ATL: HELENE - Post-Tropical - Discussion

[Steve]

Can you believe this guy?

Huge News! Helene Will Not Be A Major Hurricane On Landfall!
https://www.youtube.com/watch?v=GG3nS9ZAxZ4&lc

The amount of weather YouTubers these days with clickbait thumbnails just spewing cliffnotes of what they've learned from others is alarming. Especially when they make bold claims to try and be different

What always annoys me are the thumbnails of an obviously photoshopped image of a past storm (usually a cat 5 or something) on top of the forecast track for the storm that the video is actually about.

I love those. Cars flying through the air, people flipping upside down. Haha. I never click on them though but appreciate the vigor to put crap like that out there. “USA in ruins!!!”

[xironman]

The dry slot is getting quite pronounced on visible. Probably gonna take the rest of the day to mix out. Good news is that potentially lowers Helene’s ceiling…bad news is a delay in RI could mean RI-to-landfall like Michael.

If you look at the water vapor imagery it is not that dry, it is an area lacking visible convection.

[ScottNAtlanta]

They just issued a Tropical Storm Watch for Metro Atlanta. Don't see that very often
Peak winds 45-55mph with gusts to 75mph. Lots of trees gonna come down with that along with 8-10 inches of rain.

[Teban54]

I don't know enough about the dry air intrusion (if at all) to comment on it here, but let's recall that at this time yesterday, people were saying Helene may not even become a major because it was just a naked swirl running away. That tone changed within 12 hours or less.

As another example, Ian was a much drier, almost pathetic-looking Cat 1 when in the NW Caribbean. Then it landfalled in Cuba as an MH, did an EWRC after land interaction, and still became a Cat 5 in the Gulf. Obviously Helene has much less time than Ian, but...

[Travorum]

The dry slot is getting quite pronounced on visible. Probably gonna take the rest of the day to mix out. Good news is that potentially lowers Helene’s ceiling…bad news is a delay in RI could mean RI-to-landfall like Michael.

FWIW this is something that hurricane models have been picking up on for a few runs now, most show the drier air getting mixed out within 6 hours.

[WeatherBoy2000]

From what I've seen with these gulf storms in the past several years is that they either come in as strong as predicted or stronger, rarely weaker. Hurricane Helene is shaping up to be this season's second major and another bad storm for the gulf coast.

[thickiminaj]

The dry slot is getting quite pronounced on visible. Probably gonna take the rest of the day to mix out. Good news is that potentially lowers Helene’s ceiling…bad news is a delay in RI could mean RI-to-landfall like Michael.

FWIW this is something that hurricane models have been picking up on for a few runs now, most show the drier air getting mixed out within 6 hours.

https://i.imgur.com/zmHicLC.png

https://i.imgur.com/xoxwecG.png

I was just coming to post this exact thing. All hurricane models picked up on this, and modeled it well. They also very clearly have it not mattering in the slightest and it mixes out quickly.

[xironman]

Looks like the mesos on the south side are creating an inner eyewall

[ScottNAtlanta]

Last 24hrs Microwave

[Kazmit]

The dry slot is getting quite pronounced on visible. Probably gonna take the rest of the day to mix out. Good news is that potentially lowers Helene’s ceiling…bad news is a delay in RI could mean RI-to-landfall like Michael.

It could take a full 24 hours to get mixed out or longer, due to the size of the eye and it's current struggles I think C5 is already off the table, C4 will be next if it can't get the dry air out today. I don't think Helene can RI until landfall like Michael did, too much forward speed when it nears land. I think the best bet is a max of C3 with weakening near landfall to a low end C3/C2.

None of the hurricane models show weakening before landfall.

[Jr0d]

The last center pass is still showing a NNW motion so I was premature in thinking the north to north north east turn started.

SFMR showed winds at 75 kts so we might see the NHC bump up the strength at the 2pm advisory

[Pipelines182]

I don't see that. Outflow is unrestricted and whatever pulsing of convection and cloud tops appear to me as nothing greater than part of the diurnal processes still at hand until the systems core is fully developed (getting close). I will say this though, there is a small upper level of dry air capping between 200mb and 300mb that might be slightly affecting the system's vertical vorticity where RH is showing to be only 40-50% within that layer (as depicted on the 6Z GFS vertical cross-section graphs - https://www.tropicaltidbits.com/analysi ... 92506&fh=1 ) from about Longitude 85.5 and points westward. I'm sure that NHC clearly sees that, and why RI is practically a given after the hurricane begins moving northward and especially as it begins motion east of due north.

If you look back to 12z you can see a large outflow boundary coming out of the CDO on the SW half, that's a sign of a large collapse of thunderstorms. The feathery cirrus outflow that's continuing is another sign of continued dry air entrainment. The diurnal process isn't that significant for hurricanes, and they don't cause drastic warming/cooling of the CDO in short time frames.

Feathery cirrus outflow is NOT a sign of dry air entrainment. Outflow boundaries indicative of collapsing CB's ARE a sign of dry air. In actuality the diurnal process is practically insignificant for any well developed or mature hurricane but where other conditions mitigate the hurricane from sustaining it's vertical structure the diurnal process can and will enhance the low-level convergence, updraft and vertical velocity, and ultimately upper air heat & energy dispersion. Helene is now a hurricane however still in the final stages of organization and remains short of having a very well-developed structure. That will change soon enough.

You only see feathery cirrus outflow when the CDO is warming, you don't see it during deepening phases or when a storm is maintaining their CDO. There's a difference between healthy outflow and the feathery outflow we've been seeing from Helene recently.
What do think it looks like when there's a cycle of updrafts being continually collapsed by dry air at the mid levels? The cloud pattern has to react to that and it's something you're going to be able to see. The solid cloud line of an outflow boundary is what happens when a thunderstorm collapses (single event) but that's not what it looks like when thunderstorms are continually forming and also collapsing over and over (continuous event). You're going to have some type of cloud cover streaming out from the CDO and the only thing I've ever seen that fits that scenario is the feathery cirrus outflow, and it happens every single time dry air gets entrained into a cyclone.

[InfernoFlameCat]

If you look back to 12z you can see a large outflow boundary coming out of the CDO on the SW half, that's a sign of a large collapse of thunderstorms. The feathery cirrus outflow that's continuing is another sign of continued dry air entrainment. The diurnal process isn't that significant for hurricanes, and they don't cause drastic warming/cooling of the CDO in short time frames.

Feathery cirrus outflow is NOT a sign of dry air entrainment. Outflow boundaries indicative of collapsing CB's ARE a sign of dry air. In actuality the diurnal process is practically insignificant for any well developed or mature hurricane but where other conditions mitigate the hurricane from sustaining it's vertical structure the diurnal process can and will enhance the low-level convergence, updraft and vertical velocity, and ultimately upper air heat & energy dispersion. Helene is now a hurricane however still in the final stages of organization and remains short of having a very well-developed structure. That will change soon enough.

You only see feathery cirrus outflow when the CDO is warming, you don't see it during deepening phases or when a storm is maintaining their CDO. There's a difference between healthy outflow and the feathery outflow we've been seeing from Helene recently.
What do think it looks like when there's a cycle of updrafts being continually collapsed by dry air at the mid levels? The cloud pattern has to react to that and it's something you're going to be able to see. The solid cloud line of an outflow boundary is what happens when a thunderstorm collapses (single event) but that's not what it looks like when thunderstorms are continually forming and also collapsing over and over (continuous event). You're going to have some type of cloud cover streaming out from the CDO and the only thing I've ever seen that fits that scenario is the feathery cirrus outflow, and it happens every single time dry air gets entrained into a cyclone.

Thats the storm mixing out the dry air. The models showed this. Give it time the storm will quickly recover once that small lick of dry air is gone. Then its off to the races.

[Frank P]

Water vapor certainly shows dry air up in the top left of the image.

[Zonacane]

Based on radar, I wonder if Helene will attempt to close off a much smaller core. It would suck if the dry air kills off an unwieldy/oversized core and we get a more compact and stronger storm instead.

[wxman57]

I was running SLOSH this morning. A large Cat 3 moving into Apalachee Bay from the south or SSW would produce a 30 ft surge, not 10-15. Forward speed 25 mph at landfall.

Wxman57. What is the slosh for Tampa Bay?. They backed it down to 3 to 6 feet but living here as long as I have I feel that 6 ft. Will be the minimum. Thanks.

Just have a brief second. Tampa Bay would be on the edge of 50kt (58 mph) sustained wind. Possibly 10ft into the bay.

[Travorum]

The dry slot is getting quite pronounced on visible. Probably gonna take the rest of the day to mix out. Good news is that potentially lowers Helene’s ceiling…bad news is a delay in RI could mean RI-to-landfall like Michael.

FWIW this is something that hurricane models have been picking up on for a few runs now, most show the drier air getting mixed out within 6 hours.

https://i.imgur.com/zmHicLC.png

https://i.imgur.com/xoxwecG.png

One notable caveat to this as Andy Hazelton notes is that this dry slot has been trending stronger over the past few model runs:
 https://x.com/AndyHazelton/status/1838980813704368505

[CycloysisNegative]

The last center pass is still showing a NNW motion so I was premature in thinking the north to north north east turn started.

SFMR showed winds at 75 kts so we might see the NHC bump up the strength at the 2pm advisory

Can you share the source?

[aspen]

The dry slot is getting quite pronounced on visible. Probably gonna take the rest of the day to mix out. Good news is that potentially lowers Helene’s ceiling…bad news is a delay in RI could mean RI-to-landfall like Michael.

FWIW this is something that hurricane models have been picking up on for a few runs now, most show the drier air getting mixed out within 6 hours.

https://i.imgur.com/zmHicLC.png

https://i.imgur.com/xoxwecG.png

One notable caveat to this as Andy Hazelton notes is that this dry slot has been trending stronger over the past few model runs:
 https://x.com/AndyHazelton/status/1838980813704368505

It wasn’t enough to stop a fairly quick recovery on this run. Helene has it mixed out by tonight on the 12z HAFS-B.

[ScottNAtlanta]

I think it might be time for me to hit the store for some non perishable food. I don't think people here have figured out what is coming for the next 2 days and longer for power outages.