Hurricane Irene, a category one storm with maximum winds of 80 mph, continues to intensify as it moves west-northwestward at 13 mph. Currently located 105 miles west-northwest of San Juan, Puerto Rico, hurricane warnings are in effect for the southeast Bahamas, Turks and Caicos islands, and north coast of the Dominican Republic. A hurricane watch is posted for the north coast of Haiti and the central Bahamas.
Current model track guidance is nearly unanimous in steering Irene towards the southeast U.S. coast by Friday or Saturday, with landfall occurring somewhere between central South Carolina and southern North Carolina. However, track errors at this range can be as large as 200 to 250 miles so landfall could occur considerably further south or north of that area or the storm could even take enough of a right turn to go out to sea.
Somewhat ominously, once Irene separates itself from the influence of Puerto Rico and Hispaniola and moves over open ocean, significant intensification is possible as it nears the Southeast coast. The National Hurricane Center (NHC) projects Irene’s maximum winds of 115 mph in 120 hours, around the time U.S. landfall may occur - although there are large possible errors in intensity forecasts.
Irene’s present appearance
Irene exhibits a presentation in the satellite imagery which is in some ways similar to that of a large and well-developed hurricane. Although an eye is not apparent, high-altitude outflow is solidly established in most directions, with evidence of a clockwise (anticyclonic) turning in the cirrus cloud streams near the edges of the circulation.
Irene’s appearance in the satellite imagery indicates the storm itself has duly responded to its new surroundings characterized by moist atmosphere and high ocean heat content. Though the presence of dry air, particularly to Irene’s northwest (note the sharp cutoff to Irene’s cloud pattern in that direction), may still obstruct its maturation to some degree, the primary factor that seems to stand in its way of becoming a major hurricane soon is its nearness to land
In the short term, the NHC official track pulls Irene away from the Antilles and into the central Bahamas by midweek as a Category 1 hurricane before strengthening it into a major category 3 hurricane.
As for a potential U.S. impact, the experts at NHC appropriately advertise the still considerable uncertainty in even the near term forecast for Irene. But the basic idea is this:
During the course of this week, a series of troughs in the midlatitude westerlies will gradually tug Irene out of the tropics. One of the more influential ones (outlined in red in a forecast valid Wednesday night) is expected to bring Irene northward across the central Bahamas Wednesday and Thursday.
Unfortunately, it is becoming alarmingly apparent that this trough will not be strong enough to pull the storm out to sea. Instead, Irene may very well be left behind in an environment that, as suggested by a growing opinion in a wide array of prediction models and their ensemble systems, will guide her to the U.S. Coast by the weekend – as a category 2 or stronger hurricane.
We will have a forthcoming post that looks at potential scenarios and impacts along the U.S. East Coast.
Irene developed from a large and strong rotary circulation in the African Easterly Jet - a strong band of east winds (shaded in bright colors to the right) centered at about 10,000 ft altitude that flows from Sub-Saharan Africa across much of the tropical Atlantic.
Its robust presence in the tropical Atlantic as a well-defined low-level swirl in this easterly wind was never really surprising to many of the weather prediction systems. Most of them guided the coherent vortex all the way into the Caribbean intact.
A large question –though not so in the eyes of a few models- was whether or not it would overcome unfriendly interactions with its environment and become a named storm. For much of the time Irene spent in this relatively shallow easterly flow over the Atlantic, it was surrounded by exceptionally dry air aloft from the desert region of Africa.
Unable for many days to sustain deep convection (tall thunderstorms) near its center, the system remained nameless for most of its cross-Atlantic trek– as the interaction with the dry air tended to oppose the spinup process.
In the last day or so, however, the environment surrounding Irene has, at least ostensibly, become more humid and, in general, more supportive of tropical cyclone development.
Irene’s effects on Puerto Rico
Here’s a video on Irene from the Associated Press:
Description: Irene reached hurricane strength early Monday after it began moving across Puerto Rico, pounding the U.S. Caribbean territory with torrential rains and winds.