Despite Isaac, Weekend Looks Sunny

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By Erica Grow on August 30, 2012, 10:51am

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The slowly-meandering of Tropical Storm Isaac has made it difficult for our forecast models to predict its next move. Early in the week, it appeared that Isaac's remnants might give us a messy Labor Day weekend, but as the week wore on and the storm remained nearly stationary, it became obvious that this was not going to happen. 

One of the reasons that Isaac has moved so slowly is because it doesn't have a strong steering mechanism. Big weather pattern shifts, like an upper-level trough, can push a tropical storm in one direction or another. The strong winds of a jetstream can guide a tropical system along its path, while also creating the type of shear that destroys these maritime systems. In Isaac's current position, it is not subject to either of these influences; instead, the storm moved into the Gulf of Mexico right under a big bubble of high pressure that has been nearly stationary all summer long (remember the crop-destroying drought in the Midwest this summer?) This stagnant airmass is devoid of steering mechanisms, and provided no influence for Isaac's movement.

In the absence of a steering flow, tropical systems often become nearly stationary in the lower latitudes, but once they make a move northward, they can rapidly accelerate. You might have noticed that tropical systems tend to move faster when they head north; this is not a coincidence! The curvature of the Earth helps make the systems move faster as they head north. 

A consequence of Isaac's slow movement is that the storm is losing a lot of its moisture! As it continues to pour buckets in Louisiana and Mississippi, there is less available water in those tropical clouds as the remnants make their way toward Connecticut. Our model data is finally starting to pick up on this. By the time Isaac finally arrives in New England on Tuesday (according to GFS), the precipitation amounts are around 1" or less in a 24-hour period. This is a nice soaking rainfall, but certainly not a flooding rainfall! Compare this to Louisiana, for example, which has parishes that have gotten more than a foot of rain since Tuesday night. 

We're certainly getting lucky as far as Isaac is concerned. Instead of ruining the Labor Day weekend, we will be on the "good side" of the storm; i.e, the side with compensating subsidence. The weather on the outskirts of a tropical system is usually fantastic, and that's what we'll get to enjoy on Saturday, Sunday, and even into Labor Day Monday! 


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Erica Grow

Town: Branford, CT  

Reporting for WXedge since January 2012.

Articles: 21

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