Prognostic Meteorological Discussion
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000
FXUS02 KWBC 221518
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1117 AM EDT FRI SEP 22 2017

VALID 12Z MON SEP 25 2017 - 12Z FRI SEP 29 2017

...HEAVY RAIN LIKELY FOR PORTIONS OF TEXAS AND OKLAHOMA NEXT
WEEK...


...OVERVIEW...

BIG PATTERN CHANGE FOR THE LOWER 48 NEXT WEEK -- INITIAL
TROUGH/RIDGE SETUP WILL FLIP AS RIDGING ENTERS THE WEST COAST
WED-THU AND TROUGHING DIGS INTO THE EAST ON FRIDAY. THIS WILL PUT
AN END TO THE RECORD HEAT FOR THE NORTHEAST MON-WED AS
TEMPERATURES FALL BACK TO MORE TYPICAL LEVELS FOR LATE SEPTEMBER.
TROUGH PROGRESSION TO THE EAST WILL ALSO SET UP AN AXIS OF HEAVY
RAIN OVER TEXAS AS GULF MOISTURE FEEDS NORTHWESTWARD INTO THE HILL
COUNTRY AND POINTS NORTHWARD INTO WESTERN OKLAHOMA. HURRICANE
MARIA IS FORECAST TO MOVE NORTHWARD BETWEEN THE EAST COAST AND
BERMUDA BEFORE LIKELY TURNING NORTHEASTWARD OUT TO SEA LATE NEXT
WEEK. PLEASE CONSULT THE NATIONAL HURRICANE CENTER FOR THE LATEST
FORECAST TRACK.


...GUIDANCE EVALUATION/PREFERENCES...

ENSEMBLE MEANS SHOWED GOOD AGREEMENT AND RELATIVELY LOW SPREAD
THROUGH NEXT WEEK WITH RESPECT TO THE LARGE SCALE PATTERN. EARLY
IN THE PERIOD, DAYS 3-4 (MON-TUE), AGREEMENT AMONG THE
DETERMINISTIC GUIDANCE REMAINS HIGH. THE 00Z ECMWF MADE A FAIRLY
SIGNIFICANT CHANGE FROM ITS PREVIOUS RUN BY DAYS 5-7 (WED-THU),
KEEPING MUCH MORE ENERGY CONSOLIDATED IN AN UPPER LOW ACROSS THE
SOUTHWESTERN U.S., RESULTING IN A WEAKER SHORTWAVE TRAVERSING THE
MIDWEST/GREAT LAKES/NORTHEAST THAT MOVES QUICKER. ENSEMBLE
SOLUTIONS REMAIN LARGELY AGAINST THIS 00Z ECMWF SOLUTION, WITH THE
00Z ECENS AND 06Z GEFS CONTINUING TO SHOW A SOLUTION MUCH MORE
SIMILAR TO THE GFS - WHICH BUILDS A RIDGE MORE QUICKLY ACROSS THE
WEST, WITH A MORE AMPLIFIED AND SLOWER TROUGH ACROSS THE CENTRAL
U.S. THE ONLY ENSEMBLE THAT SEEMS TO PROVIDE EVEN MODEST SUPPORT
FOR THE 00Z ECMWF WAS THE 00Z NAEFS. ADDITIONALLY, TAKING A LOOK
AT ENSEMBLE SPAGHETTI PLOTS, ONLY 2 OR 3 ECENS MEMBERS SUPPORT THE
OPERATIONAL ECMWF BY DAY 7, WITH AN ADDITIONAL 5-6 NAEFS MEMBERS.
AS A RESULT, THE 00Z ECMWF WAS CONSIDERED AN UNLIKELY SOLUTION BY
DAYS 6-7, AND THE WPC FORECAST WAS SHIFTED AWAY FROM THE 00Z ECMWF
BY THAT TIME.

MEANWHILE, THE GFS IS ON THE FASTER SIDE OF THE SPREAD WITH
RESPECT TO THE EVENTUAL TRACK OF MARIA DURING DAYS 6-7, AS THE
SYSTEM IS STILL FORECAST BY MOST GUIDANCE TO REMAIN OFFSHORE,
MOVING BETWEEN THE U.S. AND BERMUDA. A TREND TOWARD A SOMEWHAT
SLOWER PROGRESSION WAS NOTED AMONG THE ENSEMBLE GUIDANCE THIS
MORNING, AND THUS A PROGRESSION A BIT SLOWER THAN THE GFS (BUT
STILL A LITTLE FASTER THAN THE ECMWF), IS FAVORED AT THIS TIME.

THE WPC FORECAST WAS INITIALLY BASED PRIMARILY ON A BLEND OF THE
DETERMINISTIC 00Z ECMWF/06Z GFS FOR DAYS 3-4. FROM DAY 5 ONWARD,
WEIGHTING OF THE 00Z ECMWF WAS SUBSTANTIALLY REDUCED IN FAVOR OF
INCREASED 06Z GFS AND ENSEMBLE (00Z ECENS/00Z NAEFS/06Z GEFS)
WEIGHTING. ENSEMBLES COMPRISED A MAJORITY OF THE FORECAST BLEND BY
DAYS 6-7.


...WEATHER HIGHLIGHTS...

ONGOING LATE SUMMER/EARLY FALL HEAT IN THE GREAT LAKES AND
NORTHEAST WILL END BY WEDNESDAY AS THE COLD FRONT MOVES EASTWARD.
TEMPERATURES WILL DROP ABOUT 20 DEGREES BACK TO NEAR AVERAGE
VALUES BEHIND THE LEAD FRONT, WITH A REINFORCING COLD FRONT BEHIND
IT. MAIN AREA OF PRECIPITATION WILL BE ASSOCIATED WITH THE
SOUTHERN PORTION OF THE COLD FRONT AS IT SINKS THROUGH TEXAS.
SOUTHEAST FLOW AT LOW LEVELS WILL DRAW IN GULF MOISTURE FOR
SEVERAL DAYS BEFORE THE COLD FRONT PUSHES INTO MEXICO AND THE
WESTERN GULF. NEARLY ALL OF THE GUIDANCE INDICATE THE POTENTIAL
FOR SEVERAL INCHES OF RAIN MON-FRI FROM THE HILL COUNTRY NORTHWARD
INTO OKLAHOMA (GENERALLY WEST OF I-35). THIS MAY SPREAD
SOUTHEASTWARD TOWARD THE NW GULF COAST LATER NEXT WEEK.


FRACASSO/RYAN


$$




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