Felicia refuses to give up and intensifies even further due to a southward jog. The movement provides a higher sea surface temperature for further intensification. By the Latest Force Thirteen Analysis at 12:00UTC we believe Felicia has intensified into a category 5 hurricane with winds of 165mph. However, the latest NHC update statement has Felicia at 145mph, both by satellite estimates. In this article, the reason behind the 20mph difference will be explained.

Current Storm Information

As of the latest Force Thirteen Analysis (12:00 UTC July 17), Felicia is located at 14.5N 124.3W, with maximum sustain winds of 165mph (265km/h) and a minimum central pressure of 932mb. Felicia is moving WSW at 9mph (15km/h).

Meanwhile, as of the latest NHC update statement at 13:30UTC, Felicia is located at 14.5N 124.3W, with maximum sustain winds of 145mph (230km/h) and a minimum central pressure of 947mb.

Felicia has been going all out in the last few hours. Due to the southwestward movement, it has reached a lower latitude where the sea surface temperature increases from 27C to 28C. Although it’s just 1C, it is a world of difference in terms of energy supply. Given other environmental conditions remain favorable, after maintaining its intensity during sunset, it developed as an annular hurricane with overshooting tops and surface gravity waves, along with a very warm (20C) and dry eye, which such characteristics in cloud tops can only be seen in extreme intensity. In short, that is the reason Force Thirteen has given it a 165mph rating.
Yet it will weaken soon due to decreasing sea surface temperature and the latest microwave imagery shows an eyewall replacement cycle is imminent.

Full forecast discussion from Force Thirteen Cyclone Analysts.

Eye

Felicia is simply enough expressing all the hallmarks of an extremely intense tropical cyclone. The storm has maintained a structure that is actually analogous to past 145kt tropical cyclones, like Hurricane Isabel of 2003, for several hours, along with various other 145kt+ cyclones that would help aid in concluding this storm’s current intensity of 145kts. Further analysis of this storm’s structure finds an eye that is currently perfectly symmetrical, and hovering at around 7-8C below the current sea surface temperature, at 19-20C, which is a deficit only in more intense tropical cyclones because they normally have the lift modes and such to cause such subsidence to clear and maintain it. This system has maintained such an eye for 12+ hours and counting. Little storms have done this with such success.

Central Dense Overcast (CDO) structure

The storm is currently maintaining and improving a full-fledged gravity wave-type CDO structure with several clustered overshooting tops, which can assist the vertical transport of momentum within the atmosphere, and gravity waves can also mean that there is more localized latent heat release in the storms, which more latent heat release signified by these gravity waves would lead to a thermal expansion of the atmospheric column, and deepening of the storm, therefore having a better structure for both rapid deepening and mixing. The storm also had many overshooting tops in the north quadrant during peak, which is another good thing for mixing of stronger winds, signifies a greater depth of stratospheric cooling being achieved through adiabatic lofting and increases the intensity of turbulent mixing across the troposphere, which would assist mass transport across the area of the cloud tops. This also easily implies higher cloud tops, which would signify greater lapse rates and a more intense updraft to help the storm while rapidly intensifying. Such a CDO is really only seen in 145-150kt+ cyclones, and this pattern is fairly analogous to the likes of even Hurricane Irma of 2017. The symmetry and the lack of variability of cloud tops in the -74-75C CDO also highlights the presence of neutral buoyancy which shows an extremely stable and healthy storm. Microwave imagery also showed a full red ring, which along with the fully initiated CDO provides great support for a higher intensity than 140kts, as many systems with 150-155kt+ strength enter an axisymmetric quasi-isentropic mode, which is when lift symmetrizes due to the theta e surface between the eye and eyewall. That would help keep the cloud tops of similar temperatures, with more uniformity, once again due to the theta e difference between the lower theta e eye, and higher theta e eyewall. This all allows idealized isentropic lift which is something many 150kt-155kt+ cyclones exhibit due to their maturity.

Microwave and Water Vapour Analysis

This also relates to MW symmetry because of even stronger updrafts throughout the entire eyewall, which would scatter ice to around the same temperatures. The storm is also currently exhibiting an azimuthal arch in its northwest quadrant, which as of lately, hasn’t been seen in any cyclones at all below 150kt intensity, which helps boost the intensity. Water vapor imagery has also revealed likely extreme intensification with a rapidly drying eye, drying to around -15C, which is also only seen in more intense tropical cyclones. Although the intensity correlation spread is somewhat large, once again -15C and below hasn’t been seen in storms below 145kts. The storm is also expanding its CDO actually, which isn’t in the form of an eyewall replacement cycle due to microwave imagery confirming a singular eyewall. Many structural regimes of 150kt+ cyclones show great similarities to Felicia, and the intensity is adjusted as such. Taking into account the satellite presentation and the fact that many 145-150kt+ systems have exhibited structural similarities to Felicia, leading to the fix of 145kts at this time.

To summarize, the sheer impressiveness of this structure, signs showing that it’s not near finished yet, analogs and correlations contribute to the storm’s 145kt strength.

The reasoning behind the official estimates

The NHC, as an official meteorological agency, has to follow protocol. Without direct observations from reconnaissance plane, surface, or radar observations, the  Subjective and Objective Dvorak Technique are their only tools for intensity estimates. The latest Dvorak estimate from the Satellite Analysis Branch (SAB) and Tropical Analysis and Forecast Branch (TAFB) has estimated a Final-T number of 6.0 (WMG eye surrounded by B embedded by LG) and 6.5 (WMG eye embedded by B), respectively. Though they also notice the annular structure and an eye with significant stadium effect, which both factors may lead to a higher intensity.

The ineffectiveness of the Dvorak Technique in this specific case

Dvorak Technique, invented in 1984, is considered to be one of the most effective estimation methods by infrared enhanced satellite imagery. It has been extensively using in all meteorological agencies. But why it is considered to be less effective in this case?
The first reason is the overall size. Felicia is a very tiny storm with a CDO of only around 90km in radius. However, step 2C in the Dvorak Technique has stated that the ring of B and W spectrum (representing -63 to -69, -69 to -75C cloud tops respectively)  to be at least 0.5 degrees wide (around 55km) to be classified as the eye number. Given that, such a small system is hard to generate such a cold ring that matches the threshold thickness to be considered as the eye number, only the warmer ring, light gray (-53-63C cloud tops) can be counted, which resulted in a lower estimate from Dvorak by SAB and TAFB.
The second reason is the low tropopause height and annular characteristics which resulted in generally warmer cloud tops but a higher intensity in actuality. The coldest temperature in the tropopause is estimated to be around -73C, which means -73C is more or less the limit of the cloud tops and falls under just the W category in Dvorak. So, it is hard for Felicia to generate thick enough overshooting tops and maintain it as a T7.0, which resulted in the underestimation from Dvorak. What is more, Felicia has exhibited annular characteristics, which in most past cases like Felicia’s structure, even with warmer cloud tops overall and surface gravity waves, recon found significantly higher winds than what Dvorak Technique estimated, which Felicia probably is not an exception. Hence from the above two reasons, it’s highly likely that Felicia is being underestimated by the Dvorak Technique which is the main tool that the NHC uses.

2 Comments

  1. I used to work directly with Vern Dvorak in NESDIs from 79-86. CIMMS and NHC have far expanded the initial technique developed on hardcopy images into the digital data. Agree it is a CAT 5 now but cat 5’s are inherently unstable and generally weaken by 24-36 hours out. Good reasoning in your writeup

  2. Good review on whats going on and using past data to reason why this is a C5.

    I also saw a few tweets that said the same thing to this as well, but this goes more in-depth on how Felicia is being underestimated by the NHC.

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