Charles E. Roop giving his own forecasts, weather discussions, photos and adventures for the Starkville, MS area, Florida, and beyond.

Friday, May 22, 2009

Is it tropical or not?

You know that low pressure system that has been pestering us with the rain for the last five days. If you live in Florida, you know you know it. Well, the NHC has raised some suspicions in the last few hours of making this at least a tropical depression soon.

Wait. What? A TD? It's not even hurricane season yet (starts June 1). However, that's not the reason why I am questioning it.

For the last five days, there has been some chatter in the weather community whether this could become a tropical system. It seemed real unlikely due to some technical and other factors. Today, it came up again when the NHC decided to reissue special outlooks regarding the system and giving it less than a 30 percent chance of forming. I've been saying all along that this storm could not do so at this point (and I'll get to why soon).

While watching Weather Center on The Weather Channel this evening, Meteorologist (and UF Geography alumna - WoOt!) Stephanie Abrams read a fresh statement from the NHC changing the likelyhood of formation from low to medium. Wha?

This is why I thought it couldn't. 1) For the last few days this low has been accompanied by a upper level low. Why is this important. Tropical systems are usually aided by high pressure aloft (if you ever see the outer spiral of a hurricane and how it kind of looks like it's going clockwise? That's a clue). It's a way it can vent. But it's not the case with this storm. The upper low would make it more of a cold core system.

2) Broad convection. For the last few days, the storm lacked any convection around the center. Instead, the heaviest rain has mostly been far from the center of circulation. Tropical Cyclones (will be noted as TCs for now on) have a closed-center circulation with most of the convection wrapping around it.

3) The wind shear is high over the storm (25-30 kts, the threshold is usually 20 kts or less). Further explained by an excerpt from Divine Wind (Emanuel, 2005, Oxford Press):
"[I]f wind shear is present, the storm tries to move along at an average wind speed, and at some altitudes wind must blow through the storm. This storm-relative flow can import dry air from outside the cloud cluster, destroying the humid column of air that is needed for genesis."
So, what is the National Hurricane Center thinking? A senior hurricane specialist from the NHC was on The Weather Channel via phone to talk about it. Jack Beven had some very valid points that I am now realizing.

1) There is still a upper-level low in place. HOWEVER, it is weakening and not completely for the hell of it. What's happening is that the low is becoming more organized to the point where it is releasing more latent heat in the atmosphere which is helping to weaken the colder upper low.

2) Evidence of strengthening is the amount of convection that has blown up in the last few hours (as shown below from the latest IR satellite at 10:45 PM EDT).

The convection was not this crazy at around 7 p.m.

3) The heaviest convection doesn't always have to completely wrap around the center of circulation. In fact, Beven said that there are some full-blown tropical depressions that do not have all of the thunderstorm activity completely wrapped around it, but it coverage and intensity do play a role. The increased convection will send off more latent heat and modify the system even more.

Both NAM and GFS models have the cut-off upper low disappearing by 8 a.m. Saturday.

Buoy reports have winds near the center at 30 mph, but there is a chance winds might be higher, Beven said.

However, things to consider. If it's strengthening, will it become a TD before landfall Saturday? The models have this system making landfall sometime Saturday afternoon or evening. If it does, will it matter? No matter what happens, the results will be the same: rain, rain, rain in and near the landfall area. There might be a little more wind, but that's it.

Beven was pretty awesome in the interview. He explained things pretty well. I learned some stuff from that interview on TWC. I should have a little more faith in the guys and gals with the Masters and Ph.D's. They might see things that others can't.

Lets see what this thing does.

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