Aug 26, 2011: Final Hurricane Irene Forecast

Notes:

– Even though the 5-Day Forecast was not changed, the forecast is still valid and was used for this update. The forecast for each part of the area was described in more details below.

– The poll for Irene’s impact in NYC is still open. This poll will close on Saturday afternoon.

– Frequent updates will be posted in this page as well as the tropics page throughout the day tomorrow and on Sunday, which will include any potential changes to the forecast.

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Hurricane Irene has been a very difficult storm to forecast up to this point. Constant changes in the model guidance resulted in uncertainty with Irene’s track, and Irene’s unexpected weakening today could result in a slightly better scenario for the East Coast than previously thought. Regardless of these issues with Irene and its forecast, Irene is a very large hurricane with its impacts being felt far away from the center of the storm, and will be the most significant tropical cyclone to affect the area since Hurricane Floyd in 1999.

Latest Information/Forecast For Hurricane Irene

As of NHC’s 8 PM update, Irene is located south of North Carolina with 100 mph sustained winds and a minimum pressure of 950 millibars. Irene is moving to the NNE at 14 mph. Irene’s outer rain bands are currently affecting the eastern half of North Carolina with heavy rain along with isolated tornadoes, and Irene’s center is starting to appear in the North Carolina radar.

Irene’s Intensity: Since yesterday, Irene has failed to improve in organization. This morning, Irene unexpectedly weakened into a category 2 hurricane while east of Florida; Irene was expected to maintain category 3 intensity until reaching North Carolina. This trend continued throughout the afternoon hours as Irene’s core became disorganized and an eye was no longer evident. Dry air and shear affected Irene, and it weakened at 2 PM to a weak category 2 hurricane with 100 mph sustained winds while east of South Carolina; Irene was not expected to reach this intensity until reaching Delaware’s latitude. As of this evening, Irene is attempting to reorganize its structure, and its pressure dropped by 1 millibar, but Irene has failed to reorganize its core and form a well defined eye. Due to Irene’s large size, any minimal drop in pressure is unlikely to result in any large increase in wind speed, and as a result of the above and Irene running out of time over water, Irene is expected to maintain its intensity, if not slightly weaken to a strong category 1 hurricane with 90 mph sustained winds, by the time that it makes landfall in North Carolina early on Saturday afternoon.

After making landfall in North Carolina, Irene will encounter colder sea surface temperatures, and is expected to begin weakening. At the same time, Irene will be interacting with a trough and will begin to become extra-tropical. Due to Irene’s very large size, only slow weakening is expected, and Irene is expected to maintain category 1 intensity throughout Saturday. There is some uncertainty on Irene’s intensity when making landfall near the NYC area, which will depend on how close to the Mid Atlantic coast Irene tracks as well as its intensity during the day tomorrow. At this time, Irene is expected to make landfall in western Long Island or NYC as a strong tropical storm, with sustained winds around 70 mph, but it is possible that Irene could be slightly weaker or stronger, with sustained winds between 65 and 75 mph possible when Irene makes landfall.

Irene’s Track: Irene’s track has been uncertain over the last couple of days, but a much clearer model consenus has finally emerged today, which allows for a higher confidence forecast for Irene’s impact in the region and the area. From Irene’s current location, a NNE movement is expected, which will take Irene into eastern North Carolina early on Saturday afternoon. From there, Irene is expected to move NNE right up the coast, making landfall in western Long Island. Some models are still showing an eastern Long Island landfall scenario or a storm moving through western New Jersey, but there is a much clearer model consensus taking Irene through western Long Island, with a decreasing number of models showing other scenarios. Even though the models agree on a specific track, these are only computer models, and it is possible that Irene may slightly move to the west or east of the expectation, but regardless, no significant change in the forecast track is expected.

Timing Of Irene’s Impact In NYC Area

At this time, Irene’s track has been nailed down for the most part, moving up eastern New Jersey before moving over NYC or western Long Island. There is no doubt that Irene will cause heavy rainfall over 6 inches, which will cause major flooding and lead to record breaking rainfall in the area, and that Irene will cause windy conditions in the area. The main uncertainty at this time is Irene’s intensity when it reaches the area, which will determine how strong the winds will be across the area, and the exact spot where Irene makes landfall, which will determine which place has the strongest winds.

Saturday: Mostly cloudy skies are currently covering the area. These clouds are already a part of Irene, whose center is still south of North Carolina, showing how large the hurricane is. Irene’s outer rain bands are expected to begin affecting the area on Saturday afternoon as scattered showers and thunderstorms begin to reach the area. These storms will mainly be scattered, but will be capable of causing heavy rainfall. These bands will be moving in from the ESE, or from the Atlantic Ocean, moving WNW and then west, as they rotate around the center of Irene. The wind will not be much of an issue tomorrow with a light SE/ESE wind expected, though breezy conditions should begin to develop by the late afternoon and evening hours. Rainfall amounts between 1/4 and 1/2 inch are expected across parts of the area by tomorrow evening, with locally higher amounts in places that get affected by these storms.

Saturday Night – Sunday: Deteriorating conditions are expected by Saturday evening after 8 PM, with increasing ESE/east wind. Moderate to heavy rain bands ahead of Irene will begin to move into the area after at least 10 PM-12 AM, and will steadily increase through the rest of the overnight hours, with the heaviest rain focusing from NYC and further west. By the early morning hours, a steady heavy wind-driven rain is expected across the area.

The worst of Irene will take place during Sunday morning into at least 2-4 PM. During this time, winds will continue to increase coming out of the east/NE. The rain is expected to continue intensifying from NYC and further west, and in Long Island/S CT, which should be east of Irene’s center, the rain should still be heavy but not as heavy as in places west of NYC. Irene’s center will make landfall in western Long Island or NYC around the early-mid afternoon hours, and once it does so, the rain should begin to weaken from north to south across the area. By the evening hours, with Irene’s center already in New England, the rain should end across the area with diminishing winds.

Irene’s Rain/Wind Impact

Irene’s Rain Outlook: The rain from Irene will be a major concern, as parts of the area have already seen a foot of rain this month, and an additional foot of rain from Irene alone could fall in the same areas, bringing rain totals this month over 20 inches, a very unusual event for NYC. Looking at Irene’s rain totals, most of the area should see over at least 4-6 inches of rain, with the lightest amounts in the easternmost and westernmost parts of the area. At least 3 to 7 inches of rain should fall in NW NJ and Orange county in NY, 6 to 12 inches in the immediate NYC area, and 5 to 10 inches of rain in Long Island and southern Connecticut. These rain totals will produce major flooding, especially near rivers and in the coastal areas, where Irene’s storm surge will also result in coastal flooding.

Wind Outlook: Irene’s wind forecast is more uncertain, and depends on Irene’s uncertainty when it makes landfall in the area. Given the current expectation of a 70 mph landfall in western Long Island or NYC, I am expecting sustained winds between 35 and 50 mph in western New Jersey and Orange county in NY, with gusts up to 55-65 mph possible. In the immediate NYC area, sustained winds between 50 and 65 mph are expected, with gusts up to 65-80 mph, especially in NYC. Long Island and southern CT should see windy conditions as well, with sustained winds between 50 and 70 mph. Gusts between 65-85 mph are expected. Near the location where Irene makes landfall, wind gusts up to 90 mph may be possible. This wind outlook has a slightly lower confidence level than the rain outlook, and it is possible that the wind forecast may have to be adjusted tomorrow to show more or less wind depending on Irene’s intensity.

Irene is a very large and dangerous storm, and should not be taken lightly. Impacts will be felt in the area, ranging from large waves to extreme rainfall amounts to strong wind gusts, with results being significant damage in the area including downed trees and widespread power outages. Stay tuned for frequent updates on Irene in the main page and in the Tropics page throughout the day tomorrow and on Sunday, which will include any potential changes to the forecast.

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