Invest 90L to Develop

If you’ve been following my tweets over the last day or so, you would know that Invest 90L was reactivated yesterday after it temporarily fell apart. Conditions are much improved tonight, and I believe this will be Don within 48 hours. The National Hurricane Center gave 90L a 40% chance of development at 8pm, which I whole heartedly agree with. The early models from the NHC are out now, and I will talk about those and my forecast track and intensity below.

Synopsis —

Invest 90L is still a tropical wave, and is clearly not closed off as of the 10pm hour.  All winds are out of the SE, E, or NE near the wave with absolutely no evidence of a westerly wind.  The winds in the Caribbean are generally out of the east, so there must be cyclonic turning in the atmosphere in order for westerly winds to occur.  The tropical wave fired off amplified storms in Cuba and Florida today, which shows me that the moisture envelope is well in tact.  The lower and upper level air flow is building, in other words 90L is breathing enough to sustain itself, and is breathing with less effort. This is a sign that the cyclone is organizing. Below I look at each of the conditions that affect cyclones:

  • Convergence: Seems to be coming together quite nicely and is growing in the low levels.  Condition: good
  • Divergence: Divergence aloft is also building. Condition: good.
  • Shear: I see very little evidence of shear taking thunderstorms away from the core. I do see a flatness on the western side of the storm that may be shear induced. This should go away. Condition: fair.
  • Vorticity: The spin in the atmosphere is ever building, not only at the surface but also in the middle layers. Condition: great!
  • Humidity: What dry air?  Condition: great!
  • Water Temperature: Hot as always in the Caribbean. Condition: great!
Forecast —
Two things will dictate where 90L/future Don goes: Intensity and Size.  The bigger and stronger 90L is, the more of a westerly component it will have because of the Bermuda High. The surface ridge is building over Florida, and 90L will be going around the edge of it. Stronger storms are forced to be farther away from higher pressure, while weaker and smaller systems can get closer. 90L is a small to moderate sized system…and that probably will not change a whole lot. The main challenge is how much 90L will intensify. Here are a few of my intensity highlights:
  • Weak Tropical Storm by mid to late-afternoon tomorrow
  • Maximum intensity of 60-70 mph on Thursday or Friday near landfall
  • There is a chance of a stronger than anticipated intensification if every variable becomes very favorable, which I believe is possible.  I think there is a 20% chance of this happening.
A stronger storm would head towards the La Pesca, Mexico or Tex/Mex border area.  A weaker storm would likely head toward the Freeport, Texas area. The area I encompassed here as possible landfall areas is quite large due to the uncertainty involved.
Further comment: The Hurricane Hunters will most likely fly out tomorrow during the afternoon…and I expect no less than them to find a tropical depression during their flight.  If convection keeps up, the National Hurricane Center may have evidence enough to pull the plug before then.  I will be updating during recon tomorrow when they go, and on any significant updates before then at

Posted on Tuesday, July 26th, 2011, in Tropical Weather and tagged , , . Bookmark the permalink. Leave a comment.

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