WTNT42 KNHC 282041

500 PM EDT SAT MAY 28 2016

A 1431Z ASCAT-B overpass indicated two 34-kt wind vectors existed in the northwest quadrant of the tropical cyclone in a region of deep convection that was not sampled during the earlier reconnaissance mission. Convection briefly weakened, but has redeveloped and persisted in that same part of the storm circulation for the past 5 hours. Furthermore, NOAA Doppler velocity radar data from Charleston and Jacksonville have indicated winds ranging from 50-55 kt between 15,000 and 20,000 feet in the same area of the 34-kt ASCAT wind vectors. Based on these data, the depression has been upgraded to Tropical Storm Bonnie.

The initial motion estimate is 320/09 kt. The exposed low-level center near the southeastern edge of the deep convective cloud canopy has been easy to track over the past several hours, and has essentially been moving along the previous forecast track. The NHC model guidance remains in good agreement on Bonnie gradually turning toward the north-northwest as it moves around the west side of a deep-layer ridge, and moving onshore between Charleston and Beaufort, South Carolina, in about 18-24 hours. After landfall a mid-level shortwave trough moving northeastward out of the Mississippi Valley region is expected to significantly weaken the ridge, causing the steering to collapse. The result is that Bonnie is forecast to stall or meander along the coastal region of South Carolina in 24-36 hours before drifting off to the east or northeast by 48 hours. The NHC forecast track is similar to the previous advisory track, and closely follows a blend of GFS and ECMWF models.

Bonnie is currently moving over the axis of warmest Gulf Stream sea-surface temperatures of 27-28 deg C. Although slightly cooler shelf water lies ahead of the cyclone, those ocean conditions do not appear to be sufficient to significantly weaken Bonnie based on rather vigorous convection that has developed just offshore of South Carolina today. However, southerly vertical wind shear of at least 20 kt is expected to prevent any rapid or significant intensification before landfall. After 24 hours, land interaction and the aforementioned wind shear should induce slow weakening, although there could be some convective rain bands over water producing wind gusts to tropical-storm force until about 48 hours. The official intensity forecast is similar to the previous advisory and follows the Decay-SHIPS model.

The primary impact from Bonnie is expected to be locally heavy rainfall.


INIT 28/2100Z 31.1N 79.4W 35 KT 40 MPH 12H 29/0600Z 31.9N 80.1W 40 KT 45 MPH 24H 29/1800Z 32.8N 80.5W 35 KT 40 MPH...INLAND 36H 30/0600Z 33.2N 80.1W 30 KT 35 MPH...INLAND 48H 30/1800Z 33.5N 79.4W 30 KT 35 MPH...INLAND 72H 31/1800Z 34.5N 77.8W 25 KT 30 MPH...INLAND 96H 01/1800Z 35.0N 76.5W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 02/1800Z 35.7N 75.3W 25 KT 30 MPH...POST-TROP/REMNT LOW

$$ Forecaster Stewart