Preliminary Forecasts
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000
FXUS02 KWNH 300641
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
240 AM EDT TUE AUG 30 2016

VALID 12Z FRI SEP 02 2016 - 12Z TUE SEP 06 2016

...OVERVIEW...

GUIDANCE CONFIDENTLY SHOWS THE MEAN PATTERN SETTLING INTO AN ERN
PAC RIDGE/WRN U.S. TROUGH/ERN RIDGE CONFIGURATION BUT WITH SOME
EMBEDDED UNCERTAINTIES.  THERE ARE VARIED SOLNS FOR SHRTWV DETAILS
WITHIN THE WRN TROUGH WHILE FARTHER EWD THERE IS A FAIR AMOUNT OF
SPREAD FOR HOW ENERGY IN THE SRN PART OF THE DEPARTING LATE WEEK
EAST COAST TROUGH MAY LINGER OVER THE AREA.  AS FOR TROPICAL
FEATURES THE NATIONAL HURRICANE CENTER IS ISSUING ADVISORIES ON
T.D. EIGHT WHICH SHOULD BE OUT OF THE PICTURE BY FRI...AND T.D.
NINE WHICH IS FCST TO EMERGE OVER THE WRN ATLC AFTER CROSSING NRN
FLORIDA IN THE LATTER PART OF THE SHORT RANGE PERIOD.
COMPLICATING THE EAST COAST FCST IS HOW THE LINGERING ERN TROUGH
ENERGY MAY INTERACT WITH T.D. NINE.


...GUIDANCE/UNCERTAINTY ASSESSMENT...

WITH VARIOUS DETAIL DIFFS ARISING FAIRLY EARLY IN THE PERIOD THE
LATEST MANUAL FCST STARTS WITH SOMEWHAT MORE THAN USUAL GEFS/ECMWF
ENSEMBLE MEAN WEIGHTING VS OPERATIONAL RUNS EARLY-MID PERIOD
BEFORE GOING ENTIRELY TO THE MEANS BY DAYS 6-7 SUN-TUE.

WITHIN THE WRN MEAN TROUGH... SOME RECENT GFS RUNS INCL THE NEW
00Z VERSION ARE SOMEWHAT ON THE SRN SIDE OF THE SPREAD WITH THE
TRACK OF UPR LOW ENERGY THAT PROCEEDS FROM THE PAC NW TOWARD
HUDSON BAY AND SUPPORTS VIGOROUS CANADIAN LOW PRES WITH A TRAILING
COLD FRONT THAT REACHES THE NRN PLAINS.  AMONG OTHER SOLNS THERE
ARE STILL SOME TIMING/DEPTH/AMPLITUDE DIFFS ALOFT THAT HAVE AN
INFLUENCE ON FRONTAL TIMING.  FOR THE AMPLIFIED TROUGH THAT
CONTINUES INTO THE WEST SAT ONWARD THE 12Z/18Z GEFS MEAN AND 12Z
ECMWF MEAN HAVE ARRIVED AT VERY SIMILAR SOLNS AFTER THE FORMER
TRENDED SOMEWHAT FASTER AND THE LATTER SHARPER AND SLOWER COMPARED
TO 24 HRS AGO.  THE MOST PROMINENT UNCERTAINTY DURING DAYS 5-7
SUN-TUE WILL BE THE EVOLUTION OF EMBEDDED SHRTWV ENERGY ACROSS THE
NRN HALF OF THE WEST AND INTO THE NRN PLAINS... WHICH WILL
DETERMINE WAVE DETAILS ALONG THE NRN PLAINS FRONT.  OVERALL PREFER
AN INTERMEDIATE SOLN SLIGHTLY CLOSER TO THE ECMWF/ECMWF MEAN
DURING THE FIRST HALF OF THE PERIOD THEN TRENDING TO AN ENSEMBLE
MEAN BLEND WHILE AWAITING IMPROVED AGREEMENT SFC/ALOFT ACROSS THE
NRN TIER LATER IN THE PERIOD.

FCST CONFIDENCE OVER THE EAST COAST/WRN ATLC IS LOWER THAN DESIRED
GIVEN UNCERTAINTY OVER WHETHER/HOW T.D. NINE AND WEAK UPR ENERGY
BEING LEFT BEHIND BY THE DEPARTING ERN TROUGH MAY INTERACT... AND
EVENTUALLY THE UNCERTAIN DEGREE OF WWD PROGRESS/EXTENT OF A CNTRL
ATLC RIDGE BUILDING TOWARD BERMUDA.  THE COMBINATION OF FEATURES
INVOLVED IS MAKING IT VERY DIFFICULT FOR MODELS AND ENSEMBLE
MEMBERS TO COME TO ANY SEMBLANCE OF REASONABLE AGREEMENT FOR T.D.
NINE ONCE IT REACHES BEYOND THE NERN GULF OF MEXICO.
EXTRAPOLATION OF THE 0300 UTC NHC ADVISORY BEYOND 00Z SUN YIELDS
FAIRLY GOOD CONTINUITY WITH A WELL OFFSHORE TRACK AS AGREED UPON
IN YDAYS DAYTIME COORDINATION.  NOTE THAT RECENT GFS RUNS AND THE
18Z CYCLE OF THE GEFS MEAN ARE AMONG A PORTION OF THE WIDE
GUIDANCE SPREAD THAT WOULD YIELD GREATER INFLUENCE ON CONDS NEAR
THE EAST COAST.  FAVORING A SOLN CLOSER TO CONTINUITY LED TO USE
OF THE 12Z GEFS MEAN OVER THE 18Z RUN FOR THAT COMPONENT OF THE
FCST BLEND.


...WEATHER HIGHLIGHTS/THREATS...

THOSE NEAR THE EAST COAST... AND THE SOUTHEAST/SRN MID ATLC COAST
IN PARTICULAR... SHOULD CONTINUE TO MONITOR FCSTS FOR T.D. NINE
WHICH WILL LIKELY BE ACCOMPANIED BY HVY RAIN/STRONG WINDS.  THE
EXTENT TO WHICH THE REST OF THE EAST COAST SEES ANY EFFECTS FROM
T.D. NINE IS VERY UNCERTAIN AT THIS TIME.  FROM THE NORTHWEST INTO
THE PLAINS... MAJORITY OF PAC NW RNFL SHOULD BE EARLY IN THE
PERIOD IN ASSOC WITH THE DEEPEST CORE OF ENERGY ALOFT.  THE NRN
ROCKIES SHOULD SEE PERIODS OF PCPN.  AS THE LEADING SFC FRONT
REACHES THE PLAINS AND LIKELY DECELERATES THERE ARE CONTINUED
SIGNALS FOR HVY RNFL POTENTIAL FROM THE NRN-CNTRL PLAINS INTO THE
UPR MS VLY/UPR GRTLKS.  SOME CONVECTION WITHIN THIS AREA MAY ALSO
BE ON THE STRONG SIDE.  ELSEWHERE EXPECT SOME LINGERING RNFL OVER
FL AND VICINITY WHILE THERE MAY BE ONE OR MORE PERIODS OF
SHWRS/TSTMS NEAR THE GULF COAST AND OVER THE SRN HALF OF THE
ROCKIES.

THE NRN PLAINS WILL SEE THE MOST DRAMATIC CHANGE IN TEMPS DURING
THE FCST PERIOD AS ADVANCING COOL AIR OVER THE WEST DISPLACES LATE
WEEK HEAT.  NRN PLAINS HIGHS SHOULD BE 10-20F ABOVE NORMAL FRI BUT
THEN FALL TO 5-15F BELOW NORMAL BY SUN-MON.  HIGH PRESSURE
BUILDING INTO THE EAST WILL BRING NEAR TO SLIGHTLY BELOW NORMAL
TEMPS DURING THE FIRST HALF OF THE PERIOD BUT WARM FLOW ON THE WRN
SIDE OF THE HIGH WILL LIKELY PUSH HIGHS 10-15F ABOVE NORMAL FROM
THE OH VLY/GRTLKS INTO NORTHEAST BY EARLY NEXT WEEK.

RAUSCH

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