Weather Ready Nation: Tornado Warnings by CWAs
Tonight’s blog post depicts the number of tornado warnings issued by year by CWA. I should add a disclaimer: I am using the current CWA boundaries for all prior years. Thus, it one runs the risk of making incorrect assessments if one directly compares the specific increase in CWA tornado warnings counts. Instead, the idea is to look at the general trend in the number of warnings issued by forecast offices.
Data for 2007 and later years need to be understood in the context that WFOs have switched from county-based warnings to storm-based warnings. Thus, it is possible to have situations where multiple storm-based warnings are in effect when in the past a single county-based warning would have been issued. The flip side to that is, a single storm-based polygon can now capture 2 or more counties, which would have required multiple county-based warnings in the old system.
As I mentioned last night, the number of warnings issued can be thought of as a cumulative measure of the number of times the NOAA Weather Radios would have sounded in a specific CWA. Thus, the increasing number of warnings issued can be thought of in terms of the National Weather Service requesting more responses from the residents in their CWAs.
Weather Ready Nation: Counties Warned for Tornadoes by CWAs
In the wake of last night’s post, several questions were raised regarding the time periods of the two images. The biggest concern was that you can’t compare a 22-year average and a 3-year average, as the 3-year average is more easily biased by one or two high years than the 22-year average. Tonight I’ve added a plot that shows the number of counties under a tornado warning by National Weather Service County Warning Areas (CWAs) broken down by year.
Starting with 1986 and advancing toward 2010, one can easily see that the number of counties warned by CWA has increased substantially, with most of the increase occurring in last few years. Although this isn’t the same as the number of tornado warnings (because a single tornado warning could affect multiple counties), it is still an important number. This is because it represents the number of times NOAA Weather Radios would have sounded in each CWA. Why is that? This is because the NOAA Weather Radios, as they currently stand, cannot activate based on the storm-based warning. They are activated based on the county-level warning. Thus, two small storm-based warnings issued for the same county would require two activations of the NOAA Weather Radio for that county, even if the warnings were issued only minutes apart. Thus, for a large portion of the population, the county-level warning is what they would receive.
Tomorrow I’ll post similar maps but instead of counting the number of counties warned, I’ll count the number of tornado warnings.
Weather Ready Nation: Tornado Warning Frequency
Today kicked off the first day of the Weather Ready Nation: A Vital Conversation. (OU is recording and posting the presentations on the web.) Dr. Harold Brooks of the National Severe Storms Laboratory really got things going with a presentation on our known challenges. One of his main take-away points was that the number of tornado warnings issued has dramatically increased in the recent era. To illustrate this point, I provided two county-level heat maps (below). The top figure is the average number of tornado warnings per county per year from 1986-2007. The bottom figure is the same figure except for 2008-2010. As you can see, the average number of tornado warnings per county per year has increased almost uniformly across the county, although the increase is much larger in some areas and almost non-existent in others. Whether or not this is an improvement in National Weather Service “service” is one of the topics open for discussion in the following days.
I’m sure I’ll create more figures in the coming days.
Tornado Emergencies: The Map
Below is a map of all tornado emergencies issued since 1999. Keeping with my definition of a tornado emergency, the polygons plotted are the polygon described by the “LAT…LON” tag at the bottom of either a) the first severe weather statement (SVS) that contained the “Tornado Emergency” phrase, or, if no “LAT…LON” tag was present in the SVS, b) the parent tornado warning (TOR). If the “LAT…LON” tag was not present in either the SVS or the TOR, I used the county listed in the original TOR. This only happens 3 times: Ochiltree County, TX (2000); Johnson and Marion County, IN (2002); and Hancock County, IN (2002).
I’ve done more analysis, but will save that for later.
And, since I know I’ll be asked, here is the same map but without 2011.
Data Mining Tornado Emergencies
Last night I posted a call for help from fellow meteorologists and weather nerds to help me acquire the tornado emergencies that I was missing. Wow! Thanks to the help of Jason Kaiser, Daryl Herzmann from Iowa State, and Rick Smith from National Weather Service (NWS) forecast office Norman, I now believe I have all tornado emergencies in my possession! Now the fun part of analyzing them begins! Since many people are already aware of my project, I decided to go ahead and post two quick graphics that I think will be of interest.
First, a bit about my method. For this analysis I treat the tornado warning and all subsequent severe weather statements associated with the tornado warning as a single episode. Since a tornado emergency can be issued in the text of either a tornado warning or a severe weather statement, all tornado emergencies issued within a single episode is consider a single tornado emergency. Consider the following scenarios:
Scenario 1: A tornado warning is issued for 2 counties and does not contain the tornado emergency text. Subsequently 4 severe weather statements are issued. The second severe weather statement contains the tornado emergency language for county A but not county B. The fourth severe weather statement contains the tornado emergency for county B but not county A. How many tornado emergencies will this count as?
Using my definition, even though 2 tornado emergencies were technically issued, they are both contained within a single tornado warning and thus this is treated as a single tornado emergency.
Scenario 2: A tornado warning is issued for county A and does not contain the tornado emergency text. Subsequently 2 severe weather statements are issued. The second severe weather statement contains the tornado emergency language for county A. As the tornado moves into county B, the NWS issues a new tornado warning, without the tornado emergency language, for county B. Subsequently, the NWS issues three severe weather statements for county B, all three containing the tornado emergency language. How many tornado emergencies will this count as?
Using my definition, even though 4 tornado emergencies were technically issued, they are all contained within the framework of only two tornado warnings. Thus this is treated as two tornado emergencies — even though this is for the same tornado.
Scenario 3: A tornado warning is issued for county A and does contain the tornado emergency text. Subsequently 2 severe weather statements are issued, both of which contain the tornado emergency language. How many tornado emergencies will this count as?
Using my definition, even though 3 tornado emergencies were technically issued, they are all contained within the framework of a single tornado warning. Thus this is treated as one tornado emergency.
Does that make sense?
With that said, there have been 143 distinct tornado emergencies issued since the first one on 3 May 1999 for south Oklahoma City, OK. The chart below breaks them down into yearly counts:
Examining tornado emergencies by NWS forecast office, the top three are
- BMX (Birmingham, AL)
- HUN (Huntsville, AL)
- JAN (Jackson, MS)
This might be a bit surprising to many who typically think of “tornado alley” as being in the central United States. Unfortunately, or fortunately, you decide, many things go into the decision to issue tornado emergencies. Some of these things include population potentially impacted and local office policies. Remember, the tornado emergency is not a formal product. It wasn’t until the last year or two that local forecast offices has official guidance in place. Bottom line, not every office uses tornado emergencies. (Also, keep in mind the magnitude of the 27 April 2011 tornado outbreak and where that was located! However, to be fair, I will say that all of the top three offices had issued tornado emergencies prior to 27 April 2011.)
The rest can be found in the chart below:
In the coming days and weeks I will be doing more analysis on tornado emergencies and will post findings as they are completed. If there is something you would like to see, leave me a comment and let me know. If I don’t already have plans to examine your request, I’ll add it to my list!
7 November 2011 Tornadoes
The first tornado of the day is ongoing across portions of southwest Oklahoma. The tornado vortex signature (TVS) is located just to the northeast of Tipton, where the brighter reds and greens are side-by-side. Additionally, I should point out that the Oklahoma Mesonet has lost communication with the Tipton Mesonet station. With visual confirmation of a tornado in the vicinity, this would seem to suggest the tornado moved very close to, if not hitting, the Tipton Mesonet station. This would not be the first tornado to hit an Oklahoma Mesonet station this year. Back on 24 May 2011, the El Reno, OK EF-5 tornado moved close enough to the El Reno Mesonet station that debris wrapped around the Mesonet’s observing site. The strongest measured wind gust in Oklahoma history was recorded by the El Reno Mesonet station during the tornado’s passage.
Meteorological Detective Work: Using All Your Tools
Tropical cyclones are giant, yet complex, heat engines driven by the release of latent heat. In a simplified context, here is how this heat engine works:
- Thunderstorms develop over the warm tropical waters in an area of weak vertical wind shear. This results in thunderstorm updrafts being nearly 100% vertical.
- As thunderstorms continue to develop, latent heat is released in the middle troposphere. As a result of this mid-tropospheric warming, updrafts become stronger.
- Because air is rising faster than it is being replaced at the surface, the pressure at the surface decreases and a surface low-pressure develops.
- As a consequence of the developing surface low-pressure, thunderstorms begin to congeal and rotate around a central point. Additionally, air at the surface begins to converge into the center of the low-pressure.
- The increased surface convergence results in additional rising motion, meaning more thunderstorms, more latent heating, and further decrease in pressure.
Throughout all of this, the center of the tropical cyclone is located at the same horizontal location as a function of height. This is often referred to as a “vertically stacked cyclone”.
The processes described above continue until a balance is achieved, or something changes in the environment. Some negative environmental changes are cooler water temperatures (resulting in cooler, drier air being lifted), landfall, or an increase in shear. The reason an increase in shear is bad is because it tilts thunderstorm updrafts which acts to weaken the updrafts, in turn weakening the amount of latent heating.
The environment around Hurricane Irene has changed completely from a few days ago. Irene moved over land, is moving into cooler waters, and is experiencing an increase in vertical wind shear. How can I tell the latter? From utilizing the radial velocity from area Doppler radars.
In a radial velocity image, the wind is either toward or away from a point (in this case the point is the Doppler radar). In the image below the doppler radar is the black dot in the center of the image. Pixels that are green to blue in color depict air that is moving directly toward the radar with green pixels indicating slower motion than blue. Pixels that are red to orange in color depict air that is moving directly away from the radar with red pixels indicating slower motion than orange. Pixels that are grey in color indicate air that has no component of motion toward the radar. This does not mean that the air is not moving!. It simply means that the air is not moving toward the radar. It might be moving very quickly, but is completely parallel perpendicular to the radar beam! Utilizing this fact, the giant grey “S” like shape down the middle of the image means that the wind is predominantly parallel perpendicular to the radar beam at that location. The black arrows indicate the wind direction along the grey “S” like shape.
One other fact to remember about radar interpretation is that because the earth is curved, the radar beam actually increases in height as it moves away from the radar itself. Thus in the image below, areas near the periphery of the image are at a higher altitude than areas near the center. Combining this fact with the wind directions from the black arrows, we can infer that the wind is changing direction from east-northeast at the surface to almost due south at some higher altitude.
Before I'm bombarded with complaints, I do not mean to imply that the center of Irene is located at each of the L's exactly. It is merely an approximation of where the center may be with increasing height based on the cyclostrophic balance. Other forces are at play, especially since the cyclone is transitioning from tropical to extra-tropical.
Now, let’s think back to the heat engine process described above. If the tropical cyclone is “vertically stacked”, and we assume the wind is cyclostrophic (meaning it is perfectly circular) about the center of the tropical cyclone (which is a good first order approximation), the wind would be in the same direction no matter what height we examined! If we looked at a radial velocity image, the grey “line” would be a straight! This is not the case with Irene. In fact, using the cyclostrophic balance, we can determine the approximate tilt with height of the center of Irene. This is denoted by the giant “L” on the image above. (The short arrows between “L” locations indicates the path from surface to higher altitudes.)
This tilt with height indicates that Irene will most likely not strengthen (at least not significantly) as it moves back over the ocean. Furthermore, this tilt with height probably indicates that Irene is undergoing a transition from a tropical cyclone to an extra-tropical cyclone. However, discussion of the differences in the types of cyclone and the transition process will be left to a future blog post.
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