Preliminary Forecasts
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FXUS02 KWNH 240641
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
240 AM EDT FRI APR 24 2015

VALID 12Z MON APR 27 2015 - 12Z FRI MAY 01 2015

...OVERVIEW...

THE DOMINANT FOCUS OF THE MEDIUM RANGE FCST IN TERMS OF COMBINED
SENSIBLE WEATHER IMPACTS AND UNCERTAINTY WILL BE ON THE EVOLUTION
OF CLOSED LOW ENERGY EJECTING FROM THE SRN HIGH PLAINS AND ONE OR
MORE BUNDLES OF NRN STREAM ENERGY FLOWING INTO AN ERN CONUS MEAN
TROUGH.  IMPORTANT DETAILS ARE OF THE TYPE THAT CAN BE DIFFICULT
TO RESOLVE VERY FAR IN ADVANCE BUT WILL HAVE SIGNIFICANT INFLUENCE
ON SENSIBLE WEATHER.  UPSTREAM THERE IS FAIR AGREEMENT THAT A WRN
CONUS MEAN RIDGE SHOULD GRADUALLY BUILD TOWARD THE PLAINS AS AN
ERN PAC MEAN UPR TROUGH HEADS TOWARD THE WEST COAST... THOUGH
GUIDANCE SHOWS INCREASING DIVERGENCE FOR SHRTWV DETAILS THAT WILL
INFLUENCE TIMING/SWD EXTENT OF NWRN CONUS PCPN.


...GUIDANCE EVALUATION AND PREFERENCES...

FOR CNTRL-ERN CONUS EVOLUTION THERE IS DECENT CLUSTERING INTO DAY
4 TUE.  AFTER THAT TIME THE UNCERTAINTY IN FCST DETAILS INCREASES
FAIRLY RAPIDLY GIVEN THE MODEL/ENSEMBLE SPREAD IN HOW NRN
PLAINS/MS VLY ENERGY MAY INTERACT WITH THE SRN PLAINS FEATURE.
THE APPROACH OF ONE OR MORE UPSTREAM SHRTWVS FURTHER COMPLICATES
THE FCST MID-LATE PERIOD.  THUS FAR THE GEFS/ECMWF MEANS HAVE BEEN
THE MOST STABLE GUIDANCE SOURCES OVERALL WITH THE TRACK/TIMING OF
ASSOC LOW PRES FROM THE GULF COAST INTO THE WRN ATLC... WITH ONLY
TYPICAL RUN TO RUN VARIATIONS.  AT THE MOMENT LATEST OPERATIONAL
MODELS ARE SIMILAR TO OR A LITTLE SLOWER THAN THESE MEANS AND IN
THE SLOWER PORTION OF THE VERY BROAD ENSEMBLE SPREAD FOR TIMING.
RECENT 12-HRLY GFS/ECMWF RUNS HAVE OFFERED QUITE VARIED EVOLUTIONS
SFC/ALOFT THUS PROVIDING FURTHER SUPPORT TO MAINTAINING MORE OF AN
ENSEMBLE MEAN EMPHASIS UNTIL THE MODELS STABILIZE.  LATE IN THE
PERIOD CONFIDENCE IS FAIRLY LOW IN THE PAST TWO 00Z GFS RUNS THAT
BRING CNTRL CANADA CLOSED LOW ENERGY INTO THE NORTHEAST AND AS A
RESULT PULL WRN ATLC LOW PRES TOWARD NEW ENGLAND.  THAT SAID...
SUCH AN EVOLUTION ALOFT WOULD STILL FIT WITHIN THE EXPECTED MEAN
PATTERN SO CANNOT FULLY DISCOUNT SUCH A SCENARIO.

SHRTWV DETAIL DIFFS WITHIN THE TROUGH OFF THE NWRN COAST BECOME
APPARENT ALREADY BY DAY 4 TUE.  LATEST GFS RUNS AND ESPECIALLY THE
00Z VERSION ARE NOTICEABLY SLOWER THAN MOST OTHER SOLNS WITH A
LEADING SHRTWV.  THEREAFTER THE 12Z/23 AND 00Z/23 ECMWF RUNS
BECOME PROGRESSIVE EXTREMES WITH TRAILING ENERGY... BRINGING IT
INTO THE DOWNSTREAM MEAN RIDGE MORE AGGRESSIVELY THAN OTHER MODELS
AND THE 12Z/18Z GEFS MEANS AND 12Z ECMWF/CMC MEANS.

A BLEND OF THE 18Z GFS/12Z GEFS MEAN AND 12Z ECMWF/ECMWF MEAN BEST
REPRESENTS EXISTING CONSENSUS FOR THE MON-TUE PORTION OF THE FCST.
 THEN FOR DAYS 5-7 WED-FRI PREFER A 70/30 WEIGHTING OF THE 12Z
ECMWF MEAN/GEFS MEAN.


...SENSIBLE WEATHER HIGHLIGHTS...

COMBINATION OF UPR LOW ENERGY EJECTING FROM THE SRN HIGH PLAINS
AND ASSOC LOW PRES TRACKING ALONG THE GULF COAST INTO THE ATLC
SHOULD GENERATE SOME AREAS OF HVY PCPN FROM PORTIONS OF THE SRN
HALF OF THE ROCKIES OR HIGH PLAINS THROUGH THE SRN TIER STATES.
SOME AREAS LIKELY TO BE AFFECTED HAVE ALREADY RECEIVED SIGNIFICANT
RNFL TOTALS OVER RECENT WEEKS.  CONSULT SPC OUTLOOKS FOR LATEST
INFO REGARDING SEVERE WEATHER THREATS... CURRENTLY INDICATED OVER
THE SRN PLAINS EARLY IN THE PERIOD.  COVERAGE AND INTENSITY OF
PCPN OVER THE CNTRL-NRN PARTS OF THE EAST IS MUCH MORE UNCERTAIN
AND DEPENDENT ON DIFFICULT TO RESOLVE ASPECTS OF MULTIPLE FEATURES
ALOFT.  MEANWHILE THE PATTERN FARTHER WWD WILL SUPPORT MULTIPLE
EPISODES OF LGT TO PERHAPS LOCALLY MDT PCPN OVER THE PAC NW AND
VICINITY.  THE MOST NOTABLE EXTREMES IN TEMPS WILL BE FROM THE
CNTRL-SRN HIGH PLAINS INTO THE SOUTHEAST WHERE CLOUDS/PCPN SHOULD
HOLD DAYTIME HIGHS TO 5-20F BELOW NORMAL ONE OR MORE DAYS... AND
FROM THE WEST INTO THE NRN PLAINS WHERE SOME LOCATIONS MAY SEE
READINGS UP TO 10-20F ABOVE NORMAL.

RAUSCH

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