Why do many hurricanes weaken before landfall?

[Dave Lewison]

It seems that in recent memory I can think of at least three hurricanes that looked very formidable right up until several hours before landfall, only to be knocked down a couple Saffir-Simpson categories upon actual landfall. Ivan, Dennis, & Katrina (thank goodness) come to mind, although I know there are others.

Does direction of movement play a role? All three of these storms had a dominant north component to their movement. Frances & Jeanne last year were west-movers and retained much of their strength a full day before landfall. I've heard that a northward movement implies a trough is to the west of the storm, increasing the liklihood of shear and/or dry air entrainment.

Or is it more a factor that these three storms were all making landfall on a continental landmass, where there is a larger pool of dry air, plus the fact that half of the storm is over land before landfall actually occurs. (Don't know if S. FL is considered a continental landmass, since it's much smaller & flatter...similar to an island).

Thoughts on this phenomenon? Can anyone think of storm examples where intensity was maintained right up to landfall on a northward-moving storm (i.e., Camille)?

 

[B Ozanne]

I don't want to answer the original part but I can think of two storms that strengthened right before landfall. I'm sure there are many other examples of this as well.

When Katrina originally made landfall in Florida it was rapidly intensifying. That partially explains the extensive damage the category 1 storm produced. Hurricane Andrew was also intensifying as it made its initial strike in Florida.

 

[Justin Walker]

If you watch WV imageries of landfalling hurricanes, you can see that the hurricanes start entraining drier continental air. This effect usually rapidly weakens the inner structure of the core of the Hurricane thus you see degenerated eyewalls and wobbling in foward direction. As far as florida is concerned, it is a very tropical environment and does not have much in the way of a continental airmass, like the northern gulf.

As for a northern moving storm having more shear, I don't necessary agree that it implies a trough to the west. The storm could be moving around the southern end of a ridge, then around the west side. The storm would be moving north then. There are many more factors that can weaken a hurricane before landfall. One such is water depths on the continental shelf are a lot shallower and this does not give the hurricane as much energy to work with.

So when hurricanes get close to making landfall in areas such as the northern gulf coast, there are a lot of factors that inhibit strengthening and maintaining current intensities. So they usually weaken before landfall.

 

[Jeff Snyder]

Originally posted by Justin Walker

As for a northern moving storm having more shear, I don't necessary agree that it implies a trough to the west. The storm could be moving around the southern end of a ridge, then around the west side. The storm would be moving north then.

Technically, if you have a ridge to your east, and you're experience southerly flow, then the pressure gradient force is oriented to the west. So, there has to be some sort of trough or low to the west, otherwise heights/pressures wouldn't decrease in westward extent (increase in eastward extent). A "break" in the ridge can be viewed as a trough or inverted trough. Regardless, still a trough (or low). If there wasn't a trough/low to the west, why would heights be higher to the east? Remember, troughs/ridges are relative in that they are local lows or local highs (well, troughs/ridges don't have closed circulations else they'd be lows or highs). I know what you're saying, however...

Originally posted by Justin Walker

There are many more factors that can weaken a hurricane before landfall. One such is water depths on the continental shelf are a lot shallower and this does not give the hurricane as much energy to work with.

So when hurricanes get close to making landfall in areas such as the northern gulf coast, there are a lot of factors that inhibit strengthening and maintaining current intensities. So they usually weaken before landfall.

Agreed. There are times when deep tropical air overspreads the southeastern US, so dry air entrainment doesn't always occur at landfall. More often than not, however, one of the weakening factors is present to at least inhibit strengthening.

 

[Jim Tang]

A northward movement could be caused by a weakness in the ridge instead of a trough, and that's probably why some hurricanes (like Frederic and Camille) did not weaken prior to landfall.

Usually, however, a trough picks up a storm and carries it to the Gulf Coast, in which shear, while disrupting the structure of a storm, entrains dry air into it. However, before it does that, it enhances outflow on the northern side of a hurricane, thus allowing a hurricane to strengthen somewhat before weakening.

And, of course, waters near the Gulf coast are shallow and thus have less oceanic heat content, whereas water off the Florida coastline are deeper, and thus, more oceanic heat content.