SPC Day 3 Moderates In Context

Posted by pmarshwx on 8 Apr 2011 | 2 comments

This morning, weather enthusiasts woke to a Storm Prediction Center (SPC) Day 3 Moderate Risk outlook for severe thunderstorms. Most weather enthusiasts already know a Day 3 Moderate Risk outlook is a pretty rare occurrence, but just how rare is it? The simple and misleading answer is that since 2000, and including today’s, only 10 Day 3 Moderate Risk outlooks have been issued. The more precise answer is a bit more complex.

The SPC is continually refining their products based on the state of the science and user feedback. As such the criteria for a Day 3 Moderate Risk outlook has changed over time. Currently, it takes an “Any Severe” probability of 45% and a “Significant Severe” probability of 10% to reach Day 3 Moderate Risk category. Previously it was possible to ascertain the Day 3 Moderate Risk with as little as an “Any Severe” probability as low as 30%, which has happened several more times since the change in probability criteria. Therefore, it is difficult to compare old Day 3 Moderate Risks to current Day 3 Moderate Risks. However, I’ve attempted to break them down below.

Below is a table of the date of issuance for Day 3 Moderate Risk outlooks:

Broken down by year:

For additional information regarding SPC Outlooks, this time focusing on Day 1, please check out the following posts from earlier this year:

• Average Location and Date of First SPC Day 1 Moderate and High Risks
• SPC Day 1 Moderate and High Risk Climatologies
• SPC Day 1 Slight Risks Climatology

Drought and Tornadoes In 2006

Posted by pmarshwx on 3 Apr 2011 | 0 comments

As many already know, the southern plains is in the midst of a fairly pronounced drought. The drought reminds me of the drought we experienced in Norman during 2005-2006. During the drought, the rain stopped in late summer of 2005 and remained that way through November of 2006, when a very intense cyclone helped to alter the prevailing storm track.

As we head into the spring storm season, I’ve been getting asked more and more frequently how the lack of precipitation in the southern plains will affect storm season. The answer to this question is highly complex, and poorly understood at this time. However, I’ll offer one possible outcome.

The lack of southern plains precipitation, and more importantly, the dry soil, might allow the dryline to mix eastward faster than if the soil held more moisture. This, coupled with warm air aloft being advected over the southern plains from the higher elevations of New Mexico (which has also been extremely hot and dry of late), will help suppress thunderstorm and tornado development across much of western Oklahoma and the Texas Panhandle.

This is what happened in 2006, and you can see based on the graphic below, most of the tornadoes were to the north and east of western OK.

April Tornadoes

Posted by pmarshwx on 3 Apr 2011 | 0 comments

As we head into April, it’s time to update the plot of tornadoes by county for the current month. These data have been updated to include tornadoes in 2010. (Please note the modified color scale compared to January, February, and March.)

As you can see, the southern plains is becoming more as we approach late spring!

March Tornadoes

Posted by pmarshwx on 6 Mar 2011 | 1 comment

After a long hiatus from blogging for most of February, I’m back at it as things appear to be slowing down just enough to let me have coherent thoughts again. This is fortunate since we’ve entered a new month and I need to post the March tornado graphics! This post should do the trick!

Above is a choropleth map of tornadoes by county. As we head into spring, warmer temperatures and more moisture begins making its way farther north and west than January and February. This allows for surface-based thunderstorms to develop farther north and west, and consequently we see a considerable expansion north and westward in the number of tornadoes. However, numerous tornadoes have been reported in the central valley of California.

If we break the tornado numbers down by state, we can see that Texas is the big winner (or loser depending on your point of view), with the other southern plains states having high numbers of tornadoes. The southeastern states have little change with the February numbers, making this area one favored area for tornado development.

The actual counts by state are broken down below

What about injuries? Keeping the same state ordering above (namely, in decreasing order of tornadoes), we can see the breakdown of injuries by state. For the third month in a row, Mississippi leads the nation in injuries, with Arkansas not far behind.

Examining the number of fatalities, Mississippi no longer leads the nation. Arkansas has the most number of March tornado fatalities, thanks in part to the 1 March 1997 tornado outbreak.

Lastly, here is a summary figure combining the last three figures.

February Tornadoes

Posted by pmarshwx on 12 Feb 2011 | 1 comment

Since I forgot to post a “Question of the Week” this week due to continued winter weather and because the snow battered center of the United States has begun its long awaited warm-up, I thought I’d go ahead and share the statistics and figures relating to February tornadoes. The plots and statistics are similar to those I computed for January that can be found in posts here and here.

Above is a figure depicting the number of February tornadoes per county for the years 1950 to 2009. Most tornadoes tend to occur in the southeast United States, however tornadoes have been reported as far west as California and Hawaii, and as far north as Oregon and western New York!

If we sum up the number of unique tornadoes from the country data and turn it into the number of tornadoes by state, it would look like this:

Florida and Mississippi lead the way with over 180 tornadoes, followed by Texas and then the rest of the southeastern states. Breaking down each states numbers, the tallies look like this:

Now, the number of injuries per state break down as follows (please note the order of the states listed on the left is in descending order of number of tornadoes per state):

The number of fatalities per state break down as follows (please note the order of the states listed on the left is in descending order of number of tornadoes per state):

And putting everything into a single, cluttered figure, we have this:

SPC Slight Risk Climatology

Posted by pmarshwx on 4 Feb 2011 | 6 comments

Next up in the list of Storm Prediction Center (SPC) outlook climatologies is the yearly climatology of slight risks. Between 1990 and 2008 there were 24,455 slight risk polygons issued. This does not mean there were 24,455 slight risk outlooks issued as it is possible to have more than one slight risk polygon drawn on a given outlook. Furthermore, note that each day consists of multiple outlooks and so it is possible for a grid point to receive multiple “hits” for being located in a slight risk outlook on the same day. In other words, if a location was located within 2 slight risk outlooks, this does not guarantee that two days of slight risks. The location might simply be contained within a slight risk from two separate outlooks issued for the same day.

Below is the slight risk climatology that employs the Kernel Density Estimation technique. Notice how it is smoother along the edges than the raw outlook which is located at the end of this post. The black solid lines are contour intervals of outlooks per year in steps of 25. The color fill contours are in increments of 1 outlook per year. As you can see, the central United States is once again the leader in the number of slight risks to expect per year. This has to do with the repeatability and predictability of the ingredients necessary for severe thunderstorm development. In other words, ingredients come together more frequently and more reliability across “Tornado Alley” than anywhere else in the world. This doesn’t mean that other areas aren’t prone to severe thunderstorms. Some of the largest tornado outbreaks in history have actually occurred well east of traditional Tornado Alley (e.g., 21 January 1999 and 3-4 April 1974).

Below is the raw graphic, with now Kernel Density Estimation applied. Because the edges are not smooth line, rather they are intersections from thousands of polygons, I did not plot any black line contours.

All SPC Moderate and High Risk Climatologies

Posted by pmarshwx on 3 Feb 2011 | 0 comments

After posting the climatology of where the first moderate and high risks occur, I’ve received a couple of requests for additional graphics. One that was extremely easy to produce, and also one most frequently requested, is a climatology of moderate and high risks. Using the same Kernel Density Estimation technique described in the original post, I’ve calculated the number of moderate and high risk outlooks an area might expect during a given year, based on data from 1990 through 2008.

Please note that each day consists of multiple outlooks and so it is possible for a grid point to receive multiple “hits” for being located in a Moderate or High risk outlook on the same day. In other words, if your location is located within 2 high risk outlooks, this does not guarantee that you will have two days of high risks. The location might simply be contained within a high risk from two separate outlooks issued for the same day.

For moderate risks, central Oklahoma appears to be the clear winner with nearly 30 moderate risks averaged per year. Increase probabilities extend both north and east from here.

As expected, the average number of high risks per years is considerably less than the average number of moderate risks. (In fact, I had to change the color scale!) Northeast Kansas, northern Missouri, and west-central Illinois are the most likely areas to experience a high risk in a given year with slightly more than 2 expected. A minor axis of increased probability extends southward from the eastern edges of this highest probability band, reaching portions of eastern Arkansas and far northern Mississippi.

Notice how in both of these climatologies, the maximum probabilities are centered in the central United States — east of the Rocky Mountains and west of the Appalachian Mountains. We’ll leave discussion as to why this is for another blog post.