10/10/2018 - Hurricane Michael

Dan Robinson

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So, we have a little bit of an El Reno-esque controversy brewing on Michael's intensity. I'm not as well versed in the nuances of hurricane ratings as I am with tornadoes, so I've been trying to keep up with the discussion between the more "in the knows" about whether or not Michael will be upgraded to Category 5 status in NHC's post-season analysis. Here is what I've been gleaning so far:

Arguments in favor:

- The worst of the observed damage is consistent with Cat 5 winds

- Hurricane Hunter aircraft SFMR instrumentation measured 138kts (158mph) in the *southwest* quadrant of the eyewall during landfall, that's exceeding the Cat 5 threshold on the *weaker* side of the eyewall.

- The storm's satellite/radar presentations (Dvorak) strongly suggest a strengthening Cat 5 at landfall, even post-landfall intensification is noted

- Chaser/research group pressure readings suggest a minimum at or below 918mb at landfall, lower than the official reading.

- Since the one "solid" recon measurement of 135kt (155mph) is just below the threshold, it is highly likely that some other part of the storm exceeded this.

Arguments against:

- SFMR readings can be contaminated by two factors: 1.) "shoaling", which is breaking waves on the ocean surface due to shallow water, and 2.) heavy precipitation rates. Either of those can artificially increase the SFMR readings, and if it is found that either/both of those were present, the 138kt reading must be disqualified as a factor in the final analysis.

- Satellite/radar presentations are not considered in official ratings.

- Ratings are rounded to the nearest 5kt, so the evidence will need to show at least 137.5kts before the storm will be reclassified at 140kts (160mph).

- In the modern era, most of the relevant data used for ratings is available to the NHC in real-time. In other words, it is rare that some rating-changing factor is missed by the official evaluation at the time of landfall. It's not impossible for something extraneous to surface later, but the bar is very high to overturn the operational rating.

Relevant Twitter thread:

https://twitter.com/JustonStrmRider/status/1050783750932566019
 
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rdale

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I don’t think the ASOS units have been interrogated... I’m under the impression that they still record data as long as they are powered - but obviously unable to transmit that out.


Sent from my iPad using Tapatalk Pro
 
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MClarkson

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This storm was well sampled by the eglin radar and doppler velocities were consistently in the high end cat 4 range during the landfall with winds ~4000-6000 feet up of about 160kt. Following a standard flight level reduction this places surface winds in the ~130kt range.

There were isolated occurrences of stronger pixels, so brief cat 5 winds could have occurred. I would personally call it a 135kt cat 4. But its close enough that I don't have a very strong opinion... measurement error could easily cross the threshold between 4/5 in this case.

Interestingly the traditionally much weaker left side of this storm was only about 5kt weaker than the right side. Stronger convection in the left eyewall looks like it almost overcame its geometry.
 

Dan Robinson

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This "Preliminary Virtual Assessment" (VAT) report was released yesterday and is making the rounds. It is by the "StEER" or the Structural Extreme Event Reconnaissance Network.

http://windhazard.davidoprevatt.com/wp-content/uploads/2018/10/StEER_-Hurricane-Michael-P-VAT-Report.pdf

I have never heard of this group before and I am not sure what role their report plays in the final analysis, but this report maps the maximum *gusts* at 140mph in the SE quadrant of the storm at Mexico Beach (low-end Category 4 to high-end Category 3) and 110mph max gust in Panama City (low-end Cat3 or high-end Cat 2).

Those seem really low based on all of the other information, observed damage and chaser reports, including what I witnessed myself in Panama City. Although I will admit I have no experience with visually estimating winds at that speed (110mph+). How El Reno-ish would *that* be if Michael is re-analyzed as a Cat 3!
 

Mark Blue

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The Tweet is prefaced by *preliminary* so it isn’t written in stone just yet. It does feel like there is a contingent pushing for the Cat 5 rating based on the damage they witnessed on the ground. Is there room for two flies on the wall? ;)
 
Not sure of the source, but I saw an analysis on TWC this morning that suggested sustained wind of 149 mph just offshore from Mexico Beach. It was suggested that once the storm was onshore, peak gusts in Mexico Beach may have been around that intensity. I think the surface wind offshore was estimated from a measurement higher up, but am not sure of the exact methodology. This again seems pretty close to what others have said about the "maximum stained wind" in the NHC bulletins approximating the peak gusts once the storm is over land.
 

MClarkson

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According to radar the peak winds were just northwest of Mexico beach and hit unpopulated territory. Even accounting for that, the VAT report above is likely too low on max wind speeds. Other people have pointed out some obvious inconsistencies with their report. I would like to add that one should probably take with a very large grain of salt any report that mistakes strait wing propeller planes for "fighter jets". See page 16.
 
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rdale

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Radar doesn't observe winds at the surface though, and in hurricanes it is not an exact correlation.

And I'm unsure what you are suggesting about the fighter planes - many F22s were damaged at the base.
 
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Dan Robinson

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I don't know, but they cited a method covered in a journal paper "W&F in 2009 by Powell, Kepert and Uhlhorn". Will see if I can find it.

Sudduth had an RM Young station mounted on the Hathaway bridge that recorded 93mph sustained winds with a 110mph peak gust. Although it appears that the station was in the middle of the bridge (its highest point). I couldn't find a height stat for the bridge, but the vertical clearance for ships is listed at 65 feet. That would probably make the weather station around 80-90 feet or so. I asked Mark if he knows the elevation. EDIT: Mark said the station was in the middle of the bridge.
 
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This is probably a stupid question but what is the “official” determinant of wind speed for purposes of assigning a category? With tornado ratings, as we all know, it’s damage, to the exclusion of all else really, for example El Reno where the mobile Doppler reading is literally disregarded because of the lack of damage indicators. So what is the overriding factor with hurricanes? If the damage indicators clearly show Cat 5 wind speed, will that take precedent over obs, radar, etc.? What criteria take precedence?
 
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Dan Robinson

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Some interesting info RE the recon data, including maps plotting the locations of all of the Hurricane Hunter data points:

https://canetalk.com/2018/10/1539295892_1539287470.shtml

I could not find the sources of those Google maps with the data points. The 138kt SFMR reading may have been far enough offshore to not be affected by shoaling, but the NHC still considers it suspect. Something that the reanalysis will have to cover.

James, from what I understand, there are several factors used in ratings including:

- Hurricane Hunter flight-level winds (ground-level winds are calculated at 90% of flight level)
- Hurricane Hunter SFMR wind instrument readings (though these can be contaminated by shallow water surf/waves and heavy rain)
- Direct on-land anemometer readings
- Barometric pressure
- Observed damage

All of those are taken into account, though some less than others. The flight-level wind values, uncontaminated SFMRs and direct anemometers are the higher-weighted factors. The damage rating process is similar to the EF scale but uses the Saffir-Simpson damage descriptors:

https://www.nhc.noaa.gov/aboutsshws.php