ZCZC MIATCDEP2 ALL TTAA00 KNHC DDHHMM Tropical Storm Greg Discussion Number 18 NWS National Hurricane Center Miami FL EP072017 200 PM PDT Fri Jul 21 2017 Greg has thrown me a curveball today with the center unexpectedly becoming exposed southwest of the main ball of convection. While there is clearly an impressive low-level circulation, as seen by both conventional and microwave images, Greg has become less organized overall. Satellite intensity estimates, consequently, have dropped, and the initial wind speed is reduced to 45 kt. While Greg is forecast to remain over warm waters for the next couple of days, dry air in the mid-levels and some westerly shear is likely to impede the strengthening process, despite an intact low-level core present on microwave data. Thus the intensity forecast is held steady for about a day. Thereafter, the regional hurricane models suggest Greg will have a chance to reintensify, although this is far from a slam dunk with an uncertain environment. I have elected to show some intensification on day 2 of the forecast, but reduced by only about 10 kt from the previous prediction due to continuity constraints. The guidance continues to indicate a weaker storm, so it would not be surprising if later shifts had to lower the forecast winds more. The initial motion has shifted to 275/11. The subtropical ridge should hold to the north of Greg for the next few days, steering the storm generally westward. Thereafter, Greg should turn to the west-northwest due to a small low- to mid-level trough dropping out of the mid-latitudes, weakening the periphery of the aforementioned ridge. Model guidance has trended southward, following the lead of the 00Z ECMWF. With a weaker storm expected, it makes sense that Greg would feel less of the trough, so the official forecast is shifted to the southwest, close to a blend of the 12Z ECMWF and model consensus forecasts. FORECAST POSITIONS AND MAX WINDS INIT 21/2100Z 14.9N 120.7W 45 KT 50 MPH 12H 22/0600Z 15.1N 122.4W 45 KT 50 MPH 24H 22/1800Z 15.3N 124.6W 50 KT 60 MPH 36H 23/0600Z 15.5N 126.8W 55 KT 65 MPH 48H 23/1800Z 15.6N 129.0W 55 KT 65 MPH 72H 24/1800Z 15.8N 132.4W 50 KT 60 MPH 96H 25/1800Z 16.8N 135.0W 45 KT 50 MPH 120H 26/1800Z 18.0N 138.0W 35 KT 40 MPH $$ Forecaster Blake NNNN
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