Prognostic Meteorological Discussion
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000
FXUS02 KWBC 301600
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1159 AM EDT MON MAR 30 2015

VALID 12Z THU APR 02 2015 - 12Z MON APR 06 2015

...OVERVIEW...

PROGRESSIVE MEAN FLOW OVER THE LOWER 48 SHOULD EVOLVE TOWARD
TROUGHING IN THE VICINITY OF THE WEST COAST WITH RELATIVELY FLAT
FLOW DOWNSTREAM... A PATTERN THAT HAS NOT BEEN VERY COMMON IN
RECENT MONTHS.  BROADLY SPEAKING THERE ARE TWO PROMINENT
UNCERTAINTIES DURING THE FCST PERIOD.  THE FIRST IS WITH A
POTENTIAL WAVE TRACKING FROM NEAR THE FOUR CORNERS REGION INTO THE
PLAINS AND MID ATLC OR NORTHEAST AS A SHRTWV EJECTS FROM THE WEST.
 FARTHER WWD SOLNS DIVERGE WITH IMPORTANT ASPECTS OF PACIFIC
FLOW... LEADING TO INCREASING DIFFS FOR THE FCST FROM THE ERN PAC
THROUGH AT LEAST THE WRN CONUS.


...GUIDANCE EVALUATION/PREFERENCES...

AS SHRTWV ENERGY EJECTS FROM THE WEST IT APPEARS THAT GUIDANCE IS
IN THE PROCESS OF TRYING TO REFINE THE DETAILS OF A PSBL SFC WAVE
TRACKING E/NE FROM THE FOUR CORNERS REGION THU-FRI.  OVER THE PAST
DAY THERE HAS BEEN SOME NARROWING OF THE ENSEMBLE SPREAD FOR THE
SFC WAVE BUT SPREAD IS STILL BROADER THAN DESIRED ESPECIALLY BY
DAY 5 SAT.  AS OF 12Z DAY 4 FRI THE PRIMARY CLUSTER EXTENDS FROM
NRN TX THROUGH MO WHILE THE FAVORED AREA FOR THE TRACK BY EARLY
SAT IS OVER THE NRN MID ATLC/NEW ENGLAND... OR A LITTLE OFFSHORE.
THE FULL ENSEMBLE SPREAD HAD EXTENDED MUCH FARTHER SWD YDAY.  AN
AVG OF LATEST GUIDANCE LOOKS REASONABLE FOR TRACK BASED ON THE
ENSEMBLES BUT THERE IS MINIMAL SUPPORT FOR A WAVE AS STRONG AS
DEPICTED IN THE 00Z ECMWF.

IMPORTANT DETAIL DIFFS ARISE WITH W-CNTRL/CNTRL PAC FLOW FAIRLY
EARLY IN THE PERIOD AND THESE DIFFS ULTIMATELY IMPACT THE
AMPLITUDE AND TIMING OF THE TROUGH EXPECTED TO DEVELOP NEAR THE
WEST COAST.  OVERALL 00Z-06Z GFS/GEFS SOLNS TREND MORE PROGRESSIVE
WITH THE FLOW AND THE 06Z GEFS MEAN ENDS UP WITH THE FLATTEST
PATTERN ALONG THE WEST COAST FOR MOST OF THE MID-LATE PART OF THE
FCST PERIOD.  IN CONTRAST... TO VARYING DEGREES THE PAST TWO
12-HRLY ECMWF/ECMWF MEAN RUNS ARE FARTHER WWD WITH THE TROUGH
VERSUS PRIOR RUNS.  A FURTHER ASPECT OF UNCERTAINTY WITH THIS
EVOLUTION IS REFLECTED IN THE 00Z CMC/CMC MEAN AS THEY SHOW
POTENTIAL FOR THE INITIAL TROUGH TO BE EJECTED WHILE UPSTREAM FLOW
REINFORCES THE LARGER SCALE MEAN TROUGH.  SUCH DETAIL CAN BE
DIFFICULT TO RESOLVE 6-7 DAYS OUT IN TIME.  D+8 MULTI-DAY MEANS
AGREE FAIRLY WELL WITH A CORE OF NEGATIVE HGT ANOMALIES JUST E OF
HUDSON BAY WITH TELECONNECTIONS SUPPORTING A MODEST TROUGH JUST
OFF THE WEST COAST AND WEAK RIDGE OVER THE NRN HALF OF THE
INTERIOR WEST.  IN THE LARGE SCALE THIS RELATIONSHIP IS MORE
SUPPORTIVE OF THE ECMWF/CMC AND THEIR MEANS IN PRINCIPLE BUT
PERHAPS WITH A TROUGH A LITTLE LESS AMPLIFIED THAN THE ECMWF.  A
70/30 BLEND OF THE 00Z ECMWF MEAN/NAEFS MEAN OFFERS THE BEST
BALANCE OF TELECONS AND CONTINUITY.

THIS PREFERENCE FROM THE PAC INTO WRN NOAM... ALONG WITH A
TENDENCY FOR ERN CANADA TROUGHING TO EXTEND INTO NRN CONUS...
SHOULD LEAD TO MORE SUPPRESSED SFC PATTERN ACROSS THE NRN TIER
VERSUS RECENT GFS/GEFS MEAN RUNS FROM SAT ONWARD.

FROM DAY 3 THU INTO DAY 5 SAT THE FAVORED BLEND STARTS WITH MORE
00Z ECMWF/ECMWF MEAN THAN 00Z NAEFS MEAN AND 00Z-06Z GFS RUNS TO
YIELD AN INTERMEDIATE SOLN FOR THE FOUR CORNERS TO NERN CONUS WAVE
AND TO LEAN AWAY FROM THE FLATTER GFS SCENARIO FOR THE TROUGH
DEVELOPING NEAR THE WEST COAST.  OPPOSING
TRENDS/TELECONS/CONTINUITY LEAD TO USE OF THE 70/30 BLEND OF THE
00Z ECMWF MEAN/NAEFS MEAN FOR DAYS 6-7 SUN-MON.


...SENSIBLE WEATHER HIGHLIGHTS...

OVER THE ERN 2/3 OF THE CONUS AN AREA FROM THE MID MS VLY THROUGH
THE TN VLY/OH VLY INTO PARTS OF THE MID ATLC/NORTHEAST SHOULD SEE
THE BEST POTENTIAL FOR SIGNIFICANT RNFL... DUE TO THE SFC WAVE
TRACKING OUT OF THE FOUR CORNERS/S-CNTRL PLAINS AS WELL AS A
PERIOD OF SLOW MOVEMENT FOR THE LEADING FRONT ANCHORED BY CANADIAN
LOW PRES.  EARLY IN THE PERIOD A FOCUSED AREA OF UPSLOPE FLOW TO
THE N OF THE SFC WAVE MAY GENERATE MOSTLY SNOW OVER A LOCALIZED
AREA OVER THE CNTRL ROCKIES/HIGH PLAINS.   LATER IN THE PERIOD LOW
LVL RETURN FLOW COULD HELP RNFL TO EXPAND FROM THE RIO GRANDE FLY
INTO THE LWR MS VLY.  MEANWHILE TROUGHING THAT DEVELOPS NEAR THE
WEST COAST SHOULD AT LEAST INCREASE PCPN OVER THE PAC NW.
CONFIDENCE IS LOWER WITH THE EVENTUAL SWD EXTENT AS SOLNS RANGE
FROM DRY IN GFS RUNS TO FAIRLY WET IN ECMWF RUNS.  CURRENT PREFS
WOULD LEAN SOMEWHAT MORE TOWARD THE ECMWF SCENARIO IN PRINCIPLE
BUT WITH LIGHTER AMTS GIVEN THE POSSIBILITY THE ECMWF TROUGH ALOFT
MAY BE A LITTLE DEEP.  SHRTWV DETAILS TYPICALLY NOT WELL RESOLVED
SEVERAL DAYS OUT IN TIME WILL DETERMINE PCPN COVERAGE/INTENSITY
INLAND THROUGH THE ROCKIES.

PROGRESSIVE MEAN FLOW ALOFT WILL SUPPORT CHANGEABLE TEMPS
EARLY-MID PERIOD OVER CNTRL-ERN AREAS.  EXPECT ABOVE TO WELL ABOVE
NORMAL READINGS WITH UP TO PLUS 10-20F ANOMALIES ESPECIALLY FOR
MINS AHEAD OF THE COLD FRONT EVENTUALLY REACHING THE EAST/SOUTH.
EVOLUTION OF THE BEST DEFINED SFC WAVE WHICH MAY REACH THE
VICINITY OF NEW ENGLAND BY EARLY SAT WILL DETERMINE THE POTENTIAL
FOR SOME SNOW IN THE COLD SECTOR... WITH GREATEST PROBABILITIES
FOR WINTER WEATHER INDICATED OVER NWRN NEW ENGLAND.  BEHIND THE
FRONT ANTICIPATE BELOW AVG READINGS BY ABOUT 5-15F.  TEMPS OVER
THE NORTHWEST SHOULD BE BELOW NORMAL MOST DAYS WHILE MORE VARIABLE
FARTHER S.  THE PLAINS SHOULD EXPERIENCE A WARMING TREND BY
SAT-MON.

RAUSCH

$$




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