ZCZC MIATCDEP4 ALL TTAA00 KNHC DDHHMM Tropical Storm Hilary Discussion Number 4 NWS National Hurricane Center Miami FL EP092023 300 AM MDT Thu Aug 17 2023 Hilary is a large tropical storm. An elongated band of deep convection curves around the southern and eastern side of the circulation, and a Central Dense Overcast has begun to develop due to new convection near Hilary's center, with possibly the formative stages of an eye. Subjective Dvorak estimates from TAFB and SAB have jumped to a consensus T3.5/55 kt, and given the improved satellite presentation since 06 UTC, the initial intensity is set at 60 kt. Hilary is moving toward the west-northwest (300/11 kt), to the south of a mid-tropospheric high stretching from the U.S. Rockies southward into northern Mexico. The ridge is expected to shift eastward to the Central Plains during the next 48 hours, while a mid- to upper-level low moves eastward to the California coast. This steering pattern evolution should cause Hilary to recurve toward the northwest and then north during the next 3 days. Model guidance is in generally good agreement on Hilary's future path, with a fairly tightly packed guidance envelope for much of the forecast period. Therefore, the new NHC track forecast is very close to a blend of the previous forecast with the HFIP Corrected Consensus aid (HCCA) and the TVCE consensus. Despite the high confidence in the track, Hilary's oblique angle of approach to the west coast of the Baja California peninsula does make it nearly impossible to know at this point if the center will remain just offshore or move over the peninsula before reaching the southwestern United States. Negligible shear, very warm waters of 30 degrees Celsius, and plentiful atmospheric moisture mean the environment is ripe for Hilary to rapidly intensify during the next couple of days. In fact, many of the SHIPS Rapid Intensification (RI) indices are showing a 100 percent chance of RI during the next 48 hours. The NHC intensity forecast follows this thinking and shows Hilary becoming a hurricane very soon and then a major hurricane by tonight or early Friday, more or less a blend of the HCCA aid and the SHIPS solutions. Hilary should reach waters colder than 26 degrees Celsius soon after 72 hours, and fast weakening is indicated after that time, especially if the storm crosses over the Baja California peninsula. The surface circulation is likely to be dissipated by day 5, but a day 5 forecast point (as a remnant low) is still provided to maintain a forecast track north of the Baja California peninsula. KEY MESSAGES: 1. Hilary has the potential to bring significant impacts to the Baja California Peninsula and portions of the southwestern United States this weekend and early next week, including after it becomes post-tropical. Although it is too soon to determine the location and magnitude of wind impacts, interests in these areas should monitor the progress of Hilary and updates to the forecast. A Tropical Storm Watch is now in effect for southern portions of Baja California Sur, and additional watches or warnings will likely be required later today. 2. Heavy rainfall associated with Hilary may produce areas of flash flooding and result in landslides over portions of the Baja California peninsula from late Friday into late Sunday. Rainfall impacts from Hilary within the Southwestern United States are expected to peak on Sunday and Monday. 3. Large swells from Hilary will spread northward along the coast of southwestern Mexico and the Baja California Peninsula during the next several days. FORECAST POSITIONS AND MAX WINDS INIT 17/0900Z 14.6N 106.3W 60 KT 70 MPH 12H 17/1800Z 15.5N 108.0W 75 KT 85 MPH 24H 18/0600Z 16.7N 110.1W 95 KT 110 MPH 36H 18/1800Z 18.1N 111.7W 110 KT 125 MPH 48H 19/0600Z 19.9N 113.0W 115 KT 130 MPH 60H 19/1800Z 22.0N 113.8W 110 KT 125 MPH 72H 20/0600Z 24.6N 114.3W 95 KT 110 MPH 96H 21/0600Z 31.6N 116.0W 50 KT 60 MPH...INLAND 120H 22/0600Z 39.1N 118.6W 25 KT 30 MPH...POST-TROP/INLAND $$ Forecaster Berg NNNN