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Tropical Storm DON
ZCZC MIATCDAT5 ALL TTAA00 KNHC DDHHMM Tropical Storm Don Discussion Number 2 NWS National Hurricane Center Miami FL AL052017 1100 PM AST Mon Jul 17 2017 Before completing its mission in Don, the Air Force Reserve reconnaissance aircraft twice measured reliable surface winds around 40 kt via the SFMR instrument, and that is set as the initial intensity for this advisory. A burst of deep convection which began just after 2000 UTC has persisted for the past several hours, and small clusters of lightning have periodically occurred near the estimated center. Don is embedded within a narrow east-west zone of low shear, and that shear is expected to remain low for another 24 hours or so. Since the cyclone has a compact circulation, significant changes in intensity--both up and down--become more likely. Therefore, given that the environment looks favorable for the next 24 hours, the new NHC intensity forecast shows strengthening up to 50 kt as Don approaches the Windward Islands. After 24 hours, increasing westerly shear should induce weakening while Don moves through the southeastern Caribbean Sea, and the cyclone is expected to degenerate into an open wave by 72 hours, if not sooner. Since model guidance varies widely between only showing an open wave (most of the global models) to Don reaching hurricane strength (HWRF and some of the statistical models), it cannot be stressed enough that confidence in the intensity forecast is very low at this time. Don's initial motion is 280/16 kt. There is also a bit of uncertainty in the track forecast, especially in Don's future forward speed, since most of the global models don't show a strong enough vortex to provide a tracker. However, the general trend is for Don to move just north of due west and speed up by 24 hours. The updated NHC track forecast is a little faster than the previous forecast after 24 hours, mainly based on the latest GFS and ECMWF global model fields. FORECAST POSITIONS AND MAX WINDS INIT 18/0300Z 11.3N 54.6W 40 KT 45 MPH 12H 18/1200Z 11.6N 57.0W 45 KT 50 MPH 24H 19/0000Z 12.0N 61.2W 50 KT 60 MPH 36H 19/1200Z 12.3N 65.4W 45 KT 50 MPH 48H 20/0000Z 12.5N 69.6W 40 KT 45 MPH 72H 21/0000Z...DISSIPATED $$ Forecaster Berg NNNN