Preliminary Forecasts
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FXUS02 KWNH 190631
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
130 AM EST THU JAN 19 2017

VALID 12Z SUN JAN 22 2017 - 12Z THU JAN 26 2017

...OVERVIEW AND GUIDANCE EVALUATION...

GUIDANCE CONTINUES TO SHOW A FAIR AMOUNT OF SPREAD FOR THE
SIGNIFICANT STORM SYSTEM FCST TO TRACK EAST/NORTHEAST FROM THE
LOWER HALF OF THE MS VLY INTO THE MID ATLC AND THEN MOST LIKELY
CLOSE TO THE NEW ENGLAND COAST.  IN SPITE OF THE RIDGING OVER ERN
CANADA CONSENSUS TREND OVER THE PAST 12-24 HRS HAS BEEN SOMEWHAT
FASTER... FARTHER EWD EARLY IN THE PERIOD AND THEN FARTHER NWD
NEAR THE COAST.  ANOTHER MAJOR STORM AFFECTING THE WEST AS OF SUN
WILL MOVE INLAND DURING THE FIRST PART OF NEXT WEEK.  IF ANYTHING
THE FCST FOR WHAT SYSTEM MAY TRACK OUT OF THE WEST AND AFFECT
AREAS E OF THE ROCKIES MID-LATE PERIOD HAS BECOME MORE NEBULOUS
THAN WAS THE CASE YDAY.  EXPECTED EWD PROGRESSION OF THE
SUPPORTING POSITIVELY TILTED UPR TROUGH REPRESENTS A PATTERN
CHANGE THAT WILL BRING A PERIOD OF CALMER/DRIER CONDS TO THE WEST
AND A TREND TOWARD COOLER DOWNSTREAM TEMPS BY NEXT WED-THU.

OVERALL IT REMAINS DIFFICULT TO LATCH ONTO ONE OR TWO SPECIFIC
SOLNS AS BEING MORE PROBABLE.  BY MON THE 12Z ECMWF/ECMWF MEAN/CMC
SHOW SOME LEADING WRN ENERGY HEADING INTO/THROUGH THE PLAINS
RIDGE... LEADING TO SOMEWHAT FASTER PROGRESSION OF THE ERN SYSTEM.
 THE AMPLIFIED NATURE OF THE PATTERN SUGGESTS TAKING AT LEAST SOME
ACCOUNT FOR THE GFS/GEFS MEAN RUNS THAT MAINTAIN A STRONGER RIDGE.
 HOWEVER EVEN THE GFS/GEFS MEAN HAVE TRENDED A LITTLE FASTER WITH
THE ERN SYSTEM FROM THE LAST DAY OR SO PLUS THE GFS AMPLITUDE OF
THE MID LVL TROUGH CROSSING THE GULF OF MEXICO/FL PENINSULA LEADS
TO SOME GREATER THAN MINUS 4 STDEVS FOR MID LVL HGTS.  THUS WOULD
FAVOR TONING DOWN THE GFS SCENARIO SOMEWHAT AS WELL.  NOTE THAT
THERE IS STILL CONSIDERABLE ENSEMBLE SPREAD FOR SFC LOW TRACK TUE
ONWARD.  BEST CONSENSUS WOULD HAVE THE SFC LOW NEAR THE COAST TUE
AND THEN EITHER INTO THE CANADIAN MARITIMES OR FARTHER S BUT
OFFSHORE BY WED.

THERE IS DECENT CLUSTERING FOR THE STRONG SFC LOW OFF THE WEST
COAST AS OF EARLY SUN.  THE NEW 00Z GFS HAS STRAYED NWD OF
CONSENSUS THAT NOW INCLUDES THE 00Z UKMET/CMC.  COMPARED TO THE
GUIDANCE AVG FROM 12Z/18Z THERE IS NOW A MORE OF A SIGNAL TOWARD
MAINTAINING DEFINITION OF THE SFC LOW INTO TUE AS IT DROPS SWD
ALONG THE CA COAST.

MEANWHILE THE LEADING SHRTWV WHICH THE 12Z ECMWF EJECTS INTO THE
PLAINS MON LEADS TO A WEAK/SUPPRESSED WAVE BTWN THE TWO LARGER
SCALE SYSTEMS FROM ESTABLISHED CONSENSUS AND CAUSES THE MODEL RUN
TO LOSE THE PLAINS INTO UPR GRTLKS SYSTEM TUE-THU.  IN CONTRAST TO
THE OPERATIONAL 12Z CMC... THE CMC MEAN DAMPENS OUT THE PLAINS
SYSTEM IN A MANNER SIMILAR TO THE ECMWF MEAN SO THIS IDEA CANNOT
BE FULLY DISCOUNTED.  GIVEN THE SPLIT IN GUIDANCE THE BEST OPTION
IS TO MAINTAIN A SLIGHTLY WEAKER VERSION OF THE PLAINS/GRTLKS
SYSTEM FROM CONTINUITY.  NEW 00Z SOLNS THUS FAR REMAIN VARIED.  BY
DAY 7 THU MOST GUIDANCE SEEMS TO TREND TOWARD SOMEWHAT BETTER
AGREEMENT IN PRINCIPLE SFC/ALOFT OVER THE GRTLKS.  THERE ARE
SIGNALS THAT THE GRTLKS PART OF THE UPR TROUGH MAY INCLUDE SOME
LINGERING ENERGY FROM THE ERN SYSTEM.  BEHIND THIS SYSTEM THE
MODELS/MEANS ARE VERY SIMILAR WITH STRONG/COOL SFC HIGH PRESSURE
BUILDING INTO THE WEST WED-THU AS THE AXIS OF A BUILDING UPR RIDGE
REACHES THE WEST COAST.

WHILE NOT PARTICULARLY ELEGANT... A MODEL/ENSEMBLE MEAN BLEND
INCORPORATING AN AVG AMONG 18Z GFS/GEFS MEAN AND 12Z ECMWF/ECMWF
MEAN/CMC SOLNS PLUS A LITTLE MANUAL EDITING YIELDED A GOOD
STARTING POINT TO REFLECT THE BEST COMBINATION OF CONTINUITY AND
LATEST DATA WHILE DOWNPLAYING THE LESS CONFIDENT ASPECTS OF
INDIVIDUAL SOLNS.


...WEATHER HIGHLIGHTS/THREATS...

EXPECT SUN-MON TO BE THE MOST ACTIVE WEATHER DAYS OVER THE WEST
WITH SOME AREAS OF STRONG WINDS ALONG THE WEST COAST AND HIGHEST
PRECIP TOTALS ALONG THE CA COASTAL RANGES AND SIERRA NEVADA RANGE.
 FAVORED TERRAIN OVER THE SOUTHWEST SHOULD ALSO SEE A PERIOD OF
ENHANCED ACTIVITY WITH RAIN OVER LOW ELEV/SRN LOCATIONS AND SNOW
ELSEWHERE.  SOME PCPN SHOULD LINGER INTO TUE OVER THE INTERIOR
WEST/ROCKIES.  DRIER CONDS WILL MOVE INTO THE WEST BY MIDWEEK AS
COLD HIGH PRESSURE BUILDS OVER THE REGION... WITH A TENDENCY
TOWARD GREATER COVERAGE OF TEMPS 10F OR MORE BELOW NORMAL WITH
TIME.  GUIDANCE SPREAD FOR DETAILS SFC/ALOFT OVER THE CENTRAL U.S.
EARLY-MID WEEK MAKE IT DIFFICULT TO PINPOINT PRECIP
LOCATION/INTENSITY/TIMING.  AT THE VERY LEAST THERE IS POTENTIAL
FOR AN AREA OF SNOW EXTENDING ACROSS THE NRN-CNTRL PLAINS INTO THE
GRTLKS.  THERE IS GREATER CONFIDENCE IN A DECLINING TREND FOR
TEMPS OVER THE PLAINS WITH SOME NEAR TO MODESTLY BELOW NORMAL
READINGS BY MIDWEEK.

WITH THE STRONG ERN U.S. SYSTEM EXPECT A POTENTIALLY SIGNIFICANT
EPISODE OF SEVERE CONVECTION OVER PARTS OF THE SOUTH/SOUTHEAST
DURING THE WEEKEND.  CONSULT LATEST SPC PRODUCTS FOR MORE INFO ON
THIS SEVERE THREAT.  DYNAMICS ALOFT AND A PERIOD OF STRONG INFLOW
FROM THE ATLC WILL PROMOTE AREAS OF HVY RNFL OVER AND INLAND FROM
THE EAST COAST INTO TUE.  WINTER WEATHER SHOULD BE CONFINED TO THE
MORE NRN LATITUDES AND PERHAPS WITH THE UPR LOW AND HIGHER ELEVS
OF THE APLCHNS.  EXPECT THE FRONTAL SYSTEM REACHING THE EAST LATE
IN THE PERIOD TO BRING MOSTLY LGT RNFL THOUGH PSBLY WITH SOMEWHAT
HIGHER TOTALS OVER PARTS OF THE SOUTH.  TEMPS WILL BE ABOVE TO
MUCH ABOVE NORMAL WITH MINS GENERALLY MORE ANOMALOUSLY WARM.  THE
UPR MS VLY CONTINUES TO SHOW THE GREATEST FCST MIN TEMP ANOMALIES
OF PLUS 30F OR GREATER ONE OR MORE DAYS WITH RECORD WARM LOWS PSBL
EARLY IN THE PERIOD.

RAUSCH

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