Tropical Storm Leslie Track Still In The Air

Tonight we are going to start by looking at some real time info from ADT...

The blue square is where ADT is placing the center and this matches up fairly well with what RECON fixes show as the following:
 Coordinates:  23.5N 62W

             ----- Current Analysis ----- 
     Date :  03 SEP 2012    Time :   061500 UTC
      Lat :   23:47:30 N     Lon :   61:53:55 W

     
                CI# /Pressure/ Vmax
                3.2 / 997.0mb/ 49.0kt

     
             Final T#  Adj T#  Raw T# 
                3.0     2.9     2.9

 Center Temp : -63.4C    Cloud Region Temp : -60.3C

 Scene Type : UNIFORM CDO CLOUD REGION 

 Positioning Method : FORECAST INTERPOLATION 

 Ocean Basin : ATLANTIC      
 Dvorak CI > MSLP Conversion Used : ATLANTIC  

 Tno/CI Rules : Constraint Limits : NO LIMIT 
                   Weakening Flag : ON    
           Rapid Dissipation Flag : OFF   

This would show very little north movement with slightly more of a westerly component. Movement essentially WNW . You can also see that there really has been no strengthening but rather the weakening flag is on..which means it is in a stage where it is weakening.

From here we are going to go to some images including the wide water vapor view and closer water vapor as well as the visible satellite.

These images already are beginning to show the future of Leslie with the sheer size of the system. The coldest cloud tops are south and south west of the center as the center lies on the Northwest side of the convection via ADT.

Now why is Leslie not really gaining any strength? First of all..the larger a cyclone is the longer it will take to vertically align the mid level center with the lower level center or become vertically stacked. This has been the case from the get go with leslie and at first it was due to the fast speed, then the decoupling and it still remains not vertically stacked as you can again see by looking at the 200,500, 700 mb level vorticities..

Now another reason that we have not seen her strengthen any is because of the atmosphere. So many weather outlets when forecasting tropical systems fail to look at the surrounding environment and that can lead to many false assumptions and makes one more prone to error...

We listed two of these things in our previous outlooks:

A. Dry air being entrained into the center. Looking at water vapor wide view and close in view you can still see we are dealing with dry air but it is not quite as bad as just 24 hours ago.

B. The second thing we mentioned was wind shear. While shear is beneficial to the Severe thunderstorms it is detrimental to Tropical Development..

Now we do see the eastern side of the system still under some heavy shear but you can see by looking at the tendency image (shear) it is relaxing somewhat in the direction it is moving so it should be going into a slightly better atmosphere.

Currently movement is off to the west north west and this fits well with the current steering flow:

Convergence and divergence continue to not be an issue with this system as it is in decent convergence and divergence .

Now here is where things become a little interesting. All guidance is in agreement on this system slowing down and perhaps even stalling and showing significant intensification and then moving towards the North or North North west depending on the guidance looked at and even west (after moving North)..

So lets look at the guidance tonight and see what has happened. First would be the GFS and the GFS would appear to be the right side outlier tonight...

This is at 144 hours above and ironically it is in the same position essentially as the 00z NOGAPS. Now that raises a flag on the GFS..because the rest of the guidance is further west..yet the GFS is in line with the NOGAPS which is not a trustworthy model despite it being a global model.

Interestingly enough you can see that the GGEM or the CMC is posing a threat towards the SNE area into Nova Scotia region. Now if it was just the GGEM i would be left scratching my head and not thinking to much about the GFS being where it is ..However..

Now we have the ECM's little brother the UKMET also shifting well to the west from prior runs. Matter of fact UK is kind of interesting as from 120 to 144 it is almost as if its moving due west from the east.  But wait what about the ECM?

As you can tell the ECM is showing a extremely large system and also close enough to the Southern New England area to Nova Scotia  that we can not say that this will not have an effect on the east coast. 

Three things when looking at guidance tonight:

1. We are going to be dealing with a Large system -perhaps the first major of the year.

2. With models (all but GFS ) trending west this has to be continued to be watched.

3. The west track is starting to happen more because the ridge to the west is going to be starting to push to the east (ala why the UK comes almost from the east due west) to its spot at 144. The other thing would be the trough that is shown over the lakes with the closed low (when looking at the ECM) you can see this feature clearly. This trough "could" end up pulling Leslie further west and it would NOT take that much further of a west relocation that then we would be talking problems for SNE...

As is now the most likely target zone will be Nova scotia as we would put that at a 60 % landfall but Southern New England can ill afford to stop paying attention.. We would place a SNE landfall at about 40 % right now.

In the mean time look for slow movement with direction being west northwest to NW and a system that should maintain for now its status as a tropical storm. 

Next up date 24 hours from now (if not sooner) and will include a tracking map with a cone of uncertainty.. 

Happy Labor Day to those of you who celebrate the holiday from the staff here at Down To Earth Forecasting!

Tropical Storm Leslie Update #4

Once again we are going to get into the heart of the matter right away in regards to Leslie. Above is the steering currents assuming that the pressure on Leslie is less then 999 mbs. This shows a general west to northwest steering with a break or weakness in the ridge. This will allow Leslie to take a turn more towards the Northwest and then eventually to the north...
As we mentioned on our facebook page earlier in the afternoon yesterday things have not been going to plan (at least according to NHC plan) as this was suppose to be a hurricane at one point and time already but as we pointed out that the shear and the dry air and the fact that earlier in the day the decoupling had occurred between the mid level vortex and the lower level center. 

As we look at some recent images of Leslie with the visible at the top and then the wide view water vapor in the middle and then a closer look at the water vapor. Water vapor images continue to show dry air off to the west of the system and also into or just north of the system. Visible shows a blow up of some colder cloud tops but all this convection is occurring south of where they feel that the center is located. 

We are curious to see , especially when viewing the 700, 500, 200 vorticity levels if by chance because the convection is further south and the system is over all a weaker system and still disorganized if the center is not trying to relocate further to the south. At this point and time this is speculation but it is something we need to keep an eye on.

Again, you can see that Leslie is still NOT a vertically stacked system and this is just one of the reasons why leslie has not become a hurricane yet. Another reason is because of the Anti cyclone on her western side which is producing shear across the system. National Hurricane Center had placed the intensity at 55 knots but based on latest water vapor and satellite images this may actually be to generous.

As has been the case the Divergence and convergence have been decent for this system.. This is normally a positive factor for developing cyclones but there are two things that are presently working against this strengthening.
1. Its forward speed... (this should slow down once the North turn commences)
2. The Shear.. Remember shear is good for Severe thunderstorms but in Tropical cyclones it hinders and slows development..

As you can see to the north , west and east and even to the south the shear is increasing across the system. This very fact alone should keep the strength of the system in check for the immediate future. Matter of fact we would not be surprised to see it weakening more with shear expected to even increase further over the next 24 or so hours..

Lets look at tonights guidance and one common theme seems to be in place tonight with the Global Models and that is track is slightly further west and in some cases such as the ECM it takes this system very close to Nova Scotia...

This is well west and well stronger compared to its 00z run...

Here we have the GGEM which has also come west..followed by the GFS ..followed by the NOGAPS.. As you can see pretty much all the models are expecting Leslie to regain strength after the environmental conditions improve across the Atlantic. They also continue to indicate a very large system down the road by about 120-144 hrs.

One thing that we like to mention is that the guidance is starting to build back a ridge to the east of the system in the medium range..

Depending on how strong that ridge gets will determine how far west Leslie will be able to get to the USA . Right now ..It would appear that Nova Scotia may be a reasonable point for where this is heading. However we are still a good 7-10 days away before it would even be a threat towards that region and as we know these model runs can change in 6-12 hrs. 

Latest on Leslie from NHC as of 5 AM is as follows:

..LESLIE STRENGTHENS SLIGHTLY AS IT CONTINUES TO MOVE NORTHWESTWARD AWAY FROM THE LEEWARD ISLANDS...

5:00 AM AST Sun Sep 2 Location: 20.8°N 59.5°W Moving: NW at 15 mph Min pressure: 994 mb Max sustained: 70 mph

We will continue to monitor the situation thru the days ahead ..and the next update will be with in 24 hrs...

From all of us to all of you and your loved ones a very Happy Labor Day! We will be here tracking Leslie and keeping you aware of the latest current conditions..

Leslie Update # 3 .. Model Mayhem

Lets get into the heart of the matter... Tropical Storm Leslie.. What is going on with Leslie? We mentioned yesterdays update that this system was still not vertically stacked and this was just one of the reasons we thought development would be slower to occur. We also mentioned dry air as well as shear effecting the system. These three things have really not changed any..

1. We still do not have a vertically stacked system... Lets look at the 200,500, 700 mb vorticity charts..

You can see very clearly from the images at 06Z that we are still not vertically stacked. Matter of fact the mid level center is still discoupled from the lower level center. This is because the system is still under the influence of shear from the North.. While the shear over the system itself is not bad it is all being northerly directed and you can see the shear tendency map showing an increase around Leslie..

Only 5-10 knots across the southern part of Leslie but you can see 10-20 knots across the northern part and to the NW shear as high as 40 knots. This is something that you DO NOT want to see happen if you would like to see Leslie develop into a hurricane.

Just as last night the convergence and the divergence are actually decent across the system..

Now still yet another problem for Leslie to over come is the dry air that is also surrounding the system.

So at the present time what we need to see happen for strengthening of this system is as follows:
1. Dry air has to abate..
2. Shear has to decrease (most likely not with an Upper Level low moving off the east coast)
3. The system must become vertically stacked and pull the convection over top of the Center of circulation...

Looking at the guidance tonight does not really give a clear answer as to what is going to happen with Leslie...

You have the GFS (Global Forecasting System) which is so slow with the movement of the system that it takes to roughly 300 hrs for the system to be in the vicinity of Nova Scotia..

However..on the other side of the spectrum and a much weaker and progressive solution is the European Computer Model.. Now generally the ECM would reign supreme over the rest of the guidance but the model has been changing from run to run that there is not much consistency and the same thing can be said for the GFS. After all we just had both these major models not more then 24 hrs  ago have the Southern New England region in play..

Still yet there is the GGEM which is also a stronger system then what the ECM is showing.

Then perhaps another reason to give pause to the ECM with the weaker system would be the NOGAPS which is the most progressive model which is stronger and actually a touch slower then the ECM..

When you look at the wide view of the Atlantic you can really see the size of this system.

We are actually dealing with a very large system and it is hard to believe that the ECM is going to be correct with its weaker version that it is showing.
Though visible satellite really shows the effects of the shear across the NW part of the system ..

The water vapor image shows that the system is slowly strengthening...

The other thing that you can clearly see on the wide view water vapor is the upper level trough between the US and Bermuda. This upper level trough is going to essentially "cut off" and this upper level trough "should " create enough of a weakness in the ridge that Leslie should start going towards the NW...then eventually to the North..

Finally the steering currents that are in place and there is still ridging to the north of Leslie. So this means that Leslie should continue on a West to West Northwest path at least for the next 18-24 hrs ..it is at that point when the Upper level trough should form a cut off low and then allow Leslie to seek out that weakness.

Keep in mind that because of the shear size of this system it takes longer for these type of systems to become vertically stacked. Also while Leslie is getting close to a CAT 1 hurricane ..keep in mind that shear will be INCREASING due to the upper level trough (cut off low) from the NW

Until we can get an exact center location on Leslie..and with such radical swings being seen in the guidance , and increasing shear expected ... if Leslie does not become a Cat 1 in the next 12-24 hours she probably will not until after 96 hrs. 
With such uncertainty we are not going to be making a track at this point and time but we will continue to monitor the situation and our next update can be expected another 24 hours from now!