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By: wxchaser97 , 6:49 AM GMT on October 21, 2012
The tropical wave near South America has been increasing in organization today. It has went from 20% to 60% today in the NHC tropical weather outlook. I give 99L an 80% chance of developing in the next 48hrs due to trends in organization. 99L's circulation and structure has improved greatly today and convection was flourishing earlier today. I'll explain why it has diminished and how it is already trying to come back later. The latest ATCF update had 99L with 25kt winds and a 1006mb pressure. The pressure continues to fall from earlier today as shown in surface observations. Invest 99L has a high chance to develop over the next day and aircraft recon will be investigating 99L later today.
There is so much that needs to be, and can be, said about 99L. So I'll first mention on what I'll be talking about. First I will do an analysis of 99L and its environment, then I will go over the models and their forecast, finally I will talk about my forecast and the threat areas. I know this will be a rather lengthy discussion but 99L could be a real threat to some areas.
Analysis of 99L
Right now looks are deceiving with invest 99L, but in a different way than you would think. Earlier in the year we would have storms look good in the E & C Caribbean but actually be disorganized. Right now the opposite is true, 99L doesn't look organized to a non-meteorologist stand point but in fact it is pretty organized. Convection is lacking right now but that is not a problem and to be expected. DMIN is affecting invest 99L with really limiting convection around the center. Some popcorn convection is already popping up in and around 99L signaling a comeback. Earlier today there was a good amount of thunderstorms around the center. When we get later into the morning you will see convection sprout all round 99L. This is due to DMAX affecting 99L and that is expected too. These daily cycles are a part of a developing depressions life and as 99L gets stronger it won't depend on DMAX for it's convection. Most likely tomorrow 99L should gain enough organization so DMIN doesn't cause the convection to fade a lot. The structure and circulation of invest 99L is actually really good. Satellite reveals a good cloud pattern and a surface circulation. This surface circulation could be closed, a trait of life for a tropical cyclone, but we won't know until an ASCAT pass or recon confirms this. All the invest needs to be declared a tropical depression is persistent convection.
The environment around invest 99L is conducive for development. An anticyclone is trying to build over 99L as we speak. This will fight off some shear that is currently impacting the invest. Coincidentally the shear is being caused by the anticyclone trying to cover 99L. The CIMSS wind shear map shows the anticyclone is still just to the south of 99L. This should change today as it slowly slides on to 99L and then wind shear would relax to more favorable levels. Storms earlier this year have struggled in this part of the Caribbean, but not 99L. That is because 99L is not moving as fast and the trade winds are slower than earlier. This is allowing the LLC to keep up with the convection. Another factor for 99L is the high TCHP and SST values in the Caribbean. They will allow 99L to get as much energy as it wants. It will be getting in an area where tropical cyclones of the past have bombed out. I don't think 99L will bomb out as it won't have as good of an environment as those storms. In fact I don't think we will see much more than a category 1 hurricane due to wind shear. If wind shear wasn't going to be present then we could be dealing with a dangerous situation. Also vertical instability is below average in the Caribbean. This won't be too much of a factor as other storms in the past have done fine with the amount of instability present. The environment will be conducive for some strengthening in the open Atlantic too. Once 99L gets there it could strengthen again after getting impacted by land. I still don't think it will be good enough for significant strengthening but some intensifying is definitely possible. Invest 99L would then eventually get absorbed by a trough and turn cold-core. All in all 99L should have a decent environment for strengthening but it shouldn't bomb out and become a major hurricane.
Models on 99L
The models have had a relatively decent handle on 99L so far. They at least have been doing better than some complex storms from earlier in the year. Nevertheless, there are disagreements on track and intensity. Some models have been flip-flopping where they take 99L and how strong it gets. Others have been consistent of where it takes 99L and how strong. The timing of a trough That is forecasted to come out of the Great Lakes is key. If it comes too early or too late it would affect how close it comes to the US. You can find all the model runs talked about in this blog here. The CMC has been consistent with sending 99L to the SW Caribbean before looping it back to the NNW and then toward FL. The intensity has been pretty consistent from a strong TS to a cat2 range. It then sends it up the east coast as a strong nor'easter. While there isn't much more model support for this it should be noted the most recent GFS run had something similar to this. Speaking of the GFS, it has been forecasting 99L to develop for over a week now. Lately it loops it in the Caribbean before taking it NNE toward Hispaniola and then further NE. It has had it up to a strong TS and a weak TS with multiple competing lows. It is a little hard to buy the multiple lows but the track isn't too bad. You have to remember it has been a stellar model this year and it does deserve credit. The other high preforming model, the ECMWF, has been forecasting a storm for several days. It is pretty similar to the GFS in track and and develops 99L into a TS. However, it has had a few runs were it has trended stronger and closer to the US. The NOGAPS model and the UKMET both develop 99L into a TD/TS. They don't run too far out to know exactly how strong or where they take 99L. The hurricane models are now running on 99L since it is an invest. While they overdo the intensity with 99L I do like their track. They take the invest in a loop and northward near Jamaica and Cuba. They then continue northward and slowly strengthen 99L over the open Atlantic. I am somewhere in the middle of these models and the GFS and have a track near Cuba and Hispaniola. We need to watch the models for a wide spread consensus on track and intensity which will happen in the coming days.
My forecast on 99L
My forecast on 99L hinges on the forecasted trough that will impact the Great Lakes. I don't think 99L will hit the USA as a tropical storm or hurricane. While there is some model support I am waiting for more support/consistency before moving toward the US. Invest 99L should continue to move west over the next few days before curving to the NNE. I think we should be dealing with a strong TS to a cat1 hurricane in the northern Caribbean to the open Atlantic before weakening. As noted already shear should prevent 99L from bombing out in the Caribbean like the HWRF and GDFL say. After the Caribbean 99L should continue through the open Atlantic without impacting much land. Of course if a consensus shifts toward the US this could change.
I have introduced a threat area category to go along with my rating category. The threat level shows the risk of getting impacts from a TS or hurricane. I have put a low impact risk in parts of Central America. I have done this because of some ensemble models and non-global models bringing 99L close to CA so to cover every aspect I did this. Parts of the Bahamas and Cuba have a low risk due to showers, uncertainty, and waves from 99L. The Cayman Islands and parts of Hispaniola, Cuba, and the Bahamas have a medium risk. These areas could get heavy rain, some wind and sea action from 99L, plus there is still uncertainty with track. A portion of Cuba, most of Hispaniola, and all of Jamaica are under a high risk. There is pretty high confidence that they will receive heavy rain, wind and waves from 99L. Everyone in invest 99L's path needs to be ready for tropical storm/hurricane impacts and they need to monitor the latest conditions.
An upper level low combined with a surface low is currently over the open Atlantic. This AOI has been slowly organizing throughout the day and has been tagged invest 90L. the NHC gives 90L a 30% chance to develop while I give it a 40% chance in the next 48hrs. The latest ATCF update has 90L at 30kt winds and a 1012mb pressure. Right now it is trying to fire up more convection since it is limited. Upper level winds aren't favorable for tropical development right now but that should change. Shear is expected to decrease from a high range to a low/medium range. Combined with decent SST 90L could develop into a tropical or subtropical storm. 90L has a subtropical look to it due to that upper level low interacting with it. The two lows should build some distance from each other which could also allow for tropical development. I think 90L could develop into either Sandy or Tony depending on when 99L becomes a named storm. 90L would likely remain a TS as shear would increase to high levels in a few days. 90L will likely be picked up by a cold front and remain out to sea. It is still interesting to track if it becomes a named storm as it could put us in 2nd place for named storms. Also you never know what it could do as Michael and Kirk have rapidly intensified in this general area. We will have to watch it and see what happens no matter what.
Have a great day everyone and I will have my local forecast up tonight and a tropical update tomorrow.
The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.
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