING IN THE SUBTROPICS SW OF CALIFORNIA. THE ECMWF MEMBERS HAVE
ONLY SUDDENLY INDICATED MORE TROUGHING OFF THE WEST COAST RATHER
THAN ALLOWING THE LEAD SHORTWAVE TO LIFT NORTHEASTWARD WHICH WOULD
RAISE HEIGHTS ALONG 130W. GIVEN THE TENDENCY TOWARD MAINTAINING
RIDGING IN THE WEST OVER THE PAST SEVERAL MONTHS COUPLED WITH THE
FACT THAT THE ENSEMBLES ARE IN A STATE OF FLUX... FELT A MODEST
WEIGHTING TOWARD THE 00Z ECMWF ENSEMBLE MEAN BALANCED BY WPC
CONTINUITY... WHICH FAVORED THE 12Z/16 ECMWF ENSEMBLE MEAN... WAS
WARRANTED. ALL MEASURES OF RELATIVE OR NORMALIZED SPREAD INDICATED
HIGH UNCERTAINTY IN THE FORECAST IN THE WEST BY NEXT WEEK... BUT
COMPARATIVELY MUCH LESS UNCERTAINTY IN THE EAST WITH THE EXITING
TROUGH.


...SENSIBLE WEATHER HIGHLIGHTS...

RAINFALL SHOULD FOCUS OVER THE CENTRAL/SOUTHERN PLAINS AS THE
REMNANTS OF ODILE MERGE WITH THE COLD FRONT. THE 00Z MODELS
YESTERDAY WERE LACKING THIS ROBUSTNESS. THE UPPER LOW INITIALLY
OVER CALIFORNIA SHOULD ACT AS ANOTHER FOCUS FOR PRECIPITATION OVER
THE CENTRAL ROCKIES AS IT EASES EASTWARD. FINALLY... FLORIDA
SHOULD SEE AN INCREASE IN RAINFALL WITH THE INITIAL BOUNDARY
WASHING OUT AS PW VALUES REMAIN NEAR TO ABOVE AVERAGE AHEAD OF THE
NEXT COLD FRONT THAT SHOULD STALL SOUTH OF 30N. TEMPERATURES WILL
BE VARIABLE BUT STILL OVERALL COOLER THAN AVERAGE IN THE
CENTRAL/EAST AND WARMER THAN AVERAGE IN THE WEST... IN PARTICULAR
THE NORTHWEST.


FRACASSO