Weakening Factors and Chasing Rita
Happy Friday everyone,
I will not be chasing this one as I already intercepted her as she side-swiped the lower FL keys and I am rather burned out right not chasing so many hurricanes in one year!
Remember when we would chase one hurricane every other YEAR or so?
Now its chasing a hurricane every other WEEK or so!
One thing to consider about the intensity, since I see this happen in MANY landfalling Gulf storms. They almost always weaken just prior to landfall, especially the upper category storms (Cat3 and higher). Hurricanes Lili, Ivan, Dennis, even Katrina all did this to some extent.
What usually happens is that the west side of the storm pulls in dry air from the "continental" air mass, and causes evaporative cooling which erodes the western eyewall of the system. This allows weakening to occur, even when vertical wind shear is no where to be found.
Also, the Gulf coast has a long and sloping continental shelf, go 30 miles offshore, and the water is only 20 feet deep in some places! The bedrock and sand BELOW the water column does not contribute much heat to the water above it since it is on average 50 to 60 degrees (like in a cavern). Only the shallow water has the heat content to fuel the storm.
In practice, a 78.8 degree F SST with a depth of at LEAST 100-200 feet is necessary to fuel a hurricane. Shallow water easily has its heat content "used up" and weakening may ensue.
Eyewall replacement is where an outer eyewall develops around the original "inner" eyewall and chokes off its inflow causing the inner eyewall to dissapate and the outer eyewall becoming the dominant main eyewall there after. The main final eyewall is wider, and like a spinning skater slowing down putting her arms out, has a lower windspeed. Excessive eyewall replacement can weaken a hurricane from Cat4 to Cat2 (remember Lili in October 2002)?
Lastly, there is usually more vertical wind shear over the central USA (that's why we chase there in the spring)! "Shear" is a favorite word for supercells and tornado formation - But NOT for tropical systems. A measly 30 knot wind at 500 mb can destroy a hurricane. The thunderstorms in a hurricane MUST remain OVER the surface low (called a CONVECTIVE VORTEX) to allow the thermal updraft processes to "remove" air from the low and make it stronger (pressure drop). Shear will move the convection away from the low, and weaken it.
Right now, there is little shear on Rita, but some models are bringing shear over the system in 12 hours or so (from time of this writing). The storm HAS been replacing its eyewalls too, and such internal dynamics of a hurricane are the most difficult to ascertain since they are governed by mesoscale fluid dynamics, and as we all know, are very complicated. My judgement is the dry air at low and mid levels issuing into the storm from the west, so I expect a cat3 at landfall.
Remember, a Cat3 is still EXTREMELY DANGEROUS - Complacency kills! This can have 111 to 130 MPH sustained winds, and most likely with Rita, will lean towards the upper end of this range. Also, if the eyewall does erode, it will be on the LEFT side of the eye. The RIGHT side will still have its full marine boundary layer air driving directly on shore (examples: Ivan and Katrina). I also noticed that even a weakening system, that was formerly a Cat5, may have less winds, but the surge, created WHILE it WAS Cat5, takes more time to dissapate (it's a dome or wave of water, and once created, it continues for a while). This is why Katrina had Cat3 winds but a 30 foot surge like that of a strong Cat5! This is why Ivan last year had a 20 foot surge (also like a Cat5) but was Cat3 at landfall. Just something to consider while in that parking garage on the waterfront.
Good luck to all out there and be safe as possible.
