Thursday, August 23, 2012

Modeling Consensus Gathers Up On Isaac

Tropical Storm Isaac is undergoing a bit of an evolutionary process this morning, with its center of circulation reforming a bit farther south under a stronger complex of thunderstorms that has taken over as a slightly more dominant part of the storm's circulation. One of the reasons Isaac hasn't developed quickly out of the gate despite a pretty large circulation is the broadness of the circulation center. The storm at one point was sporting two circulation centers, with the southern of the two now dominating over the old northern center. This, combined with the mid level center not aligned with the surface center, has resulted in a sloppy mess of a storm.  Dry air coming in from South America into the broad centers of circulation is not helping the cause either.  The dry air factor should begin to lessen over the next day or so as the southern center takes over.

Development will continue to be slow...but it should begin to organize further in about a day or so.  One of the concerns that continues to be in the offing for Isaac is potential land interaction with Cuba or Hispaniola.  Earlier computer modeling from the GFS suggested a track that would take the storm over Hispaniola and then Cuba before bringing it up across or just near Florida's west coast.  That guidance has shifted a bit westward over the past 24 hours, as the Euro's more western track has nudged eastward.  The focus point suggests a landfall somewhere between Mobile and Apalachicola, FL in about five days time.  There are outliers in the guidance from some of the more exotic and funky tropical models but generally Isaac is poised to track west, then turn northwest.  The reforming of the storm's center to the south will make a more easterly track (east coast of Florida or even up Florida's spine) less likely as the storm will need to gain additional latitude more quickly to compensate for what it lost.  Parsing through what I just said, it means I think the more western solutions (Florida Panhandle, perhaps even towards Mobile) may be a bit more likely for a landfall point.

It doesn't mean Florida will be skating out of this will likely rain later Monday into Tuesday across Florida...and if the GFS track is a bit more closer to reality than the Euro it does mean it will be a rather wind-swept rain across the Sunshine State.   However, the brunt of the storm may pass offshore of Tampa and avoid them.   I posted a graphic below that shows the positional differences between the Euro (black for last night's run, red for yesterday afternoon's run) and the GFS (last night in blue)...the spread between last night's Euro and GFS is not that much geographically -- about 100 miles for six days out.

In terms of "impact" up this way, the Euro's track would likely keep the heaviest rains to our west as the remnant circulation pushes slowly north and then northeastward after landfall.  The GFS would bring the remnant low track nearby and could result in a rainy Friday a week from now -- potentially steadier rains would result.  Most tropical systems post-landfall bring heavier rainfall along and northwest of the track of the low (not always the case but typically that way) so a track from the Euro would bring humidity, moisture, scattered thunderstorms that could produce tropical downpours but the heaviest rains would pass across the Corn Belt (who needs the rain) perhaps into the Great Lakes.  The GFS brings the heaviest rains up through the Carolinas and towards us.  In both scenarios, note that the system post landfall moves very, very slowly so the issue of flooding rains will be a part of the weather landscape to our south and southwest.

More:  Tropics Tracker