Simon could dump up to 25 inches of rain on parts of Michoacán, Colima and Jalisco as a hurricane strengthening toward Category 4

Simon’s rain forecast is the largest number attached to the storm: 8 to 15 inches across parts of southwestern Mexico through Sunday, with isolated totals of 15 to 25 inches possible in Michoacán, Colima and Jalisco. The National Hurricane Center put that in its 9 p.m. CST Public Advisory 7 on Thursday, Oct. 8, and described the flooding as potentially life-threatening, with mudslides a particular danger on steep terrain. The advisory locates the center about 235 miles south of Manzanillo.

The same advisory made Simon a hurricane, with 75 mph sustained winds and a minimum central pressure of 985 millibars, and the forecast calls for rapid strengthening toward Category 4 before the core approaches the coast on Saturday.

Eight to 15 inches, with 25 as the isolated ceiling

The advisory’s hazards section sets out two tiers. The broad tier, 8 to 15 inches, covers portions of southwestern Mexico through Sunday. The isolated tier, 15 to 25 inches, is limited to parts of Michoacán, Colima and Jalisco, the three states the advisory names. Isolated means a handful of spots under the heaviest bands, not a statewide average, and the advisory gives no breakdown of which of the three states is likeliest to see the top figure. Michoacán, Colima and Jalisco are listed together because the totals could land in any of them as Simon approaches from the south.

For scale, NHC’s hazards guidance describes tropical cyclone rain “sometimes over 6 inches” as capable of causing deadly floods. The broad Simon forecast starts above that line, and the isolated maximum is more than four times it. The NHC adds on the same page that river and stream flooding can last several days after a storm passes, so the water problem outlasts the forecast window.

That guidance also explains why Simon’s track matters more than its category for rain. The NHC hazards page says rainfall totals depend more on a storm’s speed and size, and on local geography, than on its strength, and that slower and larger storms produce more rain. Simon was crawling west-northwest at 6 mph when Advisory 7 went out, a pace that leaves its outer bands over the same stretch of coast for long periods.

Steep terrain, mudslides and debris flows

The NHC hazards page notes that mountainous terrain enhances rainfall from a tropical cyclone, and the advisory specifically warns of life-threatening flooding and mudslides, especially in steep terrain. Heavy totals on steep ground are the combination that moves water downhill fastest, which is why the mudslide language sits in the same sentence as the rain figures.

The U.S. Geological Survey describes the mechanics on its landslide overview. Any area with weak or fractured material on a steep slope is at risk, landslides can move from inches per year to tens of miles per hour, and debris flows commonly occur in small steep watersheds during intense rainstorms. The agency also says landslides are very difficult to predict, and that some strike without warning.

From 75 mph to Category 4 in the intensity forecast

Forecaster Roberts wrote in Discussion 7 that rapid intensification is expected to begin Friday, with the statistical guidance giving a near 70 percent chance of a 30-knot gain in 24 hours. The forecast peak is 120 knots, or 140 mph, early Saturday, 36 hours out.

On the Saffir-Simpson scale, Category 4 starts at 130 mph, and the 140 mph peak sits inside it. The scale rates only sustained wind, and the NHC states that it does not account for storm surge, rainfall flooding or tornadoes, so a Category 4 label says nothing about the 25-inch rain figure. By NHC’s own reasoning, forward speed and size shape the rain total more than the category does, and Simon was moving at 6 mph at 9 p.m. CST Thursday.

Timing: Friday’s approach, Saturday’s coast, Sunday’s totals

The rain window in the advisory runs through Sunday, while the core of Simon is expected to approach the coast of southwestern and west-central Mexico on Saturday. The NHC’s earlier tropical weather discussion, issued at 1605 UTC Thursday, carried the identical rainfall language and expected the storm to move inland early Monday.

Discussion 7’s track table has Simon at 100 knots at 6 p.m. CST Friday, 120 knots at 6 a.m. CST Saturday and 110 knots at 6 p.m. Saturday, then weakening to 75 knots inland by 6 a.m. Sunday and 35 knots by Sunday evening. The advisory’s rainfall window runs to the end of Sunday, so the forecast has water still falling after the wind has dropped.

Simon’s rise has been quick. The tropical weather discussion put the storm at 55 knots and 995 millibars at 1500 UTC Thursday, and Public Advisory 6 at 3 p.m. CST listed 70 mph winds for a tropical storm. Six hours later Advisory 7 had 75 mph, 985 millibars and a hurricane. Advisory 7 also expects hurricane conditions in the warning area Saturday afternoon or evening, and dangerous surge with large destructive waves near and east of the landfall point, which means the rain arrives alongside wind and water pushed ashore, not ahead of them.

The next NHC updates are an intermediate advisory at midnight CST and a full advisory at 3 a.m. CST. The numbers that can still move are the isolated 25-inch maximum and the three-state footprint, both of which depend on where Simon finally turns north.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

More from Morning Overview