The 06Z GFS has trended slightly slower with the Upper Air features in the SW as well as the northern stream. That model run has also come in a bit 'cooler' behind the front. This trend would tend to be more in line with what the Euro has shown. That said, this is still a very complicated and volatile forecasting challenge, so expect further adjusting as the weekend evolves. The concern in the near term is heavy rainfall chances. This will need to be monitored throughout the weekend as we see just how this all plays out. Some much for boring hot and dry weather...
PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
742 AM EST FRI DEC 02 2011
VALID 12Z TUE DEC 06 2011 - 12Z FRI DEC 09 2011
A DEEPENING/BROADENING MID-UPPER LEVEL TROF ACROSS CANADA INTO THE
CONUS IS FORECAST TO EJECT AN ELONGATED MEAN TROF INITIALLY
EXPECTED TO EXTEND FROM HUDSON BAY SWWD INTO THE SWRN CONUS. THE
EXACT DETAILS OF HOW ENERGY WITHIN THE LEADING TROF WILL EVOLVE AS
IT CROSSES THE LOWER 48 WILL LIKELY TAKE SOME TIME TO BE RESOLVED
GIVEN CURRENT GUIDANCE SPREAD AND RECENT CONTINUITY CHANGES. THE
BEST CLUSTERING OF SOLUTIONS LIES WITH THE 00Z
ECMWF/UKMET/CMC/ECMWF ENSEMBLE MEAN AND TO A LESSER EXTENT THE GFS
ENSEMBLE MEAN WHICH ARE ALL SLOWER THAN THE 00/06Z GFS RUNS.
THESE GFS RUNS HAVE NOT TRENDED MUCH SLOWER WITH THE NRN PART OF
THE TROF OVER THE PAST DAY...BUT HAVE TENDED TO HOLD BACK MORE
ENERGY IN THE SRN PART. VERIFICATION AND TRENDS WITH SHORT RANGE
FEATURES OVER THE WEST GENERALLY FAVOR THE SLOWER WRN HALF OF THE
OVERALL SOLUTION ENVELOPE. THUS A NON-GFS CONSENSUS IS THE MOST
LIKELY SCENARIO. THE SAME IS THE CASE FOR FLOW AMPLIFYING FROM
CANADA...WITH THE GFS ON THE FAST SIDE OF THE SPREAD FOR THE
LEADING COLD FRONT THAT SHOULD REACH THE US NRN TIER BY DAY 5 WED.
SPREAD IN NERN PACIFIC DETAILS BY DAYS 6-7 LOWERS CONFIDENCE IN
ANY ONE PARTICULAR SOLUTION...BUT THERE IS A GENERAL THEME TOWARD
BRINGING LESS ENERGY INTO THE ERN PAC MEAN RIDGE THAN FORECAST BY
THE 00/06Z GFS RUNS. THIS PREFERENCE LEADS TO LOWER HEIGHTS
DOWNSTREAM OVER THE WRN US. TELECONNECTIONS RELATIVE TO THE CORE
OF POSITIVE HEIGHT ANOMALIES OVER THE ERN PAC AS OF D+8...EVEN
WHERE FORECAST BY THE 00Z GFS...FAVOR LOWER WRN OR W-CENTRAL CONUS
HEIGHTS THAN FORECAST BY THE GFS AND TO A LESSER EXTENT THE GFS
ENSEMBLE MEAN. THE ECMWF HAS THOUGH MODERATED ITS WWD EXTENT OF
ITS LATE PERIOD WRN TROF COMPARED TO ITS PREVIOUS TWO RUNS WHILE
THE CMC IS A LITTLE FARTHER EWD AND THE ECMWF ENSEMBLE MEAN NOW
SOMEWHAT BROADER AND FLATTER WITH ITS TROF. AS SUCH UNCERTAINTY
IS NO BETTER THAN AVERAGE FOR THIS TIME RANGE GIVEN THE UNCLEAR
NATURE OF EMBEDDED UPSTREAM KICKER IMPULSE WAVELENGTH SPACING.
ACCORDINGLY TO REFLECT CONSENSUS PREFERENCES THAT LEAN AWAY FROM
THE 00Z GFS FOR SIGNIFICANT FEATURES DURING THE PERIOD...HPC DAYS
3-7 MON-FRI GUIDANCE WAS PRIMARILY DERIVED FROM A BLEND OF THE 00Z
ECMWF AND ECMWF ENSEMBLE MEAN BUT HEDGED IN 20% GFS ENSEMBLE MEAN
AMID LINGERING UNCERTAINTY.