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Prognostic Meteorological Discussion
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000
FXUS02 KWBC 241502
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1102 AM EDT SAT JUN 24 2017

VALID 12Z TUE JUN 27 2017 - 12Z SAT JUL 01 2017

...PATTERN OVERVIEW AND MODEL PREFERENCES...

THE MEDIUM RANGE PERIOD WILL BEGIN ON TUESDAY WITH AN UPPER LEVEL
TROUGH OVER THE EASTERN U.S. THAT DEAMPLIFIES AND MOVES OFF THE
COAST.  AN UPPER LOW OVER ALBERTA 12Z TUE IS FORECAST TO MOVE
SLOWLY EAST...WITH  SHORTWAVE ENERGY ROTATING THROUGH THE BASE OF
THE TROUGH LOWERING HEIGHTS ACROSS THE NORTHERN ROCKIES TUE AND
NORTHERN HIGH PLAINS WED, FURTHER EAST IN THE NORTHERN PLAINS
THU...UPPER MS VALLEY FRI AND THE UPPER LAKES OVER THE WEEKEND.

THE MAJORITY OF THE 00 GEFS AND 00Z ECMWF ENSEMBLE MEMBERS SHOW A
BROAD CENTRAL US TROUGH WITH LOW AMPLITUDE WAVES DEVELOPING WITHIN
THE TROUGH. THE LAST FEW RUNS OF THE GFS, INCLUDING THE 00Z RUN,
HAVE INDICATED A MORE AMPLIFIED TROUGH ACROSS THE NORTHERN ROCKIES
AND NORTHERN PLAINS, WITH THE ECMWF AND ECMWF MEAN LESS AMPLIFIED.
 THE GFS A FEW DAYS AGO WAS FASTER TO SHOW POTENTIAL FOR LOWER
HEIGHTS SO WE DON`T WISH TO UNDERWEIGHT THE GFS SOLUTION.
UNTIL WE SEE WHICH WAY THE SITUATION BREAKS...WE THOUGHT EQUALLY
WEIGHTING THE 00Z GFS/GEFS MEN AND 00Z ECMWF AND ECMWF ENSEMBLE
MEAN WOULD BE A GOOD APPROACH TO THE FORECAST FOR DAYS 3-7.

THE OPERATIONAL 06Z GFS WAS NOT USED FOR DAY 5 AND BEYOND GIVEN
THAT ITS UPPER LEVEL TROUGH BECAME MUCH MORE AMPLIFIED OVER THE
NORTHERN PLAINS AND UPPER MS VALLEY/UPPER GREAT LAKES.

ONE AREA SUBJECT TO CHANGE IS THE 00Z CANADIAN GLOBAL WAS MORE
ASSERTIVE IN DEVELOPING WEST COAST TROUGHING NEXT WEEKEND VS THE
FLATTER/MORE PROGRESSIVE ECMWF AND GFS.  SEVERAL CANADIAN AND
ECMWF ENSEMBLE MEMBERS DO HAVE A HIGHER AMPLITUDE TROUGH SO AN
APPROACH USING THE MEANS WAS USED TO MITIGATE THE UNCERTAINTY.

...WEATHER HIGHLIGHTS...

A WAVY SURFACE FRONT EXTENDING FROM THE CENTRAL PLAINS TO THE
UPPER MS VALLEY WILL HELP TRIGGER NUMEROUS SHOWERS AND STORMS.
THERE IS A GROWING SIGNAL IN THE QPF GUIDANCE THAT AN AXIS OF
HEAVY RAIN COULD FALL FROM THE CENTRAL PLAINS TO THE GREAT LAKES
REGION.
AREAS OF EXCESSIVE RAINFALL CAPABLE OF PRODUCING FLASH FLOODING
WILL BE POSSIBLE.

A SLOW MOVING FRONTAL BOUNDARY NEAR THE GULF COAST AND OVER
FLORIDA THROUGH WEDNESDAY WILL PROVIDE A FOCUS FOR PERIODS OF
SHOWERS AND THUNDERSTORMS, MAINLY DURING THE AFTERNOON HOURS.
THIS BOUNDARY SHOULD DISSIPATE BY THE END OF THE WEEK AS RETURN
FLOW FROM THE GULF OF MEXICO DEVELOPS.

THE HEAT WAVE ACROSS THE DESERT SOUTHWEST IS FORECAST TO ABATE
EARLY NEXT WEEK AS THE RIDGE GRADUALLY SINKS SOUTH INTO MEXICO,
BUT TEMPERATURES ARE WILL STILL LIKELY BE SLIGHTLY ABOVE AVERAGE
FOR MANY AREAS.  TEMPERATURES IN THE OHIO VALLEY ARE FORECAST TO
BE SEVERAL DEGREES BELOW THE 30 YEAR MEANS ON DAY 3 BUT BY THE END
OF THE WEEK, EXPECT TEMPERATURES TO BOUNCE BACK TO NORMAL OR
SLIGHTLY ABOVE NORMAL WITH SOUTHERLY FLOW INCREASING AHEAD OF THE
APPROACHING FRONTAL SYSTEM TO THE WEST. TEMPERATURES ARE FORECAST
TO RISE ABOVE NORMAL OVER THE MID ATLANTIC STATES DAY 6 FRI 30 JUN
AND DAY 7 SAT 01 JUL IN FAVORABLE SOUTHWEST FLOW WITH A WARM CORE
OF 850 MB TEMPS NEAR 18 DEGREES C.

PETERSEN

$$
National Weather Service Weather Forecast Office
