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Tropical Storm Gonzalo Forecast Discussion Number 9
2020-07-23 22:42:05| National Hurricane Center (Atlantic)
Issued at 500 PM AST Thu Jul 23 2020 000 WTNT42 KNHC 232041 TCDAT2 Tropical Storm Gonzalo Discussion Number 9 NWS National Hurricane Center Miami FL AL072020 500 PM AST Thu Jul 23 2020 Scatterometer data received just after the release of the previous advisory only showed winds of 40-45 kt, but given Gonzalo's small size, it's unlikely that the resolution of the instrument was able to capture the maximum winds. Even correcting for that, Gonzalo's winds were lowered to 50 kt on the 2 PM intermediate advisory, and that still looks good now with satellite intensity estimates ranging from 35 kt to 55 kt. Structure-wise, deep convection has redeveloped on top of Gonzalo's center, so the storm is not ready to give up its fight quite yet. There is still greater-than-normal uncertainty in Gonzalo's forecast intensity due to its small size and how it will behave in an environment of relatively light shear and warm sea surface temperatures counterbalanced by a lot of dry air. The statistical and dynamical hurricane models continue to show strengthening, while several of the global models, in particular the ECMWF and UKMET, still show Gonzalo weakening and opening up into a trough near or after it passes the Windward Islands. Based on the lower initial intensity and the latest guidance, the new official forecast has been lowered slightly, but it still brings Gonzalo to the hurricane threshold in 36-48 hours. Either way, Gonzalo's small size makes it susceptible to short-term changes in intensity that cannot be reflected in the official forecast. There is more confidence that weakening will occur after Gonzalo moves over the Caribbean Sea, with even the GFS showing the system becoming a trough. Given that, dissipation is now shown in the forecast at day 5. Gonzalo's initial motion is westward, or 275/11 kt. There has been no change in the track forecast reasoning, with Gonzalo expected to accelerate toward the west and west-northwest through the forecast period. The track guidance is fairly tightly clustered, showing Gonzalo's center crossing somewhere through the Windward Islands between 48-60 hours. Some of the models shifted northward a bit on this cycle, and the NHC track forecast has therefore been nudged in that direction close to the multi-model consensus aids, HCCA, and the Florida State Superensemble. However, users should not focus on these relatively small shifts in the forecast track from cycle to cycle, and for the Windward Islands in particular, consider that 48-60 hour forecast points can be off by an average of 60-80 nm. Key Messages 1. The risk of wind and rain impacts from Gonzalo in portions of the southern Windward Islands this weekend continues to increase, 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 or tropical storm conditions are possible across portions of the southern Windward Islands Saturday and Saturday night. Hurricane and Tropical Storm Watches are currently in effect some of the islands, and additional watches or warnings could be required tonight or early Friday. 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 23/2100Z 9.8N 49.4W 50 KT 60 MPH 12H 24/0600Z 10.0N 51.2W 55 KT 65 MPH 24H 24/1800Z 10.5N 53.8W 60 KT 70 MPH 36H 25/0600Z 11.1N 56.6W 65 KT 75 MPH 48H 25/1800Z 12.0N 59.7W 65 KT 75 MPH 60H 26/0600Z 13.0N 62.6W 60 KT 70 MPH 72H 26/1800Z 13.8N 65.6W 55 KT 65 MPH 96H 27/1800Z 14.5N 72.0W 40 KT 45 MPH 120H 28/1800Z...DISSIPATED $$ Forecaster Berg
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