EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1133 AM EDT SAT SEP 23 2017

VALID 12Z TUE SEP 26 2017 - 12Z SAT SEP 30 2017

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


...OVERVIEW...

TROUGH/RIDGE SETUP OVER THE WEST/EAST TO START THE PERIOD ON
TUESDAY WILL TRANSITION TO A RIDGE/TROUGH BY FRIDAY. AN AXIS OF
HEAVY RAIN IS ANTICIPATED TO SET UP OVER TEXAS AS GULF MOISTURE
FEEDS NORTHWESTWARD INTO THE HILL COUNTRY AND POINTS NORTHWARD
INTO WESTERN OKLAHOMA. HURRICANE MARIA IS FORECAST TO MOVE
NORTHWARD JUST OFF THE EAST COAST BEFORE LIKELY TURNING
NORTHEASTWARD OUT TO SEA LATE NEXT WEEK, BUT MAY BE CLOSE ENOUGH
TO THE OUTER BANKS OF NORTH CAROLINA TO SPREAD RAINFALL AND GUSTY
WINDS OVER THE AREA.


...GUIDANCE EVALUATION/PREFERENCES...

THE UPPER TROUGH INITIALLY IN PLACE ACROSS THE WEST ON DAY 3 (TUE)
IS FORECAST BY THE MAJORITY OF MODEL SOLUTIONS TO SEPARATE INTO
TWO COMPONENTS, WITH ONE NORTHERN STREAM SHORTWAVE MOVING
RELATIVELY QUICKLY EAST ACROSS THE UPPER MIDWEST ON WED ON INTO
THE NORTHERN GREAT LAKES BY THU. MODELS/ENSEMBLES HAVE SHOWN A
DEFINITIVE TREND COMPARED TO THIS TIME YESTERDAY OF KEEPING A MORE
CONSOLIDATED AREA OF UPPER ENERGY IN THE FORM OF AN UPPER LOW
ACROSS THE SOUTHWEST THROUGH LATE NEXT WEEK. THE RESULT HAS BEEN A
SOMEWHAT LESS AMPLIFIED NORTHERN STREAM WAVE - WHICH MAY BE
PARTIALLY RESPONSIBLE FOR THE WESTWARD SHIFT IN THE GUIDANCE WITH
THE TRACK OF HURRICANE MARIA. MODELS CONTINUE TO SHOW SOME TIMING
DIFFERENCES WITH THIS NORTHERN STREAM SHORTWAVE, WITH THE 00Z
ECMWF ON THE FAST SIDE OF THE SPREAD, AND THE 06Z GFS/00Z CMC ON
THE SLOW SIDE. A MIDDLE OF THE ROAD SOLUTION WAS FAVORED HERE.
MODELS/ENSEMBLES HAVE ALSO TRENDED A BIT STRONGER WITH THE UPPER
RIDGE EXTENDING FROM THE MID-ATLANTIC EASTWARD INTO THE NORTH
ATLANTIC OCEAN ON TUE-WED, WHICH IS LIKELY ALSO PLAYING A
SIGNIFICANT ROLE IN THE WESTWARD SHIFT AMONG THE GUIDANCE FOR THE
TRACK OF MARIA. THE COMBINATION OF THESE TWO FACTORS - THE
STRENGTH OF THE THE RIDGE NORTH OF MARIA AND THE SPEED/AMPLITUDE
OF THE NORTHERN STREAM WAVE CROSSING THE GREAT LAKES WILL DECIDE
THE EVENTUAL TRACK OF HURRICANE MARIA. PLEASE REFER TO PRODUCTS
ISSUED BY THE NATIONAL HURRICANE CENTER FOR FURTHER DETAILS AND
FORECAST INFORMATION PERTAINING TO MARIA.

BY DAYS 6-7 (FRI-SAT), UNCERTAINTY INCREASES SIGNIFICANTLY IN THE
FORECAST. DETAILS ARE HIGHLY UNCERTAIN SURROUNDING A VIGOROUS
SHORTWAVE/UPPER LOW ORIGINATING IN THE ARCTIC THAT IS FORECAST BY
MOST GUIDANCE TO AMPLIFY ACROSS EASTERN CANADA, AND ITS
INTERACTION WITH SHORTWAVE ENERGY ORIGINATING IN THE NORTH
PACIFIC/GULF OF AK ON TUE. ADDITIONAL UNCERTAINTY ARISES OFF THE
PACIFIC NORTHWEST BY LATE NEXT WEEK. ENSEMBLES SHOW GENERAL
AGREEMENT ON TROUGHING OFF THE NORTHWEST COAST BY FRI-SAT, HOWEVER
INDIVIDUAL SOLUTIONS VARY WIDELY, FROM AN AMPLIFIED LONG WAVE
TROUGH TO A RELATIVELY LOW AMPLITUDE SHORTWAVE THAT MOVES SOUTH OF
AN UPPER LOW OVER THE GULF OF AK. GIVEN THE INCREASED UNCERTAINTY
BY THIS TIME FRAME, ENSEMBLE SOLUTIONS WERE FAVORED OVER
DETERMINISTIC GUIDANCE LATE IN THE MEDIUM RANGE PERIOD.

THE WPC FORECAST WAS INITIALLY BASED HEAVILY ON A BLEND OF
DETERMINISTIC GUIDANCE, INCLUDING THE 00Z ECMWF/06Z GFS FOR DAYS
3-4. A TREND TOWARD HEAVIER ENSEMBLE GUIDANCE WEIGHTING, INCLUDING
THE 00Z ECENS/00Z NAEFS/06Z GEFS WAS SHOWN FROM DAY 5 ONWARD, WITH
A MAJORITY OF THE BLEND COMPRISED OF ENSEMBLE GUIDANCE BY DAYS 6-7.


...WEATHER HIGHLIGHTS...

ONGOING HEAT IN THE GREAT LAKES AND NORTHEAST WILL END BY
WEDNESDAY AS THE COLD FRONT MOVES EASTWARD AND DROPS TEMPERATURES
BY ABOUT 20 DEGREES (BUT ONLY TO NEAR AVERAGE VALUES). MAIN AREA
OF PRECIPITATION WILL BE ASSOCIATED WITH THE SOUTHERN PORTION OF
THE COLD FRONT AS IT SINKS THROUGH TEXAS DURING THE WEEK.
SOUTHEAST FLOW AT LOW LEVELS WILL DRAW IN GULF MOISTURE FOR
SEVERAL DAYS BEFORE THE COLD FRONT PUSHES INTO MEXICO AND THE
WESTERN GULF, SETTING UP A MULTI-DAY RAINFALL EVENT. NEARLY ALL OF
THE GUIDANCE INDICATE THE POTENTIAL FOR SEVERAL INCHES OF RAIN
TUE-SAT FROM THE HILL COUNTRY NORTHWARD INTO OKLAHOMA (GENERALLY
WEST OF I-35). MEANWHILE, MARIA'S FORECAST TRACK HAS SHIFTED
CLOSER TO THE U.S. EAST COAST, AND IT IS BECOMING INCREASINGLY
LIKELY THAT SOME DIRECT IMPACTS, INCLUDING RAIN, GUSTY WINDS, HIGH
SURF, AND DANGEROUS RIP CURRENTS, WILL OCCUR ALONG PORTIONS OF THE
COAST NEXT WEEK. 


RYAN/FRACASSO