TS Debby Likely To Form Later Today

Invest 96L:

Invest 96L has continued to slowly organize over the past 24 hours. The once broad center has begun to contract and there is now a dominant low-level circulation center. At the 8am EDT Tropical Weather Outlook issued by the National Hurricane Center, Invest 96L had a 90% chance of becoming a tropical cyclone over the next 48 hours. The NHC also mentioned that 96L is producing winds of tropical storm force so the system will most likely be classified as a tropical storm in its first advisory. To be honest 96L is most likely a tropical storm already, but the NHC is waiting for aircraft reconnaissance to investigate the system before it is named. Recon will arrive in the center of the system around 2pm EDT today and will not only determine if 96L has a closed LLC, but how strong the storm is.

Satellite Imagery shows a very large and lopsided system. All of the system’s thunderstorms are limited to the East side of the circulation:

Visible Satellite Image of Invest 96L

This is due to a more stable airmass to the west of the storm as well as westerly wind shear of about 20 knots. Banding features are evident in the western semicircle where strong convection is not blocking the view of the lower atmosphere.

Forecast for Invest 96L:

Unfortunately, there is still a lot of uncertainty in the forecast for Invest 96L/Debby. The only thing that appears likely is that recon will find enough organization for 96L to be classified as TS Debby later this afternoon. The forecast reasoning remains the same: Invest 96L is currently located in a region of weak steering currents due to a ridge that is blocking its motion to the North. Invest 96L should slowly drift to the north over the next 24-36 hours and it is at this time where the models begin to diverge…. Big time.

The models are unsure of whether or not a trough over the Eastern US will be deep enough to allow a weakness for 96L to escape to the NE over Florida. There is a currently a shortwave trough over the Great Plains that will move eastward towards the Atlantic. In about 48 hours the shortwave will help the trough over the East Coast deepen as depicted by the GFS model. The GFS has consistently shown enough of a weakness for future Debby to move over the Big Bend/Tampa area of Florida in around 96 hours.

The other picture that is being painted by the majority of the models is that the trough will not be deep enough to pick up Debby, and instead the ridge will send her back to the west towards Texas or Mexico. This what arguably the most reliable computer model, the ECMWF, has been saying. This line of thinking is also backed by the NOGAPS, UKMET, and HWRF. The CMC and GFDL are leaning towards an in-between scenario by taking the system straight North into Louisiana in the case of the CMC, and Alabama in the case of the GFDL.

To better illustrate the uncertainty here is the 12Z spaghetti plot:

Model Plot for Invest 96L

The intensity forecast a little easier… Hopefully. The system is under 20 knots of wind shear and in a pretty moist environment. However there is an upper-level low pressure system in the Western Gulf of Mexico that is helping induce the wind shear as well as creating a more stable environment to the west of 96L. A large part of the intensity forecast depends on storm track. The GFS upper air forecast shows an anticyclone developing over the SE Gulf of Mexico in 48 hours, which would provide great outflow channels when combined with the ULL if future Debby is located close enough to the center of the anticyclone. Here is the 48 hour 200mb (upper atmosphere) forecast from today’s 12Z GFS which shows the clockwise curved wind barbs over the Eastern Gulf representing the anticyclone:

12Z GFS 200mb Forecast

Chances are that future Debby will not be close enough to the center of this anticyclone and instead will still be under 20 knots of westerly wind shear. This will most likely result in our typical June Gulf of Mexico tropical cyclone: a sheared tropical storm with all of the convection remaining on one side of the storm. Virtually all of the computer models keep future Debby at tropical storm strength and I still think the chances of Debby reaching hurricane strength are low.

Personally, I like the forecast provided by the GFDL model. I think the wind shear will prevent future Debby from intensifying past a 50 knot tropical storm, but at the same time we may see the center reform to the NE under the deeper convection enough times that the storm just barely catches the trough. I think we should not overlook the consistency of the GFS model even if it may be an outlier at this time.

All residents of the Gulf Coast, from Northern Mexico to Florida need to closely monitor the progress of this storm as there are still a lot of question marks associated with its future. It is a very large system and capable of producing tremendous amounts of rain, which will probably be the main threat.

I will have an update again when conditions warrant.

-Mike

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