Major Hurricane Delta

[STxVortex]

Given the NHC's current discussion, Delta could be a Cat 5 [157mph sw] when it hits the Cozumel, Cancun Yucatan, Quintana Roo areas. The 5PM Discussion only needs a 2mph bump at 12H mark.

NHC forecast [models synthesis] has also gradually bumped the northern GOM approach and landfall intensity up as they move the projected track westward, now showing 'Major' status at landfall, it had previously lost that rating mid-GOM.

Hurricane Delta may be yet another record-setter. And the season still has seven weeks to run.

"064 WTNT41 KNHC 062034 TCDAT1 Hurricane Delta Discussion Number 9 NWS National Hurricane Center Miami FL AL262020 500 PM EDT Tue Oct 06 2020...

FORECAST POSITIONS AND MAX WINDS

INIT  06/2100Z 18.9N  84.1W  125 KT 145 MPH
 12H  07/0600Z 20.2N  86.1W  135 KT 155 MPH
 24H  07/1800Z 21.8N  88.8W  105 KT 120 MPH
 36H  08/0600Z 23.0N  91.1W  110 KT 125 MPH
 48H  08/1800Z 24.4N  92.6W  115 KT 130 MPH
 60H  09/0600Z 25.9N  93.2W  115 KT 130 MPH
 72H  09/1800Z 28.0N  92.9W  110 KT 125 MPH

[Doc Jon]

23 minutes ago, MUWX said:

Pretty decent uptick in lightning on the SW side. Eye seems slightly more apparent on visible as the sun sets.... *ducks* 

I noticed that too.  Lightning's been almost nonexistent 'till the last half hour or so and now the eyewall's flashing like a bad strip club.

[JasonOH]

Eyewall looks much improved on the last few scans from the Cayman Islands. It’s 175+nm away but the eye appears to be 10-15nm wide.  Also, recon just took off. Should arrive near 0z.

[e46and2]

1 hour ago, brianc33710 said:

The pressure is still very high for a 145 mph hurricane. 

does it have to do with it being october?  since gradient is what creates wind, i imagine overall atmospheric pressure is higher in october compared to august, but not sure if this is a contributing factor.

[cptcatz]

Man if this reaches cat 5 in the next ~12 hours that would make 5 straight years of a cat 5, and even more scary/impressive/sad would be 4 straight years of a hurricane making landfall at cat 5 intensity. 

[Orangeburgwx]

Eyewall looks much improved on the last few scans from the Cayman Islands. It’s 175+nm away but the eye appears to be 10-15nm wide.  Also, recon just took off. Should arrive near 0z.

10-15 nm is right in the Cat 5 golden zone

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[Cheeznado]

I agree this does not look anything like a borderline cat 5 on the sat pic. Odd storm.

 

[Amped]

Massive convective blob well southeast of the center.  Thats gonna mess things up.

[Moderately Unstable]

Advance warning: this is long. If you don't like long posts, skip this, I'm not about to say anything earth shattering, and carry on with your discussion.

Just to be clear, the NHC is staffed by human forecasters--not wizards that set the actual hurricane's wind speed--and forecasting when it comes to intense, high-profile, high-impact events, is an art. The NHC provides an update every 3 hours, they may get an aircraft measurement every 3-6 hours (sometimes they get the benefit of having one or two aircraft in the storm for the entire 3 hour period, other times, like now, there are none). This means, like us, the NHC have to put forth their own best estimate for intensity. Sometimes, they are marginally wrong. I strongly doubt they are ever VERY wrong, but, to be blunt, based on what I'm seeing *right now*, I would bet money that there are not sustained 145 mph winds in the eyewall. To the same degree, storms don't magically become hurricanes, or dissipate, coincidentally always at 3 hour intervals. To further this concept:  hurricanes are not single points, and they aren't singular instances of time. When the NHC issues an update, it is for the next 3 hours, and reflects those data received and analyzed over the preceding 3-6 hours (depending on whether it's a completed or intermediate update, early vs late cycle model guidance etc).

 

A couple hours ago, even an hour ago, a very clear, tight eye was seen that would make a reasonable forecaster conclude persistence to very slight strengthening from the most recent aircraft data, hence 145. Recall basic forecasting principles--your most basic forecast is persistence--things should continue to do what they did before. Tweak that slightly: the eye looks a bit better, the clouds a bit colder (etc etc), and you can as a forecaster bump up your wind guess without an objective measure. I think the NHC has the additional strengthening forecasted pre-Mexico due to the underlying dynamics--very warm SSTs, and modest shear (but it seems possibly a touch more than was expected at this time yesterday). It should be noted that models do not do a great job with compact, intense hurricanes and they wouldn't necessarily show, say, cat 5 winds, well, in this setup. Hence why the NHC is looking at the SHIPS rapid intensification progs to estimate probabilities for further intensification. There has been a trend this season, and over the last few years, for storms to outperform model guidance (given a certain amount of shear, ssts, etc), and the NHC likely factors that into their forecasts as well. Big picture: more heat, low shear, climate change (please don't shoot the messenger!), stronger storms. The NHC knows they're often better betting high than low versus the model guidance. Thus, the main message I'm conveying is--this storm is clearly messy right now. We really need another aircraft to get truly objective data and actually *know* what the current wind speed is, and what the current pressure is. Otherwise, it is a subjective to quasi-objective estimate based on IR/microwave/visible imagery, persistence from earlier aircraft data, mesoscale hurricane model data, and forecaster experience. The NHC's word is not gospel--they do not set the wind speeds/pressure, they provide their best guess. So can you or I, but the NHC is better at it. I look at 156mb and that satellite presentation and (personally) say that the onus for a claim of 145 rests with the claimant--I would need objective measurements from some source for that, otherwise, I'm not buying what you're selling. I also don't jump to cat 5 forecasting either--156 is low, it is not LOW. The storm has not been deepening much for most of the afternoon, it's come up a couple mb. That is not a sign of cat 5 at this time. You can yourself estimate winds subjectively with the right textbook and knowledge to calculate Dvorak numbers and extrapolate estimated winds based on pressure/eye size and other things. The NHC likely correctly assume however that, even if there was or is a temporary slight drop in maximum winds, 145 is likely right in the 3-hour time frame before 8pm, and we are currently seeing conflicting data vis a vis intensity (lots of lightning and good banding but loss of some symmetry and spatial extent of the coldest cloud tops). 155+ therefore remains quite possible before landfall, and the NHC forecast shows that. It should be noted that for anyone who isn't a weather weenie, being WRONG, by being LOW on your wind forecast, is an order of magnitude worse for the public and preparedness than being a touch too high.

 

Also note that the NHC deliberately tries to make changes slowly. Their job is communicating risk to the public and public safety officials. Meteorologists know how to read models, interpret satellite imagery and understand charts--so the NHC is not necessarily writing explicitly minutia-level changes. No one, sans a weather weenie, stands outside in strong winds with an anemometer, pushes up on their glasses and exclaims: "these winds are 10mph less than the NHC said they'd be!", and after the end of the season, the NHC goes back and reviews the data from all the storms and confirms intensities and other things. The last hour has seen a notable reduction in the symmetry of the coldest -80-90 degree cloud tops, and a clear eye is NOT visible. If the storm continues to be disorganized, they may well lower the winds a touch on the next advisory or hold them steady. Conversely, shear could abate a bit, and a bit more intensity could be added. Almost all intensity guidance now says that the storm should maintain intensity or weaken slightly (sans two models which show a cat 5 peak, but are out to lunch as they keep the storm extremely strong once inland in the US...I question any other things they're projecting as a result). I'm more concerned, personally, about the fact the models lately are keeping the storm at major status to landfall, and the nhc track hints at that. If you overlay a cat 3 landfall slightly west of New Orleans, and then have that strong right side onshore flow into that area, you're gonna have some problems, though the new protection system should help that...in theory.

 

Cheers all!

 

 

[Windspeed]

Great post. Welcome to the forum.

 

Some kind of internal structural change has been underway. I expect recon to find a temporarily weaker vortex but larger RMW. That being said, it's still a small hurricane. For Cancun/Cozumel's sake, perhaps it weakens and levels off instead of actually pushing a Cat 5. But it's still got a lot of time left for a better organized eyewall to rapidly deepen beyond what the syatem has already experienced. Likewise, there was talk of mid-level en-easterly shear. Unsure how legit 10 kts is having on it since it is moving with flow but then again, unsure how much gyre motion with Gamma's remnant guts is taken into account here.

 

 

[Brian5671]

2 minutes ago, Windspeed said:

Great post. Welcome to the forum.

 

Some kind of internal structural change has been underway. I expect recon to find a temporarily weaker vortex but larger RMW. That being said, it's still a small hurricane. For Cancun/Cozumel's sake, perhaps it weakens and levels off instead of actually pushing a Cat 5. But it's still got a lot of time left for a better organized eyewall to rapidly deepen beyond what the syatem has already experienced. Likewise, there was talk of mid-level en-easterly shear. Unsure how legit 10 kts is having on it since it is moving with flow but then again, unsure how much gyre motion with Gamma's remnant guts is taken into account here.

 

 

yeah i'd have to wonder what that area of disturbed weather out ahead if it will do as to having an effect on the system?

[Windspeed]

¯\_(ツ)_/¯

GFS and ECMWF trend the past few days didn't really have any upper or mid level shear until the 12z run. Past few days had very light VWS beyond the steering flow. So I guess using models to forecast is chaotic at best these days. Might as well pound me head against an anemometer.

[STxVortex]

10 minutes ago, Brian5671 said:

yeah i'd have to wonder what that area of disturbed weather out ahead if it will do as to having an effect on the system?

My SWAG regarding the residual effects of TS Gamma are that the huge amounts of precip Gamma recently dumped on the Yucatan will cause Delta to experience a much-lessened 'land effect' weakening period than might be expected or modeled [do any models even factor in something like that?].

And perhaps possibly even an intensity increasing "Brownwater Effect"...  Of course, having previously traveled the Yucatan extensively, I suspect the mostly limestone and sandy terrain won't hold much standing water for long.

[hlcater]

Shear appears to reduce later tonight and through landfall. Should have a 8hr window with negligible shear prior to landfall. Not likely enough time to go sub 900, but may be enough time to clear the eye out. The ongoing convective burst is probably inducing pressure falls again, symmetric or not.

[Moderately Unstable]

1 hour ago, Windspeed said:

Great post. Welcome to the forum.

 

Some kind of internal structural change has been underway. I expect recon to find a temporarily weaker vortex but larger RMW. That being said, it's still a small hurricane. For Cancun/Cozumel's sake, perhaps it weakens and levels off instead of actually pushing a Cat 5. But it's still got a lot of time left for a better organized eyewall to rapidly deepen beyond what the syatem has already experienced. Likewise, there was talk of mid-level en-easterly shear. Unsure how legit 10 kts is having on it since it is moving with flow but then again, unsure how much gyre motion with Gamma's remnant guts is taken into account here.

 

 

 

1 hour ago, Brian5671 said:

yeah i'd have to wonder what that area of disturbed weather out ahead if it will do as to having an effect on the system?

http://www.atmos.millersville.edu/~adecaria/ESCI344/esci344_lesson10_TC_structure.pdf

(1) Thank you :). 

 

2) If you buy almost any of the current guidance, I haven't looked at a couple of "proprietary" hurricane models yet...you expect a larger hurricane with a bigger wind field, and perhaps, no, you don't get the cat 5 peak. I don't bet against cat 4's over warm ocean's though. Using a wind speed equation from the above, Vmax=A sqrt(ambient pressure-minimum central pressure), with A here as, per the above, 6.3--- Vmax= 6.3*sqrt( 1016mb- 956mb) = 48.79 m/s --> *2.23 mph / m/s--> 109.162 mph. That seems low (although to be blunt if you told me 956mb, gave me a sat image of Delta right now and said it's a 110-115mph storm I'd say I believe you). Regardless, I did something wrong in that analysis which is why my reply took so long. Either I chose a bad value for A, or I incorrectly estimated the ambient pressure outside of the storm. I considered that pa is the unit of pressure vis a vis SI units, so I tried that conversion first but that was, well, let's just say that didn't work.That however is just one empirical equation so it doesn't mean all that much. I spent the last thirty minutes reading through the above link, see btm pg 11-13 for the whole lack of currently visible eye thing (but I'm still digesting it). Specific things I wanted to pull out:

 

-The intensity of the cyclone can [] react rapidly to fluctuations in diabatic and latent heating, as well as fluctuations in vertically integrated divergence.

-At some distance near the center of the vortex, the dissipation of angular momentum can no longer keep up with [] horizontal advection. This leads to a horizontal convergence of angular momentum.

--> This horizontal convergence of angular momentum leads to an increase in the winds above the value that the pressure gradient can support (i.e., they are super-gradient).

--> The super-gradient winds develop a radially outward component, since the pressure gradient cannot supply the required centripetal acceleration.

-->The consequences of this are (see figure): ◼ There is radial convergence at distances outside of the radius of maximum winds. ◼ There is radial divergence at distances inside the radius of maximum winds.

 

--

 As the air subsides, the resultant compressional warming actually works against subsidence (through buoyancy).*********

 All that is needed in order to keep the eye relatively clear is enough subsidence to balance the buoyancy.********

 Thus, in the steady state, there doesn’t have to be strong subsidence in the eye in order to maintain the eye.******

 The maintenance of the eyewall is a balance between the horizontal and vertical advection of angular momentum. The radius of the eye can expand or contract depending on this balance. *********

 

So, taken together, there is not enough subsidence to balance the buoyancy being seen, or, in english, the SSTs are very warm, and supporting vigorous convection already (which we noted above with the -80-90 C cloud tops and tons of lightning). So, that in itself isn't necessarily an indicator of a lack of strength, it indicates extremely vigorous convection and at least cursorily that compressional warming is hindering sinking motion in a piece of the eye in a manner that isn't *weakening* the storm but is precluding the eye from clearing out (if anyone who wants to correct me has a better response to this please feel free to chime in). What is more of an indicator is symmetry--which we already know--since that's basically telling us about the overall shear situation and organization of the system. There were a bunch of other things I got out of the paper/lecture I was just reviewing but that is the most salient part. 

 

One final piece I did already know but the paper noted as well, and I do actually remember with a couple intense storms in the gulf the last couple of years is that having good upper level outflow becomes important to get truly monster-level cat 5 type dynamics, usually with a "dual jet" outflow. 

3) The biggest impact of Gamma's remnants is that the airmass ahead of Delta is wet. Minimal fujiwhara effect, minimal impact to shear. Several earlier season gulf storms suffered when running into dry air, including Gamma. Gamma's remnants create a moist airmass aloft out ahead of Delta so it is unlikely to entrain any dry air before landfall (that's my guess). So, the best way to interpret Gamma, I think, is that it will not HURT Delta. The best way to figure out what will happen is to look at the analogs, but, I have always been horrible at analoguing so I'm currently looking through the data trying to find a good comparison. 

 

4) The latest hurricane hunter aircraft just entered the storm and is now making observations which you can follow in real time and I'm sure we'll start seeing the posts for that on here soon. I look forward to eating crow at being wrong on half of whatever I've just said.  I will say, in the span of me writing my first post, and this one, and editing this one, the storm has regained more symmetry and I'm seeing more pockets of 85-90 c tops (which to be clear, you don't see a lot even on cat 4s or 5s...that's insanely cold). I'm sure those eyewall adjacent cells are hot towers. 

 

 

[SnowLover22]

37 minutes ago, Windspeed said:

¯\_(ツ)_/¯

GFS and ECMWF trend the past few days didn't really have any upper or mid level shear until the 12z run. Past few days had very light VWS beyond the steering flow. So I guess using models to forecast is chaotic at best these days. Might as well pound me head against an anemometer.
 

What do you think of the HWRF. It weakens Delta just below major status before the 1st landfall because of shear. 

[It's Always Sunny]

9 minutes ago, SnowLover22 said:

What do you think of the HWRF. It weakens Delta just below major status before the 1st landfall because of shear. 

I still think it's pretty on point. 12Z winds did really well so I buy into what it is selling. I am interested to see how much Delta is affected once it moves over the YP because that will set the baseline of how much/fast it can recover once it re-enters the GoM.

[Doc Jon]

wow those are some cold cloud tops!

[tiger_deF]

[brianc33710]

1 hour ago, e46and2 said:

does it have to do with it being october?  since gradient is what creates wind, i imagine overall atmospheric pressure is higher in october compared to august, but not sure if this is a contributing factor.

But Mitch's pressure fell below 910 mb, if I remember right. 

[SnowLover22]

Recon says this storm has weakened a bit.  

[Orangeburgwx]

82 kt surface in NW eyewall

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[Orangeburgwx]

959.8

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[SnowLover22]

1 minute ago, Orangeburgwx said:

959.8

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So yeah steady-state/ a little bit of weakening.

[Floydbuster]

Not looking good

 

[the ghost of leroy]

Cmon naso, help rebuild my faith. We need weenie prayers 

[JasonOH]

No eyewall noted on VDM. 

[CheeselandSkies]

What is the deal with shear always being there when the models said it won't be (Delta today, Sally through most of its trek through the GOM prior to the 11th hour before Panhandle landfall) and not being there when it's supposed to be (Laura, Michael at landfall)?

[Floydbuster]

I think it's easterly shear.

[Orangeburgwx]

I think it's easterly shear.

Yeah, remember Florence a few years back? It RIed into a 4, was forcasted to be a 5, and as it was doing it's first EWRC shear destroyed the core. This reminds me of that

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