Forecast Discussion
National Hurricane Center
Issued:
11:00pm EDT -- October 17, 2008
TROPICAL STORM OMAR
THE CONVECTION ASSOCIATED WITH
OMAR HAS DECREASED AGAIN THIS EVENING...WITH THE REMAINING
CONVECTION IN A RAGGED BAND WELL SOUTHEAST OF THE CENTER.
HOWEVER...AN AMSU OVERPASS NEAR 20Z SHOWS THAT THE CYCLONE HAS A
DEEP-LAYER WARM CORE...WHICH SSM/I IMAGERY SHOWS IS SURROUNDED BY
MID/UPPER-LEVEL DRY/COOL AIR. A QUIKSCAT OVERPASS NEAR 22Z
SHOWS A SIGNIFICANT DECREASE IN THE WINDS FROM THE EARLIER
OVERPASS...WITH NO RELIABLE-LOOKING VECTORS SHOWING WINDS OF MORE
THAN 45 TO 50 KT. AMSU INTENSITY ESTIMATES AT 20Z WERE 55 TO
60 KT...WHILE REGULAR SATELLITE INTENSITY ESTIMATES WERE
TROPICAL 65 KT FROM TAFB AND SUBTROPICAL 35-40 KT FROM SAB.
THE INITIAL INTENSITY IS DECREASED TO 55 KT BASED ON AN AVERAGE OF
THESE ESTIMATES...AND THIS MIGHT BE A BIT GENEROUS. THE
INITIAL WIND RADII HAVE BEEN REVISED BASED ON THE QUIKSCAT DATA.
OMAR HAS SLOWED ITS FORWARD MOTION DURING THE PAST FEW HOURS AND THE
INITIAL MOTION IS NOW 040/14. IN THE SHORT TERM...OMAR SHOULD
CONTINUE A GENERAL NORTHEASTWARD MOTION BETWEEN A DEEP-LAYER RIDGE
OVER THE EAST CENTRAL ATLANTIC AND THE MID-LATITUDE WESTERLIES TO
THE NORTH. THE FIRST 48 HR OF THE FORECAST IS DOWN THE MIDDLE
OF THE FAIRLY TIGHTLY CLUSTERED GUIDANCE. AFTER 48 HR...THE
GFS...ECMWF...UKMET..AND HWRF SHOW OMAR BEING ABSORBED INTO A
BAROCLINIC LOW MOVING EASTWARD FROM THE WESTERN ATLANTIC...THIS
AFTER OMAR TURNS MORE NORTHWARD IN ADVANCE OF THE LOW. THE
NOGAPS...THE BAMD...THE BAMM...AND THE LBAR FORECAST OMAR TO TURN
EASTWARD AND EVENTUALLY SOUTHWARD AROUND THE EASTERN ATLANTIC RIDGE.
THE NEW FORECAST TRACK WILL LEAN MORE TOWARD THE GFS/UKMET/ECMWF/HWRF
SOLUTION...CALLING FOR OMAR TO TURN MORE NORTHWARD AFTER 72 HR.
HOWEVER...OUT OF DEFERENCE TO THE OTHER MODELS AND THE PREVIOUS
FORECAST...THE NEW FORECAST IS NOT AS FAR NORTH AS THAT OF THOSE
MODELS.
OMAR IS MOVING OVER SEA SURFACE TEMPERATURES OF LESS THAN 26C...AND
IS LIKELY TO ENCOUNTER INCREASING WESTERLY VERTICAL WIND SHEAR AFTER
24 HR. WHILE ANOTHER CONVECTIVE FLARE-UP CANNOT BE RULED
OUT...THESE CONDITIONS INDICATE THE STORM SHOULD GENERALLY WEAKEN
THROUGH THE FORECAST PERIOD. THIS IS SUPPORTED BY ALL THE
INTENSITY GUIDANCE. OMAR SHOULD BECOME EXTRATROPICAL IN 48-72
HR AS IT MERGES WITH A FRONTAL SYSTEM OVER THE NORTH ATLANTIC.
IF THE
GFS/UKMET/ECMWF/HWRF SCENARIO VERIFIES...OMAR WILL BE ABSORBED BY
THE WARM FRONT OF THE ONCOMING BAROCLINIC LOW...WITH THE FORMER
TROPICAL CYCLONE SHOWING LITTLE RE-INTENSIFICATION AS AN
EXTRATROPICAL LOW. THIS IS REFLECTED IN THE 48-96 HR PORTION OF THE
INTENSITY FORECAST.
