Tropical weather analysis - June 28, 2010

Tropical Storm Alex has only gradually intensified today. At the 7:00 PM CDT intermediate advisory from the National Hurricane Center, Alex's maximum sustained winds estimated at 60 mph, its central pressure at 990 mb, and its movement stationary. Additionally, it was located at 20.6N 91.6W. The last of the visible satellite animations depict that Alex may be beginning to overcome the dry air evident on water vapor animations, as deep convection is attempting to wrap cyclonically into the western half of the circulation, something that has been considerably lacking today.

The likely causes for the cyclone's slow intensification today is 20 kt of northwesterly vertical shear, analyzed by University of Wisconsin CIMSS, the aforementioned dry air punching into the storm from the west, and cold water upwelling, as the longer strong winds persist over a given area of ocean, particularly along shallow shelf waters a la offshore the western Yucatan Peninsula, the more cold water is stirred up from the depths, and brought to the surface. I expect that later tonight and into Tuesday morning, as Alex begins to actually move, it will resume intensification. Indeed, I find it quite likely that Alex will be a hurricane by mid Tuesday morning.

In any case, it's time to focus on the track forecast for Alex. Fortunately, the computer models have come into much better agreement today on Alex's final landfall location, with the majority of them calling for a landfall in northeastern Mexico. The GFS and HWRF are the only models, as per 18z, that are calling for a Texas landfall, with the former depicting a landfall between Port Mansfield and Corpus Christi, and the latter depicting one just north of Port Mansfield. An averaging out of the model forecasts leads to a landfall near Brownsville, Texas. CIMSS steering analysis from 500 to 400 mb indicates that a NNW motion should begin within the next few hours, albeit a slow one.

The models also generally agree with this. By around 24 hours, it is generally agreed upon by the models that Alex will begin moving WNW as the ridge builds back in the wake of an unseasonably strong longwave trough currently moving across much of the central and eastern United States. Water vapor imagery indicates that the trough isn't digging far enough southward as to give Alex a significant poleward movement, and any poleward movement will be short-lived (24 hours), and Alex will not be able to get very far in that direction, due to its slow movement.

After 24 hours, as I said, the cyclone should turn back to the WNW, and possibly NW in the wake of a building ridge. I'm going with WNW though, because I don't see the rebuilt ridge having any significant weakness in it. How far north Alex gets before steering currents restrengthen is critical to just where it will make its final landfall. Right now, I think the most likely location is Brownsville, but residents from Tampico to just north of Corpus Christi should carefully monitor the progress of Alex over the next couple of days.

As of 4:00 PM CDT, a hurricane watch is in effect for the following:

- The Texas coast south of Baffin Bay to the mouth of the Rio Grande

- The coast of Mexico from the mouth of the Rio Grande to La Cruz

As of 4:00 PM CDT, a tropical storm watch is in effect for the following:

- The Texas coast from Baffin Bay to Port O'Connor.

There are currently no hurricane warnings posted, but this could change later tonight or early Tuesday morning, as the landfall location of Alex becomes more clear. Residents within the watch area should rush their preparation plans to completion. Also, always heed local law enforcement officials during natural disasters such as this.

The intensity forecast is fairly straightforward. By 24 hours, Alex could be located over SSTs as high as 29C. More importantly, oceanic heat content increases sharply along the forecast track beginning at 23N. Oceanic heat content is a fancy term for warm water that extends to great depth. These waters, not the shallow waters that Alex is currently sitting over, are crucial to intensification of a tropical cyclone.

Alex will be entering an area of oceanic heat content in about 24 hours. This high octane fuel will continue to be present up until just before landfall, which, based on a general consensus of the models, should be sometime on early Thursday morning. The farther north Alex goes, the longer landfall will be delayed, and vice versa. However, landfall will certainly occur no later than Thursday evening, and no earlier than Wednesday night.

Based on the continued forecast of low shear from the GFS, I find it likely that Alex will steadily intensify to a Category 2 hurricane, and make landfall at that intensity. There also remains an outside chance of Alex becoming a Category 3 hurricane.