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Tropical Storm Irma Forecast Discussion Number 2
2017-08-30 22:36:26| National Hurricane Center (Atlantic)
Issued at 500 PM AST Wed Aug 30 2017 000 WTNT41 KNHC 302036 TCDAT1 Tropical Storm Irma Discussion Number 2 NWS National Hurricane Center Miami FL AL112017 500 PM AST Wed Aug 30 2017 Irma is displaying increasingly organized symmetric and interlocking rainbands with a well-defined outflow as seen in the visible and infrared satellite imagery this afternoon. Dvorak classifications from both TAFB and SAB have gone up at 18Z, suggesting 45 and 35 kt, respectively. Given their low bias earlier compared with the ASCAT scatterometer passes, the initial intensity is bumped up to 50 kt at advisory time. Irma is embedded within very low tropospheric vertical shear currently and for the foreseeable future, as the tropical storm lies underneath the upper-level anticyclone. However, in about three days the SSTs and the mid-level humidities that Irma should start to encounter will become cooler and drier. The intensity guidance shows steady intensification for the next two to three days, then diverges in response to the low shear/not-as-favorable thermodynamical environment. The official intensity forecast is closest to the ICON consensus technique through three days, then shows no change to day five. This new forecast is higher than the previous advisory, but lower than ICON at day 4 and 5 (and thus may be conservative). Irma is moving toward the west at about 13 kt along the south side of the deep-layer Azores high. A slight turn toward the west- northwest at a slower rate of forward speed is expected during the next three days. As the high strengthens, Irma is expected to turn toward the west-southwest around day 4. There is a moderate amount of spread among the normally reliable global and mesoscale models in this scenario. The official track forecast is based upon the HFIP Corrected Consensus Approach and is nearly the same as that from the previous advisory. Despite the large overall envelope of the Irma's circulation, the earlier ASCAT passes showed a very small area of tropical-storm- force winds - only 30 nm radius. The official size forecast is based upon the RVCN - variable consensus technique. FORECAST POSITIONS AND MAX WINDS INIT 30/2100Z 16.4N 31.2W 50 KT 60 MPH 12H 31/0600Z 16.8N 32.7W 60 KT 70 MPH 24H 31/1800Z 17.3N 34.5W 70 KT 80 MPH 36H 01/0600Z 17.8N 36.2W 80 KT 90 MPH 48H 01/1800Z 18.2N 38.2W 85 KT 100 MPH 72H 02/1800Z 18.2N 43.0W 90 KT 105 MPH 96H 03/1800Z 17.3N 47.8W 90 KT 105 MPH 120H 04/1800Z 16.5N 52.5W 90 KT 105 MPH $$ Forecaster Landsea
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