Tropical weather analysis - May 21, 2012
Alberto is hardly a threat, and continues off the coast of Georgia as a minimal tropical storm. As of the latest NHC advisory, the following information was posted on the storm:
Wind: 40 mph, with higher gusts
Location: 30.5°N 80.1°W
Movement: S at 5 mph
Pressure: 1007 mb
Category: Tropical storm (Saffir-Simpson Hurricane Scale)
Alberto is getting ready to recurve, as satellite and radar data suggest that the tropical cyclone is moving south. Presumably, this is in response to the beginnings of the anticipated deterioration of the blocking pattern that is currently draped across the eastern United States. While it's true that I had forecast a landfall, I was at least fortunate enough to be correct that the storm would make it perilously close to the coast before recurving.
The cyclone peaked shortly after formation, and has been on a slow decline ever since. Convection is limited to the eastern semicircle due to westerly shear. And indeed, what convection exists is fairly shallow. With the shear forecast to follow Alberto on its seaward trek, reintensification later in the period seems unlikely despite slowly warming sea surface temperatures.
Figure 1. Latest infrared satellite image of Tropical Storm Alberto, courtesy of NOAA's Satellite Services Division (SSD).
In the meantime, the cyclone can be expected to slowly weaken due to the extremely subsident airmass in which it is embedded. It is possible that if current convective trends persist, Alberto will weaken to a depression by morning. In fact, the cyclone could weaken even faster than what I have indicated below. Most of the models lose the circulation in about three days, and Alberto is forecast to be extratropical by that time.
All coastal watches were discontinued earlier today.
5-day intensity forecast
Initial 05/21 0300Z 35 KT 40 MPH
12 hour 05/21 1200Z 30 KT 35 MPH
24 hour 05/22 0000Z 30 KT 35 MPH
36 hour 05/22 1200Z 25 KT 30 MPH
48 hour 05/23 0000Z 25 KT 30 MPH
72 hour 05/24 0000Z 25 KT 30 MPH...extratropical
96 hour 05/25 0000Z...dissipated
The area of low pressure we have been tracking across the eastern Pacific for several days has finally developed enough persistent thunderstorm activity to be considered a tropical depression. As of the most recent NHC advisory, the following was posted:
Wind: 35 mph, with higher gusts
Location: 9.3°N 99.6°W
Movement: WNW at 5 mph
Pressure: 1005 mb
Category: Tropical depression (Saffir-Simpson Hurricane Scale)
There isn't much convection over the center, as easterly shear continues to inhibit conglomeration of the shower activity. Instead, satellite photos portray a sheared system, with most of the thunderstorms confined downstream from the center, or in simpler terms, the western quadrant.
Figure 2. Latest infrared satellite image of Tropical Depression Two-E, courtesy of NOAA's Satellite Services Division (SSD).
A large ridge over the western Caribbean has been the catalyst with regards to the shear over the system. Careful examination of water vapor imagery shows that this ridge hasn't really moved much, and shear is expected to remain in at least the 15 kt range through 24 hours. This should at least slow intensification, given the system's relatively slow movement. Thereafter, upper-level winds are forecast to slowly become more anticyclonic as a broad 200 mb ridge builds over the cyclone. This should allow a greater degree of intensification beyond that time. Later in the period, the system is forecast to be moving across cooler waters. Although these waters will still be warm, they are cool relative to what the cyclone is experiencing now. In addition, oceanic heat content degrees markedly north of 15N. This should at least slow intensification. Although the NHC mentions stronger shear afflicting the storm by that time, I see mostly southeasterlies in the GFS, which is not conducive to weakening of a storm moving northward. In the event the system develops a well-defined inner wind core and moves underneath the warm ridge, rapid intensification is possible over the very warm eastern Pacific waters, which are warmer than normal this year due to the emerging El Nino. Of course, we have little skill in predicting these episodes, but the bottom line is that southwest Mexico should prepare for a hurricane.
A more primary ridge is building over the central United States in the wake of an upper-level trough/attendant cold front, the same one that will recurve Alberto. Large scale guidance suggests this should continue around Wednesday evening, when the cyclone is forecast to slowly get picked up by a developing trough off the west coast. This trough is forecast to be of particularly large amplitude, so it should prove sufficient to recurve the system. The GFDL and HWRF are faster, but like the NHC, I have ignored those for now. The system is forecast to be just off the coast in five days. However, there are three possible scenarios I see for this system as it approaches the coast in the long-range: a) the system moves inland a little faster than anticipated, b) it moves as forecast, or c) it briefly moves inland, then moves westward back over water due to difficult to predict local wind patterns downslope from the mountains.
Whatever the case, interests in and along the southwest coast of Mexico from Puerto Vallarta to Manzanillo should closely monitor the progress of this tropical cyclone, which is expected to strengthen.
5-day intensity forecast
Initial 05/21 0300Z 30 KT 35 MPH
12 hour 05/22 1200Z 35 KT 40 MPH
24 hour 05/23 0000Z 40 KT 45 MPH
36 hour 05/23 1200Z 50 KT 60 MPH
48 hour 05/24 0000Z 60 KT 70 MPH
72 hour 05/25 0000Z 75 KT 85 MPH
96 hour 05/26 0000Z 80 KT 90 MPH
120 hour 05/27 0000Z 85 KT 100 MPH