Prognostic Meteorological Discussion
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FXUS02 KWBC 280643
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
242 AM EDT TUE APR 28 2015

VALID 12Z FRI MAY 01 2015 - 12Z TUE MAY 05 2015

...OVERVIEW...

GUIDANCE IS MAINTAINING SIMILAR IDEAS FOR THE LARGE SCALE
EVOLUTION DURING THE PERIOD BUT WITH CONTINUING DIFFS FOR SOME OF
THE DETAILS.  BEHIND A VIGOROUS ERN CONUS/WRN ATLC SYSTEM THAT HAS
BEEN UNUSUALLY DIFFICULT TO FCST THUS FAR... EVEN INTO THE SHORT
RANGE... LOW AMPLITUDE FLOW ACROSS THE NRN CONUS/SRN CANADA SHOULD
EVENTUALLY SHARPEN SOMEWHAT BY THE LATTER HALF OF THE PERIOD.  AN
UPSTREAM NERN PAC TROUGH/PSBL EMBEDDED UPR LOW WILL LIKELY SETTLE
TOWARD THE PAC NW BY NEXT TUE.  SOME WEAK MEAN TROUGHING SHOULD
EXIST OVER/NEAR CA FOR A TIME WITH SRN STREAM ERN PAC ENERGY PSBLY
FLOWING INTO THE SWRN CONUS DURING THE FIRST HALF OF NEXT WEEK.


...GUIDANCE EVALUATION AND PREFERENCES...

MOST MODEL/ENSEMBLE MEAN SOLNS OFFER DECENT AGREEMENT OVER THE
WEST... AT LEAST WITH FEATURES THAT ARE ON A SCALE THAT CAN
TYPICALLY RESOLVED A FEW DAYS OUT IN TIME.  THE NORTHWEST SHOULD
BE JUST BE MODESTLY BRUSHED BY ERN PAC FLOW CONTINUING ALONG THE
US-CANADIAN BORDER.  UPSTREAM ERN PAC UPR LOW SHOWS UP IN NEARLY
ALL MODELS AND FOR A TIME IN THE GEFS/ECMWF MEANS.  THE 18Z/00Z
GFS RUNS BRING THE UPR LOW TOWARD THE PAC NW COAST ON A FARTHER
SWD TRACK THAN MOST OTHER SOLNS SO PREFERENCE IS TO LEAN AWAY FROM
THE GFS AFTER EARLY MON.  THIS FEATURE IS PASSING THROUGH A MEAN
RIDGE DURING THE LATTER HALF OF THE PERIOD AND THERE ARE DIFFS IN
HOW MUCH INFLUENCE THERE MAY BE FROM WRN CANADA FLOW THEREAFTER...
SO IT WOULD NOT BE SURPRISING TO SEE FURTHER VARIABILITY IN THE
GUIDANCE.

OVER/NEAR CA AND THE SWRN STATES... UKMET RUNS ARE INITIALLY THE
NOTABLE EXTREMES WITH THE DEPTH OF A PSBL UPR LOW NEAR CA.  FOR A
DAYS 6-7 FCST GFS/ECMWF RUNS AND ENSMEANS SHOW BETTER THAN AVG
AGREEMENT REGARDING SRN STREAM ERN PAC ENERGY STREAMING INTO THE
REGION.

DURING THE FIRST HALF OF THE PERIOD THE MDLS ARE STILL TRYING TO
SORT OUT SHRTWV DETAILS AND ASSOC SFC FEATURES IN THE PROGRESSIVE
NRN TIER FLOW.  BY THE LATTER HALF OF THE PERIOD MOST
MDLS/ENSMEANS SHARPEN TROUGHING ALOFT WITH TIMING OF THIS TROUGH
AND THE LEADING COLD FRONT BECOMING THE PRIMARY UNCERTAINTIES.
DOWNSTREAM RIDGING IS NOT ESPECIALLY IMPRESSIVE BUT ENOUGH TO
FAVOR A CONSENSUS/INTERMEDIATE TIMING VERSUS THE 18Z GEFS MEAN
THAT BECOMES A LITTLE FASTER.  THE 00Z GFS IS ALSO ON THE FASTER
SIDE WITH THE FRONT BY MON-TUE.

ONGOING CHALLENGES IN RESOLVING DETAILS ALOFT OVER THE ERN CONUS
AND WRN ATLC EXTENDING FROM THE SHORT RANGE INTO THE MEDIUM RANGE
ARGUE FOR AN INTERMEDIATE TRACK/STRENGTH OF WRN ATLC LOW PRES FRI
INTO THE WEEKEND.  THIS WOULD PLACE THE TRACK BTWN THE GFS/GEFS
MEAN THAT HAVE TENDED TO BE ON THE WRN SIDE OF THE SPREAD IN
RECENT RUNS AND THE ECMWF THAT PROGRESSES FARTHER OFFSHORE BY SAT.
 THE ECMWF HAS TENDED TO BE IN THE ERN HALF OF THE SPREAD BY THE
WEEKEND... WITH AN UPR SYSTEM THAT IN MOST RUNS HAS BEEN EVEN
FASTER THAN THE ECMWF MEAN.  PREFERRED COMPROMISE WAS AN AVG AMONG
THE 12Z NAEFS/ECMWF MEANS AND 18Z GFS/12Z ECMWF.

A BLEND OF THOSE SOLNS JUST MENTIONED OFFERS A REASONABLE FCST FOR
UPSTREAM FEATURES AS WELL INTO EARLY DAY 6 MON.  THEREAFTER THE
18Z GFS COMPONENT OF THE BLEND WAS REPLACED BY THE 12Z GEFS MEAN
IN LIGHT OF LOWER CONFIDENCE IN THE GFS UPR LOW NEARING THE PAC NW.


...SENSIBLE WEATHER HIGHLIGHTS...

ERN CONUS-WRN ATLC SYSTEM SFC/ALOFT MAY BRING SOME UNSETTLED
WEATHER TO THE EAST COAST EARLY IN THE PERIOD BUT CURRENTLY EXPECT
A SFC LOW TRACK SUFFICIENTLY EWD TO KEEP THE MAJORITY THOUGH
PERHAPS NOT ALL SIGNIFICANT PCPN OFFSHORE.  FARTHER WWD A COUPLE
FRONTS CROSSING THE NRN TIER FRI INTO THE WEEKEND MAY BE
ACCOMPANIED BY SCT RNFL.  THEN FROM SUN INTO NEXT WEEK A BETTER
DEFINED FRONT CROSSING THE NRN TIER/GRTLKS AND EXTENDING INTO THE
WEST SHOULD BEGIN TO FOCUS AN EXPANDING AREA OF SHWRS/TSTMS WITH
INCREASING POTENTIAL FOR EMBEDDED HEAVIER ACTIVITY AS LOW LVL FLOW
FROM THE GULF EVENTUALLY INTERACTS WITH THE FRONT.  AMTS MAY ALSO
BE ENHANCED OVER PARTS OF THE ROCKIES/HIGH PLAINS WHERE LOW LVL
UPSLOPE FLOW BECOMES ESTABLISHED BEHIND THE FRONT.  EXPECT A
FAIRLY BROAD AREA OF ABOVE NORMAL TEMPS AHEAD OF THE FRONT... FROM
THE WEST THROUGH THE NRN-CNTRL PLAINS AND THEN INTO THE
NORTHEAST... WITH VARYING DEGREES OF COOLING BEHIND THE FRONT.
ERN STATES SHOULD BE ON THE COOL SIDE FRI BEFORE TRENDING WARMER
WHILE APPROACHING HGT FALLS SHOULD BRING COOLER TEMPS TO THE PAC
NW BY NEXT TUE.

RAUSCH

$$




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