Tropical weather analysis - June 29, 2010

Alex

Tropical Storm Alex is on the verge of becoming a hurricane, with recentAMSU-B microwave satellite data measuring a partial eyewall, though this is still about half open, according to the aforementioned AMSU-B pass. As of the 7:00 PM CDT intermediate advisory from the National Hurricane Center, Alex is located near 23.2N 94.5W, moving WNW at 12 mph. Maximum sustained winds were near 70 mph, and the central pressure at 980 mb. Alex has a rather low pressure for a mere tropical storm, but this is because larger storms such as this one tend to not produce much of a pressure gradient, because their energy spans a much greater distance (remember, pressure gradients generate wind speeds, not central pressure).

This was most recently the case with 2008's Hurricane Ike, also an exceptionally large storm. Infrared satellite loops also indicate a gradual increase in overall organization, though upper-level outflow still appears to be restricted to the northwest. This is probably due to dry air entrainment, as there are several noticeable gaps within the convection, particularly along the northwestern side.

However, water vapor animations suggest that any dry air affecting Alex will not produce any appreciable negative effect upon the cyclone's intensity, though the dry air currently seen over Texas and Mexico might begin to slow intensification as Alex gets closer to the coast. My forecast track, based on CIMSS steering data for 400 mb, along with water vapor animations, steering data from PSU, and model data, indicates a landfall north of Tampico, but well to the south of Brownsville, Texas. This forecast is in best agreement with the 18z run of the GFS model, taking the cyclone NW for the first 6-12 hours, then WNW, then W as it nears landfall. It is also similar, though not identical, to the NHC's official forecast. However, this could still be too far to the north, as the ridge appears to be quite strong -- stronger than previously forecast, even.

Upper-level winds appear conducive for steady intensification of Alex, until landfall occurs in just over 30 hours. However, as I said earlier, dry air ingestion may be a problem as Alex gets closer to the coast. Nonetheless, I am not expecting this to be able to deter the cyclone from intensifying entirely. At most, it should only slow intensification as the cyclone nears the coast. Alex appears poised to pass underneath a warm eddy located from about 22.5N to 24N. This should occur within the next several hours. TCHP will decrease as Alex pulls away from this region, but will still be sufficiently abundant as to aid in continued intensification up until landfall. I am still forecasting Alex to attain Category 2 status before landfall, and there is still a small chance (10%) that it could attain major hurricane status.

As far as watches and warnings goes, as of 7:00 PM CDT, a hurricane warning is in effect for:

- 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 7:00 PM CDT, a tropical storm watch is in effect for:

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

- The coast of Mexico south of La Cruz to Cabo Rojo.

As far as watches and warnings goes, as of 7:00 PM CDT, a hurricane warning is in effect for:

- 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 7:00 PM CDT, a tropical storm watch is in effect for:

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

- The coast of Mexico south of La Cruz to Cabo Rojo.

Elsewhere

Elsewhere in the tropics, the GFS and GFDL are forecasting that an extratropical surface low will break off from the unseasonably strong longwave trough currently moving across the United States. This is forecast to occur in four days somewhere off the northern Gulf Coast from the western Florida panhandle westward to the mouth of the Mississippi River. Upper-level winds do not appear all that conducive for strengthening, and the system will probably be too close to land to form a deep warm core, anyway.