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.gov‘5$$