Bryan Norcross' Official Blog

Bad and Even Worse

By: bnorcross, 3:20 AM GMT on August 30, 2012

I was looking over last night's blog and realize now all of the hazards I didn't list. I was concentrating on the effects in southeast Louisiana, which all came to pass except that Grand Isle seems to have lucked out... or we haven't heard yet.

My rainfall estimates look low... I said up to 20 inches and that much fell just today at Audubon Park in New Orleans. It looks like some areas will get 30 inches of rain or more!

And then there's the unrelenting severe weather in Mississippi and some in Alabama as well. I expected heavy rain and wind there, but a band set up near Gulfport and Biloxi and arced up to then north all day. Communities in that channel of storms got pounded. To a somewhat lesser degree the same thing happened near Mobile.

Major storm surge flooding hit the western Mississippi coast and the area east of New Orleans outside the levee. And... another thing I didn't mention, it looks like there will be significant flooding around Lake Ponchartrain as well.

A couple of days ago, I talked about the nightmare scenario of people not evacuating or taking protective measures because Isaac was a disorganized tropical storm. To make it worse, it was a no-show in Key West. Indeed, that happened in Plaquemines Parish. It's incredibly lucky that nobody died when the levee there was topped and the water went up to people's roofs.

No doubt untold tens or hundreds of thousands of people are going to wish they had more batteries and food and other storm supplies since they'll be trapped at home with no power and trees down all around them. Travel isn't going to be possible in many areas for at least a few days due to downed trees and flooding.

Isaac is a monstrous vortex with a tremendous amount of momentum (think a huge, heavy spinning top). Even with the friction of the ground, it's going to take a long time to spin down. Making it even worse, in the Gulf of Mexico the storm had more rain on its back side than its front due to the dry air it was encountering. That is still the case. Hopefully some of that will weaken as the storm moves north, but there is a LOT of the tropical circulation still to come ashore.

The last 20% of a storm like this is always the worst. Trees, bridges, sea walls, levees, and anything else subject to the pounding wind and/or rising water is more likely to fail or be topped at the end of the storm. I'm expecting more and more evacuations and reports of neighborhoods isolated because of the wind, rain, and surge effects of the storm.

I'm hoping for the best... but fearing the worst.

Hurricane

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Isaac as Scheduled... Unfortunately

By: bnorcross, 4:27 AM GMT on August 29, 2012

Hurricane Isaac is pretty much on schedule and delivering as feared. Storm surge east of the New Orleans proper, outside of the hurricane levees is extremely high, and major damage is expected. The western end of the Mississippi Gulf coast is also getting extremely high water, with lesser amounts farther east. The surge flooding is comparable to a healthy Category 2 hurricane due to Isaac's large size and slow movement.

The parishes near the Gulf of Mexico are going to get pounded by the worst of Isaac. We may see significant damage in Grand Isle on the Gulf coast.

Coastal flooding is still expected as far east as Apalachee Bay at the eastern end of the Florida Peninsula. In addition, significant storm surge flooding in some of the non-protected areas around Lake Ponchartrain is expected, since the winds will blow for a long time from the same direction, which will get the water moving.

If the track is anything like the computer models are forecasting - including the RPM Future Radar (an in-house Weather Channel model) - there will be an extended period of persistent rain, very heavy at times, over New Orleans and southeastern Louisiana. The center of Isaac will likely track to the west of the city, and the comma-shaped tail of tropical moisture will align over or near the metropolitan area and areas to the east of the center's track. Rainfall forecasts of 12 to 20 inches may be exceeded in some areas, so significant flooding from the rainfall is guaranteed.

The highest winds in the storm bands should decrease through the day tomorrow (Wednesday), though it will take a long time to wind down this monstrous wind machine. With a pressure of 968 millibars, it's like a deep well that has to be filled to calm the wind, and it can only be filled so fast. Expect windy weather to continue in the Louisiana and Mississippi impact zone on Thursday as the system moves toward Arkansas and points north.

Let's hope that people behaved well and got out of the surge zones. Otherwise, they are in a seriously bad situation.

See you on The Weather Channel.

Hurricane

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Isaac as Scheduled... Unfortunately

By: bnorcross, 4:17 AM GMT on August 29, 2012

Hurricane Isaac is pretty much on schedule and delivering as feared. Storm surge east of the New Orleans proper, outside of the hurricane levees is extremely high, and major damage is expected. The western end of the Mississippi Gulf coast is also getting extremely high water, with lesser amounts farther east. The surge flooding is comparable to a healthy Category 2 hurricane due to Isaac's large size and slow movement.

Coastal flooding is still expected as far east as Apalachee Bay at the eastern end of the Florida Peninsula. In addition, significant storm surge flooding in some of the non-protected areas around Lake Ponchartrain is expected, since the winds will blow for a long time from the same direction, which will get the water moving.

If the track is anything like the computer models are forecasting - including the RPM Future Radar (an in-house Weather Channel model) - there will be an extended period of persistent rain, very heavy at times, over New Orleans and southeastern Louisiana. The center of Isaac will likely track to the west of the city, and the comma-shaped tail of tropical moisture will align over or near the metropolitan area and areas to the east of the center's track. Rainfall forecasts of 12 to 20 inches may be exceeded in some areas, so significant flooding from the rainfall is guaranteed.

The highest winds in the storm bands should decrease through the day tomorrow (Wednesday), though it will take a long time to wind down this monstrous wind machine. With a pressure of 968 millibars, it's like a deep well that has to be filled to calm the wind, and it can only be filled so fast. Expect windy weather to continue in the Louisiana and Mississippi impact zone on Thursday as the system moves toward Arkansas and points north.

Let's hope that people behaved well and got out of the surge zones. Otherwise, they are in a seriously bad situation.

See you on The Weather Channel.

Hurricane

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Today Is the Day

By: bnorcross, 11:33 AM GMT on August 28, 2012

Isaac looks like a hurricane this morning, with an eye-like feature apparent on the satellite. Dry air is continuing to wrap into the center, so its a balance between the many factors that are favorable for strengthening, the dry air, and the limited time the system has left over the water. As I noted last night, the coastline effect can add a little as well. It's looking like a Category 1 at landfall.

The track is pretty clear now. The center should come ashore near the Mississippi River Delta, the far southeastern tip of Louisiana. This track will put the highest winds over New Orleans that the city has seen in decades. And, it will be relentless. Winds will pick up during the day today with the strongest winds in the city this evening and overnight. Then the pounding will only slowly recede tomorrow.

Strong onshore winds will also affect the Mississippi, Alabama, and Florida Panhandle, but the track of the center into Louisiana will mean the winds farther east won't be perpendicular to the coast, which will keept the storm surge heights a little less that they would have been with a track farther east. Still a maximum of 4 to 8 feet of water above the land is forecast for the Alabama coast, and 3 to 6 feet in the Panhandle. Storm surge flooding is continuing to affect low lying areas in the Florida Peninsula due to the large area covered by the circulation.

The highest storm surge - the forecast continues to be 6 to 12 feet - will be on the Gulf side of southeast Louisiana and in Mississippi. This won't be Katrina, but life-threatening high water will be pushed over the coast today and tonight. The onshore winds will keep the water high for an extended period.

Low-lying areas inland will be especially dangerous due to flooding rains that can't drain at their normal rate. Do NOT drive through flooded roadways. The water will be moving faster than normal, and it only takes six inches of water to move a car.

Let's hope we get lucky... but the best evidence is that luck is going to hard to find. See you on The Weather Channel.



Hurricane

Updated: 11:52 AM GMT on August 28, 2012

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Ready for Isaac

By: bnorcross, 2:10 AM GMT on August 28, 2012

Isaac is less than 24 hours from its center's landfall on the Louisiana or Mississippi coast. The main computer-forecast models are finally in reasonable agreement on the location, although the small differences would change exactly who gets the worst storm surge.

The center has jogged a bit to the right tonight, likely due to the asymmetry of the system. If that trend continues, the surge will be higher in Mississippi, and the surge on the south-facing coast of Louisiana would be much less. This far out, however, more slight variations are possible.

The somewhat good news is that dry air continues to invade the core of the system, which is inhibiting or at least tempering the intensification process. Large-diameter storms like this generally intensify more slowly, as well. On the other hand, the pattern aloft looks very favorable for strengthening tomorrow, the heat content of the water will go up some, and the friction of the circulation's initial interaction with the coast can turn the winds in toward the storm's center kicking up the wind speed. Given the short window of time left before landfall, it looks most likely that Isaac will come ashore as a Category 1 storm, though Category 2 can't be ruled out.

The bad news is that the models are in lockstep on the LONG duration that the storm will batter areas near the coast, including the city of New Orleans. Tropical-storm force winds may continue over southeast Louisiana and Mississippi for 24 hours or more, with hurricane force winds for 8 hours or more. The storm is expected to move very slowly once it makes landfall, with tropical-storm force winds, at least in gusts, continuing into Thursday as the storm center moves away to the north.

If the storm tracks over or just west of New Orleans as is forecast, the heart of the city will experience stronger winds than it did in Katrina. And the duration of the strong winds will be far far longer.

The dry air in the circulation may temper the rain totals a bit, though there's no way to know it won't mix out before landfall. in any case, a foot or more of rain will fall along the coast and inland.

The levee and pumping system in New Orleans is NOT designed to keep the city dry in all eventualities. It's designed to keep the city safe. If a foot or more of rain falls on the area inside the levee, there will be flooding, just like there would be just about anywhere else. The difference is that the water is PUMPED out of New Orleans - one inch in the first hour and one-half inch each hour after. So people still need to use common-sense precautions in low-lying areas.

Elsewhere, the combination of heavy rain and storm surge is going to create an enhanced flood threat. The storm surge will push Gulf water up rivers, creeks, and inlets where the wind is onshore. The rainwater will want to drain out those same waterways. The storm surge will win and the rain water won't have anyplace to go.

Life-threatening storm surge will affect the coast for hundreds of miles east of where the center comes ashore. And, the slow movement is expected to hold the Gulf water over the coast and into inlets and bays for at least two high-tide cycles. The amounts at any one location will vary with the exact final track, but be ready for water higher than 6 feet above the land from Louisiana to Alabama, and dangerous surge in the Florida Panhandle as well.

The best we can hope for is that Isaac doesn't intensify, although at least some intensification is expected.

There is very little time to take precautions that will make a tremendous difference in how uncomfortable the period during and after the storm will be. Follow emergency instructions, use common sense, and stay safe.

Hurricane

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Glimmer of Good, Mostly Bad

By: bnorcross, 11:49 AM GMT on August 27, 2012

The only glimmer of good news this morning is that Isaac is still strengthening very slowly. Although an eye is evident on the satellite - and in the radar aboard the NOAA research aircraft - the winds are not increasing dramatically in the huge sprawling circulation. Still, steady strengthening is likely as the center heads toward the Louisiana or Mississippi coast. The NHC is now forecasting Isaac to be a Cat. 1 at landfall, though a Cat 2 certainly can't be ruled out. (The NHC gives it a 5% probability of reaching Cat 3.) Huge circulations take longer to spin up, and this one doesn't have a lot of time.

The nightmare scenario of a long-duration landfall I wrote about last night continues. The forecast consensus is that the storm will slow to a crawl near southeastern Louisiana keeping southeast to east winds into the corner between Louisiana and Mississippi, and southerly winds against southern Alabama and the western Florida Panhandle, for an EXTENDED period of time. This will continue to push Gulf water over the coast and into Mobile Bay and other inlets for longer than normal in a landfalling hurricane.

New Orleans has decided NOT to order an evacuation, trusting the new levees. It's important to remember, however, this storm could well put much worse weather over New Orleans than Katrina. If you recall, dry air was filtering into the left side of Katrina when it made landfall. There was very little damage from wind in the city. Top readings were mostly of tropical storm strength, except on the far east side. Issac may put higher winds over the city proper meaning people will have to take precautions in their homes that were not required in Katrina.

Other areas outside the massive new levee system WILL require evacuations, however. It's extremely important that people take action today to get to a safe, comfortable place with plans and supplies to ride out a relentless attack of wind, rain, and high water from Isaac.

See you on The Weather Channel.

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A Nightmare Scenario

By: bnorcross, 3:07 AM GMT on August 27, 2012

What a horrendous confluence of events. Tropical storms never get on people's radar, and the fact that Isaac passed Key West with less effect than a gusty afternoon thunderstorm made the situation worse. Even with a Cat 1 hurricane, will people take action tomorrow that may save their life Tuesday night?

Isaac's extremely large circulation is one of the factors that should make it intensify relatively slowly, and a track that misses the high heat-content pools in the Gulf should help in that direction as well. Plus it has less than two days over the water before landfall. But, the water ahead is still plenty warm - over 85 degrees - and the atmospheric pattern looks very favorable for strengthening. Taken together, the NHC's Cat. 2 forecast looks reasonable, though preparations should be made for a Cat. 3, given the lack of skill inherent in intensity forecasting.

In any case, that same large circulation will move a lot more water toward the coast than an average hurricane. The entire Hurricane Warning area - from Louisiana to the western Panhandle - is extremely vulnerable to storm surge flooding. The NHC is forecasting 6 to 12 feet of water ABOVE THE GROUND in spots along that stretch of coast.

Normally, the exact amount of surge will be dependent on the exact track and where in the tide cycle the storm comes in. In Isaac's case, however, there will be such a long duration of onshore winds, due to the storm's size and slow forward speed, the water may stay high for more than one tide cycle.

In fact, that size and slow forward speed will make this a hurricane experience like none in memory, if it comes together as forecast. The weather will deteriorate tomorrow (Monday) in the Florida Panhandle, and tomorrow night across the rest of the northern Gulf coast. Then Tuesday the storm approaches the coast and a whole day later it is just inland on the current timetable. Even Thursday there is still onshore wind over part of the coast.

People will experience strong, howling winds and torrential rain for 24 to 36 hours or more in this scenario. They will be trapped at home, many without power.

And then there's New Orleans. The storm as currently forecast should NOT exceed the capabilities of the new super-strong levees, but what if the storm comes in stronger? What a decision to have to make! Should they call an evacuation or not? In addition, there are populated areas outside of the levee-protection system.

Tomorrow (Monday) will be a day for big decisions from Louisiana to the Florida Panhandle. With life-threatening storm surge expected well away from the landfall point, widespread evacuation orders are likely.

And then there's the inland flood threat from the torrential rain.

Unless something unexpected happens, it's truly a nightmare scenario.

Continuing coverage, of course, on The Weather Channel.

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Isaac On Track and On Schedule, But Questions Remain

By: bnorcross, 11:44 AM GMT on August 26, 2012

Isaac is behaving as expected so far today. Bands of increasingly windy squalls will move across the Keys and southeast Florida through the day. The storm surge against the Keys will peak in the late afternoon when the center come over or near Key West. The tide will be near high tide then, so the water will rise in the areas that were flooded by Hurricane Georges in 1998. Perhaps not to the same level, but be ready. Heed all emergency advice.

Waves of squalls with some winds to 60 mph or more will also move over the Miami-Ft. Lauderdale metro area. The best advice is to stay inside until the storm passes.

Luckily, Isaac will only slowly organize, so no surprises in intensity are expected.

The storm surge questions remain for the west coast. The NHC has lowered their maximum estimate for southwest Florida to 4-6 feet... still extremely threatening to anyone near the water. The 3-5 foot estimate remains for areas in and around Tampa Bay. The severity of the surge in these areas will be highly dependent on timing. If the peak surge comes at high tide, it will be significant and dangerous. The question of the surge being enhanced by a shelf wave is also still open. Heed emergency advice along the west coast!

There is no obvious reason why Isaac shouldn't strengthen once it gets into the Gulf. How much it strengthens depends a bit on the exact track. If it tracks just west of the coastal shelf, the water gets very deep so that cooler water can be churned up by the storm. Over the shelf or farther off into one of the warm pools and the cooling effect won't occur. In any case, the central northern Gulf coast should be thinking about a Category 3 hurricane under the theory that you always prepare for one category higher than forecast, knowing that intensity-forecasting skill is not high.

The track spread in the models continues from last night.

See you later today on The Weather Channel.

Hurricane

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Isaac's Forecast Challenges

By: bnorcross, 2:51 AM GMT on August 26, 2012

Isaac is on track tonight, and the center should be in the vicinity of the Key West tomorrow (Sunday) afternoon. The tropical core is embedded in a huge rotating circulation that extends hundreds of miles across, which has been a distinguishing feature of Isaac since its pre-depression days. This is why warnings and watches have been posted for most of the Florida peninsula.

The strength of Isaac when it gets to the Keys in tricky. In favor of strengthening: it's over very warm water, moving away from land, strong thunderstorms are wrapping around the center, and the upper air is moderately okay. Working against it: big circulations intensify more slowly, part of the circulation is over land, thunderstorms have developed each night and weakened, and the upper air is only moderately okay. It's likely to be at or near hurricane strength when it gets to the Keys, but no rapid intensification is expected.

This storm brings back memories of Hurricane Georges in 1998, though Georges was a bit stronger and more organized when it ran into eastern Cuba. The center went over Key West as a Cat 2, and pushed damaging storm surge against the Keys. Folks in the Keys should be ready for a Georges-type event, and hope Isaac doesn't get that strong, which it probably won't.

Then there's the storm surge forecast for the Florida west coast. Storm surge science is excellent. Given a certain intensity, track, and radius of maximum winds... the surge in affected locations can be forecast accurately. The problem is, you rarely know those things very far in advance, and slight differences in any of the parameters can make a big difference in the surge.

The NHC is forecasting a worst case of 5 to 7 feet of surge on the southwest Florida coast and 3 to 5 feet in and around Tampa Bay. If the storm arrives at high tide, and the storm intensifies as forecast, these worst-case values could be reached. That's seriously high water for low-lying areas near the water, of which there are many. This is a potentially very dangerous situation.

But, as discussed yesterday, if the storm takes just the right (wrong) track, like Hurricane Dennis in 2005, a wave can set up in the shelf water accentuating the surge along the west coast and even more so in the Panhandle. This will have to be watched very closely. Again, little variations and it doesn't happen. Here's a simulation from the NHC that shows Dennis' surge-enhancing wave develop over the coastal shelf and travel north.

The storm surge from Dennis is below.


And then there's landfall. The trend in the computer forecast models has been to move west toward Mississippi and Louisiana, with the European now the outlier to the east. Previously it was the outlier to the west. This is why the cone is wide at 3 days out.

There is a big high pressure system over the Rockies and Plains that is forecast by the American GFS model to build across the South just as Isaac gets near the northern Gulf coast. That blocks the storm and pushes it westward. The European doesn't build the high as strongly or quickly east. Normally we would trust the European model for atmospheric-pattern forecasts, but it's been especially flakey this year, so we take the average as a good forecast, and admit that we don't know.

The NHC is forecasting a Cat 2 storm at landfall on the northern Gulf coast, but it's easy to see how things could come together to make it much stronger. Everybody from the Florida Panhandle to Louisiana should be on alert. Pay attention... and be prepared!

See you tomorrow on The Weather Channel.

Hurricane

Updated: 3:27 AM GMT on August 26, 2012

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Extended Period of Bad Weather in Florida Begins

By: bnorcross, 11:44 AM GMT on August 25, 2012

Not unexpectedly, Isaac seems to be surviving its trek across Haiti with it's circulation intact. In moved just far enough west for its core to miss the highest mountains, although the Hispaniola and Cuban land masses will have some tempering effect for the next day or so. The upshot is... the Florida Keys need to get ready for hurricane conditions immediately, including 1 to 3 feet of storm surge flooding at high tides. The worst of this flooding will not occur until Monday when the storm has past and the backside winds push the water into Florida Bay.

For Miami and the entire metro area, be ready for an extended period of very bad weather with winds gusting to hurricane force... likely through Monday. Some flooding around Biscayne Bay is expected at high tide.

The bad weather has already begun in South Florida due to an upper-level feature that is helping pull Isaac north. Well more that a FOOT OF RAIN may fall between today's rain and that directly related to Isaac.

Be prepared to stay indoors in a safe building through Monday. When the wind starts blowing in earnest, stay away from windows.

More on farther up the west coast later.

See you on The Weather Channel.

Hurricane

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Andrew Anniversary & Isaac

By: bnorcross, 3:01 AM GMT on August 25, 2012

This evening 20 years ago the sun set on the horrendous first day after Hurricane Andrew. I was in downtown Miami at the studios of the NBC station. We knew that there was "total" destruction in South Dade County, but even that didn't describe it. Here's to the people that went through it... and held their families together in a situation that most people can't imagine.

And now there's Isaac. A number of developments today. It's organizing and intensifying... which would likely continue steadily and significantly if it weren't interacting with the landmass of Hispaniola. The system has a tropical-looking core for the first time, and the upper air pattern on the satellite imagery looks quite favorable. The intensifying trend will likely slow or stop, depending on how much land the core moves over.

Forecasting the exact track of the core and the intensity of Isaac is tricky. If the core of the system only passes over the narrow western Haiti peninsula and the eastern tip of Cuba, it will have more time over water and could be stronger than the NHC is forecasting. This is a case where the intensity is tremendously dependent on the exact track the core takes. We aren't able to forecast the minor difference in track that could result in a stronger storm when it approaches South Florida and the Keys. The NHC has issued a Hurricane Watch for the Keys for that possibility.

And then there's the forward speed. Isaac stalled today and is moving a bit more slowly tonight than it had been. Everybody in peninsula Florida needs to be ready for an extended period of heavy rain and strong winds - two days or more. In South Florida this begins late tomorrow (Saturday). Power outages and flooding in low-lying, flood-prone area are expected.

Another complicated issue, looking ahead, is the possibility of storm-surge flooding on the west coast of Florida. If the storm stays near the coast, after the center moves north of a given location and the winds come in from the southwest and west, the water will pile up along the coast and push storm surge into the rivers and bays, including Tampa Bay.

If the storm moves north farther offshore, like Hurricane Dennis in 2005, an effect called a "Kelvin wave" can set up in the relatively shallow water over the coastal shelf, which make the surge higher than it would be without that shelf effect. Just the right factors have to come together to get this enhancing effect, but it can't be ruled out. Whether Isaac is strong enough at that point so the storm surge is disruptive to the Republican Convention in Tampa remains to be seen... but based on what we know, it's going to be close.

All indications are for landfall on the northern Gulf coast as a Cat 1 or 2 hurricane, but there are too many moving parts to know exactly where that will occur.

See you tomorrow on the Weather Channel.

Hurricane

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Significant Changes Overnight to Isaac

By: bnorcross, 5:04 PM GMT on August 24, 2012

Some significant changes overnight. As suspected... the center the NHC is tracking is farther south and the winds being measured by the Hurricane Hunters and NOAA researchers are increasing. The pressure is also dropping... all indications that Isaac is finally strengthening. The big data dump that the NHC ordered up for last night has refocused the computer forecast models, so at least for now the crap shoot is over. The best indications are that the strengthening storm will move over eastern Cuba tomorrow and be in the vicinity of the southern part of Florida or the Keys on Sunday. The circulation is quite large, so even if the center is near the Lower Keys, for example, the weather will be quite bad in Miami. The big question remains how bad is "quite bad", since we don't know how strong Isaac will be at that time. The NHC is forecasting a moderate tropical storm figuring that the land interaction will disrupt the circulation, at least some. It's a tough call. If the system organizes and strengthens significantly today, that forecast may have to be adjusted upward. The future track possibilities for the center are focused offshore of the west coast of Florida, with the storm strengthening and hitting the northern Gulf coast as a hurricane late Tuesday or early Wednesday. Any plausible track shows impacts in Ft. Myers and Tampa, but it's too early to know how severe they will be.

Hurricane

Updated: 5:11 PM GMT on August 24, 2012

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About bnorcross

This is the official blog for Bryan Norcross, Hurricane Specialist at The Weather Channel.

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