94L Getting Its Act Together

94L is in one of the classic tropical development stages where it transforms from being part of the ITCZ into a tropical low entity all on its own. The system has been traversing northwestward through the night, has intensified its convection, and developed a more considerable circulation. The circulation is not yet at the surface, at least per QSCAT imagery, but could reach the surface within 24-36 hours at its current development pace. We will likely see the convection wane through the day as the diurnal minimum approaches over water, but with the separation from the ITCZ commencing, a low trying to develop, high pressure building over the system, warm sea surface temperatures, and low shear...development looks very likely at this point. I wouldn't be surprised to see the NHC up the development probabilities to a high chance in the next update or later today (NHC).

Here is the current visible satellite image of 94L:

Do you notice the wave axis? How about the ITCZ convection streamers? It is difficult on a still image, so here is an image that should help you:

The red curved lines help define the wave and are convection along the ITCZ. The Green line is the wave axis (which actually should be tilted the opposite direction, to the south-southeast). The yellow line indicates the storm's motion so far (to the northwest), while the red arrows indicate the present visible rotation, with a guessed low location indicated by the 'L'. 94L is currently moving towards the northwest, breaking off from the ITCZ which is a very important step in tropical development. Once the break is complete, convection will likely wane a bit until the low level circulation is well established. This should occur over the next 24-36 hours as the system approached Hispaniola and Cuba. At that point 94L should have a low level circulation, and I think will be our 7th depression of the season. Considering the potential here with high sea surface temps and low shear, I think 94L could be a 'major' influence in the Caribbean to Mexico...possibly even the Gulf later this week.

94L will continue its northwest track as long as the trough to the north is in place. However, this trough will pull out, allowing another ridge to build in, pushing 94L back west again in 48 hours or so. This should place 94L near Jamaica, or in the heart of the Caribbean Sea. Hopefully Hispaniola or Cuba can do a number on the system, but at this point I am very concerned that we could see a strong hurricane later in the week.

94L and 95L

(NOTE: I would like to apologize for the spacing in this post. The blogger system doesn't want to cooperate and space properly, so this will have to be how it is presented.)

Fay is still the headline story, but the main threats and track are fairly understood at this point. Fay will move westward, possibly over water again today, hold weak tropical storm strength, and then begin dissipation over Mississippi or Louisiana. The storm will produce heavy rainfall through Thursday...at least. This heavy rainfall will lead to flash flooding and river flooding in the areas that receive the heaviest rainfall. However, other tropical stories are currently playing out in the Atlantic, and this update is going to touch on these.

Currently there are two potential tropical systems, 94L (Red circle on water vapor below) and 95L (Blue circle). Each system currently contains robust convection, much more than yesterday, and are in areas of little shear. Both have a weak cyclonic rotation, but no closed surface low. This discussion will go into each system's potential and try to come to some sort of conclusion or prediction.

Water Vapor Imagery (Red: 94L, Blue: 95L)

Current Shear (Low in Tropical Development Region)

850 mb Vorticity (Highest Orange and Yellows)

95L - The Little Guy

Needless to say by the title, I am not quite as worried about this system as 94L. Yes, broad circulation is currently seen on the visible satellite image and convection has increased today, but its location doesn't make me as nervous as 94L's does. It is currently near 20N, 49W which is north of the islands. The system is currently in an area with 5 knots of shear, which is favorable for development, but continued westward motion will march it directly into 30 knot shear, which would destroy the system. The SHIPS model forecasts shear to stay below 10 knots until 72 hrs, where it only briefly brings it to 17 knots and then back to 5 knots. This seems a bit under estimated, considering the high shear environment to the west, which likely explains why the SHIPS strengthens 95L to a 85 knot storm in 120 hours while most models dissipate the system. The trough which is producing the high shear should pull out some, but not completely over the next 120 hours.

Beyond the shear, 95L has to deal with a very dry environment. Dry are is surrounding 95L to the east, north, and west. Even if the storm were to start getting its act together, the system would likely entrain this dry air which would help to weaken it once again. Currently a surface circulation is not visible, and not present on QSCAT images either, so the prospects of rapid strengthening or development look unlikely. 850 mb vorticity is also very low in the vicinity of 95L.

QSCAT for 95L: No Circulation Noted

Considering all of the negative issues with 95L, I would give it a very low probability of development, which is about what the NHC is currently saying (Low Chance). Even if it did develop, the track of 94L tells me that 95L would interact with 94L resulting in an even greater chance of the system not developing. Below are the current models for 95L, with the Canadian (Bold Green Line) being the only 'reliable' tropical model currently forecasting 95L to develop.

Model Tracks (Bold Green: Canadian Model)

94L: The Threat

94L is currently located near 12N, 57W which is very close to the continent of South America. Normally when a system is located in this region it would be considered a non-threat because it would interact with South America, entraining drier air from the interior of the continent, killing the circulation. However, looking at the mid-level steering maps, the system looks like it will be steered to the north and west over the next few days. This would bring it away from South America giving it a much better chance of development.

Steering Currents

Additionally, the highest water temperatures and tropical cyclone heat potential waters in the deep Atlantic are located just off of the South American coast, which likely indicates why such explosive thunderstorm development has occurred in the last 12-24 hours. Low shear is present over the system, and low shear continues (and is forecasted to continue) throughout the Caribbean and into the Gulf of Mexico. The SHIPS model predicts shear to stay at 10 knots or lower for the next 120 hours, which this time seems reasonable, and develops the system into a 83 knot storm, again reasonable for the shear forecast. 850 vorticity is the highest, in the entire tropical Atlantic (besides Fay) in the region around 94L and strong upper-level divergence and lower-level convergence is already present with the system. However, one major factor has not yet come together for 94L, and that is a low level circulation.

The QSCAT images currently show a low level wind shift boundary, but no closed circulation with 94L. This wind shift boundary is a starting point, and if convection can continue to occur, a surface low could develop along this boundary with time. However, with 94L so far south, the Coriolis force is not quite as strong as if it were at 15N (or further north). Thus, as the system follows the steering currents to the northwest towards the islands, the Coriolis force will increase, helping to spin up a system in a few days. Of course, if convection were to subside before the system can move north, this would reduce the chances of a surface circulation, but the low level wind shift boundary should continue to enhance thunderstorm development.

QSCAT for 94L: Notice the boundary parallel to the South American coast.

Currently 30 knot surface winds have been detected by QSCAT, which would be strong enough to designate the system a tropical depression if a surface low was present. Thus, whenever this system gets a low level closed circulation, we will see a tropical depression designated. The system will then track northwest into the Caribbean towards Hispaniola, as forecasted by the European (Red Line) and Canadian (Green Line) in the image below. Some of the less reliable models forecast 94L to move more westward, but this is most likely only if 94L stays weak. A trough will weaken the ridge to the north of Hispaniola over the next 72-96 hours, which should allow 94L to drift north before the ridge re-builds. I think 94L should get near Hispaniola before the ridge builds back in and forces it back westward. The system will act much like Fay did over Florida when it stalled out and moved back towards the west.

94L Model Tracks (Red Line: European, Green Line: Canadian)

78 Hour GFS Forecast

I give 94L a high chance of development, which is a bit stronger than the NHC (Medium Chance). Everything is coming together for a system in the Caribbean, a strong surface boundary is already present, and many of the models are predicting a system to develop from this wave. I think we will likely see a named storm (Gustav) from this system in 2-4 days. It will be a threat to the islands of the Caribbean before moving further west. If the system does reach Hispaniola, it could get weakened by the mountainous island, but should keep its circulation intact to traverse towards Florida or the Gulf as a strong ridge of high pressure will build off of the southeast coast of the United States.

Beyond 94L and 95L:

The 'wave train' that often occurs this time of year is starting to come together. The Cape Verde season will be very active over the next two to three weeks, with two strong waves already present over Africa (shown in the satellite below). Many of the models, including the Canadian, European, and GFS are indicating these waves could develop into tropical systems, so it will be important to watch them carefully over the next few weeks.

Tropical Waves in Africa

Philip, SD Weather Update

Only because I have been posting about Philip for a while now, and have been amazed that we had not hit 100F this entire year, I thought I would put in a quick update:

We hit 101F! Yes, two days ago we finally reached the illusive number, and were the only town in the state to reach it on that day. However, the heat was short lived and last night, yes only 36 hours later, the low was 43F. Thats a 58F temperature change! Fall is approaching and the cold fronts are starting to get stronger once again.

But now I am going to be moving to Wichita, KS...so Philip, SD weather isn't going to be as much of an interest to me. Expect to see more about Wichita in the coming weeks.

Now back to the tropics....

Fay Re-forms?

Earlier this morning I looked at the radar and could have sworn Fay was a little further south than it is now. The original center location would have tracked Fay into the Gulf of Mexico, giving it time to slightly strengthen and live a bit longer before weakening to a depression. However, the new location makes it much less likely that Fay will get a chance to strengthen at all. The hurricane possibilities are now at 0%, as far as I am concerned. There is the possibility that it moves back out over the water for a very short time near Pensacola, but even then it will be only for a few hours, not allowing Fay to strengthen.





The 'L' on the radar indicates the center of Fay, with the lines representing the old track, jump and then new track of the center. This is great news for the southeast, which before this jump was in line for many, many days (possibly another week!) of heavy rainfall. This threat still exists, but with Fay not able to re-energize, the storm should slowly start to weaken to a depression by the end of tomorrow (if it doesn't go over water at all). I think a remnant circulation will still be present over Mississippi in 5 days, but not the strength the GFS is forecasting, which should hold back on the extreme flooding that could occur. I'm not saying the threat is gone, but it sure will be less than originally thought.

Addition: Wait...what? That circulation broke down now too? Its broad, its tough to see, and now it looks like its going southwest. Basically, we are just going to have to watch this over the long hall because Fay is so weak the circulation can't center itself. Maybe some warm sea surface water in the Gulf will cause it to organize better, but right now, its tough to say. Will Fay even get over water?!? If so, 5 landfalls, if not, the dissipation will be much faster than anticipated.

Beyond Fay

We are moving into the busiest period of tropical development, with the peak of the hurricane season coming on September 10th. So what can we expect over the next few weeks?

First ENSO:

Currently ENSO is slightly positive, which is often correlated to higher levels of wind shear in the tropical Atlantic. However, it is only slightly positive and coming out of a strong negative phase. A lag is often seen in effects reaching the Atlantic, and thus the higher shear is not impacting the Atlantic as you might expect. Also, the long range forecasts predict ENSO to stay near neutral, so no real inhibiting factors should be felt from ENSO on the tropical Atlantic. Its impact on tropical development in the Atlantic will be near zero.

MJO:

Not forecast to be in a conducive (positive) or inhibiting phase (negative). Near neutral conditions means little influence on tropical development.

SST and TCHP:

Sea Surface Temperatures are now supportive of tropical development throughout the tropical Atlantic. The warmest areas are in the western Caribbean, Gulf of Mexico, Bahamas and surrounding waters, and off the South American coastline into the central Atlantic. Tropical Cyclone Heat Potential is greatest in the same locations. Thus, any storm that gets into the northwestern Caribbean into the Gulf of Mexico look to be the most dangerous.

Shear forecasts:

Shear is forecasted to be conducive for development throughout most of the tropical Atlantic development region, especially the Caribbean and into the Gulf over the next week.

Models and Predictions:

The models are continuing to be aggressive with their tropical development. The European and Canadian models develop the current 94L wave just east of the islands in the Caribbean in the next 2-3 days. They bring this system towards Hispaniola north into the Bahamas and stall it in this area. Again, just like with Fay, a trough tries to pull it north (the one currently moving through the mid-western United States) but pulls out before it can completely grab the system. 94L then stalls and begins to move westward as a ridge builds in! Sounds very familiar. If this were to happen, the only hope would be for the teleconnection from Asia working out, which supports a trough digging off the eastern seaboard in about 9-10 days. This would hopefully grab 94L and pull it north.

However, if 94L does not develop quickly, it could continue to track west through the Caribbean and miss both troughs, moving towards Mexico or the Gulf. I think it might be slow to develop, and thus not move as far north, but this should definitely be watched.

Beyond 94L, a second area of interest (95L) is in the central Atlantic. A broad area of circulation is seen and low levels of convection are present. However, no models predict this system to develop and the system is not sitting over the warmest sea surface temperatures. Shear is near 10 knots and will stay near that value for the next few days, likely keeping it from developing before it pulls north and out to sea.

Further waves are forecasted to move off the African coast over the next two weeks, and some of the models forecast one or more to develop. Considering ENSO and and MJO are not going to be inhibiting or helping factors, shear is to be low, and TCHP is decent, I think an average period of tropical development should occur over the next two weeks. This would mean 1-2 tropical systems should develop in this time frame, which is reasonable.

Fay Still Alive and Kicking

Fay has made the turn west after making it into the Atlantic as a moderate Tropical Storm. Fay intensified only minimally to 60 mph before making landfall once again in northeast Florida. Currently it is in the middle of the peninsula moving towards the Gulf of Mexico, shown below:






A ridge of high pressure has built in from the north and east forcing Fay to move westward. This ridge should hold on for the next few days continuing this motion. Any southward component looks unlikely as an upper-level low (Orange 'L' below) is situated over northwest Arkansas funneling the upper-level winds northward over Arkansas and Mississippi. The ridge (Yellow line) is built in a southeast to northwest fashion from the Atlantic into the Ohio Valley. Thus, the steering currently should force Fay either to the west or slightly north for the next 3 days while Fay moves very slowly. Once Fay reaches Mississippi however, it will begin to be impacted by the upper-level low in Arkansas which has been cut off and is not moving very much. This will force Fay to the north and likely northeast as a trough of low pressure (Purple arrow in MN) dives in from the northwest breaking apart the ridge.




Fay never became a hurricane in its lifetime, and even with another two landfalls (thus two more times over the Gulf of Mexico) forecasted, it shouldn't strengthen enough to reach hurricane status in the future. However, Fay will be remembered throughout the state of Florida as EVERY location will be impacted by it before its departure. Flooding rainfall approaching 30 inches has been estimated by doppler radar, and Melbourne, Florida was above 18 inches of rain in their rain gauge. Lucky for Florida, the soil type, flat surface, and proximity to the ocean, allows the rainfall to be mostly soaked up or flow into the ocean before catastrophic flooding can occur. Also, without steep hills or valleys, flash flooding and fast flowing water is not as likely. Thus, damage and deaths have been on the low side even though the rainfall has been so high. For instance, if this storm had parked over Houston, it would be been devastating (remember: Allison).



The tropical season has not even reached its peak yet, and we are to 6 named storms! With 10 being the average, we are definitely above normal. The European and Canadian models are forecasting more systems to continue to move west from Africa and show a new system in the Caribbean or Bahamas within the week. I will go more into this in a future blog later today.

Update coming...

Tomorrow morning. Wed and Thur are the days my wife is off work, so we are travelling around. Check in tomorrow.