Moderately Unstable

I try not to focus on minute-by-minute forecast changes based on how the storm looks at any given moment. I said this yesterday when I said I thought this would become a cat 2, and I'll say it now: I do not see dry air entrainment. The radar from klix shows a well organized system with several well formed curved bands. Better organized than some stronger wind-speed storms earlier in the season. Right now the eye is starting to show again. A hurricane is a well organized collection of thunderstorms. They pulse. Look at trends over the course of a few hours and don't get too bogged down by looking one way now another way now. Landfall intensity I think will be a function of shear. So far, shear has stayed low. 81-84 in this environment for water temps is fine for maintenance with low shear. Right now the storm is still over slightly warmer waters. I am guessing that that is what the 06z dynamical models picked up on because the structure of the system in said models depicted a relatively symmetrical storm, which implies low shear, and we do still see that now. The thing isn't on land yet and it will inevitably look different in 2 hours than it does now. For that reason, don't count the storm out without a clear eye, and don't assume if an eye clears now that it means RI. It's still strengthening right now, it will level out at some point close to landfall. I still say Cat 2 is likely as a peak intensity--low end Cat 2. Good upper divergence, low shear, not doom level water temps. Marginal 100mph storm. This is a good example of the difference between fast uniform flow and strong shear. Overall the storm is very short on time to change much. It isn't going to become a cat 3 and it isn't going to collapse into a tropical storm. What IS important to note is that a strengthening or steady state tropical system does mix down winds easier. So there's a difference between a weakening 100mph storm and a strengthening 90mph storm. The strengthening 90mph does more overall damage (potentially, I'm postulating here so if anyone thinks that's incorrect please let me know)...my logic is: you have the back side to contend with too and it takes longer to swing the pendulum the other way. We should be able to see the eyewall circulation on radar in an hour or two. MU

I mean, it's relative. The purpose of the flights is to provide data to the nhc and for forecast models to improve predictions. Hurricane forecasting is a national security issue. Hurricanes cause potentially billions in damages, and can kill a lot of people if the wrong forecast is given and people aren't prepared. Aircraft are useful when storms are changing intensity, speed, direction, and when the track is uncertain. Zeta is not really in those categories. Because of Zeta's speed, the models are in very good agreement on the track. We know 95% CI where this is going to go. Separately, whether this is a 90mph Cat 1, a 100 mph Cat 2, or an 85 mph Cat 1 at landfall...the impacts to New Orleans will be the same. Finally, they *are* flying enough missions such that there is basically an updated in situ measurement for every 3 hour forecast update. They don't have a plane in every moment because it isn't needed. It won't affect the forecast that's being issued. As the storm closes in on the coast, buoys and land based radar also helps provide more data. There is no budget cap for hurricane hunter missions. They just fly what they need to fly based on what nhc asks for. They'll have an updated in situ measurement before 2pm and that's really what's important at this point. MU

I have a few thoughts this am. #1 As the storm approaches the coast we'll start being able to see the eyewall on KLIX radar. As always this is another tool in our toolbox to see the structure and intensity of the eye and any associated mesovortices and their qualities. #2 The physics suggest that as we know, shelf waters cool near the coast. They are however still warm enough to support tropical systems, 80-81 near the coast. However, the storm will be moving very fast, and upper level divergence is solid. As a result, weakening will not be substantial. I am curious why both the 06z HWRF and HMON *both* show strengthening to landfall. That's odd. To ping off of Windspeed--at the moment, the data suggested a pause in the strengthening process. The storm has ~10-11 hours to go before landfall. The eye has become harder to see on recent satellite imagery, and convection is currently more asymmetric. Hurricanes go through pulses of convection so this isn't an immediate flag that strengthening is over. The storm may wrap convection around the eye again and become more symmetric. Moreover, the storm had excellent sat presentation overnight that would make you think it was a cat 2, but the hhunters didn't find those winds. Conversely, delta didn't look good on satellite a lot of the time but had potent winds. It looks to me based on the absolute most recent pass by the aircraft that winds have come up, and fixed geopotential derived surface pressures are down a touch from the last pass. And, to confirm that, just now, yeah, the plane shows pressures are down again now to 977. New vdm, closed eye, 24 miles, max fl winds steady at 89kts, estimated max surface winds of 70kts (previous vdm had those at 58kts). The sonde in the NE eyewall has 75kt winds in the lowest 150 meters. This is down by a knot from the last sonde through the NE eyewall and approximately steady. MU

I agree with that--the general conclusion. I was more pointing out that earlier posts here and in other threads made fun of the low ace score and my point was, nah, this season will be a history book one regardless of ace score. But you're right--the low score does indicate that generally storms have had trouble staying long and strong. MU

Hhunters data is back up again now and they appear to be heading back through the right front quadrant right now so expecting this to be the highest windspeed pass of their pattern. Last pass through did bear out that wind speeds had gone up a bit. Latest VDM reported 66kts peak fl winds. I feel like 70 should be easy to exceed with the current presentation but, I suppose this is why we have the planes haha. MU

I mean ACE isn't the only useful measure of a season. Ace is strongly biased towards strong and long lasting high end hurricanes. You can debate the merits of that but ace mostly uses windspeed. There's more to a hurricane season then peak winds...though ironically it appears the winner of the forecast this season is the handle windspeed. In any case, let's take this into a scientific vacuum. If I make a rectangular ocean of 3k miles per side, as number of storms go up in a given time, intensity goes down. Why? Because each time I bring a tropical system over the area I upwell cooler water, and I transport heat and momentum out of the system. This happened earlier this year with Hurricane Teddy going over cool waters upwelled by Paulette. That likely weakened Teddys maximum strength and reduced the time it spent at peak strength. Without Paulette out front, Teddy could have been a 5. Because wind speed is squared in the calculation of ACE, that makes a difference. In the gulf, we've seen a prodigious number of storms. Did Gamma affect Delta? Hard to say. At the end of the day, yeah, ace is low, but like, who is complaining about a season in which you've had a ts to forecast literally every other week, or more? Also the ace score doesn't factor in landfall numbers, and rainfall. From the us perspective, a bunch of landfalling hurricanes matters more than a cat 5 in the open Atlantic. MU

I agree with Windspeed's statement on landfall in large part. I think as with many things, stronger peak equals stronger arrival...BUT, in the same way that I think the current environment is strongly favorable for strengthening, I think the near-coast environment is pretty clearly not. The upper level pattern will be favorable and that will keep the system stronger than it might otherwise be, but it's hard to expect it to hit at peak intensity. That said, it may not weaken as much as we think it should--so whether or not sub-970 is possible at landfall is really a question of what it peaks at. It won't be in a hostile environment, just not a great environment. The question isn't, can it strengthen, it's can it hold together. The guidance is pretty solid in saying, "hey I don't think this is going to weaken a ton". It's saying that expecting a tropical storm, not a cat 3, so take that fwiw, but I do trust models to do OK with their upper level dynamics. Either way my feeling is, follow the physics. Models are guidance, not gospel. Helpful tools, show trends and tracks...but if you see a duck and the models say it's a cow, I'm going to treat it as a duck. I buy trends...that it won't weaken close to landfall a ton, but I also think the stronger it gets, the harder it will be to sustain that strength. In other news, the current ADT CI number is 3.0, estimated Vmax 45 knots, and it has the weakening flag ON. This is why I don't put tons of stock into automated dvorak numbers. Sometimes, very useful. Other times, not so much. Separate aside: I'm not sure what's up with the AF hurricane hunters the last few days. Last night it looked like they turned back after flying out for around 45 minutes, today data reporting stopped after around 20 minutes of flying from the base. Unusual. Also unfortunate since it's a useful way to monitor the overall environment. MU

Honestly, I'm going to go crazy and forecast this thing becomes a moderate Cat 2. None of the intensity guidance shows that, and I care 0%. The environment is excellent and I don't see dry air entrainment. The eyewall that's developing is solid, and I actually think the outer bands are helpful in keeping the interior of the system more stable. Less chance for some unknown shear to work in, less chance for extra dry air to work in. Whatever happens here, it will absolutely beat the intensity guidance. As for why the guidance is low, few possible reasons but frankly my best guess is low initial intensity. I've watched the guidance forecasts all season and they rise and fall in line with the ingested initial intensity. The thing is, it does NOT take THAT much to get a storm to grow stronger in a low shear, high water temp, moist environment, and you have better than chance odds by betting above the model consensus. If the eye feature closes off, which it looks poised to do, ya know, go nuts. As I've said all along, I'm definitely most interested in what this thing does inland when it phases with the continental low ejecting from the rockies. Very Isaias/Sandy-esq in the sense of baroclinic interaction maintaining strength for a LONG time. This storm won't produce 40" in a single spot, but the phasing is going to mean tens of millions of americans are going to see significant impacts from these storms. 2, 4, 6" of rain, some areas a couple feet of snow...these are large totals and will cause widespread spatially, but not dense, power failures, flooding, wind damage, etc over a HUGE swath of the eastern CONUS. Obviously you're also ingesting a bunch of vorticity, and if you take that, add some moisture, jet streak and a strong llj, you'll see a touch more tornado potential than with your average hurricane. Whenever a barotropic system phases with a baroclinic system, things get interesting. MU

I second this sentiment. Even before this thing consolidated, the modeled moisture and vort interaction with the trough and subsequent merger into a coastal low type system seemed meteorologically-interesting. You have a potentially strong moisture rich TS interacting with a digging large scale, fall/spring type baroclinic zone. It seems a bit like a Miller A in some ways. In another way it reminds me faintly of Isaias earlier this year which stayed rather strong once inland due to baroclinic interaction. Regardless, that was and is the thing I'm really going to be most interested in watching play out with this thing. As for Zeta and its direct impacts, it really has a great environment right now. The faster it gets organized, the more it can feed off bomber ssts, and the stronger it would be at peak strength. Given the marginal environment in N. Gulf, the degree of strengthening attained in Yucatan will portend the ending strength of the likely-weakening system at landfall. Like Delta, current environment would favor RI if the storm already had its circulations well aligned. You wouldn't see Delta-level strengthening which was 75kts/24hrs, but RI is 30kts/24hrs, and the environment is clearly capable of that if a well organized system were feeding off of it. You can clearly see available energy is very high due to the persistently strong (aka high topped/cold topped) convection on imagery, shear is low, and upper level outflow is good. But, thus far, Zeta is still trying to organize and as such, its center is chasing the strongest convection and not well defined or well fixed. It will be interesting to see what hurricane hunters find on their next flight. My guess is, they find it's stronger than they think. No scientific explanation for that, just seems that whenever the hhunters go into a storm, it's "significantly stronger than satellite and other data had suggested", lol. Once it better defines its center and that center begins getting steered some direction, we'll see more steady to quick intensification until the storm reaches the gulf. MU

Subsidence? Turbulence? Overturning of the pbl as it switches to the nocturnal scheme? Dry microbursts driven by ingest of dry air? Could also be similar to other hurricanes where the segment over water maintains some intensity for a bit even when the center has moved inland. You've also got strong mid level and upper level winds, which at this point could play a role both in upper level mass transport and in the vertical wind currents in the storm. Not really sure which of those it is, if any, just sort of thinking out loud. Your basic physics for this requires sinking air to transport the momentum/energy from the 925-850mb layer to the surface, so just thinking of what would do that without precip. MU

Okay that makes more sense. What you had said in the OP with the link confused me a bit because I thought you were implying those were predicted values, e.g., forecasted, not just the regular tides. MU

That comparison is a bit misleading. Looking at the "predictions", all of those observing sites near the coast "predict" ZERO flooding. That is incongruent with the storm surge forecast. That seems like the "no storm" prediction to me, so I don't think the water levels are 4' higher than the storm surge warning indicates. Still, a good link with good data. MU

Actually, Laura was a bigger storm. TWC did a direct overlay a couple hours ago, the hurricane force wind field, and the TS wind field, were larger at landfall with Laura. Delta is still an above average sized hurricane though. The storm is tracking at a different angle and speed, that is why different areas are going to see the eye. With laura the eye came straight in, rather than diagonally. W.r.t the landfalling major thing--I don't think almost anyone thought this would hold up as a cat 3 that well given the hostile environment. The consensus was upper 2, it may end up mid or low 2, so, not that far off the mark. We are so used to seeing strengthening landfalling cyclones the last few years in pristine environments that one that behaves in a weakening manner is almost beyond comprehension and lends itself to forecasting a high intensity bias. The nhc, most people on here, figured it would be a cat 3 in the middle gulf--it was--and would drop intensity on approach to the coast--which it is doing. E.g., this isn't earth shattering and unexpected. It would have been surprising if it HAD stayed together given that the entirety of the guidance weakened it to cat 1/2 at landfall. Looking at the satellite and radar presentation though, that seems to be too little, too late in terms of impacts (which also is as expected). I think 100 at landfall is probable, though it may be marked at 105, however I think the highest observed gusts will be mid 90s because I don't think the odds are high of an observing station being perfectly situated along the coast to capture the absolute max winds this thing produces anywhere and obviously once inland you immediately deal with viscosity effects that bring any storms wind speeds down, regardless of cat + weakening trends and SFMR measurements are lower than the estimates based on FL winds. I can see from the radar that Beaumont, Cameron, and Lake Charles, are already seeing damaging winds and flooding rain probably entering into and damaging already damaged structures. MU

Well, it is 2020, so, hey, ya never know. I think this may be a bit more reflective of the inherent limitations of the ACE metric than an indicator that the season wasn't hyperactive. With any index, you almost invariably have some kind of bias. If you weight by, say, rainfall produced, you'll get a different measure than using wind speeds. If you measure by number of hurricanes landfalling on the us coast, that's again another number. For the us population, I'd say that last one probably makes the most impact. That said, it is notable that despite this season surpassing 2005, which had the 2nd highest recorded ACE since record keeping began (250), this season has seen a prodigious number of storms but very much lacked in terms of intensity. The ironic thing here is what happened is antithetical to the current going climate change hypothesis (of which I used to be a researcher). The modeling shows, generally, less storms, but more intense ones. Though lately there's some back and forth on that. This year we've seen more storms, but less intensity. I don't view that as an indicator of lack of climate change though. More that weather doesn't exist in a vacuum. Developing La Nina and various teleconnection phases play a role. In any case, I think it's likely safe (but 2020 again so, lmao, who knows), to assume that Delta is the last of the long track high-impact majors of this season. I don't think we can say with certainty it is the last major *period*, as the Caribbean is still warm as you allude to. We also can't say it is the last US impacting storm. Sandy was October, Michael was October. The reason it could be the last major impacting the us has to do with the water temps along the gulf coast and Florida. If another major hit it would need to hit Florida, probably the southern part, so it's statistically unlikely just because of the number of things that would have to go "right". You also still can't totally count out the mid-atlantic. We still have a ways to go, so a useful metric if one were able to find it, would be seeing comparable average and hyperactive ace scores *to date*, rather than for the season as a whole. That would better articulate how above/below we are tracking relative to a hyperactive season based on wind intensity. The thing that retards long track cape verde storms, and storms in general, this time forwards, is shear. Certain parts of the ocean are still warm enough to support tropical development and will remain so, likely, for a couple more months. In my research experience, things like the MJO and other teleconnection phases are useful when it comes to figuring out when the jet stream will be more zonal, more meridional, faster slower. Thus, it is conceivable to hit 152 still. One notable thing here is that the ace index biases towards intense cyclones, since the wind speed is squared in the calculation. That means the ace score is parabolic (exponential, though I like saying parabolic as it is more specific) with increasing storm intensity, not linear. I'm not sure that's the *wrong* approach to take (you have to weight things somehow), but, I think this season will be considered hyperactive regardless of the final ace score. MU

It's stronger, it's weaker, it's stronger, it's weaker, let's callll the whole thing off ba da da dum! Honestly I foresee a depression by tomorrow*. So much for Delta being a smokin' hot piece of, storm. In all sincerity this has been a very interesting storm to study and watch, from the RI into a 4 with atypical eye presentation, the sudden dramatic weakening, the wind field expansion, the models basically nailing at a 4-5 day range the track for landfall but concurrently being so awful with intensity estimates. Not a boring one to watch or forecast that much is for sure. I am hopeful/heartened by the mass evacuations from the gulf ahead of this because people were so traumatized by Laura that loss of life on this will hopefully be far reduced. At the end of the day, that's the best possible outcome here. Mets have their strong storm at sea, and people on land don't die. That is why forecasters exist at the end of the day--it is to protect the public. Well, and help stock traders know when to sell shares of Tropicana. I really feel for some of those folks. Listening to their interview segments tonight as they fled again after going through Laura was sad to listen to. I hope they get a break soon, they've definitely endured a lot. *Also sarcasm. MU

Hard to say. The pressure is down a couple more mbs, sat presentation better, flight level winds up. Not hard to think some of that mixes down somewhere. I think the sfmr data support 100 kts, and the flight level winds support 110, so they split the difference. MU

Yes we are.

Wow they were really early with the 11pm advisory tonight!

Delta must be positively glowing from all the compliments we are giving it about its figure! Clearly those at-home latent heat releasing exercise videos have been paying off! Whew, is it hot in here or is that just Delta's eye?

Nah, it works, they're live:

Wow. That's a fantastic resource, I never knew they posted the live feeds! I'm both grateful for the link and frustrated at the upcoming lack of productivity I will have at work next time there is a high risk event. Regardless, thanks!!!

Yes, it has been for hours in Brownsville. It's only so useful at the 248nm range. You can't see the velocities, not that that would be super helpful given the elevation you'd be at at that distance. I wish the mobile doppler coming in from OU would be integrated into the nws system so we could see it live, but I don't think that's possible. If I'm wrong and someone knows about that please feel free to chime in. Latest pressure in the eye reported by the way, 955mb.

Yeah, if it sounds like I am downplaying the wind threat, I apologize. I am just meaning there should be significant weakening versus what the temporary maximum intensity Delta may attain in the short-term. I would be surprised if it makes landfall as a Cat 3 or at least is able to sustain the kind of convection to mix down stronger radial winds aloft at that time. But of course you're correct. Even 60-100 mph gusts is going to be a nightmare for debris and already weakened structures, roofs, etc. Still think surge is the worst danger however due to the ever expanding size and fetch east of the core. Lol, not just you. I chimed in because it was a trend in the conversation. I agree the surge is the worst of the many impacts. I'm just saying, we are at the point in the forecast where we are looking at landfall impacts rather than the pure academic strengthening vs weakening. It's true that weakening storms don't mix winds down as well, but they do still do so. Dry air entrainment hasn't happened yet, and the models have not been perfect with this system. Not saying that won't happen, just that, the storm has defied a couple expectations already. First, shear is tracking a bit lower than forecast before, and the track is taking it over marginally warmer waters, by a degree, than previously thought, and that also does make a bit of a difference here. As you and others correctly mentioned before, a stronger initial intensity leads to a stronger final intensity, all else equal. If delta strengthens further, there's only so much time and space for the storm to weaken. It will weaken. But, it's a wind threat too. A hurricane is categorized by wind speed. It will be a hurricane. MU

Oof. Okay folks, this isn't a low impact event. Reading on here right now I'm getting-- half a cane, it's gonna be a shell of its former self, does anyone even LIVE in those areas? Yeah, actually they do. The models do weaken the storm near landfall. The shear on the latest guidance is actually a bit less and does not act until landfall. Ergo, no, this is not mostly a surge event. As was just posted above, and is also being covered on most news and weather outlets right now--there are PILES of debris, 2x4s with nails jutting out, piled up in lake Charles. Even if it only got 60mph sustained winds, those are going to fly around. This will be very messy. And they'll see stronger winds than that, cat 3 or not. The nws is estimating 100mph gusts. That is reasonable. The current trend is for a high cat 2, low 3 landfall, weakening or not. This will be very damaging, and emergency managers are treating it accordingly. ***This will be the second most impactful storm this season after Laura, albeit Sally was a prodigious rain producer. In 2021, 2020 will be remembered for Laura, Sally, and Delta, as standouts of a busy season. **. Perhaps Isaias will be remembered for its slow rate of weakening and interesting baroclinic interaction that led to the NE getting socked. Anyways, remember, hurricanes are not points. The rmi/rmw here is 30ish+ miles. Your center coming ashore east of Lake Charles matters dittily squat in that context. Though they won't get the right front quadrant of the eyewall--if the storm tracks perfectly along the tightly clustered guidance--they still get the eyewall. It isn't Laura. Laura was stronger, bigger, lasted longer. But in a normal year, this would be the storm of the year. The only thing here that is "good" is it is a fast mover. It won't linger too long and rainfall totals will be a "modest" 6-10". MU

Lol, well, in the shorter term, there's Invest 96E off the African coast that has a decent surface circulation and actually had some significantly organized outflow going on earlier this afternoon, but it's a little less vigorous right now with the deep convection more displaced from the center to the west due to shear. It has a couple days of ok conditions to strengthen but then may struggle. Plenty of possible places for epsilon.