Prognostic Meteorological Discussion
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498
FXUS02 KWBC 241517
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1116 AM EDT SUN SEP 24 2017

VALID 12Z WED SEP 27 2017 - 12Z SUN OCT 01 2017

...HEAVY RAIN FOR PORTIONS OF TEXAS, NEW MEXICO, AND OKLAHOMA THIS
WEEK...


...OVERVIEW...

UPPER PATTERN WILL TREND TOWARD INCREASED EASTERN TROUGHING AS A
LEAD COLD FRONT PUSHES ACROSS THE EAST WED-THU AND A REINFORCING
COLD FRONT DIVES THROUGH THE UPPER MIDWEST AND OHIO VALLEY
THU-FRI. THIS SHOULD HELP STEER HURRICANE MARIA OUT TO SEA BUT NOT
BEFORE IT DRAWS CLOSER TO THE OUTER BANKS OF NORTH CAROLINA. HEAVY
RAIN REMAINS IN THE FORECAST FROM THE SHORT RANGE INTO THE MEDIUM
RANGE ACROSS WESTERN/CENTRAL TEXAS AND BACK INTO EASTERN NEW
MEXICO AS WELL AS NORTH INTO OKLAHOMA. FINALLY, A PACIFIC COLD
FRONT WILL PUSH INTO WA/OR ON SATURDAY.


...GUIDANCE EVALUATION/PREFERENCES...

MODELS/ENSEMBLES CONTINUE TO SHOW GOOD AGREEMENT ON THE LARGE
SCALE PATTERN THROUGH DAYS 3-4 (WED-THU). AS A RESULT, THE
FORECAST FOR DAYS 3-4 WAS BASED HEAVILY ON A BLEND INCLUDING THE
DETERMINISTIC 00Z ECMWF/06Z GFS. FROM DAY 5 (FRI) ONWARD,
DIFFERENCES BEGIN TO EMERGE ACROSS THE MIDWEST/GREAT LAKES AS AN
UPPER TROUGH AMPLIFIES, AND ENSEMBLE WEIGHTING WAS GRADUALLY
INCREASED THROUGH TIME TO ACCOUNT FOR INCREASING UNCERTAINTY. A
COMPLEX INTERACTION OF SHORTWAVE ENERGY FROM MULTIPLE SOURCES,
INCLUDING ENERGY EJECTING FROM THE WESTERN U.S. TROUGH, A COUPLE
AREAS OF NORTHERN STREAM ENERGY ORIGINATING IN THE NORTH
PACIFIC/GULF OF ALASKA, LOOKS TO OCCUR, WITH THE TIMING AND
INTENSITY OF THESE FEATURES IMPACTING THE DEGREE OF POSSIBLE
PHASING AND THUS THE AMPLIFICATION OF THE POTENTIAL TROUGH/UPPER
LOW BY DAYS 6-7 (SAT-SUN). ADDITIONAL UNCERTAIN INTERACTIONS MAY
OCCUR WITH A POTENTIAL CLOSED UPPER LOW DEVELOPING FARTHER NORTH
OVER CANADA, WITH ENERGY ORIGINATING IN THE ARCTIC. THE END RESULT
IS THAT BY DAYS 6-7, THE ECMWF DEPICTS A LARGE TROUGH/UPPER LOW
ACROSS THE OH VALLEY/EASTERN U.S., WHILE THE GFS IS MUCH WEAKER
WITH THE FEATURE AND EJECTS IT QUICKLY INTO THE NORTH ATLANTIC.
THE ECMWF HAS SHOWN A RELATIVELY HIGH DEGREE OF RUN-TO-RUN
CONSISTENCY WITH RESPECT TO THIS FEATURE, WHILE THE GFS HAS SHOWN
A HIGH DEGREE OF VARIABILITY FROM RUN-TO-RUN. ENSEMBLE MEANS SEEM
TO BE IN GENERAL AGREEMENT ON A TROUGH THAT IS SLOWER AND MORE
AMPLIFIED THAN THE GFS, ALTHOUGH THE SPECIFIC DETAILS/EVOLUTION
ARE HIGHLY UNCERTAIN AT THIS TIME. GIVEN THE FAIRLY HIGH
CONSISTENCY IN THE ECMWF, AND SOME DEGREE OF ENSEMBLE SUPPORT, AS
WELL AS THE ORIGIN OF THE RESPECTIVE FEATURES IN DATA SPARSE
REGIONS OF THE NORTH PACIFIC AND THE ARCTIC (WHERE THE GFS SEEMS
TO GENERALLY PERFORM POORLY), A SOLUTION CLOSER TO THE 00Z ECMWF
OR ECMWF ENSEMBLE MEAN IS FAVORED AT THIS TIME.


...WEATHER HIGHLIGHTS...

LEAD COLD FRONT MOVES WILL DROPS TEMPERATURES BY ABOUT 20 DEGREES
BUT ONLY TO NEAR AVERAGE VALUES LATER THIS WEEK WHILE THE
SECONDARY FRONT WILL BRING IN COOLER THAN AVERAGE TEMPERATURES
FROM THE HIGH PLAINS AND MIDWEST LATER THIS WEEK INTO THE
NORTHEAST AND MID-ATLANTIC NEXT WEEKEND. OTHER AREA OF COOLER THAN
AVERAGE TEMPERATURES WILL BE OVER THE SOUTHERN PLAINS INTO TEXAS
AND NEW MEXICO DUE TO CLOUDS/RAIN. SOUTHEAST FLOW AT LOW LEVELS
WILL DRAW IN GULF MOISTURE FOR SEVERAL MORE DAYS THIS WEEK,
SETTING UP A MULTI-DAY RAINFALL EVENT FROM THE SHORT RANGE INTO
THE MEDIUM RANGE. TO THE EAST, IT IS BECOMING INCREASINGLY LIKELY
THAT SOME DIRECT IMPACTS FROM MARIA, INCLUDING RAIN, GUSTY WINDS,
HIGH SURF, AND DANGEROUS RIP CURRENTS, WILL OCCUR ALONG PORTIONS
OF THE OUTER BANKS WED-THU BEFORE IT PULLS AWAY TO THE EAST.


FRACASSO/RYAN


$$





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