Hurricane Irene Approaches the Bahamas

By: Dr. Jeff Masters , 7:42 AM GMT on August 24, 2011

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As of 2AM EDT, Hurricane Irene was located at 21.3N, 72.6W, 400 miles southeast of Nassau. It was moving west-northwest at 9 mph with maximum sustained winds of 100 mph, making it a Category 2 storm on the Saffir-Simpson scale. Irene has a minimum central pressure of 966 mb. Hurricane warnings have been issued for all of the Bahamas and the Turks and Caicos Islands. Tropical storm warnings are in effect for Haiti from Le Mole St. Nicholas to the Dominican Republic border.

Satellite Views
Figure 1 shows that Irene has a large eye visible in infrared imagery, (26 miles across accoring to a report from the Hurricane Hunters at 130AM) with well-defined outflow cirrus bands. Tuesday evening, TRMM, NASA's tropical research satellite, flew directly overhead Irene, getting a radar scan of the storm using it's downward pointing radar, shown in Figure 2. It is immediately apparent that Irene has well-developed bands of rain showers, with strong storms present in the eyewall.


Figure 1 IR satellite view of Irene taken at 113AM EDT, August 23, 2011


Figure 2 TRMM radar overpass of Irene at 713PM EDT, August 22, 2011. Image courtesy of the Naval Research Laboratory

Track Forecast

Irene is forecast to move to the northwest, passing over all of the Bahamas by Thursday evening, then curving to the north. Irene then makes landfall in the US near or at the Outer Banks Saturday afternoon, then traveling along the mid-Atlantic coastline of the US. After Saturday, Irene may pose a threat to Long Island and the New England coastline. However, NHC is quick to remind us that the average forecast error for day 4 is 200 miles, so don't stop your hurricane preparations if you aren't in the immediate area of landfall. It is also important to note that the windfield of Irene is expected to be large, affecting areas distant from the immediate track of Irene's center. Tropical storm forces winds are expected to be found out to at least 150 miles away from Irene's center on Friday afternoon.

NHC is forecasting for Irene to become a major hurricane (winds faster than 110 mph), within 24 hours.


Figure 3 Official track forecast of Irene at 2AM EDT.

Forecast models and adaptive observations
The different forecast models are in fairly good agreement about Irene's track through the Bahamas and along the east coast of the US. The GFDL, a dynamical hurricane forecasting model, which had been a western outlier from the other models is now agreeing with them. When a set of weather models using slighly different initial views of the atmosphere and slightly different ways of simulating how the atmosphere works all agree on a forecast, then meteorlogists tend to believe that the forecast is more likely to occur. The 00Z GFS and ECMWF (wind swath shown in Figure 4) forecasts are nearly identical, which furthur boosts forecaster confidence.

In the 11PM forecast discussion, the NHC forecaster praises the NOAA Gulfstream IV (aka Gonzo) for providing infomation about the atmosphere around Irene that will influence it's track. Looking at the plan of the day valid for today, it will be a busy day for airborne reconnaissance. Three flights for the Air Force hurricane hunters, two flights for the Gulfstream IV, and two flights for NOAA 42, a WP-3D (aka Kermit).


Figure 4 Plot of the maximum sustained winds in mph over the next week from the 00Z ECMWF forecast.

Impacts

In the immediate future, Irene will have a significant impact on the Bahamas and surrounding islands. Hurricane force winds are ongoing over the Turks and Caicos islands and southeastern Bahamas. These locations can expect storm surges that are 5-8 feet above tide levels. The northwestern Bahamas can expect hurricane force winds Thursday, and storm surges that are 7-11 feet above tide levels. The Turks and Caicos islands and all of the Bahamas can expect 6-12 inches of rain over the next two days.

I still think people living along the east coast of the US should closely monitor Irene and review their hurricane preparations. Irene will be a large storm, impacting areas far from the storm center track.

Dr. Masters will have a new blog entry this morning, and Angela Fritz will be covering the afternoon. I'll be back on third shift tonight.

Thanks for reading,

Dr. Rob Carver

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Quoting winter123:
Irene is right on track, actually.
I was speaking about the 11PM track which she was slightly west of.

That is the 2 AM track.
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Invest 90L




The next tropical wave to watch after Irene passes is a wave just south of the Cape Verde islands. An ASCAT pass from 23z (9 hours ago) found a closed (although broad) surface circulation. Convection is firing nicely with this wave, however, the mid level center and low level center (according to the ASCAT pass) are not aligned with each other at the moment.

Looking at the rest of the environmental conditions, shear is light to moderate and shouldn't be a big deal. Upper divergence is a bit of an issue, however. The upper level wind field over 90L is almost completely out of the east, preventing air from spreading out aloft. Dry air and the oceanic environment are both slightly favorable, but not exactly ideal for strong development.

Whether or not it will actually develop is no guarantee. The NHC currently gives it a 40% chance of development in the next 48hrs. However, given the ASCAT pass and nice convection, I'd say the chances of it ever developing are more likely than not. Looking at the models, the GFS develops it and peaks it out as a 997mb tropical storm, while the ECMWF drops it off. Both models forecast it kind of just sitting around off the Cape Verde islands under light trade winds (due to a weak pressure gradient) and then eventually being pulled out to the north by a trough next week.
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Quoting Scottcenfla:
I guess my concern is the forward motion of Irene is slowing down a little and I surely don't want to see it slow down to the point I've seen so many storms do in the past and have a very slow forward motion or even stall out.

We had, I think it was Fay, a few years back that stalled off the east coast of Florida for 3 or 4 days dumping almost 2 feet of rain in some places.

Couple of questions for some of the more experienced bloggers on here. It does appear that a UUL is circulating off the coast of Georgia/SC. Will this UUL get stronger? And if so, how will this effect the steering pattern of Irene?


Its an inverted upper level trough that was created by the void left behind while the mean longwave trough flattened out and the mid/upper high to the E built back in and detached a piece of that energy. I don't expect it to cause problems to the track since it will most likely be sheared apart by the next shortwave impules riding down the mean trough.
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Irene is right on track, actually.
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Quoting Scottcenfla:
I guess my concern is the forward motion of Irene is slowing down a little and I surely don't want to see it slow down to the point I've seen so many storms do in the past and have a very slow forward motion or even stall out.

We had, I think it was Fay, a few years back that stalled off the east coast of Florida for 3 or 4 days dumping almost 2 feet of rain in some places.

Couple of questions for some of the more experienced bloggers on here. It does appear that a UUL is circulating off the coast of Georgia/SC. Will this UUL get stronger? And if so, how will this effect the steering pattern of Irene?


I had noticed it last evening around 10pm on the other blog post, and no one answered until I raised it again a couple hours later... then everyone started to take notice of it. It did show up very suddenly, as it had not been there earlier in the evening, thus the models had not picked up on it at all. It is hard to say at this point what impact, if any, it will have on the strength/direction of Irene at this time. I know the ridge has been building WWD since yesterday evening, hence some of the WWD direction of IRENE overnight.
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Quoting StarnzMet:
Nothing about a Florida impact! Put that pipe dream to bed wishcasters!!! Thank you Dr. Carver


He only wanted to mention official information. Not go of course with a possible theory which is perfectly understandable.
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Quoting emguy:
Let's just nope the G-IV Jet notices the very solid cutoff low that has developed and settled north of Irene (south of the Carolinas) when they go out there tomorrow. Might also be smart for the NWS to expand 4 times a day baloon operations further west to sample the diving trough, since the Texas ridge has collapsed and the weakness is diving further west. Something that was not measured...


G4 is already out their now getting data for advisories later today.
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Quoting emguy:
Let's just nope the G-IV Jet notices the very solid cutoff low that has developed and settled north of Irene (south of the Carolinas) when they go out there tomorrow. Might also be smart for the NWS to expand 4 times a day baloon operations further west to sample the diving trough, since the Texas ridge has collapsed and the weakness is diving further west. Something that was not measured...


There is already a G-IV out there this morning dropping sondes out east of FL.
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IDK about no FL impacts... severe beach erosion is likely. Squalls with very heavy rain and gusty winds likely along the east coast. This per the 345am AFD out of Melbourne, FL. Also mentioned that per the 11pm advisory, Irene will parallel the ECFL coast about 200 miles out. As it is, TS winds extend out 205 miles, and can only expect that they will expand some as it deepens and lifts northward. They do have a Tropical Storm Watch (for marine interests ONLY) at this time from Sebastian Inlet south to Ocean Reef, FL. And of course, all that could change IF she tracks farther west than currently progged.

I would say minor impacts. Landfall, not likely (I would say less than 10% chance at this point)... but not 0%. Most likely scenario at this point is NC.
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I guess my concern is the forward motion of Irene is slowing down a little and I surely don't want to see it slow down to the point I've seen so many storms do in the past and have a very slow forward motion or even stall out.

We had, I think it was Fay, a few years back that stalled off the east coast of Florida for 3 or 4 days dumping almost 2 feet of rain in some places.

Couple of questions for some of the more experienced bloggers on here. It does appear that a UUL is circulating off the coast of Georgia/SC. Will this UUL get stronger? And if so, how will this effect the steering pattern of Irene?
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Quoting StarnzMet:


Black Hawk Down



We have a winner!!!!!
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Let's just nope the G-IV Jet notices the very solid cutoff low that has developed and settled north of Irene (south of the Carolinas) when they go out there tomorrow. Might also be smart for the NWS to expand 4 times a day baloon operations further west to sample the diving trough, since the Texas ridge has collapsed and the weakness is diving further west. Something that was not measured...
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Quoting atmosweather:


Turn on the tropical forecast points and you will see that she is literally exactly on the forecast track. All of the wobbles that she has been through still average out to a track that is right in line with the model consensus and the NHC forecast.


Gotcha
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Quoting HimacaneBrees:
Mission launch codeword is "Irene". Anyone know what movie that's from?


Black Hawk Down
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Quoting HimacaneBrees:


Irene will have to make a fairly quick northerly turn to the to be at the 8 am forecast point.


Turn on the tropical forecast points and you will see that she is literally exactly on the forecast track. All of the wobbles that she has been through still average out to a track that is right in line with the model consensus and the NHC forecast.
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Thanks Doc!!
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Mission launch codeword is "Irene". Anyone know what movie that's from?
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Irene will have to make a fairly quick northerly turn to the to be at the 8 am forecast point.
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Thanks Rob, great summary.
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Guys I am having a feeling this will track much closer to the Georiga and South Carolina coast than they have been expecting recently. It will be very interesting to see it all play out but I am becoming more concerned about the Georgia/South Carolina coast. That trough is not as strong and deep so far and the ridge is very strong and much further to the west. I think this storm is still having struggles strengthening could influence it as well. We must not forget that the most westward section of the forecast path takes it into Georgia and South Carolina.
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Irene





Looking at Irene on the water vapor loop, Irene's eye has once again become partially covered up, partially obscuring the eye. Although, the eye has always been ragged and never really looked all that impressive, so this isn't a very significant observation. Also, this appears to be a result of the core of convection trying to throw around the convection evenly around the center, whereas before it was focused to the west and east. Convection with the core has leveled off some and cloud tops have warmed ever so slightly. Once again, however, the core of convection does appear to be trying to throw out the convection more evenly around the center, so although thunderstorm intensity may have dropped a little, organization has improved. Also, I expect more intense thunderstorms to fire up again soon. In the upper levels of the atmosphere, an upper level trough to the west of Irene (resulting dry air and upper convergence) has been hassling Irene for sometime now. The effects of this aren't very significant, but it has limited convection and especially outflow. Shear is also moderate over the storm as a result of this trough feature displacing the upper level anticyclone over Irene.

Despite these less than perfect upper level condition on the west side of the storm, dry air and upper convergence are not present around the rest of the storm and the oceanic environment is very warm and as a result continued intensification should occur up until Irene peaks on Thursday. From there, gradual weakening will occur until it passes the Carolinas, after which point weakening will pick up as a result of land interaction, cooler SSTs, less moisture, and greater shear.

After the Bahamas region gets blasted, people in the Carolinas, especially North Carolina, should prepare for a major hurricane. After the Carolinas, the mid-Atlantic states will receive stronger winds than the New England states, however, the New England states are looking like they may have to deal with a direct landfall, whereas the mid-Atlantic states will receive a graze. Also worth mentioning is that as the storm weakens and the peak winds drop, the wind field will expand, meaning a larger area will be affected.
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Thank you Dr. Carver for the early am update. Thank you wunderground for the coverage!
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Thanks Rob
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Nothing about a Florida impact! Put that pipe dream to bed wishcasters!!! Thank you Dr. Carver
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Wow. Late night/early morning update. Thanks Dr. Carver!
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Thank you, Dr. Carver! What a storm.
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THANKS DR. C.
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Thanks for the update, Dr. Carver! It'll be an interesting 120 hours, for sure...
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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.