Issued at 900 AM MDT Thu Aug 17 2023
000 WTPZ44 KNHC 171500 TCDEP4 Hurricane Hilary Discussion Number 5 NWS National Hurricane Center Miami FL EP092023 900 AM MDT Thu Aug 17 2023 Hilary has continued to intensify rapidly. The cloud pattern of this large hurricane is very impressive, with extremely intense deep convection and cloud tops to -80 deg C or colder. Well-defined banding features are evident over all quadrants of the circulation. The upper-level outflow pattern is quite symmetric and expanding. Satellite intensity estimates are wide-ranging and the Dvorak technique rules do not allow subjective current intensity estimates much above 65-70 kt. However based on data T-numbers the advisory intensity is set, perhaps conservatively, to 75 kt. The hurricane is moving west-northwestward, or about 300/12 kt. Over the next few days, a mid-level ridge to the north of Hilary is forecast to shift eastward while a mid-level low remains near the California coast. This evolution of the steering currents should result in a gradual turn toward the north in a couple of days, which would bring Hilary near the Baja California peninsula in 72 hours or so and near the U.S./California border in 3-4 days. The official forecast track is very similar to the previous one, and follows the HFIP corrected consensus, HCCA, guidance. Although there is fairly high confidence in the track prediction, Hilary's oblique angle of approach to the west coast of the Baja California peninsula makes 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. Hilary is in an environment of low shear and high mid- to low-level humidity, and over very warm waters. The SHIPS rapid intensification (RI) indices show extremely high probabilities of rapid intensification during the next day or so. Therefore the official intensity forecast shows a 40-kt increase over the next 24 hours. Later in the forecast period, cooler waters and land interaction should result in some weakening. The official wind speed forecast is near or a little above the latest HCCA guidance. 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 over southern California. KEY MESSAGES: 1. Heavy rainfall associated with Hilary may produce areas of flash flooding and result in landslides over portions of Baja California Peninsula from late Friday into late Sunday. Rainfall impacts from Hilary within the Southwestern United States are expected to peak this weekend into Monday. Flash, urban, and arroyo flooding, is possible with the potential for significant impacts. 2. 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. Tropical Storm Warnings and Watches are now in effect for southern portions of Baja California Sur, and additional watches or warnings will likely be required later today. 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 17/1500Z 15.5N 107.8W 75 KT 85 MPH 12H 18/0000Z 16.4N 109.4W 100 KT 115 MPH 24H 18/1200Z 17.6N 111.2W 115 KT 130 MPH 36H 19/0000Z 19.3N 112.7W 120 KT 140 MPH 48H 19/1200Z 21.2N 113.7W 115 KT 130 MPH 60H 20/0000Z 23.6N 114.4W 105 KT 120 MPH 72H 20/1200Z 26.6N 115.0W 85 KT 100 MPH 96H 21/1200Z 33.4N 116.8W 50 KT 60 MPH...INLAND 120H 22/1200Z 41.0N 119.0W 25 KT 30 MPH...POST-TROP/INLAND $$ Forecaster Pasch
