Issued at 1100 PM AST Sun Sep 24 2023
969 WTNT42 KNHC 250243 TCDAT2 Tropical Storm Philippe Discussion Number 7 NWS National Hurricane Center Miami FL AL172023 1100 PM AST Sun Sep 24 2023 Another prominent convective burst has formed over Philippe (the storm) since the prior advisory. However, indications from the last received microwave imagery (a 2033 UTC F-16 SSMIS pass) suggest that the large cirrus plume produced is mainly the product of a cluster of cells in the down-shear quadrants of the storm, with little indication of improved organization with the tropical cyclone. There is also evidence of mid-level shear undercutting the outflow layer to the west of the cyclone. Subjective Dvorak intensity estimates from TAFB and SAB remain at T3.0/45-kt, and Philippe's initial intensity this advisory is held at 45 kt, which is also close to a mean D-MINT value associated with the above mentioned microwave pass. The intensity forecast for the next 2-3 days is tricky, as vertical wind shear (especially mid-level shear under the outflow layer) already appears to be keeping Philippe in check, and preventing the deep convection from wrapping around the center. Neither the GFS or ECMWF suggest this shear will abate much for at least the next 2-3 days. In fact the primary reason why the forecast was held steady over this time span is that the cyclone will also continue traversing anomalously warm 29C sea-surface temperatures, while mid-level moisture stays about the same or increases some during this time span. However, it would not be surprising to see some weakening in the short-term either, as suggested by the regional-hurricane models, particularly HMON and COAMPS-TC forecasts. After 72 hours, assuming the storm begins to turn poleward, there is some potential the storm start moving north of this shear zone, where some gradual intensification could begin. The intensity forecast is largely similar to the prior advisory, and is closest to the HFIP Corrected Consensus Approach (HCCA). Philippe continues to move west-northwestward at 290/12 kt. For the next day or so, the guidance is in fairly good agreement on this motion continuing as an extensive mid-level ridge centered north of the cyclone remains in place. Beyond that time span, however, is where the guidance diverges quite dramatically. While most of the guidance agrees a large weakness will appear in the mid-level ridging to the north of Philippe, whether or not the system is able to turn northward into this weakness is largely a byproduct of how vertically deep the cyclone is able to remain. Assuming the storm remains at least somewhat vertically coherent, the track forecast does show a turn northwest and then north-northwest by the end of the 5-day forecast. However, the spread in the guidance at this time period remains notable, with stronger solutions turning more north or even northeast, while weaker (and solutions that dissipate Philippe) maintaining a west-northwest heading. The NHC track forecast continues to split the difference between these extremes, and lies close to both the simple and corrected consensus aids. As discussed previously, this remains a low confidence track and intensity forecast. FORECAST POSITIONS AND MAX WINDS INIT 25/0300Z 17.1N 43.3W 45 KT 50 MPH 12H 25/1200Z 17.6N 45.0W 45 KT 50 MPH 24H 26/0000Z 18.1N 47.3W 45 KT 50 MPH 36H 26/1200Z 18.5N 49.2W 45 KT 50 MPH 48H 27/0000Z 19.3N 51.0W 45 KT 50 MPH 60H 27/1200Z 20.4N 52.7W 50 KT 60 MPH 72H 28/0000Z 21.6N 54.0W 50 KT 60 MPH 96H 29/0000Z 23.4N 55.9W 55 KT 65 MPH 120H 30/0000Z 25.5N 57.0W 55 KT 65 MPH $$ Forecaster Papin