Special tropical weather analysis - April 18, 2012

By: KoritheMan , 5:41 AM GMT on April 18, 2012

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And yet again nature produces an anomaly. An area of disturbed weather over the central Atlantic located a couple hundred miles east of Bermuda was recently designated an invest ("91L"). For those wondering, this system has garnered my attention for about a week now, pretty much when the incipient cold front first entered the western Atlantic waters, since it had such a strong model consensus for development. There was no real development until today though, when the system comparatively shed its frontal characteristics.

The satellite signature is relatively unimpressive, with only shallow banding noted along the southern periphery of the low-level center.



Figure 1. Latest infrared satellite image of Invest 91L, courtesy of RAMMD satellite imagery Colorado State University (CSU).

There is still a rather large frontal-looking system to the north, but the influence of it is diminishing. There are no southwesterly winds ahead of the system, so what front was there is quickly becoming diffuse. That, coupled with 91L's southward movement, could allow for possible subtropical organization.

Model guidance suggests that this system will be short-lived, no more than perhaps two days. Given this, and the large amplitude cold front approaching the system from the west (the same one that brought the tornado outbreak to Kansas and Oklahoma on Saturday) indicating acceleration, this system is unlikely to become Alberto.

Water vapor imagery shows increasing mid to upper-level flow approaching 91L from the west, with the downstream ridge weakening in response. This synoptic regime ultimately favors recurvature into the westerlies. Model guidance insinuates that this should happen within about 36 hours.

Scatterometer data says 91L is producing a large area of tropical storm force winds, especially on the western side.



Figure 2. 0514 UTC (roughly 1:14 AM EDT) April 18, 2012 ASCAT scatterometer pass. Since ASCAT typically has a low bias, extrapolation of these values lends credibility to the likelihood of tropical storm conditions occurring on the surface with this system.

On the positive side, the well-defined surface circulation and preexisting fetch of tropical storm force winds argues that this system could quickly develop. But since area water temperatures are rather cold (23C), and the system is forecast to begin feeling the trough in about 24 hours, there seems to be little chance of significant development, even factoring in the warmer waters to the south, and the current equatorial motion. The low is likely to dissipate in about three days as it loses its upper support, currently associated with the 200 mb low sitting above the surface low.

Probability of (sub)tropical genesis in the next 48 hours: 10%

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2. nigel20
6:35 PM GMT on April 18, 2012
Thanks Koritheman...nice blog post
Member Since: November 6, 2010 Posts: 12 Comments: 8424
1. aspectre
7:02 AM GMT on April 18, 2012
One quibble, grammatical rather than substantive. Scare quotes around a word, eg "word", denote skepticism about how the word is being used (misused or abused).
I haven't gotten the impression that you think 91L shouldn't have been Invested, so "91L" seems disharmonious with the rest of your writing on the topic.
Member Since: August 21, 2007 Posts: 0 Comments: 4860

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About KoritheMan

I'm just a 23 year old with an ardent passion for weather. I first became aware of this interest after Tropical Storm Isidore struck my area in 2002.