Saturday, April 6, 2013

Special Discussion: April 9th Tornado Outbreak

Storm Prediction Center has outlined two areas of severe weather on April 9th (red) and April 10th (purple). It's looking like the April 9th outline will be more enhanced as far as the tornado threat goes, while precipitation forecasts from the American GFS model suggest April 10th will be more of a squall line event, spanning multiple states.

Multiple sounding forecasts suggest this event will be held down by a slight capping inversion. As you may recall, a capping inversion involves a situation where the air at the surface is unable to rise due to a layer of warmer air just above the surface. However, as soon as that capping inversion breaks (which is expected to occur in the evening hours of April 9th), air attempting to rise and create thunderstorms will have over 1500 j/kg of instability available. From here, this rising air will form updrafts, which will then turn into weak (and likely rotating) thunderstorms. These initial storms will feed on upper air support and continuing availability of moderate instability to mature into supercells. With elevated helicity and high amounts of atmospheric spinning in the immediate vicinity of the mature supercells, tornadoes should begin to become a real concern as the 7:00 PM (central) hour comes to a close.

Main concern for tornadoes is in much of Arkansas and into Missouri. Storms will begin in Oklahoma and quickly turn supercellular. I believe the storms will quickly allow National Weather Service offices to issue severe thunderstorm warnings, but it may take another while before tornado warnings will be needed. I expect most of the supercells to become tornadic in Arkansas before transitioning into southern Missouri as the storms quickly move off to the northeast. Louisiana will also get into the severe weather, and they have been included in the greatest risk. It is these areas that are most likely to experience tornadoes on the evening of April 9th and overnight. I only accounted for the evening portion of this event, so areas into Illinois, Tennessee and Mississippi have not been completely accounted for going into April 10th.

I outlined a forecasted sounding from the most recent American GFS model, and denoted a few select factors. The first thing to note is the presence of a loaded gun sounding. The loaded gun sounding is signified by a slight capping inversion (yellow line to the left of the red line) just above the surface, with high instability above that capping inversion (yellow line far to the right of the red line). The name 'loaded gun' comes from the idea that the 'gun' (severe weather) could fire at any time- the trigger (capping inversion) just has to be pulled (erased). As soon as that capping inversion is erased, the severe weather gets going. Typically, you will see a loaded gun sounding that is much stronger, where the yellow line is much farther away from the red line than what you see above. What you see above is an 'okay' loaded gun sounding. Regardless, the mere presence of the loaded gun sounding indicates that severe weather is targeting this area. We'll start with the PW. The PW index is precipitable water, where values above 1.00 inch mean a humid atmosphere, and 2.00 inches or more is almost like you're breathing in water because it's so humid. The projected precipitable water values for Little Rock in this sounding is at 1.63 inches, which means Little Rock will be smack dab in the middle of the warm sector. If you recall, a warm sector is the warmth and humidity pulled north from the Gulf of Mexico by a storm system's warm front, and it is in here where the cold front thrives and creates severe thunderstorms. Continuing on, we see the CAPE index just above 1500 units. The elevated values of CAPE (instability) means that thunderstorms are likely to be able to access this instability and create severe thunderstorms. The amount of instability isn't as great as it could be, but you get what you get. The index right below the CAPE value is CIN, which is stability in the atmosphere. Instability creates thunderstorms, stability suppresses them. As a part of the typical loaded gun sounding, there is a small amount of stability just above the surface. The sounding lists the amount of CIN as 7, which is essentially no stability whatsoever. That's a good sign for storms that could have trouble breaking the layer of stability, as they will have little trouble breaking the stability. The last value outlined on the sounding is something called EHI, which is the Energy-Helicity Index. This index combines instability and pure spinning in the atmosphere to create a value that is one of the best predictors of tornadoes to date. As a general rule of thumb, elevated EHI values above 1.0 tend to indicate the presence of at least slight tornado dynamics. Forecaster Jon Davies indicates values above 2.5 are conducive for supercells and are favorable for tornadoes. That said, the projected EHI value of 3.6 is quite high and great for tornado development and propagation. There are several other parameters we could look at for guidance into forecasting this event, but from the aforementioned factors, I have a pretty good idea of what to expect.

A tornado outbreak is expected over eastern Oklahoma, Arkansas, portions of northeast Texas and southern Missouri. The cells should initiate in the evening hours and mature to supercell status in the later hours of the night. Tornadoes are likely in the aforementioned areas, and more than one of them could be long-tracked and strong. Further clarification is anticipated by tomorrow's new Storm Prediction Center outlook

Things to note:

-Storm Action Day has been declared for April 9th. Upgrade to a Critical Storm Action Day may be needed.
-Storm Action Day is possible on April 10th. Decision will be made in coming days.