Hurricane forecasting question

[Angie Norris]

Hi guys, I've got a question about forecasting rainfall in hurricanes. I know that rainfall rates are at least partially dependent on the hurricane's speed, but I've forgotten what the specific numbers are. Does anybody know what the equation is? Also, does the same equation work after the hurricane is inland, or is it then more dependent on variables like terrain, distance inland and those sort of things?
Thanks, y'all!!
Angie

 

[John Massura]

One base rule of thumb is to take 100 and divide by the forward speed of the storm. So a storm moving at 10 mph, would have a max rainfall of around 10 inches or so. Granted, this would not take into account local terrain or storm size, but it can be used as a rough first guess. I'd like to believe the local NWS office have more advanced algorithms that take into account their local area.

 

[Jeff Snyder]

I've be VERY hesitant about using any forward motion - rainfall estimates, given the many factors involved in rainfall accumulation. For example, if two storms are moving at the same speed, but the first is twice the size of the other, you're going to have very different rainfalls (probably a factor of 2 difference). Topography also plays a big role in rainfall, as evidenced by Mitch's effect on Honduras and surrounding areas. In addition, the coverage and extent of intense convection would significantly affect the total rainfall. For example, there was very little precip in the western 1/2 of Ivan last year, and even Katrina had considerably less precip in the western 1/2.

 

[Angie Norris]

John, those are the numbers I was thinking of...thanks!!
I know that just gives a very rough estimate at landfall, but it is enough for a head's up guess. Topography and size play a much larger role after the storm is inland and speed of movement may play a role in training...or am I off base on this? The western side is typically the "weaker" side of the storm, but isn't that really more for wind? I wouldn't think that would necessarily directly affect rainfall amounts.
Feedback much appreciated!!! Thanks, guys!!!
Angie