[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 180853
TCDAT2

TROPICAL STORM BRET DISCUSSION NUMBER   3
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL022011
500 AM EDT MON JUL 18 2011

THE AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT INVESTIGATING BRET
OVERNIGHT FOUND THAT THE CYCLONE HAS CONTINUED TO SLOWLY STRENGTHEN
...WITH THE CENTRAL PRESSURE FALLING TO 1001 MB.  THE INITIAL
INTENSITY IS RAISED TO 45 KT BASED ON A PEAK 850-MB FLIGHT-LEVEL
WIND OF 54 KT AND SOME BELIEVABLE 45 TO 50 KT SFMR WINDS OUTSIDE
AREAS OF HEAVY RAIN AND SHALLOW WATERS NEAR THE ABACOS ISLANDS.
SATELLITE AND RADAR IMAGERY SHOW THAT DEEP CONVECTION HAS INCREASED
NEAR THE CENTER AND IN A PROMINENT BAND IN THE EASTERN SEMICIRCLE
OVER THE PAST FEW HOURS...HOWEVER DRY AIR IS STILL IMPINGING ON THE
WESTERN HALF OF THE CIRCULATION.  ALSO...AN UPPER-LEVEL SHEAR AXIS
SITUATED JUST NORTHWEST OF BRET APPEARS TO BE HINDERING OUTFLOW IN
THE WESTERN SEMICIRCLE AND THE AIRCRAFT NOTED A 3 TO 5 NM
SOUTHEASTWARD TILT BETWEEN THE SURFACE AND 850-MB CENTERS.  THE
INTENSITY GUIDANCE IS CONSISTENT WITH THIS MARGINAL ENVIRONMENT AND
SUGGESTS THAT BRET HAS A PERIOD OF 24 TO 36 HOURS TO STEADILY
STRENGTHEN BEFORE THE SHEAR INCREASES MARKEDLY AND THE CYCLONE
BEGINS TO MOVE OVER COOLER WATERS.  THE OFFICIAL FORECAST HAS BEEN
NUDGED UPWARD IN AGREEMENT WITH THE SHIPS MODEL THROUGH 48 HOURS
AND A BLEND OF THE SHIPS AND LGEM BEYOND THAT TIME.

BRET JOGGED SOUTH-SOUTHEASTWARD OVERNIGHT...ALTHOUGH THE LATEST
AIRCRAFT FIXES AND DOPPLER RADAR IMAGERY FROM MIAMI SUGGEST THAT THE
CYCLONE HAS TURNED TOWARD THE EAST IN THE PAST COUPLE OF HOURS...
POSSIBLY COMPLETING A CYCLONIC LOOP.  THE LATEST AIRCRAFT FIXES
YIELD AN INITIAL MOTION ESTIMATE OF 090/03 KT.  ALL OF THE GUIDANCE
INSISTS THAT BRET WILL BEGIN TO MOVE SLOWLY NORTH-NORTHEASTWARD BY
TONIGHT AS THE CYCLONE COMES UNDER THE INFLUENCE OF A BUILDING
MID-LEVEL RIDGE TO ITS SOUTHEAST.  BY 48 HOURS AND BEYOND THE
GLOBAL MODELS SUGGEST THAT A BROAD MID-LEVEL TROUGH OFFSHORE OF THE
NORTHEASTERN U.S. WILL DEEPEN ENOUGH TO CAPTURE BRET AND ACCELERATE
THE CYCLONE INTO THE WESTERLIES BY THE END OF THE PERIOD.  GIVEN
THE EASTWARD MOVEMENT OF THE CYCLONE OVERNIGHT AND A GENERAL
WESTWARD TREND IN THE GUIDANCE ENVELOPE...THE NEW OFFICIAL FORECAST
IS SIMILAR TO THE PREVIOUS ONE THROUGH 48 HOURS AND LIES CLOSE TO A
BLEND OF THE ECMWF...UKMET...GFS...GFDL...AND HWRF MODELS.  AFTER
THAT TIME...THE NHC FORECAST IS ADJUSTED TOWARD THIS MODEL BLEND
AND IS LEFT OF AND A LITTLE SLOWER THAN THE PREVIOUS PACKAGE.
FURTHER ADJUSTMENTS IN THIS DIRECTION MAY BE NECESSARY NEAR THE END
OF THE PERIOD IF THIS TREND IN THE GUIDANCE CONTINUES.


FORECAST POSITIONS AND MAX WINDS

INIT  18/0900Z 27.2N  77.7W   45 KT  50 MPH
 12H  18/1800Z 27.6N  77.7W   50 KT  60 MPH
 24H  19/0600Z 28.6N  77.2W   55 KT  65 MPH
 36H  19/1800Z 29.7N  76.5W   55 KT  65 MPH
 48H  20/0600Z 30.7N  75.4W   50 KT  60 MPH
 72H  21/0600Z 32.5N  71.5W   45 KT  50 MPH
 96H  22/0600Z 35.0N  67.0W   35 KT  40 MPH
120H  23/0600Z 38.0N  59.0W   30 KT  35 MPH

$$
FORECASTER BRENNAN



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