I try to remember to say that I’m an amateur weather geek — a hobbyist — often enough that nobody gets the wrong impression. I’m doing my best to explain these weather graphics and souces and trying to bring what I’ve learned to more people, because I believe that television weather folks tend to over-hype things, get things very late, and get things very wrong, and it contributes to people getting injured and killed. Not that I think they’re doing it on purpose. Either way, though, I’m trying to help spread more knowledge around so people can better plan for themselves.
One thing I’m trying to say: I’m never immune from learning. And especially this particular system, as I’m using some different models, I noticed some discrepancies between what the models were predicting vs. what the NHC was predicting. For example, multiple models have shown hurricane-force winds affecting my city; yet the NHC has us under a tropical storm warning. How could this be? Also, what finally made me realize I needed to ask some questions was that I realized that the models I’m using show the winds over land as strong as the winds over water, and I know that can’t be right.
After asking about it on a weather forum, it appears to me that there’s no simple answer. Some people estimate the wind over land will be 10% weaker than shown, and others estimate 10-40% weaker.
The latter seems to match more with what I’ve seen, so I’m going to experiment with trying to talk about winds on the models with that in mind.
So let’s start off with the current GFS, with a forecast showing Irma near Panama City, FL, since that’s where I am:
This forecast is valid for 6:00am UTC on Monday. That’s 1:00am Central. The scale, in knots, is on the right side of the image. Panama City is on the coast, right inside the purple area. Looking at the scale, that’s right at/above the “64” level, so that forecast calls for 64 knot winds for the Panama City metro area. And 64 knots in miles per hour is 75mph. That’s hurricane-force winds.
But when I look at the NHC forecast for our are, we’re not under a hurricane warnings; rather, we’re under a tropical storm warning, meaning the NHC doesn’t expect us to get hurricane force winds here. Is this model different from the official forecast?
Well, as I said at the beginning, the rule of thumb I’ve heard for wind speed inland is 60%-90% of the listed speed. And I have a neat cheat on how to quickly calculate that range: 90% is 10% less than 100%, and it’s easy to calculate 10% of something – move the decimal point one place to the left. So 90% of 75mph is 75 minus 7.5, which is 67.5mph.
It’s also easy to calculate the 60%. Simple divide the originl number in half and then add that 10%: 75 divided by two is 37.5, add 7.5 for 45mph.
So according to the rule of thumb, 64 knots becomes 75mph with becomes a range of 45-68mph.
And if you want a slightly easier method, if you have knots, just use the knots number as the top end for mph, and take off 1/3 for the low end. That would mean a range of 43-64mph, which really is, to me, close enough to our calculated range of 45-68mph, just bearing in mind that the range you come up with is slightly slower than the rule of thumb – which is only a rule of thumb anyway.
So let’s look at the middle of the Florida Peninsula, just north of the eye I see some white with some pink closer to the eye. Looks to me like the maximum is around 105 knots. So using our easier-cheat rule that’s a range of about 70mph-105mph. Doesn’t sound like much fun to me! But it sounds better than the 120mph that 105 knots is!
Of course, always remember that these forecasts:
1. Are showing sustained winds. Wind gusts will be moderately to significantly higher.
2. They are the best guess the model makes for size, locaton, strength, etc. Small deviations from the forecast track can bring wildly different conditions to a given area. Actual conditions may be different from the forecast. These are guidelines, meant to help with preparations
3. For planning purposes, consider if the forecast is wrong and the system comes closer to you; that it is slightly stronger than the forecast. Plan for a “worse-case” scenario, and be glad if it’s as forecast or weaker.