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Tropical Storm DARBY Forecast Discussion Number 35
2016-07-20 10:38:01| National Hurricane Center (East Pacific)
Issued at 200 AM PDT WED JUL 20 2016 000 WTPZ45 KNHC 200837 TCDEP5 TROPICAL STORM DARBY DISCUSSION NUMBER 35 NWS NATIONAL HURRICANE CENTER MIAMI FL EP052016 200 AM PDT WED JUL 20 2016 Darby has changed little in structure during the past few hours. Deep convection with cloud tops as cold as -60C continue near the center, and recent microwave images showed a nearly closed mid-level ring. Dvorak Current Intensity numbers remain 3.5 from TAFB and SAB, and the initial intensity is therefore held at 55 kt. Darby is over SSTs around 25C and is heading toward warmer waters. However, vertical shear is expected to increase in the next 24-36 hours. Interestingly, the intensity models aren't in agreement on what will happen with Darby's intensity during this period. While the SHIPS and LGEM models indicate steady weakening over the next few days, the HWRF and Florida State Superensemble actually show some re-intensification in 24-36 hours. Due to these competing signals, the NHC official forecast shows little change in intensity during the next 48 hours, followed by gradual weakening on days 3-5. This updated forecast is not too different from the previous one, except that it delays steady weakening until after 48 hours. Darby is moving westward with an initial motion of 270/11 kt. The cyclone is located to the south of the subtropical ridge, which should maintain a westward motion for the next three days. After that time, a retrograding mid- to upper-level low north of the Hawaiian Islands will create a break in the ridge, causing Darby to turn northwestward and north-northwestward on days 4 and 5. With the exception of the GFDL, there is very little cross-track spread among the model guidance. However, there are some noticeable speed differences, with the ECMWF and UKMET showing more acceleration after the turn compared to the GFS and HWRF models. For now, the NHC official forecast, which is very similar to the previous forecast, splits the difference and is near a consensus of the GFS and ECMWF models. FORECAST POSITIONS AND MAX WINDS INIT 20/0900Z 20.0N 138.9W 55 KT 65 MPH 12H 20/1800Z 19.8N 140.6W 50 KT 60 MPH 24H 21/0600Z 19.5N 142.9W 50 KT 60 MPH 36H 21/1800Z 19.2N 145.2W 50 KT 60 MPH 48H 22/0600Z 18.9N 147.5W 50 KT 60 MPH 72H 23/0600Z 19.0N 151.0W 45 KT 50 MPH 96H 24/0600Z 20.0N 152.5W 45 KT 50 MPH 120H 25/0600Z 23.0N 154.0W 40 KT 45 MPH $$ Forecaster Berg
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