Isaac's Growing Threat

Tropical Storm Isaac is picking up some steam in the Gulf tonight and could become a significant threat for the Central Gulf Coast by the middle of the week. A more Westward track also increases the threat of significant rainfall for Arkansas. This is the National Hurricane Center Forecast Track as of 10 PM CDT Sunday night.

This forecast should be taken with a grain of salt, however, because there continues to be an unusually large spread in the computer models. Computer models support landfall solutions anywhere from the upper Texas Coast to the Western Florida panhandle

The reason this is a difficult storm to forecast, even in the near term, is that the steering currents in the Gulf and Southeast U.S. are becoming ill-defined. Well defined steering currents usually lead to less spread in the models and higher forecast confidence. That is certain not the case here. Below is the upper air pattern and water vapor imagery Sunday evening.

The wildcard here appears to be a small area of high pressure in the upper levels of the atmosphere over the lower Mississippi River Valley. The strength and exact position of this high will determine how far West Isaac gets. If the high stays stronger, Isaac will make the turn slower and more gradually, allowing for it to get farther West before getting picked up by a weakness over the plains (the yellow dashed line over Nebraska/Oklahoma). A handful of computer models take Isaac more Northward toward the Alabama/Florida Gulf Coast toward a weakness in the upper atmosphere represented by the yellow dashed line near the Southeast Coast. The situation should become more clear by Monday evening, however, that does not leave much time before landfall to make any decisions. Landfall is likely Tuesday night or Wednesday.
That brings us to our second problem, which is intensity at landfall. Isaac has consistently underperformed throughout its existence as far as strengthening. Now, Isaac will find itself in an environment over warm water and low directional wind shear. There are some factors, however, that may keep intensification in check. One factor is dry mid level air. Note the brown areas on the map above. This represents dry air North and Northwest of the cyclone. Some of this dry air will continue to get wrapped into the storm which can restrict convective development around the center, which would not allow for much strengthening. Another factor is the relatively large circulation, which can increase the time it takes for the system to strengthen. It is important to remember, that intensity is notoriously difficult to predict so it has to be assumed that Isaac could become a significant hurricane before landfall. Based on the negative factors, rapid intensification seems unlikely.

Isaac will likely be a slow mover after landfall, which brings us to our next issue, which is inland flooding. Heavy rains are likely to continue well after Isaac makes landfall. The current track takes Isaac into Eastern Arkansas by late this week.

The heaviest rain will be along and to the East of the center. If Isaac takes the track above, areas such as Jackson and Memphis would likely get some torrential rain. Should Isaac take a more Westerly route, Central Arkansas would wind up near or just East of the center, raising the potential for flooding rainfall. It is too early to call for any rainfall amounts given the questionable track, but amounts greater than 6" are likely along and just East of center.