FXUS02 KWBC 211907
PMDEPD
 
EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
306 PM EDT MON APR 21 2003
 
VALID 12Z THU APR 24 2003 - 12Z MON APR 28 2003
 
...FINAL MEDIUM  RANGE DISCUSSION...

GFS MEANS FCST STRENGTHENING OF A MID LVL RIDGE OVER
NWRN CANADA WITH HGTS ALSO RISING OVER AK... PUSHING THE
LOW ORIGINALLY OVER THE BERING SEA WWD/NWWD.  MEAN TROF
OVER THE ERN PAC/W COAST WILL BE NEARLY STATIONARY
AND DEEPEN SOMEWHAT FROM D+3 TO D+5... THEN WEAKEN SOME
AS IT SHIFTS EWD INTO THE INTERMTN WEST.  DOWNSTREAM...
POSITIVE ANOMALIES ARE FCST TO REMAIN CNTRD IN THE VICINITY
OF THE DAVIS STRAIT/CANADIAN MARITIMES... WITH RISING HGTS
OVER THE ERN CONUS/CANADA SUPPORTING NWWD MOVEMENT
OF A NEG ANOMALY CNTR ACROSS HUDSON BAY.  OUTSIDE OF
SHOWING LWR HGTS OVER NEW ENG... THE ECMWF D+8 AGREES
WELL WITH THE WRN TROF/ERN RIDGE PATTERN FCST BY THE GFS
D+8 MEAN.  GREATER DIFFS ARE FOUND IN HIER LATITUDES...
WHERE THE ECMWF MEAN IS FARTHER NW WITH BOTH THE RIDGE
AND TROF OVER NRN/CNTRL CANADA. 

DAILY FCSTS OFFERED BY GUIDANCE ARE IN ABOVE AVG
AGREEMENT WITH THE PATTERN SFC/ALOFT OVER THE CONUS AND
ADJACENT WATERS FOR THE DAYS 3-7 PERIOD... WITH ONLY
GENERALLY MINOR DETAILS UP FOR DEBATE.

DAYS 3-4 (THU-FRI) OVER NEW ENG/SERN CANADA... 00Z/06Z GFS
RUNS DEPICT AN ELONGATED MID LVL TROF OVER THE REGION IN
CONTRAST TO THE CLOSED LOW SHOWN BY OTHER OPERATIONAL
GUIDANCE.  PREFER TO LEAN TOWARD THE MODEL CONSENSUS
WHICH LEADS TO ASSOC SFC LOW BEING A LITTLE DEEPER THAN
GFS FCST.  THE 12Z GFS HAS TRENDED TOWARD THE CLOSED LOW
CONSENSUS ALOFT... BUT STILL APPEARS A TAD WEAK AT THE SFC. 

MEANWHILE THE MID LVL LOW THAT STARTS OVER THE CNTRL
PLAINS THU PROGRESSES ACROSS THE CONFLUENCE OF THE
OH/MS RIVERS BY LATE FRI... REACHING THE MID ATLC OR SRN
NEW ENG COAST BY SAT NIGHT OR EARLY SUN.  EXACT TRACK OF
THIS FEATURE AS IT HEADS INTO THE ERN CONUS WILL BE
DETERMINED BY THE EVOLUTION OF THE RIDGE BETWEEN
GREENLAND AND NERN CANADA... AND LOW/TROF TO ITS S/SW. 
THE ECWMF PROVIDES THE FARTHEST N SOLN... THE UKMET
FARTHEST S.  ECMWF/GFS ENSEMBLE GUIDANCE RECOMMENDS
GOING A LITTLE FARTHER S THAN THE ECMWF... A SOLN MORE LIKE
THE 00Z GFS.  THE 06Z GFS IS NOT TOO DISSIMILAR TO THE 00Z RUN
UNTIL EARLY SUN WHEN IT BEGINS TO TREND A TAD FASTER. 
ASSOC SFC LOW SHOULD TAKE AN APPROX TRACK FROM KS INTO
KY THU-SAT... WITH A SECONDARY MID ATLC LOW BEGINNING TO
TAKE OVER BY SAT.  ENSEMBLE MEANS ARE FARTHER S THAN THE
ECMWF/GFS WITH THE TRACK OF THE SECONDARY LOW OFF THE
MID ATLC/NEW ENG COAST... SIMILAR TO THE UKMET.  AS WAS THE
CASE ALOFT... PREFER AN INTERMEDIATE SOLN THAT CLOSELY
RESEMBLES A BLEND OF RECENT GFS RUNS.  12Z UPDATE... THE
NEW GFS IS REASONABLY SIMILAR TO ITS 06Z RUN WITH ONLY
MINOR CHGS IN SHRTWV/SFC DETAILS.  THE UKMET HAS TRENDED
EVEN FLATTER THAN ITS PRIOR RUN.  STILL PREFER TO LEAN
TOWARD THE GFS UNTIL FURTHER OPERATIONAL SUPPORT FOR
THE UKMET ARISES.

ENERGY SPINNING AROUND THE CLOSED LOW FCST TO BE JUST
OFF THE PAC NW COAST AS OF EARLY THU WILL PROCEED ACROSS
THE NWRN QUADRANT OF THE CONUS INTO THE WEEKEND... WITH
A MEAN TROF PERSISTING OVER THE WRN STATES INTO NEXT
WEEK.  SFC FRONT ASSOC WITH THE LEAD BUNDLE OF SHRTWV
ENERGY IS EXPECTED TO CROSS THE INTERMTN WEST AND CA
THU INTO THU NIGHT... REACHING THE HIGH PLAINS BY EARLY SAT. 
FURTHER EWD PROGRESS INTO THE PLAINS SAT-MON WILL BE
MORE DIFFICULT AS MID LVL HGTS WILL BE SLOW TO FALL.  BY
SUN-MON... THE ECMWF IS A LITTLE QUICKER TO ROTATE SHRTWV
ENERGY ON THE WRN SIDE OF THE MEAN TROF FROM OFF THE
CNTRL PAC COAST INTO THE SW... WITH THE ECMWF JUST W OF
NRN CA AS OF EARLY SUN VERSUS OFF THE OR COAST IN RECENT
GFS RUNS.  CONSIDERABLE SPREAD AMONG ENSEMBLE MEMBERS
OVER THE ERN PAC BY THIS TIME FRAME LEADS TO LOW
CONFIDENCE REGARDING SPECIFIC TIMING/DEPTH OF THE
FEATURE.  12Z UPDATE... WITH THE LEAD SYSTEM THE NEW
GFS/UKMET SHOW GOOD CONTINUITY WITH THEIR RESPECTIVE
PRIOR RUNS.  WITH THE TRAILING FEATURE EXPECTED TO ENTER
THE SW SUN-MON... THE NEW GFS IS WEAKER AND AT LEAST 12
HRS FASTER.

...WEST...

EXPECT TO SEE CHILLY AND UNSETTLED CONDS OVER MUCH OF
THE WRN US LATE WEEK INTO AT LEAST EARLY NEXT WEEK.  CORE
OF COLDEST AIR RELATIVE TO NORMAL SHOULD LIE FROM THE
CNTRL PAC COAST TO THE N-CNTRL ROCKIES... WITH SOME
ANOMALIES IN THE MINUS 10-20 RANGE FOR DAYTIME TEMPS. 
LEADING EDGE OF COLDER AIR WILL BE DENOTED BY FRONTAL
SYSTEM OVER THE INTERMTN WEST/SRN CA THU... REACHING THE
HIGH PLAINS BY EARLY SAT.  HEAVIEST VALLEY RAIN/MTN SNOW
DURING THE PERIOD SHOULD OCCUR FROM THE CNTRL PAC COAST
AND NRN SIERRA NEVADA NEWD INTO THE NRN ROCKIES... WITH
SNOW LVLS TRENDING DOWNWARD LATE WEEK AND REMAINING
LOWER THAN NORMAL INTO NEXT WEEK.

...CENTRAL/EAST...

SYSTEM WRAPPED UP OVER EXTREME NRN NEW ENG/SERN
CANADA WILL SUPPORT CHILLY/BREEZY CONDS OVER NEW ENG
THU INTO FRI... WITH PCPN GRADUALLY ENDING FROM S TO N
DURING THE TWO DAY PERIOD.  SOME OF THIS PCPN MAY BE IN
THE FORM OF SNOW OVER WRN PORTIONS OF NEW ENG.  MOST
OF CNTRL/NRN NEW ENG SHOULD SEE DRIER CONDS AND MORE
MODERATE TEMPS FOR THE WEEKEND.   

SYSTEM OVER THE PLAINS ON THU WILL PROGRESS EWD LATE
WEEK AND INTO THE WEEKEND... PROVIDING RNFL TO MUCH OF
THE SRN TWO THIRDS OF THE CNTRL/ERN US.  PCPN SHOULD
CLEAR MOST OF THE ERN US BY LATE SAT EXCEPT FOR SRN NEW
ENG AND FL PENINSULA.  THIS SYSTEM SHOULD PROVIDE A SIZABLE
AREA OF MDT/HVY RNFL.  AT THIS TIME HEAVIEST RNFL IS FCST TO
OCCUR FROM EXTREME ERN TX ACROSS THE LWR MS VLY INTO
THE SE AND SRN MID ATLC... WITH SOME TOTALS OF AT LEAST 2-3
INCHES POSSIBLE.  OTHER FAVORED AREAS FOR RNFL SHOULD
BE FROM THE CNTRL PLAINS TO NRN MID ATLC/SRN NEW ENG IN
ASSOC WITH MID LVL DYNAMICS... AND THE FL PENINSULA WHERE
SFC FRONT SHOULD STALL AND SUPPORT POTENTIAL FOR
TRAINING OF ACTIVITY DURING THE WEEKEND.  HOWEVER GFS
AMTS OVER FL SEEM INFLATED DUE TO FEEDBACK ISSUES.

ENERGY AT THE LEADING EDGE OF THE WRN US TROF WILL PUSH
THE FRONT INITIALLY PROGRESSING ACROSS THE WEST INTO THE
HIGH PLAINS BY EARLY SAT... WITH LGT/MDT PCPN BREAKING OUT
OVER THE CNTRL/NRN HIGH PLAINS BY FRI NIGHT.  SAT-MON THE
SFC FRONT AND ACCOMPANYING RNFL SHOULD HEAD INTO THE
MID-UPR MS VLY AND EVENTUALLY WRN GRTLKS REGION. 
RELATIVELY SLOW MOVEMENT MAY SUPPORT THE POTENTIAL FOR
LOCALLY HVY RNFL AT SOME LOCATIONS.  

...ALASKA...

FRONTAL SYSTEM ANCHORED BY A DEEP SFC LOW LIFTING
ACROSS THE BERING SEA WILL BRING A BAND OF LGT TO PERHAPS
ISOLD MDT PCPN TO THE WRN MAINLAND THU.  WITH MID LVL
RIDGE PERSISTING FARTHER E... EXPECT ACTIVITY TO DECREASE
IN COVERAGE/AMTS AFTER THU.  LGT PCPN MAY LINGER IN THE
OVER THE PANHANDLE AND ERN MAINLAND DURING THE WEEKEND
INTO EARLY NEXT WEEK.  UPSTREAM... WEAK SYSTEM HEADING
ACROSS THE ALEUTIANS LATE FRI AND SAT WILL LIKELY PROVIDE
ANOTHER ROUND OF LGT PCPN ALEUTIANS/SWRN MAINLAND INTO
THE FIRST HALF OF THE WEEKEND.

RAUSCH
GRAPHICS AVAILABLE ON THE WEB AT http://www.hpc.ncep.noaa.gov5$$