Tropical weather analysis - July 24, 2012
Invest 98L
A non-tropical area of low pressure producing winds to near gale force is centered about 700 miles east-northeast of Bermuda. Earlier microwave data suggested a well-defined surface circulation, with strong convective banding west of the center. Since that time, convection has weakened considerably -- or more accurately vanished, although low-cloud lines still suggest a decent circulation. More recent microwave data confirms this, but implies that the circulation is a little less organized than this afternoon.
Figure 1. Latest infrared satellite image of Invest 98L. Image credit: NOAA
An amplifying cold front over New England is poised to increase the net westerly shear over the disturbance. This, and the rapidly deteriorating structure suggests that 98L is not long for this world. The global models suggest its small circulation will become absorbed into the aforementioned low in about two to three days. It should be noted that before the merger occurs, the fast forward motion of the system combined with the steep pressure gradient to the east should maintain relatively strong winds across that portion of the surface low.
Probability of development within 48 hours: 10%
Invest 90E
A tropical disturbance centered about 1100 miles southwest of Manzanillo ("90E") has not made much progress today. In fact, the National Hurricane Center steadily decreased development probabilities since my previous forecast, now down to 40%.
Figure 2. Latest infrared satellite image of Invest 90E. Image credit: NOAA
Interestingly, anticyclonic flow is actually starting to develop above the vertical circulation, which suggests a lessening of the shear. While this would normally presage intensification, high cloud motions and the flat outflow pattern on the eastern side of the low suggests northeasterly shear. Environmental conditions still appear conducive for some eventual development of this system, although the disorganized nature of the circulation suggests that any such potential will be slow.
I still expect 90E to move west and cross into the central Pacific area of responsibility on Saturday. The global models are in good agreement with this scenario. Subsequent to crossing 140W, cooler waters should cause weakening. Additionally, if the system moves north of 15N, strong westerly shear associated with the mid-oceanic trough over Hawaii awaits it, guaranteeing a gruesome death. Analysis of the various global model forecast fields suggest that a rather large weakness will be present near the latitude of the Hawaiian Islands by the weekend. Ordinarily this would alarm me about a possible threat to the Hawaiian Islands. However, further analysis reveals that this weakness is not nearly as strong below that level. Since 90E is expected to lack vertical integrity by that time, a more westward to west-northwestward track to the south of the subtropical ridge seems like a more reasonable prognosis. However, it could pass about 100 miles south of the Big Island through early next week, which is likely to produce an interruption in the easterly flow, as well as enhance shower activity.
Probability of development in 48 hours: 40%
Eastern Atlantic development still possible
The possible tropical wave I mentioned yesterday has emerged from west Africa, accompanied by a decent lower-tropospheric signature. This wave still needs to be watched, as upper-level conditions could improve as it moves westward over the next few days. Another wave-like entity moving across Mali may also need to be watched given the convective vigor.
The African wave train is starting, and so is the season. I think.
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