Preliminary Forecasts
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000
FXUS02 KWNH 300650
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
249 AM EDT SAT MAY 30 2015

VALID 12Z TUE JUN 02 2015 - 12Z SAT JUN 06 2015

...OVERVIEW...

GUIDANCE IS MAINTAINING SOME SIMILAR THEMES REGARDING THE LARGE
SCALE PATTERN THAT SHOULD INVOLVE A WRN TROUGH NEAR 120W AND A
BROAD MEAN RIDGE DOWNSTREAM... WHILE OVER CANADA MEAN RIDGING
SHOULD PREVAIL OVER WRN AREAS WITH A MEAN TROUGH SETTLING IN
FARTHER EWD.  SHRTWV UNCERTAINTIES WITHIN THE WRN CONUS TROUGH
PERSIST BUT INCREASED SPREAD AND/OR RECENT TRENDS WITH FEATURES
OVER THE ERN STATES HAVE LOWERED CONFIDENCE IN SOME ASPECTS OF THE
FCST COMPARED TO YDAY.


...GUIDANCE EVALUATION AND PREFERENCES...

ONGOING DETAIL UNCERTAINTIES OVER THE WRN-CNTRL STATES AND
MODERATE ACCOUNT FOR TRENDS OVER THE EAST AS VIEWED THROUGH THE
18Z CYCLE YIELDED A COMPROMISE AMONG THE 12Z GFS/ECMWF AND 18Z
GEFS/12Z ECMWF MEANS FOR DAYS 3-4 TUE-WED FOLLOWED BY REPLACING
THE 12Z ECMWF WITH THE NAEFS MEAN DAY 5 THU AND THEN USING A
50/30/20 BLEND OF THE 12Z ECMWF MEAN/12Z NAEFS MEAN/18Z GEFS MEAN
FOR DAYS 6-7 FRI-SAT.

WITHIN AND SURROUNDING THE WRN TROUGH... CONTINUE TO EXPECT MOST
OF THE IMPORTANT SMALLER SCALE SHRTWV DETAILS NOT TO BE RESOLVED
SATISFACTORILY UNTIL REACHING THE SHORT RANGE TIME FRAME.  THUS
THE EVOLUTION OF ONE OR MORE NRN TIER SFC WAVES IS LIKELY TO HAVE
LOW PREDICTABILITY.  THERE IS REASONABLE CONSENSUS IN PRINCIPLE ON
SRN CANADA FLOW SUPPORTING THE SFC FRONT EXPECTED TO DROP INTO THE
EXTREME NRN TIER THOUGH.  AFTER INITIAL ENERGY WITHIN THE WRN
TROUGH EJECTS OUT... THE MAJORITY OF LATEST GUIDANCE IS SUGGESTING
SOME DEGREE OF AMPLIFICATION/SHARPENING WITH THE ARRIVAL OF
UPSTREAM FLOW.  THE 12Z ECMWF IS SOMEWHAT ON THE EXTREME SIDE OF
GUIDANCE WITH ITS DEEPER TROUGH/CLOSED LOW OVER CA MID-LATE PERIOD
BUT THERE ARE INDICATIONS THAT FLOW COULD SEPARATE TO SOME
EXTENT... WITH BETTER AGREEMENT ON SLOWER TROUGH PROGRESSION THAN
THE NEW 00Z GFS BY NEXT SAT.

TO VARYING DEGREES OVER THE PAST DAY GUIDANCE HAS BEEN ADJUSTING
SIGNIFICANTLY SLOWER AND MORE CONCENTRATED WITH THE MID LVL SYSTEM
CROSSING NEW ENGLAND AROUND MIDWEEK.  AS A RESULT THE SFC
EVOLUTION OVER THE NORTHEAST IS QUITE DIFFERENT WITH LOW PRES NOW
INDICATED OVER OR JUST OFFSHORE NEW ENG AS OF EARLY WED VERSUS
EARLIER FCSTS OF A SFC RIDGE.  COMPARED TO GUIDANCE THROUGH THE
18Z CYCLE THE 12Z ECMWF WAS MOST AGGRESSIVE IN WRAPPING LOWER SFC
PRESSURES INLAND WHILE THE 18Z GFS WAS FARTHEST SWD WITH ITS CORE
OF MID LVL ENERGY.  24-HR TRENDS AS WELL AS JUST THE 18Z GEFS MEAN
TREND VERSUS ITS 12Z RUN ARGUED FOR SOME ADJUSTMENT TO AN
INTERMEDIATE SOLN AMONG LATEST GUIDANCE.  NEW 00Z SOLNS THUS FAR
COULD SUPPORT FURTHER ADJUSTMENT.

MEANWHILE THE FCST HAS BECOME INCREASINGLY UNCERTAIN AND
COMPLICATED ACROSS THE SRN TIER.  THROUGH THE 18Z CYCLE...
GFS/GEFS-BASED SOLNS HAD BEEN PERSISTENT IN CLOSING OFF AN UPR LOW
NEAR THE LWR MS VLY WHILE ECMWF/ECMWF MEAN RUNS AS WELL AS CMC/CMC
MEAN SOLNS WERE TRENDING MORE OPEN AND PROGRESSIVE WITH THE ENERGY
OVER THE PAST DAY.  GFS/GEFS PERSISTENCE AND SLOW MOVING NATURE OF
SRN TIER FLOW ALOFT OFFERED SOME ALLOWANCE FOR THE GFS SCENARIO.
HOWEVER THE 00Z GFS/GEFS MEAN HAVE TRENDED MORE PROGRESSIVE THUS
LOWERING THE POTENTIAL FOR A WWD SOLN.

THEN THERE IS THE FURTHER COMPLICATION OF HOW THIS FEATURE MAY
INTERACT WITH ONE OR MORE IMPULSES THAT MAY FLOW NNEWD FROM THE
WRN CARIBBEAN OR VICINITY... AND WHAT KIND OF SFC EVOLUTION WILL
RESULT.  FOR NOW PREFER A CONSERVATIVE APPROACH WITH A BROAD SFC
TROUGH AS IT REMAINS TO BE SEEN WHETHER MID LVL ENERGY WILL EVOLVE
IN SUCH A WAY AS TO SUPPORT A WELL DEFINED SFC SYSTEM.  ALSO NOTE
THAT 12Z GEFS/ECMWF ENSEMBLE PROBS OF COMBINED HIGH RH/LOW WIND
SHEAR ARE FAIRLY LOW FROM THE SERN GULF NEWD.


...SENSIBLE WEATHER HIGHLIGHTS/THREATS...

EXPECT ONE OR MORE EPISODES OF RAIN/TSTMS FROM THE NRN ROCKIES
THROUGH THE NRN HALF OF THE PLAINS AND INTO THE UPR MS VLY WITH
SOME LOCALLY HVY TO EXCESSIVE AMTS PSBL.  LOW PREDICTABILITY OF
IMPORTANT SHRTWV DETAILS MEANS IT WILL LIKELY TAKE UNTIL THE SHORT
RANGE TIME FRAME TO PINPOINT THE MOST FAVORABLE LOCATIONS FOR
HEAVIEST RNFL.  CONSULT SPC OUTLOOKS FOR INFO REGARDING SEVERE
POTENTIAL.  THE SOUTHEAST/FL SHOULD BE ANOTHER REGION LIKELY TO
SEE ENHANCED RNFL... AT LEAST WITH SOMEWHAT HIGHER CONFIDENCE THAN
IN WHAT THE EXACT SFC EVOLUTION WILL BE.  RECENT TRENDS TOWARD
PROGRESSION OF SRN TIER ENERGY YIELD LESS POTENTIAL FOR
SIGNIFICANT RNFL OVER THE LWR MS VLY.  MEANWHILE RECENT TRENDS
ALOFT BRING MORE MSTR INTO THE NORTHEAST DURING THE FIRST PART OF
THE PERIOD.  TEMPS SHOULD BE MODERATE AS A WHOLE WITH LESS THAN
TYPICAL COVERAGE OF ANOMALIES GREATER THAN 10F DURING THE PERIOD.

RAUSCH

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