Tropical Cyclone Discussion
624 WTPZ42 KNHC 210848 TCDEP2 Tropical Storm Polo Discussion Number 3 NWS National Hurricane Center Miami FL EP172026 300 AM CST Mon Sep 21 2026 Deep convection associated with Polo has been gradually increasing over the past several hours. A recent GMI microwave pass indicates that Polo is developing a small core, which is an early indication that significant strengthening could be beginning. The latest satellite intensity estimates range from 35 to 45 kt, and based on that data and an earlier ASCAT pass, the initial intensity is increased to 40 kt. Environmental conditions appear very conducive for strengthening with the vertical wind shear expected to be less than 10 kt, mid-level humidities near or above 70 percent, and SSTs above 30 C during the next several days. These conditions, along with Polo's improving and compact structure, support rapid intensification. In fact, the SHIPS Rapid Intensification indices indicates that there is a near 90 percent chance of an increase in winds of at least 55 kt over the next 48 hours. The NHC intensity forecast is quite aggressive and calls for rapid intensification, and shows Polo becoming a hurricane later today and a major hurricane on Tuesday. This prediction is higher than the previous one, but below the latest HCCA guidance which shows Polo reaching category 5 status in about 48 hours. The storm will likely drift eastward or southeastward during the next day or so in weak steering currents. However, a sharp turn to the west-northwest or northwest is expected by late Tuesday when a mid-level ridge becomes established to the northeast of Polo. This pattern should bring the core of the storm near the coast of southwestern Mexico on Wednesday and Thursday, and then well south of the Baja California Peninsula on Friday. There is a very large spread in the model solutions in where, when, and how sharply Polo makes the turn. The GFS model is on the far south side of the guidance envelope while the Google DeepMind ensemble mean is on the northern edge. The NHC track forecast has been nudged northward, and is in best agreement with the corrected consensus HCCA model. It should be noted that given the complex steering pattern and model spread, confidence in the details of the track forecast is quite low. Key Messages: 1. Heavy rainfall associated with Tropical Storm Polo will impact coastal portions of southwest Mexico this week. This rainfall may produce life-threatening flooding and mudslides, especially in areas of steep terrain. 2. Polo is forecast to strengthen to a major hurricane while it moves closer to the coast of southwestern Mexico during the next few days. Although the core of the hurricane is forecast to stay offshore, it is too soon to determine the exact location or magnitude of impacts. Residents in southwestern Mexico should closely monitor its progress and updates to the forecast. A Tropical Storm Watch has been issued for portions of southwestern Mexico. FORECAST POSITIONS AND MAX WINDS INIT 21/0900Z 15.5N 103.8W 40 KT 45 MPH 12H 21/1800Z 15.5N 103.1W 60 KT 70 MPH 24H 22/0600Z 15.3N 102.5W 80 KT 90 MPH 36H 22/1800Z 15.3N 102.0W 100 KT 115 MPH 48H 23/0600Z 15.9N 102.4W 115 KT 130 MPH 60H 23/1800Z 16.6N 103.0W 125 KT 145 MPH 72H 24/0600Z 17.5N 104.1W 125 KT 145 MPH 96H 25/0600Z 18.7N 106.6W 120 KT 140 MPH 120H 26/0600Z 19.8N 109.0W 115 KT 130 MPH