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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0559Z Aug 24, 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
159 AM EDT SUN AUG 24 2014

VALID 12Z WED AUG 27 2014 - 12Z SUN AUG 31 2014

...PATTERN/MODEL EVALUATION...

THE MEDIUM RANGE PERIOD WILL FEATURE A PROGRESSIVE NORTHERN STREAM
WITH ENERGY RACING EASTWARD IN A LOW-AMPLITUDE FLOW ACROSS CANADA.
TO THE SOUTH...A SLOW MOVING UPPER TROF WILL BEGIN OVER THE
CENTRAL ROCKIES EARLY WEDNESDAY WITH HEIGHT FALLS GRADUALLY
WORKING THEIR WAY EASTWARD IN TIME. THE MODELS WHICH WERE FASTER
IN PREVIOUS DAYS WITH THIS SYSTEM HAVE TRENDED SLOWER WHICH IS A
GOOD SIGN GIVEN THE TROF IS CUT-OFF FROM THE POLAR JET. BY THE END
OF THE WEEK THE TROF SHOULD REACH THE OH VALLEY/MID-ATLANTIC WHILE
POSSIBLY SHEARING APART DEPENDING UPON THE PIECE OF GUIDANCE. THE
GREATER UNCERTAINTY IN THE MID-LATITUDES IS WITH THE UPSTREAM
PATTERN EVOLVING ACROSS THE WESTERN/CENTRAL U.S...PARTICULARLY
DURING DAYS 5 THROUGH 7...AUGUST 29-31. THE TIMING OF A FEATURE
DEVELOPING OVER THE NORTHEASTERN PACIFIC PLAYS A KEY ROLE IN HOW
THE 500-MB PATTERN SETS UP ACROSS WESTERN STATES. THE 18Z GFS/GEFS
MEAN WERE QUICKER AND A BIT MORE AMPLIFIED WHICH LEADS TO STRONGER
HEIGHT RISES ACROSS THE WEST COAST. THIS BECOMES EVEN MORE
PRONOUNCED BY DAY 5/FRIDAY. MEANWHILE...THE 12Z ECMWF/ECMWF
ENSEMBLE MEAN SUITE FAVOR A SLOWER NORTHEASTERN PACIFIC TROF WITH
MORE OR LESS ZONAL FLOW ACROSS THE PACIFIC NORTHWEST AND INTO THE
NORTHERN ROCKIES. AT LEAST THROUGH EARLY FRIDAY THE 12Z UKMET
FAVORS THE HIGHER AMPLITUDE PATTERN OF THE GFS WHILE THE 12Z CMC
SUPPORTS MORE ZONAL FLOW WHICH IS ALONG THE LINES OF THE ECMWF. BY
NEXT WEEKEND THE MODEL DIFFERENCES ARE RATHER HIGH AS EVIDENT BY
THE CHAOTIC 576-DM SPAGHETTI PLOT. FORTUNATELY THE 18Z GEFS/12Z
ECMWF ENSEMBLE MEAN ARE IN HALF DECENT AGREEMENT WHICH SEEMS TO
SPLIT THE DIFFERENCE BETWEEN THE AMPLIFIED 18Z GFS AND FLATTER 12Z
ECMWF.

LOOKING INTO THE TROPICS...TROPICAL DEPRESSION FOUR MAY BE AN
IMPORTANT PLAYER DURING THE PERIOD DEPENDING UPON THE TRACK. THE
GUIDANCE SHOW A VARIETY OF POSSIBLE SOLUTIONS WITH THE 12Z UKMET
SHOWING A PAIR OF LANDFALLS IN SOUTHEASTERN FL AND EVENTUALLY
SOUTHERN AL. ON THE OTHER SIDE IS THE 12Z CMC WHICH QUICKLY LIFTS
THE TROPICAL SYSTEM UP TOWARD BERMUDA. THESE ARE PROBABLY TWO OF
THE MORE EXTREME SOLUTIONS WHICH WERE NOT UTILIZED IN THE 03Z NHC
TROPICAL POINTS. IT APPEARED SOME FORM OF 18Z GFS/12Z ECMWF BLEND
AT LEAST CAPTURED THEIR TRACK TO SOME EXTENT BUT NOT ALL THE WAY.
AT THIS POINT IN TIME...THE TRACK STAYS OFFSHORE OF THE EASTERN
SEABOARD BUT THERE IS STILL PLENTY OF UNCERTAINTY AS TROPICAL
DEPRESSION FOUR IS STILL DEVELOPING NEAR HISPANIOLA.


...WPC PREFERENCE...

FOR DAYS 3 THROUGH 5...AUGUST 27-29...WPC FAVORED AN EQUAL BLEND
OF THE 18Z GFS/12Z ECMWF. THIS WAS PRIMARILY UTILIZED TO ACCOUNT
FOR THE TRACK OF TROPICAL DEPRESSION FOUR AS THE 12Z CMC/UKMET
WERE OUTLIERS WITH THIS SYSTEM. ADDITIONALLY...DID NOT HAVE A GOOD
GRASP ON HOW TO HANDLE THE UNCERTAINTY IN THE NORTHEASTERN PACIFIC
TOWARD DAY 5 SO SPLIT THE DIFFERENCE. THEREAFTER...LEANED MORE
HEAVILY ON A 50/50 BLEND OF THE 18Z GEFS/12Z ECMWF ENSEMBLE MEANS
GIVEN THE RATHER HIGH SPREAD BETWEEN THE DETERMINISTIC MODELS.
THESE MEANS WERE FAIRLY STABLE IN THE SINUSOIDAL PATTERN EVOLVING
ACROSS THE WESTERN/CENTRAL U.S. LATE NEXT WEEK.


...SENSIBLE WEATHER HIGHLIGHTS...

THE HIGHEST ANOMALIES DURING THE PERIOD WILL BE ON DAYS
3/4...AUGUST 27-28...WHERE TEMPERATURES WILL BE APPROXIMATELY 10
TO 15 DEGREES BELOW NORMAL OVER THE NORTHERN TIER STATES. THIS
EQUATES TO HIGH TEMPERATURES GENERALLY IN THE UPPER 60S/LOWER 70S.
SOUTH OF THE BAROCLINIC ZONE IT WILL BE QUITE WARM OVER THE
SOUTHERN PLAINS AS HIGHS REACH THE MID/UPPER 90S. AS HEIGHTS BUILD
ACROSS THE WESTERN U.S. MID/LATE NEXT WEEK...IT SHOULD BE RATHER
WARM ACROSS THE WESTERN STATES WITH WIDESPREAD 90S FORECAST OVER
THE INTERIOR VALLEYS. IN TERMS OF PRECIPITATION...THE BAROCLINIC
ZONE SLIDING FROM THE CENTRAL U.S. EASTWARD WILL BE FOCUS FOR
HEAVY RAINFALL THROUGHOUT THE PERIOD. GIVEN THE CLOSE PROXIMITY OF
TROPICAL DEPRESSION FOUR TO THE SOUTHEASTERN U.S. COAST...SOME
IMPACTS MAY OCCUR...PARTICULARLY OVER THE OFFSHORE WATERS.


RUBIN-OSTER