Basement-dwelling pseudo-otaku with a thrill for forecasting on the side.
By: KoritheMan , 5:24 AM GMT on July 23, 2013
A vigorous tropical wave accompanied by a surface area of low pressure is centered about 250 miles southeast of Sal in the Cape Verde Islands. This wave has been designated "Invest 98L" by the National Hurricane Center.
Yet again we have another unusually strong tropical wave for so early in the season. Satellite images show an impressive amount of rotation, and the low-level center of 98L appears to be embedded within the cloud mass just east of the deepest convection.
Figure 1. Latest infrared satellite image of Invest 98L. Image credit: NOAA's Satellite Services Division (SSD).
Upper-level winds appear favorable for additional development. However, there are some hurdles, short-term and long-term, that could ultimately mitigate significant development of 98L even if it becomes a tropical cyclone. Firstly, most of the models are forecasting a west-northwest track, probably in response to an upper low to the northwest of the system as seen on water vapor imagery. While this low appears quasi-stationary for the moment, it is predicted to move westward and weaken; even if there is a residual weakness within the cyclonic vorticity connected with that feature after it weakens, it is unclear how much short-term latitude 98L will gain. The motion for the last twelve hours appears to be due west, and I don't see any convincing evidence of an appreciable low- to mid-tropospheric weakness until around 45W. It is my personal expectation that 98L will move generally westward for the next day or two before it moves closer to the apex of the central Atlantic trough, at which point I expect some latitude gain, attendant with a west-northwest motion.
Considering the above factors, I expect the dynamical model suite to shift southward over the next 24 hours; indeed, there has already been an observed southward shift in that suite from 18z to 0z. I do not currently expect 98L to be a Caribbean cruiser based on the pattern, but I am not buying an immediate west-northwest motion as the models are advertising. 98L is also embedded within a convectively active monsoon trough, and it is possible there could be some erratic motion at times, deviating from the overall mean.
Back to what I was saying earlier about intensity, the enunciated west-northwestward motion seen in the model fields would take the system over cooler waters sooner, probably in less than 48 hours. In addition, the models show a large increase in westerly shear beyond 48 hours as the wave moves closer to the central Atlantic upper trough. If the system were to track farther south, particularly below 20N, the shear would be much less, the waters would stay warm, and the system would have more potential to strengthen. It is my expectation that 98L will remain over waters that are at least marginal through the next couple of days, which should allow some additional development.
It is too early to tell whether or not 98L recurves. Based on the current pattern and the one forecast to evolve in the model fields, a system that gains latitude quicker over the next several days may recurve, but a system that more subtly gains latitude may not. This could also be irrelevant if 98L dies a cruel death under strong vertical shear.
At the risk of completely blowing the forecast, I am going much higher with development percentages than the National Hurricane Center.
Probability of development in 48 hours: 60%
An area of low pressure located about 550 miles southwest of Manzanillo, Mexico is producing disorganized showers and thunderstorms.
Analysis of satellite imagery this evening reveals that the disturbance hasn't really gotten any better organized, and there appears to be some northeasterly shear affecting the system.
Figure 2. Latest infrared satellite image of Invest 98E. Image credit: NOAA's Satellite Services Division (SSD).
Earlier scatterometer fixes and microwave data suggest that the low-level center is poorly-defined, as does satellite imagery. The models insist that upper-level winds will gradually become more favorable, so I have no choice but to forecast some development. The system is not forecast to threaten land, and it will encounter cooler waters in about four days.
Probability of development in 48 hours: 30%
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