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WTPZ42 KNHC 210844
TCDEP2

Tropical Storm Greg Discussion Number 16
NWS National Hurricane Center Miami FL EP072017
200 AM PDT Fri Jul 21 2017

A band of deep convection remains wrapped around the eastern half of the circulation, but little convection is present on the western side of the cyclone. Since the convective structure is more or less the same as before, Dvorak estimates from TAFB and SAB are unchanged, and the initial intensity is held at 45 kt.

Both the track and intensity forecasts are challenging. The intensity guidance spread is high, ranging from the COAMPS-TC model, which makes Greg a major hurricane in 3 days, to the LGEM, which peaks at 49 kt. Based on the SHIPS diagnostics from both the GFS and ECMWF forecast fields, the warm SST, low shear environment would seem to support strengthening in the short term. On the forecast track, Greg should encounter a drier and more stable environment that should bring a halt to any intensification after about 72 hours. The NHC forecast is close to the HWRF model for the first 48 hours, and still shows Greg reaching hurricane strength. After that, the HWRF and COAMPS-TC models show continued intensification, but that seems unlikely in what is expected to be an unfavorable environment. The NHC forecast instead follows the trend of the statistical models late in the period and shows steady weakening.

The center has been difficult to locate tonight, and the initial motion is an uncertain 280/9 kt. For the first 48 hours or so, Greg should continue on a west or west-northwest heading, below a low to mid-level ridge. Beyond that, there is tremendous spread in the model guidance. The GFS and UKMET positions vary by over 450 n mi at 120 h, with the ECMWF lying in between. The GFS and the GFS-based regional models all depict a relatively stronger Greg making a right turn around 48 h and heading into the mid-level ridge. On the other hand, the ECMWF and UKMET continue a weaker Greg westward for another couple of days. Given that only modest strengthening is forecast, the latter scenario seems somewhat more likely at this point. The official track forecast is very close to the ECMWF at days 4 and 5. Given the high spread in the guidance, confidence in the track forecast beyond 48 h is low.

FORECAST POSITIONS AND MAX WINDS

INIT 21/0900Z 14.5N 118.3W 45 KT 50 MPH 12H 21/1800Z 14.8N 119.7W 50 KT 60 MPH 24H 22/0600Z 15.3N 121.8W 55 KT 65 MPH 36H 22/1800Z 15.6N 124.0W 60 KT 70 MPH 48H 23/0600Z 15.7N 126.1W 65 KT 75 MPH 72H 24/0600Z 16.0N 130.0W 65 KT 75 MPH 96H 25/0600Z 16.4N 133.3W 55 KT 65 MPH 120H 26/0600Z 18.0N 136.0W 45 KT 50 MPH

$$ Forecaster Zelinsky

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