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Tropical Storm Gonzalo Forecast Discussion Number 8
2020-07-23 16:45:20| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Thu Jul 23 2020 000 WTNT42 KNHC 231445 TCDAT2 Tropical Storm Gonzalo Discussion Number 8 NWS National Hurricane Center Miami FL AL072020 1100 AM AST Thu Jul 23 2020 Recently-obtained GCOM and WindSat microwave data from overnight shows that Gonzalo's center is a little farther south than previously estimated. In addition, the storm's structure has become a little disheveled since yesterday, with the deep convection losing some organization. SAB's data-T number responded to this by falling to 2.5, but overall the CI numbers and SATCON support maintaining 55 kt for now. There is still an incredible amount of uncertainty in Gonzalo's intensity forecast. Sea surface temperatures to the east of the Windward Islands are warm--close to 29C--and the storm is likely to be moving through an environment of relatively light shear at least for the next 48 hours or so. The ambient environment is not particularly moist, however, with mid-level relative humidity generally around 50 percent, and it already appears that this dry air is affecting Gonzalo. Small cyclones like Gonzalo tend to succumb to any type of adverse environmental conditions quite easily, and it's possible that the system could struggle during the next couple of days. This is the solution shown by some of the global models, particularly the ECMWF and UKMET. On the other hand, the hurricane statistical and dynamical models, as well as the GFS, continue to show Gonzalo strengthening to a hurricane before it reaches the Windward Islands. Out of an abundance of caution, the official forecast continues to show Gonzalo becoming a hurricane in about 24 hours, but the uncertainty in this scenario cannot be stressed enough. There is a higher degree of certainty that Gonzalo would weaken once it moves over the eastern and central Caribbean Sea, where even the GFS shows it opening up into a wave. Even with the southward adjustment of the initial position, Gonzalo still appears to be moving westward, or 270/12 kt. The subtropical ridge to the north is forecast to push Gonzalo toward the west or west-northwest, with an increase in forward speed, for the entire 5-day forecast period. Much of the track uncertainty hinges on exactly how strong Gonzalo gets, with the stronger model representations showing the storm making more poleward process. Models such as the ECMWF and UKMET, which keep Gonzalo weak or open it up into a trough, are along the southern side of the guidance envelope. The updated NHC track forecast has been shifted southward some, to account for the new initial position and to be a little closer to the intensity consensus, HCCA, and the Florida State Superensemble. Key Messages 1. There is an increasing risk of wind and rain impacts from Gonzalo in portions of the southern Windward Islands this weekend, however there is significant uncertainty in how strong Gonzalo will be when it moves across the islands. 2. Despite the uncertainty in Gonzalo's future intensity, hurricane conditions are possible across portions of the southern Windward Islands. Hurricane Watches are currently in effect for Barbados and St. Vincent and the Grenadines, and additional watches for other islands could be required later today. Interests in the southern Windward Islands should monitor the progress of Gonzalo and follow any advice given by local officials. FORECAST POSITIONS AND MAX WINDS INIT 23/1500Z 9.6N 48.3W 55 KT 65 MPH 12H 24/0000Z 9.8N 50.0W 60 KT 70 MPH 24H 24/1200Z 10.1N 52.7W 65 KT 75 MPH 36H 25/0000Z 10.5N 55.7W 70 KT 80 MPH 48H 25/1200Z 11.2N 59.0W 70 KT 80 MPH 60H 26/0000Z 11.9N 62.3W 70 KT 80 MPH 72H 26/1200Z 12.6N 65.4W 65 KT 75 MPH 96H 27/1200Z 14.0N 71.4W 50 KT 60 MPH 120H 28/1200Z 15.5N 77.5W 40 KT 45 MPH $$ Forecaster Berg
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