Isaac: This Way and That Way

We have seen some pretty wild shifts in the forecast track of soon-to-be Hurricane Isaac, so what gives? It all boils down to the winds that are steering the storm. You have seen graphics like this posted here on the blog, Facebook, Twitter and on-air. This is a spaghetti plot showing all of the potential paths of the Isaac’s central point from a wide variety of forecast models. Some of those models are good at understanding the physics surrounding a tropical cyclone; others are terrible (for example, that thin blue line with CLP5…that’s one we usually discard as an outlier).

Spaghetti plots are useful to get an over-view of where Isaac could go; however, a meteorologist I greatly respect in the Northeast once said, “forecast the high, forecast the storm.” What he means by that is if you can forecast where the large, synoptic scale features like ridges and troughs will be, you will be a better forecaster of where a storm like Isaac will go!

In the image below, you see the latest infrared satellite image with some green contours over the cloud data. Those green contours represent the height of a particular pressure level (500 mb) that is key in forecasting tropical cyclone movement. Winds at this level generally follow right along those contours, and this helps steer storms like Isaac one way or another.

The highlighted “upper-air trough” is the key to this forecast.  It will be passing nearby on Monday and Tuesday. If it is able to influence Isaac enough to pull it northward by Tuesday evening, then the coastline from Mobile Bay west to Destin will have to worry about a Category One or Two storm headed that direction. If that trough can’t pull Issac north, then it’s on to the Mississippi or Louisiana Coast by Wednesday and Thursday.

This one, small trough that normally would not grab any weather headlines at all will be the difference in whether the Tennessee Valley gets any direct impact whatsoever. If the northward pull occurs (which is less and less likely with every model run), then we will see a good soaking rain and potential for some severe storms. If Isaac does not get pulled north, it will get shoved west by a strong ridge that builds in behind the weak trough. That is essentially what the GFS (American long-range model) is showing here on Thursday morning:

The ridge’s influence essentially blocks any northward progress and causes the storm to move farther west.

These are the two options for the approximate landfall region. What we cannot determine for sure is which option is the right one. That will have huge implications on the forecast for the Tennessee Valley and the entire region through the end of this week.

What you can expect:
If Isaac is farther east (Alabama/Florida landfall): heavy rain at times along with a strong east-southeast breeze. Some severe storms Thursday into Friday.

If Isaac is farther west into Mississippi or Louisiana: rain chances will be low and the wind will not be that strong. The worst of the weather would be far to our west.

So, the bottom line is, whatever your interest is (here, the coast, South Mississippi, Alabama or Louisiana), this is an uncertain situation. Once we get to Tuesday and see how strong the trough’s influence is, we will know with 90% certainty where the major impacts will be.

-Jason
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