As Isaac continues to churn near the north coast of Cuba, we begin to look ahead to the next couple weeks, as we approach the peak of the 2012 hurricane season. Thus far, the Atlantic tropical season has been rather quiet, with very few landfalling cyclones (Isaac is the 9th named storm; Joyce in the central Atlantic the 10th). This benign pattern is not unexpected given the enormous amount of Saharan-dry air blanketing the Atlantic, and increased wind shear in the tropical breeding grounds due in large part to the developing El Nino in the tropical Pacific.
The MJO pattern was temporarily favorable for upward motion and cyclone initiation in the latter part of June, when we saw tropical storm Debby develop in the Gulf of Mexico, bringing 1-2 feet of rain to portions of Florida.
Notice on the right hand side of the above image, the dates going forward from late June through late July. The colors covering the Gulf of Mexico and Carribbean Sea turn progressively more yellow/orange, indicative of subsidence/sinking air, and thus less potential for tropical cyclone development. These images correlate well w/ the quiet period in the tropics from July into the first half of August.
The new 40-day CHI 200 hPa forecast is below:
Notice that the initial image shows some green in the Carribbean Sea, where Isaac currently sits. However, the breeding grounds become rapidly less conducive for tropical development by next week and into the peak weeks of the hurricane season – September 1st-15th.
It’s interesting to note that tropical storm Joyce in the central Atlantic (10th named storm) has now weakened to a depression, due largely to dry air, wind shear, etc – poor environment in general for cyclone formation.
The MJO will be moving into octants less favorable for tropical development over the next 1-3 weeks, with improvement likely by the latter part of September (15th-30th).
Therefore – with respect to normals, I anticipate the peak weeks for the Atlantic season to be relatively quiet, probably until the 3rd week of September, when we may begin to see more upward motion propagating into the Atlantic via the MJO. We’re up to 10 named storms now, and a handful of those names were tropical storms that developed and died pretty quickly. My original estimate for total named storms was around average (11), but since we’re probable to have another burst of activity in mid/late September, I can see 2-4 more named storms this season, but no more than 15.
Regarding Isaac’s track, I don’t differ from the NHC’s forecast at this point. The Panhandle of FL is at highest risk for a landfall, but the region east to Tampa and west to New Orleans should not let their guard down. Additionally, given the lack of significant interaction w/ land, Isaac could potentially intensify rapidly in the zone from the FL straits northwest through the Keys/SE Gulf of mexico. Wouldn’t surprise me to see a CAT 3/major hurricane approaching the gulf coast. History says that the shallow waters often result in weakening prior to landfull, so in all likelihood we’ll see a Cat 2 or 3 on a slight weakening trend prior to landful. Difficult to say the exact intensity Isaac makes landful at, as the eye wall replacement cycles and erratic changes in inner structure are somewhat unpredicatable.