Wednesday, August 22, 2012

Isaac Update -- To Turn Or Not To Turn

Tropical Storm Isaac is throwing rain across the Windward and Leeward Island chains this morning, continuing its westward scoot towards the Caribbean as it slowly continues to strengthen and organize. Isaac has the potential to become a hurricane over the next 48 hours as wind shear is relatively modest and the environment is supportive of a slow, steady intensification for the storm.

Questions still abound regarding track and intensity down the line with the storm -- the latter having to do with the former.  Along the path of Isaac lies Hispaniola and Cuba, two islands that feature some relatively higher mountainous terrain, with Hispaniola featuring five mountains over 7400' in elevation.  These mountains and the general land masses themselves have historically been a hindrance on tropical intensification and have certainly provided their share of misery and pain to those who live on Hispaniola.

Computer modeling is largely separated between the GFS camp, which takes the storm over parts of Hispaniola and brushing it over Cuba, and the Euro, which takes the storm just south of the islands and then gradually into the Gulf of Mexico.  While the spread between the two models gradually becomes larger over time to where there's a 200 plus mile difference by Monday afternoon between the GFS and Euro projected positions, it's not a huge difference in modeling for five days out.  If we were equating this to winter storms it would be a track over the "benchmark" versus an I-95 track.  For winter weather fans, such a deviation five days out is not unusual to see when looking at computer modeling.

However, the models have a vastly different approach on what Isaac does between now and Monday in terms of intensity.

The GFS is more apt to strengthen Isaac as it approaches Hispaniola and then doesn't weaken it as markedly upon crossing the then restrengthens the storm as it comes up the east coast of Florida.  In general "intensity" terms it looks like a strong tropical storm, low grade hurricane through much of its trek. The Euro keeps Isaac relatively steady state (weaker), perhaps feeling more impacts from Hispaniola in keeping the storm weaker but also preventing as rapid a northward turn as the storm is weaker and less prone to feel a weakness in the subtropical ridge that develops near the US East Coast this weekend.  The GFS' presentation is also a bit weaker on the ridging itself and a bit more aggressive in showing this weakness in the ridge, which would allow Isaac to nudge northwestward earlier on.  The National Hurricane Center is aligning more closely to the GFS camp than the Euro camp...keep in mind that a weaker system is more prone to avoiding troughs in the atmosphere so if Isaac remains weak, the more southern track could be in play.
The resulting difference is that Isaac, according to the Euro, tracks on into the Gulf of Mexico and towards a final date in Louisiana later next week while the GFS pushes it into Florida and then gradually the US Southeast.  That GFS track would result in a copious rainmaker for the US Southeast, perhaps into Philadelphia by late Wednesday into Thursday if that track holds.

The GFS representation would result in the remnant moisture from Isaac providing a steady to perhaps soaking rain event for the Mid Atlantic in the middle or latter half of next week.  It would not be an Irene-type scenario in terms of 60-80 mile per hour winds but it would be a soaking if such a scenario were to take hold.  The amount of time that Isaac, if the GFS were right, would stay onshore would result in a less windy system although rains could certainly be in the picture.

More:  Tropics Tracker