Tropical weather analysis - August 7, 2012
Shower activity remains minimal in association with a tropical wave and accompanying area of low pressure located over the eastern Atlantic about 600 miles west-southwest of the Cape Verde Islands.
However, earlier microwave data suggested that the low-level center is fairly well-defined, albeit not closed. This is also supported by recent satellite pictures.
Figure 1. Latest infrared satellite image of Invest 92L. Image credit: NOAA
I am not sure why the global models are so down on this system; water vapor imagery does suggest dry air lies to the west of the storm, but it is not being entrained into the core at this time. In addition, a large moisture field surrounds 92L, making any such intrusions unlikely in the near term. High cloud motions do still suggest some easterly shear, though, which could certainly slow rapid strengthening. One other reason the global models may not be quite so enthusiastic about development potential with this is due to sea surface temperatures of only 26/27C along the forecast track for the next several days. Remember, we are still somewhat behind the climatological due date for the arrival of the Cape Verde season, so the eastern Atlantic is just beginning to warm.
Based on global model forecast pressure fields, 92L is expected to move westward over the next several days. Should it begin to develop, it would likely take a more west-northwest track as it nears a weakness in the subtropical ridge near 55W. This would likely occur over the weekend. It is entirely possible 92L will fail to develop significantly for the next five days, and eventually enter the Caribbean. I am in between the aforementioned tracks for now.
Probability of development in 48 hours: 30%
Tropical Storm Gilma formed today. As of the most recent NHC advisory, the following was posted on the tropical cyclone:
Wind: 60 mph, with higher gusts
Location: 15.4°N 115.0°W
Movement: WNW at 13 mph
Pressure: 994 mb
Category: Tropical storm
Gilma is a well-organized storm; a well-defined curved band is evident to the west of the center, while the center itself is composed of a steadily maturing CDO. In addition, a robust inflow jet appears to be setting up over the eastern quadrant of the tropical cyclone.
Figure 2. Latest infrared satellite image of Tropical Storm Gilma. Image credit: NOAA
An earlier SSMIS overpass at the 37 GhZ channel showed what appeared to be a small warm spot north of the strongest convection, but this was not evident on the 85 GhZ channel. The former is more sensitive to the lower-levels, but we would expect the upper-levels to begin forming an eye first due to the intense release of latent heat energy at that level of the troposphere. I am not quite sure what to make of this, but given Gilma's seemingly well-defined inner core structure, it seems likely that it will become a hurricane soon. After about 24 hours, the cyclone should be crossing the 26C isotherm, at which time a weakening trend is expected. One interesting thing to note is the GFS shows easterly shear increasing over the storm beginning in about four days. This is probably due to the influence of regenerating Atlantic Hurricane Ernesto. While regeneration of Ernesto on the Pacific side is not impossible, it would be a very rare event. The last Atlantic/Pacific crossover storm was Hurricane Cesar-Douglas of 1996.
Given the rugged terrain of the Sierra Madre Oriental mountain range over central Mexico, as well as climatology, I find it doubtful that Ernesto will actually regenerate once its remnants enter the eastern Pacific. Presumably the easterly shear just mentioned emanates from Ernesto's outflow, but again, since regeneration seems unlikely for now, I will not base my intensity forecast off the presumed vertical shear.
Given the cool waters and increasing shear at longer ranges, Gilma is expected to weaken to a tropical depression by the end of the forecast period. However, if the shear increases like the GFS says, Gilma could easily dissipate prior to the end of the forecast period.
It is notable that both the Euro AND GFS regeneate Ernesto. We saw how well they did with the storm when it was in the Caribbean, defying all of my forecasts, as well as others who hypothesized that a United States strike was the most likely course. I have learned a thing or two from Ernesto, so I may have to reevaluate the possibility of regeneration tomorrow.
Gilma is on the south side of a low- to mid-level ridge. Water vapor imagery shows a rather substantial ridge is in the process of deamplifying across the southwestern United States as a large upper trough and attendant frontal zone approach coastal California. Over the next day or so, this trough is expected by all of the global models to cause an erosion of the ridge that currently provides the steering for Gilma. This should more or less cause a continued west-northwest motion, albeit at a slower forward speed. After about 72 hours, the track becomes uncertain because the models disagree on the handling of Ernesto's entrance into the Pacific. Some of them, such as the GFS and ECMWF, forecast a fully regenerated Ernesto to slowly pull the low-level center eastward as it rides the low-level westerly flow found on the south side of the center. The remainder of the global models still slow Gilma, but do not show near as much interaction between the two systems. Because of the faint possibility of regeneration as purported by the godlike GFS/Euro duo, I will compromise between the solutions from both camps. In the end, this makes my forecast track a fair bit to the right of the National Hurricane Center's, as I show a slow northward drift near the end of the forecast period. Confidence in this track is not particularly high.
5-day intensity forecast
INITIAL 08/08 0300Z 50 KT 60 MPH
12 hour 08/08 1800Z 60 KT 70 MPH
24 hour 08/09 0300Z 70 KT 80 MPH
36 hour 08/09 1800Z 75 KT 85 MPH
48 hour 08/10 0300Z 55 KT 65 MPH
72 hour 08/11 0300Z 45 KT 50 MPH
96 hour 08/12 0300Z 35 KT 40 MPH
120 hour 08/13 0300Z 30 KT 35 MPH
5-day track forecast
Figure 3. My 5-day forecast track for Gilma.
Another area of disturbed weather, this one centered approximately 450 miles south of Acapulco, remains disorganized. Satellite images show a well-defined -- probably closed -- circulation exists, but there is virtually no convection associated with this circulation.
Figure 4. Latest infrared satellite image of Invest 93E. Image credit: NOAA
93E still has a ways to go. There appears to be some easterly shear over the system, as evidenced by the large outflow channel in the western semicircle. The SHIPS and GFS don't really show this shear decreasing too much. Also, 93E appears to be practically stationary at the moment, which would most definitely favor penetration of the fragile low-level circulation by said shear. There is also a little bit of dry air to the east that make entrain into the circulation -- indeed, that is probably why no convection exists now.
Having said all that, this low appears to possess a fairly well-defined lower-tropospheric structure, which would favor intensification if the environment becomes more favorable. It should be noted that due to the possibility of Ernesto regenerating in this part of the basin, the global models will likely have a difficult time resolving -- and thus successfully forecasting -- the future evolution of 93E.
Probability of development in 48 hours: 40%