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By: wxchaser97, 6:56 AM GMT on June 29, 2014
A disturbance off the Southeast Coast has the potential to develop into a tropical cyclone over the next five days. This disturbance was dubbed Invest 91L today by the NHC, which simply means they are watching it for tropical development. A weak surface low is associated with an area of loosely-organized convection. A few days ago, a Mesoscale Convective System (MCS) developed over the NW Gulf Coast. The large area of convection developed a Mesocale Convective Vortex (MCV). This MCV, rooted around the 850mb layer, traveled slowly from S Louisiana to the Carolina's. At this same time, a weak cold front was dropping down from the north. The MCV and the cold front began to interact as they got over South Carolina. A weak surface reflection began forming and moved off the coast last night. Currently, the weak surface low sits at 31.2°N 77.7°W, south of Wilmington and off the GS/SC border. The low is still attached to the stationary front. The system isn't fully tropical since it still has fronts connected to it. Right now, the system is pretty shallow with not much vorticity up in the 500mb level, which is expected with a system like this. An upper-level anticyclone is associated, but not aligned, with the surface low. The displaced Anticyclone is exhibiting shear in the low to moderate range. The upper-level anticyclone does mean that there is some upper-level divergence occurring near/over the system. This is good as it helps sustain convection, but only when there is enough low-level convergence to fill in the air that has been rising and spreading away. Low-level convergence is lacking with this system, as evident on UW-CIMSS analysis page. This doesn't have good impacts on the existing convection as it wouldn't be able to fully sustain itself. Consequentially, the low itself would dissipate if there isn't a balance between the upper-level divergence and the low-level convergence. The current convective structure reflects this nicely. Right now, there is some modest convection around the circulation, but there isn't as much as earlier in the day. Even with some shear, 10-15 kts, impacting the system, you can see the anticyclonic flow exhibited by the cirrus. The current satellite image and ATCF info can be found below on 91L.
AL, 91, 2014062900, , BEST, 0, 312N, 777W, 20, 1017, LO, 34, NEQ, 0, 0, 0, 0, 1018, 90, 80, 0, 0, L, 0, , 0, 0, INVEST, S,
Forecast for 91L
There are several things going for, and a couple going against, development of the invest. The disturbance is over waters that are 27°-28°C. These temperatures are warmer than normal. In fact, the entire area off the East Coast has anonymously warm waters and will be the focal point for a good amount of this seasons tropical activity. The Caribbean is more hostile as SST's a below normal and vertical shear is running above average with a strengthened subtropical jet and increased outflow from EPAC convection, typical of an El Nino year. Anyway, the warm waters off the SE Coast are more than conducive for sustaining deep convection and a tropical cyclone. Ocean heat content is modest, but this isn't a strong tropical cyclone so upwelling won't be much of a factor at all. SST's of 26°C or greater look to be in the future of 91L for the next 5 days. Another factor important to development is wind shear. Shear is running low to moderate over 91L, specifically, 10-15 kts. This isn't very bad, but it is having a modest effect on organizing the convection over the center. As we head through the rest of the weekend, the upper-level anticyclone should align itself over the suface low more, helping to reduce shear. The latest SHIPS indicates dropping below 10 kts in 18hrs and staying below 15 kts for the next 5 days. Wind shear shouldn't become a problem for 91L until it is interacting with a trough once it's pulled off to the northeast, which won't be for a while. Relative humidity in the mid-levels is looking pretty modest. It isn't very moist, but it isn't dry either. 700-500mb RH values are near 60%, and, according to SHIPS, will remain that way for the next five days. On the other hand, SLP's are running rather high across the area. The 00z ATCF analysis has 91L at 1017mb, which is pretty high. This high of a pressure isn't what would expect to see with a low pressure system. Higher pressures are also unfavorable for development as the increased subsidence in the lower levels. This isn't the worst thing and the system should be able to over come it. Really, there isn't too much going against the system in the large-scale environment, it just isn't that organzied right now. Until 91L can get good low-level convergence, promoting a formidable low-level center, there won't be much organization with 91L. This is something that will take a few days, meaning we might not see this system become a tropical cyclone until Monday or Tuesday. Also, 91L has to become a fully tropical low. Meaning, it has to shed its front and have a warm core. The convection is releasing latent heat which warms the air surrounding it, causing the system to transition to a warm-core system. This will also get rid of the stationary front it is attachted to, since it came from a baroclinic environment. The transition to a warm-core system will also take some time. Once, if it develops at all, this system become a tropical cyclone, it should be able to strengthen. The latest ECMWF brings a strong TS into NC in about 7 days. The GFS, CMC, and UKMET models are less enthusiastic with 91L. It all has to do with how long 91L is allowed to meander off the SE Coast. Steering currents are weak for now and will remain weak for the next few days. The latest ECMWF was slower with the advancement of a series of troughs going through the Great Lakes, meaning 91L got left to meander for a longer period of time. The other global models open up 91L as it meanders. Putting a lot of trust into one model at this point isn't good as they aren't going to respond well with a system in a high pressure environment, especially since it's in a region of weak steering currents. With a favorable environment, tropical cyclone development of the system is likely once it can organize itself. As mentioned earlier, the system will meander for a few days in a region of weak steering currents. After that, depending on the speed of an approaching trough, it will head off to the NE. To say exactly where the system will go is ballsy, as you can't predict how the system will react in the environment it is. We should begin to have a better idea exactly where 91L will go in a couple days. The invest should bring increased precipitation to the Carolina's, Georgia, and Florida regardless of development. Potential impacts to the Northeast will be addressed later, especially as this system will likely stay offshore. Some analogs to this system could be Alberto (2012), Alex (2004), Ana (1991), or other system like those. This is a pretty classic system breaking off of a frontal boundary and forming into a TC event.
Summary: 91L isn't very organized right now. Conditions are favorable for development and it is likely that 91L develops into a tropical cyclone in the next 5 days. It should meander off the SE coast, maybe brushing Florida even, before turning to the north and then northeast. The strength of the system, position it is in, and the speed of an approaching trough will determine exactly where 91L goes. A reconnaissance aircraft will investigate the system tomorrow to see how it is strengthening.
Chance of development in the next 48hrs: 40%
Chance of development in the next 120hrs: 70%
*Disclaimer* The arrow represents the general track of the system, NOT my forecast track of 91L and people on the SE Coast should monitor the potential for development and listen to the NHC and NWS for all warnings/preparations.
Tropical Depression 4-E
The Atlantic isn't the only place that is active. The Eastern Pacific currently has an active tropical cyclone and an invest. In fact, the EPAC has been much more active lately than the Atlantic. There have been 3 named systems, 2 of them becoming major hurricanes. The Atlantic hasn't seen it's first named storm yet, not even a legitimate invest until 91L came along. The fourth tropical cyclone of the season will very likely become Douglas soon, and has some potential to strengthen into a hurricane.
A large circulation with a modest amount of deep convection is several hundred mils off the SW Mexican coastline. The NHC began issuing advisories on TD 4-E at 21z, or 2pm PDT. A broad area of low pressure developed off the coast of Mexico a couple days ago. A lot of convection was associated with the broad area of low pressure. The interaction between a passing CCKW, the MJO being in an upward phase across the EPAC, and a healthy monsoon trough lead to the development of the abundance of convection and the low pressure system. The low moved off to the NW and slowly began to tighten up. Convection began to become more organized as well around the tightening center. Today, the convection continued to show signs of some organization, and the NHC designated the system as TD-4E. TD-4E still has a large circulation. This is evident on the latest IR imagery, and ASCAT pass from 1647z, and some microwave passes from over 6 hours ago. While the convection isn't very organized, there looks to be starting to be some attempts at banding developing. However, there hasn't been a microwave pass over 04-E for over 6 hours, Furthermore, ASCAT hasn't caught the circulation since around 1pm EDT so I can't get a look at the surface wind field to see how it's organized and if there are any TS-force winds. Speaking of microwave and ASCAT passes, I'm getting really tired of having 1, and sometimes both, miss the disturbance I'm tracking. I can't wait for Rapid-Scat to be operational as this gets pretty annoying. Anyway, 4-E has a nice anitcyclone on top of it. Outflow is looking pretty good for a developing cyclone. For the current intensity, I'm not convinced this is a tropical storm yet. The convective structure just doesn't look enough organized for the depression to be a TS yet. Without ASCAT showing if there are TS-force winds, just have to use satellite estimates and experience. ADT is at T2.3/33kts, SSD ADT is a T2.6/37kts, and TAFB and SAB are at T1.5. This all still rounds to about T2.0/30kts. TD 4-E is still likely a tropical depression and will remain so until it can organize it's convection some more. The latest information from the NHC and satellite imagery can be found below.
8:00 PM PDT Sat Jun 28
Location: 14.6°N 107.4°W
Moving: WNW at 14 mph
Min pressure: 1006 mb
Max sustained: 35 mph
Forecast for TD 4-E
Tropical Depression Four-E has several days to strengthen before having a close encounter with an unfavorable kind. The depression is located over anomalously warm ocean waters. The SST's in the region are up to 1°C above average and range from 29°C-30°. These types of temperatures are very encouraging for tropical cyclone development. The SST's should remain above 26°C for about 4 days, but the lessening of the temperature after about 48-72hrs will hurt the thermodynamics whether or not the SST's are above 26C. Going from near 30C to 27-26C is means there is less energy for the cyclone even though it is still favorable for sustaining a tropical cyclone. After 4 days, waters will cool even further, down to around 25C according to SHIPS, and have an increased adverse effect on the depression due to the increased atmospheric stability. Dry air shouldn't be a problem until it gets further to the northwest. SHIPS has 700-500mb RH values near 85% currently but brings it down to near 55% at day 5. While the depression has a large moisture shield, the drier air will begin to impact the system late in the forecast period, to a degree. The drier air would have more of an impact if dangerous wind shear was able to advect it into the circulation, but that doesn't look to happen. A solid anticyclone is atop of the depression and is nicely protecting it from any shear. It is forecast to continue to keep shear at a minimum for the next 5 days. The SHIPS keeps below 10kts for the entire forecast period and has shear at its highest at the current time. Any shear doesn't seem to be having a noticeable effect on satellite and wind shear won't impact the strength of the depression over the next 5 days. The real enemy of the depression is the depression itself. The large size will only allow gradual intensification. Unlike some of the previous EPAC storms, it will run into stabler conditions within the forecast period and probably won't strengthen as much as they did. My forecast is similar to the NHC's except I go a little stronger for the peak intensity. While I don't forecast it at the moment, there is a chance that if the depression organizes fast enough, it could develop into a hurricane.
The depression is moving to the west-northwest at 12kts from the latest NHC advisory. A strong mid-level ridge is locate north of 4-E over the EPAC and N Mexico. This ridge will be the main steering force for the next 5 days. Currently, 4-E is riding on the southern periphery of this ridge at a relatively quick pace. This west-northwest motion should continue for about the next 48 hours. After that, the depression should turn off to the west and slow down. A shortwave should eject from the northern central Pacific and move into the Western US. A weakness in the subtropical ridge will develop and cause the reduction in forward speed. The heart of the ridge is also progged to move farther off to the west. A few models take the cyclone off to the northeast due to a more significant weakness, but this isn't likely. Most of the dynamic models and global models are in agreement in the track of 4-E. Most show a WNW movement and then a turn to the west. There is some differences in speed of the cyclone, but the consensus shows a slow westward movement. My forecast is close to the NHC as there isn't much model or synoptic data to argue otherwise for track. If 4-E does get stronger than expected, it would be able to feel a weakness in the ridge more and get pulled to the NE easier. Will also have to monitor how the forward speed plays out in the later part of the forecast period and how strong/where exactly the ridge builds in. The depression isn't a direct threat to any land masses in the forecast period. Swells from the system should impact parts of Mexico, creating dangerous rip currents, but other than that the system will exhibit no impacts to land.
INIT 29/0600Z 30 KT 35 MPH
12H 29/1800Z 35 KT 40 MPH
24H 30/0600Z 45 KT 50 MPH
36H 30/1800Z 50 KT 60 MPH
48H 01/0600Z 60 KT 70 MPH
72H 02/0600Z 55 KT 65 MPH
96H 03/0600Z 50 KT 60 MPH
120H 04/0600Z 45 KT 50 MPH
Have a good early morning and I'll have a new update of TD 4-E late today. I'll update on 91L when it is necessary.
Updated: 4:02 PM GMT on June 29, 2014