Prognostic Meteorological Discussion
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000
FXUS02 KWBC 251536
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1136 AM EDT SAT APR 25 2015

VALID 12Z TUE APR 28 2015 - 12Z SAT MAY 02 2015

...OVERVIEW...

WITHIN A MEAN PATTERN THAT SHOULD EVOLVE TOWARD TROUGHS NEAR EACH
COAST AND A RIDGE OVER THE CNTRL CONUS... GREATEST UNCERTAINTIES
CONTINUE TO BE WITH ERN CONUS/WRN ATLC EVOLUTION WHILE ISSUES OF
SHRTWV AMPLITUDE/TIMING PERSIST FOR THE MEAN TROUGH SETTLING
TOWARD/OVER THE WEST COAST.

...GUIDANCE EVALUATION AND PREFERENCES...

OPTED TO MAINTAIN NEAR TOTAL WPC CONTINUITY FROM THE NIGHTSHIFT
WITHOUT SIGNIFICANT CHANGE GIVEN MINIMAL COMPELLING REASON FOR
MUCH ADJUSTMENT FROM LIMITED NEWER GUIDANCE TRENDS.

FOR ERN CONUS/WRN ATLC FCST DETAILS THE CONFIDENCE LEVEL IN ANY
PARTICULAR SOLN HAS IMPROVED LITTLE OVER THE PAST DAY.  THE ONE
AGREEABLE PRINCIPLE IS THAT NRN GULF LOW PRES INITIALLY ASSOC WITH
A MID LVL SYSTEM TRACKING ACROSS THE SRN HALF OF THE PLAINS SHOULD
PROGRESS INTO THE ATLC BY WED-THU.  INDIVIDUAL MODEL RUNS AND
ENSEMBLE MEMBERS STILL DISPLAY A WIDE RANGE OF POSSIBILITIES IN
STRENGTH/TRACK/TIMING FOR THE SFC LOW DUE TO A SIMILARLY LARGE
ARRAY OF PSBL DETAILS ALOFT.  ALREADY EARLY IN THE PERIOD DIFFS
EMERGE WITH THE EVOLUTION OF NRN STREAM ENERGY OVER THE UPR
MIDWEST WITH CORRESPONDING DIFFS IN HOW THIS ENERGY AFFECTS THE
SRN PLAINS FEATURE.  AMONG THE OPERATIONAL MDLS THE PAST COUPLE
ECMWF RUNS AND NEW 00Z UKMET SEEM TO FORM ONE CLUSTER THAT IS
QUITE AGGRESSIVE WITH THE DEPTH/CLOSURE OF ENERGY FROM THE GRTLKS
INTO MID ATLC... AND STILL WITH VERY DIFFERENT SFC SOLNS... WHILE
SOME RECENT GFS RUNS AND THE 00Z/24 ECMWF WERE MOST PRONOUNCED IN
BRINGING TRAILING NERN PAC AND/OR CANADIAN ENERGY ACROSS THE
NORTHEAST AS A CONCENTRATED UPR LOW LATE IN THE PERIOD.  THE
LATTER FEATURE WOULD BRING DEEP SFC LOW PRES CLOSE TO NEW ENGLAND
OR AT LEAST THE CANADIAN MARITIMES AS IN THE THE NEW 00Z GFS.  THE
FACT THAT MODELS TEND TO HAVE GREATER DIFFICULTY WITH HIGHER
LATITUDE FLOW FURTHER RESTRAINS ANY FCST CONFIDENCE.  PERHAPS IT
IS REFLECTIVE OF THE DISJOINTED EVOLUTION ALOFT INVOLVING MULTIPLE
FEATURES EACH WITH THEIR OWN UNCERTAINTIES THAT THE GEFS AND ECMWF
MEANS ARE ACTUALLY A LITTLE WEAKER WITH THEIR SFC SYSTEMS THAN IN
SOME PRIOR RUNS.  THE RELATIVELY HIGH UNCERTAINTY IN THIS OVERALL
EVOLUTION FAVORS MAINTAINING AN ENSEMBLE MEAN/CONTINUITY THEME.

WITHIN THE MEAN TROUGH NEARING THE WEST COAST... MULTIPLE GFS RUNS
HAD BEEN ON THE SLOW/SEPARATED SIDE OF GUIDANCE WITH A LEADING
SHRTWV NEAR THE START OF THE PERIOD BUT THE 00/06Z RUNS NOW
COMPARE MUCH BETTER TO ESTABLISHED CONSENSUS.  TRAILING SHRTWVS
STILL EXHIBIT SOME SPREAD IN AMPLITUDE AND TIMING.  CURRENTLY MOST
ENSEMBLE MEANS SUGGEST A WEST COAST TROUGH THAT IS SOMEWHAT WEAKER
AND/OR SLOWER THAN THE 18Z/00/06Z GFS AND 12Z CMC.  THE PAST TWO
ECMWF RUNS ARE FAVORABLY SLOWER WITH ENERGY IN THIS STREAM
RELATIVE TO PREVIOUS RUNS.  ECMWF/ECMWF MEAN TIMING RESPECTS THE
DOWNSTREAM RIDGE ALOFT THROUGH MID-PERIOD WHILE THE GEFS/ECMWF
MEANS BECOME SIMILAR ENOUGH TO INCLUDE BOTH BY FRI-SAT.


...SENSIBLE WEATHER HIGHLIGHTS...

THERE IS CONTINUED POTENTIAL FOR AREAS OF HVY RNFL FROM THE LOWER
MS VLY/CNTRL GULF COAST THROUGH THE SOUTHEAST/FLORIDA IN ASSOC
WITH THE SYSTEM TRACKING E/NE FROM THE GULF OF MEXICO... BUT
ISSUES WITH DETAILS ALOFT FROM EARLY IN THE PERIOD BEGIN TO CAUSE
DIFFICULTY IN PINPOINTING LOCATION/TIMING.  CONFIDENCE REMAINS LOW
REGARDING PCPN COVERAGE/INTENSITY OVER THE REST OF THE EAST.  AT
THE VERY LEAST ONE OR MORE MID LVL FEATURES MAY GENERATE SCT PCPN
WITH SOMEWHAT MORE ACTIVITY DURING DIURNALLY FAVORED HOURS.  WHILE
A LOW PROBABILITY SCENARIO AT THE MOMENT... EXISTENCE OF A
SUFFICIENTLY DEEP CLOSED LOW COULD SUPPORT HIGH ELEV SNOW FOR A
TIME.  MEANWHILE THE MEAN TROUGH SETTLING NEAR THE WEST COAST
SHOULD LEAD TO PERIODS OF MOSTLY LGT-MDT PCPN OVER THE NORTHWEST.
SOME OF THIS MSTR MAY EXTEND INTO THE NRN PLAINS AND SCT ACTIVITY
COULD INCREASE FARTHER SWD OVER/NEAR THE ROCKIES BY NEXT WEEKEND.

COOLEST TEMPS RELATIVE TO NORMAL SHOULD EXTEND FROM THE SRN HALF
OF THE PLAINS THROUGH THE SOUTHEAST TUE-WED.  EXPECT THE GREATEST
COVERAGE OF MINUS 10-20F ANOMALIES FOR HIGHS OVER THE SRN
PLAINS/LWR MS VLY ON TUE.  ON THE OTHER HAND WRN CONUS WARMTH WITH
SOME MAX TEMPS OF 10-20F ABOVE NORMAL WILL SPREAD INTO THE NRN
TIER STATES FAIRLY QUICKLY AND PERSIST.  THE AREA FROM THE NRN
GRTBASIN INTO NRN HIGH PLAINS SHOULD SEE THE HIGHEST ANOMALIES FOR
THE OVERALL TUE-SAT PERIOD.

RAUSCH/SCHICHTEL

$$




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