Analysis of the Potential Tornado Outbreak on Wednesday, April 8th

This is a post that will detail an analysis of the potential tornado outbreak on Wednesday, April 8th. If you've been watching my discussions over the past several weeks, you've noticed how I've used my 'tornado research' to look at severe weather situations. Today, you'll get the first in-depth look at that research, and why I believe significant tornadogenesis (EF3+) is likely on Wednesday.

First, we'll go over the Storm Prediction Center outlook and frontal positions for Wednesday.

Storm Prediction Center
(click all images to enlarge)

The Storm Prediction Center's outlook for Wednesday shows a Slight risk of severe weather over southern Iowa, extreme southeast Nebraska, west-central Illinois, much of Missouri, eastern and central Kansas, and central Oklahoma. It is expected that these areas will see the brunt of the severe weather threat, with a lesser severe weather threat present in the Marginal risk (shown as MRGL) area.
There is a portion of the Storm Prediction Center's text outlook that I'd like to share; for some of you, it may be a little complicated, so I've underlined the parts that sum up the paragraph best.

 ...THE MIDWEST/MID AND LOWER MO VALLEY/CENTRAL AND SRN PLAINS...
   A MOIST BOUNDARY LAYER WILL REMAIN IN PLACE ACROSS THE CENTRAL AND
   SRN PLAINS AND MID/LOWER MO VALLEY REGION AHEAD OF THE DRYLINE AS IT
   MIXES EWD THROUGH THE AFTERNOON.  AS HEIGHT FALLS ALOFT OCCUR AHEAD
   OF THE ADVANCING UPPER TROUGH...RESULTING ASCENT WILL SUPPORT
   WEAKENING OF CAPPING AT THE BASE OF THE EML LAYER -- PARTICULARLY
   FROM SRN OK NWD INTO KS.  EVENTUALLY...ASCENT SHOULD PROVE
   SUFFICIENT TO ALLOW LOCAL BREACHES IN CAPPING -- AND RESULTING/RAPID
   DEVELOPMENT OF ISOLATED STORMS.  GIVEN A VERY UNSTABLE AIRMASS/STEEP
   LAPSE RATES ALOFT...AND AIDED BY FAVORABLY STRENGTHENING/VEERING
   FLOW WITH HEIGHT...STORMS SHOULD RAPIDLY BECOME SUPERCELLS.  ALONG
   WITH LOCALLY DAMAGING WINDS...VERY LARGE HAIL IS EXPECTED.  IN
   ADDITION...THE AMPLY MOIST BOUNDARY LAYER AND FAVORABLE LOW-LEVEL
   SHEAR -- PARTICULARLY NWD INTO KS NEARER THE ADVANCING SURFACE LOW
   -- WILL LIKELY BE SUFFICIENT TO SUPPORT RISK FOR TORNADOES.  WHILE
   OVERALL STORM COVERAGE SHOULD REMAIN WIDELY SCATTERED TO
   ISOLATED...SIGNIFICANT SEVERE WEATHER IS EXPECTED WITH ANY STORM
   WHICH DOES DEVELOP.

I underlined how the SPC foresees the rapid development of storms with any storms that are able to develop (we'll discuss that a bit later on), and how the storms would quickly turn supercellular. Additionally, locally damaging winds and very large hail is expected. Perhaps the most ominous sentence in this discussion is the last one, where the SPC indicates that any storms which do develop are expected to have significant severe weather associated with them. That's some pretty alarming wording, especially two days in advance of the event. We'll work through why this is as we go on down the post.

Weather Prediction Center

Let's now go over the forecasted surface map for Wednesday morning, about 12 hours in advance of storm activity. We see a low pressure system in the Oklahoma panhandle, with a dry line extending south into Texas. A stationary front then extends northeastward into central Illinois, where another low pressure system sits. It is expected that this frontal boundary will drift north as the day goes on, and will aide in severe weather development.

Now that we know how this threat will set up, let's go over the forecasted severe weather parameters on Wednesday evening.

Twisterdata
(click all images to enlarge)

The most critical ingredient to look at, to ensure that we have thunderstorms to begin with, is forecasted Convective Available Potential Energy, or CAPE. The higher the CAPE value, the more buoyant the air is, meaning the air is able to rise easier, and form thunderstorms. In this forecast, off of the NAM model (which is notorious for exaggerating forecasted CAPE/instability), we see values on the order of over 3000 joules per kilogram of buoyancy extending from northern Texas, through central Oklahoma, and into central Kansas. Lesser, but still prominent instability then exists across the eastern portions of the latter two states. CAPE values of over 3000 j/kg are not exactly common, so when they come up on a forecast, one tends to take note. Overall, this is a very favorable environment for thunderstorms... right? Not exactly, but we'll visit that a little later.

Twisterdata

Next, let's see how the upper air pattern looks, for the same Wednesday evening timeframe as the CAPE graphic we checked out. We see the Pacific jet stream arcing over the ridge along the western coast of North America, contributing to that jet streak rounding the base of the trough in the Rockies. What really grabs my attention, however, is the roaring subtropical jet stream entering Mexico and Baja California. That jet stream is barreling into North America at over 100 knots, and will be a big factor for Wednesday's tornado potential. We see an arm of the jet stream nosing into Oklahoma by Wednesday evening, which also sets up an area of divergence, as noted by the low wind speeds, over southern Kansas and northern Oklahoma. It's no coincidence that the greatest threat for tornadoes lies within that area of divergence.

Twisterdata

Let's now turn our attention to the lower level jet stream, once again valid for Wednesday evening. Here, we see wind speeds in excess of 45 knots over central Oklahoma and southeastern Kansas, right where the best instability is. This lower level jet will be able to advect the warm, moist air from the Gulf, as well as the drier air from the west to create this potentially severe environment. This kind of set-up is a consistently favorable one for severe weather in the Central US.

Twisterdata

There is a caveat to this severe weather situation. Model guidance is indicating that all of the wind shear, instability, and helicity in the atmosphere will have to contend with a warm layer above the surface, located just over 5,000 feet above the ground. The Storm Prediction Center appears optimistic that storms will be able to break through this warm layer and allow for scattered storms to form. If storms are able to form, they have the potential to become quite dangerous, threatening life and property, as will be shown below. For now, I'll go with the SPC, but this temperature inversion could become an issue down the road.

Now that we've evaluated all of the basic ingredients for thunderstorm and tornado formation, let's move on to a couple less conventional forecasting methods.

CIPS

We begin with the forecasted CWASP chart from the GFS model, valid on Wednesday evening. In a nutshell, the CWASP parameter is able to diagnose tornado favorability from a number of indices, combined into one. As the image tells you, percentage values over 65% tend to see the probability of significant tornadoes rise. The forecasted CWASP chart for Wednesday evening peaks at over 85% in eastern Kansas, with significant values over 80% extending across Missouri and Oklahoma, even a bit into Texas. This is the first 'red flag', per se, that Wednesday evening could be a pretty tornadic environment for any storms that form.

CIPS

A method I like to look at, but not necessarily put full trust in, is analog forecasting. In this case, the CIPS agency analyzes the forecasted conditions for a given date, and spits out the top 15 days where the atmospheric pattern was similar to those forecasted conditions. This chart is valid for the morning of April 9th, and paints a pretty ominous picture. Here, all hail, damaging wind, and tornado reports from all top 15 analog dates are combined into one picture, to give a view of where severe weather is most favored. As the text shows at the top of the page, 102 significant (EF3+) and 122 long-track tornadoes occurred in regimes similar to the one forecasted on Wednesday into Thursday. Again, I would not put my full trust in this, since there remains considerable uncertainty with regards to the entire event, as well as my own precautions with using this analog forecasting, but you get the gist of how ominous this is.

Alright, now that we've gotten past all of the computer modeling forecasts, it's time to go over why I think significant tornadogenesis (EF3+) is likely with storms that break through on Wednesday, as the SPC suggests.

This past winter, I began toying around with tornado data over the past decade or so, as well as the conditions they formed in. Fast forward a few months, and I was able to make a method that had great success in hind-casting tornado strength, before the storm cell and tornado even formed. I began applying it to this season, and it has performed well so far. Seeing as my research has been performing well, this severe weather event could be potentially dangerous, and there's a need to get the word out early and often, I figured now is the best time to share the outlook with you.

Using the NAM model (which tends to exaggerate severe weather parameters), the environment over Oklahoma City, OK is favorable for tornadoes of EF5 strength on Wednesday evening. Once again, that's from the NAM, notorious for exaggerating severe weather. When I ran it for the more-level-headed GFS model, it predicted a tornado strength of EF3 to EF4 for the same area. Another mathematical model I developed indicated a pretty high chance of significant tornadogenesis, with a strength of EF3 or greater, if storms do form.
The big caveat is that this model cannot predict if storms will form; it says that if storms do form and tornadoes are produced, that is the strength that the tornado should be. It's all new research, but has performed well in initial real-time applications.

Thus, if storms are able to break through the temperature inversion, the environment (as of the forecasts at this typing) is favorable for tornadoes to form and be of EF3+ strength.


To summarize:

- A potentially significant severe weather event is forecasted over central Kansas and northern Oklahoma on Wednesday, April 8th.
- Any storms that manage to form in these areas have a rather high potential to produce significant severe weather, including strong tornadoes.
- The environment is supportive for tornadoes to be of EF3 strength, or higher.
- Considerable uncertainty still exists with this event, and it must be monitored in coming hours and days.
- Those in the SPC highlighted risk at the top of this page should review severe weather action plans, in the event that this severe weather potential comes to fruition.

Andrew