Tropical Weather

Sub Tropical Depression Seventeen

Invest 97L has now been upgraded to Sub Tropical Depression Seventeen as of the 5am advisory from the NHC. Although Sub Tropical Depression Seventeen is still classified as sub tropical, it is very close to becoming a tropical system. Again, the difference between a sub tropical versus a tropical system is that in a sub tropical system, the center if the system is cold core and the strongest winds are away from the center. In a tropical system, the center of the is warm core and the strongest winds are in the center. A warm core allow the venting of the storm to be pushed out at the top of the system.

There is an upper level low is within the area of where Sub TD Seventeen is but this is headed southeast and is forecast to weaken. This will allow Sub TD Seventeen to be in a more favorable area for strengthening. Along with warm waters and light shear, slow intensification is forecast and the transition from sub tropical to tropical is also forecast. Vertical shear is forecast in a few days so any strengthening will be halted at that time. The depression at the moment it headed NW and is being steered by the upper level low and a very large ridge in the Central Atlantic. The ridge is forecast to erode in a few days and a series of mid-latitude troughs that are near the SE US coast will force the depression on a NE track and away from any land masses.

Visible Satellite image of Sub TD Seventeen
AVN Satellite image of Sub TD Seventeen
Water Vapor Satellite image of Western Atlantic/Sub TD Seventeen
Euro Model at 48 hours
Canadian Model at 48 hours

The GFS model along with Euro model are now forecasting a developing system in the western Caribbean. Whether the other models begin see this system or not, and even if the GFS and Euro models are correct, that system will be picked up and head over Eastern Cuba and then out to sea.

GFS Model at 192 hours
Euro Model at 216 hours
Tropical Weather

Tropical Depression Sixteen

Invest 96L has been upgraded to Tropical Depression Sixteen as of the 11am Advisory from the NHC. The forecast is for TD sixteen to become a Tropical storm before it gets to South Florida BUT it will most likely begin the transition to a subtropical storm. I will try to explain the difference between a Tropical Storm and a Sub Tropical storm. In a normal tropical storm – the central portion or the “center” of a storm it is a “warm core” meaning all the strongest winds will be in the center. In a “cold core” system all the winds are displaced away from the center. Also in a cold core system – all the circulation is at all levels. In a warm core system such as a hurricane, this has a pool of warm air aloft generated by latent heating. The thermal wind changes direction at some altitude aloft due to the change in the temperature gradient and you get an anticyclonic circulation at high levels above the hurricane which you see as the wispy cirrus clouds that “push away” from the center.

I believe that South Florida will be ok as most of the convection is to the east of the center and will probably be offshore BUT everyone should keep tabs on this system as with any tropical system, things can change very quickly.

Visible Satellite image of TD Sixteen
Models for TD Sixteen