TS Erin Final Report

[David Wagener]

For those that haven't seen it yet, or have been waiting for it, the final report on TS Erin is out:

http://www.nhc.noaa.gov/pdf/TCR-AL052007_Erin.pdf

Probably the most interesting part of the storm was the even over Oklahoma. From the reading, it appears there were too many questions on how/why it gained strength and it was lacking some tropical characterists. So they settled on calling it just a "low"...not a subtropical, extratropical, or remnant low...just a low. The biggest trick here was likely the duration of the event, to where it really wasn't all that long in duration.

Probably one of the more interesting storms in recent history, and I'm sure many Okies will contiue to claim it as a tropical storm regardless.

 

[Jeff Snyder]

The part most interesting and relevant to this very interesting storm:

While the system's structure, particularly its convective organization as seen on radar, resembled and had some characteristics of a tropical storm for a few hours on 19 August, the prevailing view from the Hurricane Specialists at the National Hurricane Center (NHC) is that the system was not a tropical cyclone1 over Oklahoma. The phrase “organized deep convection” in the NHC tropical cyclone definition has both spatial and temporal components, meaning that a tropical cyclone must produce deep convection over some period of time. While it is a subjective determination, in this case the deep convection is judged to have lasted an insufficient period of time to classify the system as a tropical cyclone. The limited duration of the convection also appears to be indicative of the physical mechanisms that caused the low to briefly strengthen. It is speculated that the upper-level shortwave trough forced the deep convection to increase via upper-level difluence, while briefly superimposed above the surface low that provided a focus for low-level confluence. The upper-level forcing was apparently a dominant mechanism, which is in contrast to tropical cyclones that are maintained primarily by extraction of heat energy from the ocean. Since the system was clearly non-frontal, designating it as an extratropical cyclone2 is also not the most appropriate solution. In addition, the prevailing view among the NHC’s Hurricane Specialists is that the system’s duration over Oklahoma on 19 August was also too short to classify it as a subtropical cyclone3. Given all of the considerations described above, the system is simply designated as a “low” by NHC on 19 August.

I know the folks at the NHC spent quite a bit of time with the Erin case, as evidenced by the time it took for them to release the final report. Indeed, this was a very interesting and atypical case! However, I'm not sure I can say I agree with the decision not to call Erin a "tropical storm" while it was over Oklahoma. It seems that there are several reasons cited by the NHC for calling Erin a "low" instead of a "tropical storm", including the influence of a shortwave trough and the length/duration of the increased intensity. In regards to the latter, there have been other tropical storms that were designated as such for a similar length of time. In the case of Erin, the NHC final report shows two "advisory times" during which Erin had winds >=35kt (06z and 12z). 2005's Jose was had winds >=35kt for 3 "advisory times", and I seem to recall a couple others that were tropical storm strength for a similar duration. Regarding the former, I'm sure we've all seen myriads of tropical cyclones that have been enhanced by shortwave troughs (some storms that have approached the US were significantly enhanced by approaching shortwave and longwave troughs). In the end, though, it seems that the duration of the "enhanced" Erin over Oklahoma was too short for NHC to classify it as a tropical or subtropical cyclone. Another minor point seems to be a sense that tropical cyclones must be located over warm ocean waters ("which is in contrast to tropical cyclones that are maintained primarily by extraction of heat energy from the ocean"). In the case of Erin, the release of latent heat of water vapor that was likely derived from evaporation off the Gulf waters likely played a significant role in Erin's intensification (as it does when cyclones maintain their intensity when over other lands such as the Yucatan Peninsula, Cuba, and Florida). Of course, latent heat of the condensation of water vapor is certainly not exclusive to tropical cyclones, as many extratropical cyclones are enhanced by the same mechanism. I have no vested interests in whether Erin truly was a tropical storm over Oklahoma, but I am surprised it wasn't designated as such in the final NHC report.

There's a good radar image of Erin (a good eye completely surrounded by convection) on the last page of the report --> http://www.nhc.noaa.gov/pdf/TCR-AL052007_Erin.pdf

 

[Kenny Drake]

I believe there are ongoing studies on Erin's re-intensification over Oklahoma. It seems like the NHC just avoided the concerns and left it as a "Low" due to all the uncertainties. It definitely looked warm-core per the WSR-88D from KTLX.

 

[John Cameron]

I didnt read the whole report...did it mention the tornado spawned by the remnants in Clay county, Texas on the evening of the 18th?

EDIT: I found this on Norman's web site