Harvey has recently "weakened" from a Category 1 hurricane to a tropical storm, but there really isn't a difference between the two. In fact, this tropical storm could do as much or more damage as the hurricane did at landfall.
Can you tell the difference between the two in the graphic above?
The technical difference is just 1 mph between maximum sustained winds of 73 mph for a tropical storm and 74 mph for a hurricane. This difference is imperceptible to most of us except in our heads by the way of a different name.
Why do the categories begin at 74 mph instead of 39 mph or some other incremental wind speed? The current scale, the Saffir Simpson Hurricane Wind Scale (SSHWS), is an extension of an old British Navy scale called the Beaufort Wind Scale.
The Beaufort scale broke up winds into 12 forces or grades from calm winds up through gale force winds to hurricane strength winds. The 12th grade on the Beaufort scale is "hurricane," a grade defined by winds of 64 knots or 74 mph at its minimum bound and no maximum bound.
The SSHWS breaks up that hurricane grade into five categories. The tropical storm category is a grouping of some of the Beaufort grades.
The physical impact of a decrease from 75 mph to 70 mph is very small. Both wind speeds can do damage, and, in fact, winds as low as 40 mph can do damage to structures and even trees if the winds blow for a long enough duration or if the ground is saturated.
But, these categories have no direct relationship with how much rainfall a tropical storm or hurricane will produce.
(MORE: Three Reasons Slow-Moving Tropical Storms and Hurricanes Are the Worst)
The real story for Harvey is going to be the extraordinary flooding threat that is unfolding across southeastern Texas.
Rainfall totals are a function of forward speed rather than tropical storm or hurricane intensity.
The slower Harvey goes, the more water it can pump from its clouds to the already saturated ground over the same locations. This will lead to flash flooding from coastal southeast Texas to southwestern Louisana.