WTPZ42 KNHC 221456

Tropical Storm Greg Discussion Number 21
NWS National Hurricane Center Miami FL EP072017
800 AM PDT Sat Jul 22 2017

A large and strong burst of convection has formed on the northern side of the center of Greg during the past several hours. However, microwave data suggest the center remains mostly exposed on the southern side of the thunderstorm activity. Satellite intensity estimates range between 35-55 kt, so the initial wind speed will be kept at 45 kt.

While shear is forecast to decrease near Greg over the next 36 hours as a trough lifts out, SSTs are also likely to decrease along with more dry air in the environment of the cyclone. The net result of these competing factors is that little significant intensity change is expected over the next couple of days. After that, most of the models increase the shear again while the storm moves over colder waters. Thus Greg should weaken at long range, and will likely be a remnant low by day 5. Little change was made to the previous NHC forecast, which is near or slightly higher than the model consensus.

Greg is moving westward or 270/11 kt. A mid-level ridge to the north of the tropical cyclone is likely to maintain the westward motion for the next couple of days. In 3-5 days the ridge is predicted to weaken somewhat, and this should result Greg slowing down and turning west-northwestward. There has been little change to the model tracks, with the ECMWF/UKMET on the left side of the guidance, and the GFS-based guidance on the right side. The GFS-based guidance has been trending westward, so it makes some sense to stay on the left side of the consensus. The latest NHC forecast is very close to the previous forecast, leaning a little heavier on the ECMWF, UKMET, their ensembles, and the HFIP-corrected consensus model.


INIT 22/1500Z 15.1N 124.2W 45 KT 50 MPH 12H 23/0000Z 15.1N 126.0W 45 KT 50 MPH 24H 23/1200Z 15.0N 128.2W 45 KT 50 MPH 36H 24/0000Z 14.9N 130.3W 50 KT 60 MPH 48H 24/1200Z 15.1N 132.0W 50 KT 60 MPH 72H 25/1200Z 16.0N 134.8W 45 KT 50 MPH 96H 26/1200Z 17.3N 137.2W 35 KT 40 MPH 120H 27/1200Z 18.0N 140.0W 25 KT 30 MPH...POST-TROP/REMNT LOW

$$ Forecaster Blake