Tropical Cyclone Discussion
544 WTPZ45 KNHC 101458 TCDEP5 Hurricane Simon Discussion Number 13 NWS National Hurricane Center Miami FL EP202026 900 AM CST Sat Oct 10 2026 Simon has gone through some inner-core structural changes over the last 6 to 12 hours. After exhibiting a pinhole eye for much of yesterday on geostationary satellite, the more recent imagery this morning shows a somewhat larger eye that cleared out briefly at around 10 UTC. This structure suggests that Simon may have undergone an eyewall replacement cycle (ERC) overnight. A couple of microwave images that became available after the last advisory also show this somewhat larger eye. Since that time, the eye has become more clouded over again. The initial intensity is being held at 130 kt this advisory, given little change in the satellite intensity estimates since the intermediate advisory. An Air Force Reserve reconnaissance mission is scheduled for Simon this afternoon and will hopefully provide more in-situ data on the hurricane's structure and intensity later today. The hurricane took a bit of a northwestward jog and slowed to a crawl since the previous advisory, but the longer-term estimated motion is just west of due north at 350/3 kt. The hurricane is being primarily steered by a weak mid-level ridge centered to its east, which is providing deep-layer southerly flow. Another deep-layer trough lies well to the northwest of Simon (which is picking up Rachel) but appears too far away to impart much steering, with a narrow mid-level ridge parked just south of the Baja California Peninsula providing light northerly flow. The net effect is that Simon should continue to move northward, but at a rather slow forward speed. In fact, the track guidance this cycle has slowed down a little more compared to the previous cycle, and the NHC track is a little slower and a bit further west in the short-term due to the initial position adjustment. This current track forecast still takes the hurricane inland over the Mexican state of Jalisco in 24-36 h. However, it is worth stressing there is still a notable amount of track uncertainty, with the ECMWF, HAFS-A, and Google DeepMind Ensemble mean (GDMI) keeping Simon just offshore of the Mexican coast for the next 48 h, while the GFS, HAFS-B, HWRF, and HMON models show Simon moving inland in the next 24-36 h. The latest Google Weather Next model (WN3I) attempts to split the difference between these solutions and brushes right up along the Mexico coastline. The NHC track still remains a bit of the east side of the guidance envelope overall, as there is a pretty well-known bias that tropical cyclones in this area tend to track east of the forecast track due to interaction with the high topography of coastal Mexico. The future intensity forecast of Simon is also largely influenced by this track uncertainty. Environmental conditions remain very favorable, and Simon's intensity will likely be controlled by inner-core related fluctuations today. However, as Simon approaches the coast of Mexico, the higher terrain will likely start to impinge on its outer circulation, where downslope subsidence may start to occur. This terrain effect will likely cause its wind field core to shrink in size and ultimately result in some weakening, even before the hurricane moves near or onshore of the Mexican coastline. Thus, weakening is now shown in 24 h, with more substantial weakening occurring after Simon's core is forecast to move onshore. It is worth stressing that this intensity forecast contains a lot of uncertainty, and if Simon remains offshore, it could remain stronger than shown in this forecast. This scenario is highlighted by the 06 UTC HAFS-A forecast which keeps Simon a significant hurricane since it remains just offshore. Ultimately, the intensity forecast remains quite similar to the prior one after land interaction begins, with Simon expected to dissipate over the higher terrain of Mexico after 60 h. Key Messages: 1. Catastrophic wind and life-threatening storm surge impacts are likely along a portion of the west-central coast of Mexico, and a Hurricane Warning is in effect from Punta San Telmo to El Roblito. Preparations to protect life and property should be completed today. 2. Heavy rainfall associated with Simon will impact portions of southwestern Mexico through this weekend. This rainfall will produce life-threatening flooding and mudslides, especially in areas of steep terrain. 3. Swells will affect portions of the southwestern and west-central coast of Mexico through the weekend. These swells are likely to cause life-threatening surf and rip current conditions. Please consult products from your local weather office. FORECAST POSITIONS AND MAX WINDS INIT 10/1500Z 17.4N 104.8W 130 KT 150 MPH 12H 11/0000Z 18.1N 105.0W 135 KT 155 MPH 24H 11/1200Z 19.1N 105.2W 125 KT 145 MPH 36H 12/0000Z 20.4N 105.4W 80 KT 90 MPH...INLAND 48H 12/1200Z 21.7N 105.4W 55 KT 65 MPH...INLAND 60H 13/0000Z 23.0N 105.5W 30 KT 35 MPH...POST-TROP/INLAND 72H 13/1200Z...DISSIPATED