Preliminary Forecasts
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FXUS02 KWNH 280645
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
244 AM EDT FRI APR 28 2017

VALID 12Z MON MAY 01 2017 - 12Z FRI MAY 05 2017

...PATTERN OVERVIEW AND GUIDANCE EVALUATION...

THE ESTABLISHED LONGWAVE PATTERN OF A TROUGH OVER THE CENTRAL U.S.
AND RIDGE NEAR OR JUST OFF THE WEST COAST WILL LIKELY BEGIN TO
PROGRESS EWD BY THE LATTER HALF OF THE PERIOD, WITH THE TROUGH
HEADING INTO THE EAST AND THE RIDGE CROSSING THE WEST.  BEHIND THE
RIDGE A SHRTWV TROUGH SHOULD APPROACH THE WEST COAST LATE NEXT
WEEK.  EMBEDDED WITHIN THE MEAN TROUGH THE TWO DOMINANT SYSTEMS OF
INTEREST DURING THE PERIOD WILL BE LEADING DEEP LOW PRESSURE
CROSSING THE MIDWEST/UPR GRTLKS MON-TUE, FOLLOWED BY A SYSTEM THAT
WILL DEVELOP OVER THE ERN HALF OF THE COUNTRY IN RESPONSE TO
INCOMING PACIFIC ENERGY THAT WILL RELOAD THE OVERALL TROUGH.

THROUGH THE 12Z-18Z CYCLE MOST GUIDANCE MAINTAINED REASONABLE
AGREEMENT AND CONTINUITY FOR THE LEADING SYSTEM, ALLOWING FOR
MAJORITY WEIGHTING OF 12Z-18Z GFS/12Z ECMWF SOLNS RELATIVE TO 18Z
GEFS/12Z ECMWF MEANS FOR THE EARLY PART OF THE EXTENDED FCST.
BASED ON THE FULL SPREAD OF GUIDANCE FOR THE SECOND SYSTEM, MOST
LIKELY DEVELOPING FROM A SRN PLAINS WAVE AS OF DAY 5 WED AND
REACHING NEAR THE EAST COAST BY DAY 7 FRI, CONFIDENCE IN EXISTENCE
IS MUCH HIGHER THAN FOR DETAILS.  THIS IS DUE TO THE SENSITIVITY
OF THE SFC EVOLUTION TO SPECIFICS OF THE AMPLIFYING ENERGY ALOFT
NOT TYPICALLY WELL RESOLVED 6-7 DAYS OUT IN TIME.  THE 12Z
GEFS/ECMWF/CMC ENSEMBLE LOW ENVELOPE WAS QUITE BROAD AND RECENT
OPERATIONAL MODEL RUNS HAVE LIKEWISE VARIED WITH TRACK AND DEGREE
OF DEVELOPMENT.  THE 12Z/18Z GFS RUNS WERE IN THE SLOWER PART OF
THE SPREAD WHILE THE 12Z ECMWF ADJUSTED TO A SEEMINGLY MORE
PLAUSIBLE SOLN THAN ITS OLD 00Z/27 RUN THAT HAD SPLIT THE ENERGY
ALOFT TO LEAVE BEHIND A GULF COAST WAVE INSTEAD OF A SYSTEM
REACHING THE VICINITY OF THE MID ATLC.  EVEN WITH THE ENSEMBLE
MEMBER SPREAD THE RESULTING MEANS HAVE PROVIDED BETTER STABILITY
THAN THE OPERATIONAL MODELS THUS FAR, FAVORING GREATER WEIGHTING
OF THE MEANS TOWARD THE END OF THE WEEK.  THE MEANS HAVE ALSO BEEN
MORE AGREEABLE/CONSISTENT FOR THE UPR TROUGH/LEADING FRONT
REACHING THE NORTHWEST LATE IN THE PERIOD, WITH THE 12Z ECMWF
BECOMING SLOWER AND MORE AMPLIFIED WITH THE UPR TROUGH VS
CONSENSUS.  THE NEW 00Z GFS TRENDED FASTER THAN THE MAJORITY
CLUSTER WITH THIS TROUGH, PSBLY LEADING TO AN EVENTUAL ADVERSE
INFLUENCE ON THE ERN SYSTEM.


...WEATHER HIGHLIGHTS...

THE STRONG STORM TRACKING OUT OF THE MIDWEST WILL BRING VARIOUS
TYPES OF SIGNIFICANT WEATHER ACROSS THE ERN HALF OF THE COUNTRY
INCLUDING LOCALLY STRONG WINDS.  JUST NW OF THE LOW TRACK EXPECT A
BAND OF ENHANCED PRECIP WITH SOME POTENTIALLY FALLING AS SNOW.
MEANWHILE THE LEADING COLD FRONT MAY BRING A BAND OF LOCALLY
MDT-HVY RAIN BUT LIKELY WITH LESS EXTREME TOTALS THAN IN THE
SHORTER RANGE TIME FRAME.  SOME CONVECTION MAY BE STRONG TO
SEVERE.  MORNING LOWS ON MON IN THE WARM SECTOR COULD BE NEAR
DAILY RECORD WARM VALUES AT SOME LOCATIONS BUT IT REMAINS TO BE
SEEN WHETHER THOSE VALUES HOLD THROUGH THE END OF THE DAY.  ENERGY
SUPPORTING THE SECOND SYSTEM WILL INITIALLY BRING AREAS OF RAIN
AND HIGH ELEV SNOW TO THE NORTHWEST/NRN ROCKIES AND SEWD INTO THE
PLAINS.  COVERAGE/INTENSITY OF RNFL SHOULD INCREASE AROUND MIDWEEK
OVER THE SRN PLAINS/W-CNTRL GULF COAST WITH A BROAD SHIELD OF MSTR
SPREADING EWD/NEWD THU-FRI.  CURRENTLY SEE THE BEST POTENTIAL FOR
HIGHEST RNFL TOTALS NEAR THE GULF COAST WHILE A LARGER PART OF THE
ERN STATES ALSO MAY SEE A PERIOD OF ENHANCED RNFL DEPENDING ON
EXACTLY HOW THE SYSTEM EVOLVES.  PRECIP SHOULD BE LGT-MDT WITH THE
FRONT REACHING THE NORTHWEST LATE IN THE WEEK.

ONE AREA OF MOST EXTREME TEMPS DURING THE PERIOD WILL BE IN THE
COLD SECTOR OF THE INITIAL MIDWEST STORM WITH HIGHS LIKELY 10-25F
BELOW NORMAL ON MON OVER/NEAR THE UPR MS VLY WITH MINUS 10-20F
ANOMALIES PERSISTING/EXTENDING INTO THE UPR GRTLKS TUE.  ON THE
OTHER HAND THE UPR RIDGE BUILDING INTO THE WEST WILL SUPPORT WARM
TEMPS, OR HOT IN THE CASE OF INTERIOR CA/DESERT SW, WITH ONE OR
MORE DAYS OF HIGHS 10-20F ABOVE NORMAL.  SOME OF THIS WARMTH
SHOULD REACH THE ROCKIES LATE IN THE WEEK.

RAUSCH

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