Invest 90L continues to become better organized and may be tropical depression #4 or even possibly Tropical Storm Don later today. A recon airplane will be flying into the storm today to determine how strong the winds are and if/where circulation is located. There is really no significant change to its path from yesterday. Mid and upper-level ridging will determine where this tropical system will end up making landfall. Because of the uncertainty of the position and strength of the mid-level ridge, it is still difficult to say where this disturbance will end up. As of right now, everyone from northern Mexico towards the Texas/Louisiana border has an equal chance of getting a direct hit from this system.
There is an upper-level low, visible on water vapor imagery, that was in the Bay of Campeche yesterday, and has now made its way over Mexico. This feature is allowing wind shear to be at a minimum in the mid and upper-levels. This may play a factor in any strengthening of this tropical system. Tropical systems feed off of warm waters and very little wind shear.
Dynamic computer forecasting models are still having a difficult time developing anything in the next 24 to 48 hours. NAM, EURO, and CMC models are the only three right now that have any type of significant formation. 850mb vorticity values are visible, according to GOES-EAST. Models haven’t even been developing significant 850mb vorticity past few runs. This is a clue that they are not correct. Maybe later models will detect circulation. Figure 4, below, shows vorticity detected by satellite at 850mb.
Finally, I wanted to look at microwave 89GHz imagery that shows how convective this system is. Right now, it is showing some decent convection on the east side of the storm.