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FXUS02 KWBC 251836
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
235 PM EDT FRI SEP 25 2009
VALID 12Z MON SEP 28 2009 - 12Z FRI OCT 02 2009
THE LONGWAVE PATTERN OVER THE NEXT WEEK WILL BE ONE FEATURING
PROGRESSION AND AMPLITUDE....A SOMEWHAT STRANGE COMBINATION. A
VIGOROUS SYS DEEPENS VCNTY OF THE ERN GRT LAKES MON...FOLLOWED BY
THE DEEPENING OF A NEW TROF INTO THE INTERMOUNTAIN REGION AND
ROCKIES EARLY NEXT WEEK. MODELS HAVE ABOVE AVERAGE AGREEMENT ON
THE BASIC EVOLUTION OF THIS PATTERN BUT HAVE SOME EWD TRANSLATION
TIMING DIFFERENCES CONCERNING THE NEW TROF DEVELOPING OVER THE W.
THE 00Z/25 ECMWF AND ITS ENSEMBLE MEMBERS WERE ON THE SLOW SIDE OF
THE ENVELOPE OF MODEL/ENSEMBLE SOLUTIONS CONCERNING BOTH MAJOR
TROFS AFFECTING THE COUNTRY. AFTER VIEWING THE LATEST 12Z
GUIDANCE...WE DECIDED THAT THE 00Z/25 DETERMINISTIC ECMWF...ON
WHICH THE PRELIM PROGS HAD BEEN BASED...WAS A LITTLE TOO SLOW WITH
THE EWD TRANSLATION OF THE SYS DIGGING INTO THE WRN STATES.
CONSEQUENTLY...WE BLENDED OUR PRELIM PROGS WITH 50% OF THE
SOMEWHAT FASTER 00Z/25 ECMWF ENSEMBLE MEAN. CONSENSUS OF THE NEW
12Z/25 GFS AND UKMET WERE STILL FASTER THAN THE 00Z/25 ECMWF MEAN
WITH THIS WRN TROF...WHILE THE 12Z/25 CANADIAN WAS ALMOST AS SLOW
AS THE 00Z ECMWF. THE NEW 12Z DETERMINISTIC ECMWF WAS TRENDING
SLIGHTLY FASTER THAN ITS 00Z RUN BY DAY 4-5 MORE LIKE ITS 00Z
ENSEMBLE MEAN.
LATEST ENSEMBLE SPAGHETTI PLOTS SHOW BETTER THAN AVERAGE PATTERN
AGREEMENT OVER THE NEXT 7 DAYS. THE GFS/GEFS MEAN WERE MORE
PROGRESSIVE WITH THE LONGWAVES AMPLIFYING OVER NORTH AMERICA DAYS
3 THROUGH 7...A TYPICAL BIAS OF THE GFS WHEN THINGS GO MERIDIONAL.
THE DIFFERENCE IN OUR FINALS FROM OUR PRELIMS IS THAT WE ARE
AIMING MORE FOR THE MIDDLE OF THE ROAD THAN THE VERY SLOW 00Z/25
ECMWF SOLUTION. WE HOPE THAT A HAPPY MEDIUM CAN BE ACHIEVED BY
BLENDING THE 00Z/25 ECMWF MEAN WITH ITS OPERATIONAL DETERMINISTIC
COUNTERPART. WE MANUALLY ADJUSTED THE SURFACE REFLECTION OF THE
WRN TROF IN THE PLAINS STATES A LITTLE FARTHER E AFTER MAKING THE
BLEND.
AS WE BEGIN OCTOBER...BOTH VIGOROUS UPPER SYS AFFECTING THE LOWER
48 WILL BE COLD ENOUGH FOR SN AT HIGHER ELEVATIONS....IN NRN NEW
ENG AND THE ROCKIES...RESPECTIVELY. COOL CANADIAN AIR WILL
DOMINATE THE COUNTRY IN THE WAKE OF THE GRT LAKES/NRN NEW ENG SYS
UNTIL A WARMUP BEGINS IN THE PLAINS STATES THU/FRI. SIGNIFICANT
LAKE EFFECT CONVECTION MON DAY 3. A SLOW RETURN OF MOISTURE WILL
FUEL SOME CONVECTION IN THE CENTRAL/SRN PLAINS LATE NEXT
WEEK...WITH COLD OVERRUNNING RAIN IN THE DAKOTAS.
FLOOD
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