ATL: IDA - Remnants - Discussion

[Hypercane_Kyle]

Definitely some degradation of its IR presentation, as expected, but Ida's outflow has never been better.

[Kohlecane]

The NHC rarely make very bullish intensity forecasts. But when they do, most of the time the system ends up exceeding the already high expectation.

Iv'e always noticed that, it's like a sense of pride thing almost

[Steve]

Salute!

where is Steve? Live on Northshore, if I recall.
Katrina was a big cat3,but main thing was the immense surge to the east, and the the levee failures in New Orleans.
This storm looks more like Camille with big surge on coastal places south of NOLA, and more wind than Katrina.

I would be more worried about gasoline in three or four weeks now that Keystone addition canceled as about 25% of our petrol comes thru Fourchon where the storm is headed.

Gums sends...

What’s up Gums. I’ve been splitting time between Metairie and Pensacola until I had to move back August 1. Needless to say as much as I want to ride this out, I’ll be heading back to Pensacola in about an hour.

[ThetaE]

Ida's core looks to have stayed in steady state tracking over the Isle of Youth, though the windfield is a bit wacky now. Taking no time to set recovery into motion with a hot tower firing on the SW side of the core (SW side-- sign of lessened shear/more symmetry?).

This may help get Ida back to a healthier core structure while it can. I imagine landfall over Cuba will feel pretty similiar to what just occurred. Ida is liable to take off almost immediately afterwards, IMO.

[skyline385]

55 kt SFMR so far from center and in the E/SE quad

[aspen]

24-30 hours over the Loop Current once it crosses Cuba…this is dreadful.

[Meteorcane]

Sorry to repeat myself, here, but maybe including a picture or two will help:

https://i.ibb.co/pbsJLjs/COD-GOES-East-regional-gulf-10-20210827-192617-over-map-bars.jpg

That's a low-level water-vapor shot from the COD site. There's obviously 's a lot of dry air around Ida, although it's at a low level. Medium-levels are significantly more humid, but still on the dry side:

https://i.ibb.co/fSqgqPL/COD-GOES-East-regional-gulf-09-20210827-194117-over-map-bars.jpg

My question is about the low-level air. Does low-level dry air affect the deepening of a tropical cyclone, or does it have to be closer to mid-levels to matter? Obviously, Ida is intensifying quickly, but would it be significantly faster if the low-level dry air weren't there?

Thanks!

Hey! College of DuPage has great satellite graphics, but honestly the distinction between low, mid, and high WV has never made much sense to me. For instance, the "low" WV loop shows plenty of cirrus/outflow-related features. Outside of areas of deep convection, the low-level channel always looks super dry like that.

If you want a more reliable sense of moisture in the low levels, check out some of the dropsondes from the NOAA9 environmental missions. Most show relative humidity >80% at the surface in Ida's near environment.

As for your question: Dry air at all levels negatively impacts tropical cyclones, but environmentally, mid-level dry air is more important. Air rising in convection is sourced from near the surface, and if that air is dry then it becomes saturated less quickly when rising --> the air is less buoyant. This is rarely a problem inside a sufficiently strong TC because surface winds drive evaporation that moistens the TC's inflow. If mid-level dry air works into the TC's circulation, though, it can lead to evaporational cooling: falling precipitation will evaporate, taking latent heat out of the air (just as condensation releases latent heat). This cooling also creates negative buoyancy that strengthens downdrafts.

Downdrafts can cut off updrafts, and they also replace warm, moist surface air with cool, dry air. That leads to the original buoyancy problem I pointed out.

Very good points... One additional thing I will add is that the weighting functions for the WV channels from GOES is such that even the "low-lvl" water vapor channel (the red line) weighting function peaks at just below 600 hpa (~4km AMSL, in a standard atmosphere slightly higher in the deep tropics) and actually typically has more contributions from 300hpa (a level you will often find cirrus) than the surface. So yes you are correct that even though the "low-lvl" water vapor channel is weighted lower than the other 2 GOES channels, it is still mostly picking up a contribution of WV in the 700-500hpa lvl, which many in the meteorology community would refer to as the mid-lvls rather than the low-lvls.

Finally it can be difficult when looking at WV in the vicinity of tropical cyclones to not focus on the fact that the airmass outside of the cyclone is dry relative to the cyclone itself... remember TCs are essentially the greatest cauldrons of moisture the Earth can muster so any airmass outside of it will look dry in comparison, instead look for "absolutely low" values of WV (manifested by the WV band sensing warm brightness temperatures and "seeing" further down into the troposphere).

[tropicwatch]

Wondering if AF is going to take off at all.

[aspen]

Wondering if AF is going to take off at all.

They’ll probably wait or never take off. The NOAA plane just left because it seems that Ida is already on shore. Based on how quickly it went over that other island, it’ll be in the Gulf in a few hours.

[ConvergenceZone]

Too early to say NOLA is out of the woods, but a landfall nearer to Morgan City or even Terrebone parish, would basically imply "only" TS winds for most of the NOLA area, as unless the windfield expands significantly (this would mostly be due to hard to predict EWRC) the hurricane force winds would stay west of the city proper. Ida would have to go towards the eastern end of the envelope for NOLA to have significant wind impacts.. surge though can still be a factor well east of the center so areas of SE LA (outside of the protection system) and even MS are at risk, even with a center track closer to central LA.

Yea I definitely don't think NOLA is out of the woods....While what you said is true, they can still get some pretty gusty tropical storm wind damage...........

[Keldeo1997]

Looks like the center is getting better. Frictional convergence maybe?

[Michele B]

The NHC rarely make very bullish intensity forecasts. But when they do, most of the time the system ends up exceeding the already high expectation.

Ooooh, then this is bad! I'm listening to them now - @ 6:30 - and they are sounding very worried.

REALLY warning folks to be prepped BY TOMORROW BY DARK.

I hope BigBO and others in the path are paying attention.

[ThetaE]

Sorry to repeat myself, here, but maybe including a picture or two will help:

https://i.ibb.co/pbsJLjs/COD-GOES-East-regional-gulf-10-20210827-192617-over-map-bars.jpg

That's a low-level water-vapor shot from the COD site. There's obviously 's a lot of dry air around Ida, although it's at a low level. Medium-levels are significantly more humid, but still on the dry side:

https://i.ibb.co/fSqgqPL/COD-GOES-East-regional-gulf-09-20210827-194117-over-map-bars.jpg

My question is about the low-level air. Does low-level dry air affect the deepening of a tropical cyclone, or does it have to be closer to mid-levels to matter? Obviously, Ida is intensifying quickly, but would it be significantly faster if the low-level dry air weren't there?

Thanks!

Hey! College of DuPage has great satellite graphics, but honestly the distinction between low, mid, and high WV has never made much sense to me. For instance, the "low" WV loop shows plenty of cirrus/outflow-related features. Outside of areas of deep convection, the low-level channel always looks super dry like that.

If you want a more reliable sense of moisture in the low levels, check out some of the dropsondes from the NOAA9 environmental missions. Most show relative humidity >80% at the surface in Ida's near environment.

As for your question: Dry air at all levels negatively impacts tropical cyclones, but environmentally, mid-level dry air is more important. Air rising in convection is sourced from near the surface, and if that air is dry then it becomes saturated less quickly when rising --> the air is less buoyant. This is rarely a problem inside a sufficiently strong TC because surface winds drive evaporation that moistens the TC's inflow. If mid-level dry air works into the TC's circulation, though, it can lead to evaporational cooling: falling precipitation will evaporate, taking latent heat out of the air (just as condensation releases latent heat). This cooling also creates negative buoyancy that strengthens downdrafts.

Downdrafts can cut off updrafts, and they also replace warm, moist surface air with cool, dry air. That leads to the original buoyancy problem I pointed out.

Very good points... One additional thing I will add is that the weighting functions for the WV channels from GOES is such that even the "low-lvl" water vapor channel (the red line) weighting function peaks at just below 600 hpa (~4km AMSL, in a standard atmosphere slightly higher in the deep tropics) and actually typically has more contributions from 300hpa (a level you will often find cirrus) than the surface. So yes you are correct that even though the "low-lvl" water vapor channel is weighted lower than the other 2 GOES channels, it is still mostly picking up a contribution of WV in the 700-500hpa lvl, which many in the meteorology community would refer to as the mid-lvls rather than the low-lvls.

Finally it can be difficult when looking at WV in the vicinity of tropical cyclones to not focus on the fact that the airmass outside of the cyclone is dry relative to the cyclone itself... remember TCs are essentially the greatest cauldrons of moisture the Earth can muster so any airmass outside of it will look dry in comparison, instead look for "absolutely low" values of WV (manifested by the WV band sensing warm brightness temperatures and "seeing" further down into the troposphere).

https://www.researchgate.net/profile/Patrick-Skinner/publication/323623133/figure/fig1/AS:811095908425728@1570391742508/Vertical-weighting-function-for-a-GOES-13-and-b-GOES-16-water-vapor-bands-assuming-a.png

Thanks! That's a useful graphic to visualize those WV channels. I didn't know exactly what was being referred to as low/mid/high.

[bella_may]

Looks to be a little north and east of nhc official track. At least by my eyes

[PandaCitrus]

Am I seeing that radar correctly? That's a huge eye northeast of track?

[Zonacane]

Am I seeing that radar correctly? That's a huge eye northeast of track?

24 nautical miles wide to be exact

[StormPyrate]

cuban Radar has the eyewall just onshore
http://www.insmet.cu/asp/genesis.asp?TB ... AXw01a.gif

[ThetaE]

Wondering if AF is going to take off at all.

They’ll probably wait or never take off. The NOAA plane just left because it seems that Ida is already on shore. Based on how quickly it went over that other island, it’ll be in the Gulf in a few hours.

I'm honestly impressed the NOAA plane was able to penetrate the eyewall at all, considering how the center is wedged between two landmasses. Getting permission to fly over Cuban airspace is necessary here, and that's one thing in itself. But dropsondes? You wanna drop little projectiles out of your plane just offshore of Cuba?

[InfernoFlameCat]

I shudder to think of what the ceiling for Ida is. Dare I say, I think it might be our first sub-900 storm in years if everything goes wrong.
It's silly but I've always wanted to visit New Orleans someday. I hope I still get the chance with this in mind.

Sub-900 is too much, it doesnt have the time for that kind of RI and storms only get to sub-900 with a pinhole eye while undergoing explosive intensification. A system with an average sized eye like Ida isnt dropping that low, it would not be able to sustain it...

Rita did it with a regular eye.

[TorSkk]

I shudder to think of what the ceiling for Ida is. Dare I say, I think it might be our first sub-900 storm in years if everything goes wrong.
It's silly but I've always wanted to visit New Orleans someday. I hope I still get the chance with this in mind.

Sub-900 is too much, it doesnt have the time for that kind of RI and storms only get to sub-900 with a pinhole eye while undergoing explosive intensification. A system with an average sized eye like Ida isnt dropping that low, it would not be able to sustain it...

Rita?