SWOD48
SPC AC 180825
DAY 4-8 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0325 AM CDT SUN SEP 18 2011
VALID 211200Z - 261200Z
...DISCUSSION...
DAY 4 /WEDNESDAY/ AND DAY 5 /THURSDAY/...DETERMINISTIC AND ENSEMBLE
GUIDANCE PROVIDE SIMILAR LARGE SCALE SOLUTIONS SHOWING A HIGH
AMPLITUDE TROUGH MOVING SLOWLY EWD ACROSS THE GREAT LAKES AND MIDDLE
MS VALLEY. THE PRIMARY SURFACE LOW IS PREDICTED TO LIFT NEWD ACROSS
QUEBEC TOWARD THE CANADIAN MARITIME PROVINCES...WITH A TRAILING COLD
FRONT MOVING SLOWLY ACROSS THE ERN US. ALTHOUGH A FEW STRONG
THUNDERSTORMS MAY OCCUR ALONG/AHEAD OF THE FRONT...GENERALLY WEAK
INSTABILITY AND MARGINAL VERTICAL SHEAR OVER THE WARM SECTOR ARE
EXPECTED TO PRECLUDE A MORE ORGANIZED AND WIDESPREAD SEVERE THREAT.
DAY 6 /FRIDAY/ THROUGH DAY 8 /SUNDAY/...MODEL GUIDANCE INDICATES
INCREASING SPREAD IN PREDICTIONS OF THE LARGE SCALE UPPER FLOW
PATTERN. THE ECMWF DETERMINISTIC AND SOME GEFS/ECMWF ENSEMBLE
MEMBERS SHOW THE UPPER TROUGH PROGRESSING SLOWLY EWD TOWARD THE
ATLANTIC COAST...WHILE THE GFS DETERMINISTIC AND OTHER ENSEMBLE
MEMBERS DEVELOP A CUT-OFF UPPER LOW OVER THE MIDDLE MS VALLEY.
THERE WILL BE SOME POTENTIAL FOR STRONG/SEVERE STORMS AHEAD OF THE
COLD FRONT MOVING OVER PARTS OF THE ERN STATES...BUT CONSIDERABLE
UNCERTAINTY EXISTS IN THE SEVERE THREAT OWING TO THE LARGE VARIANCE
IN THE PREDICTED SYNOPTIC PATTERN EVOLUTION.
..WEISS.. 09/18/2011
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