Sunday, April 5, 2015

Wednesday, April 8th Potentially Tornadic Severe Weather Outlook

There appears to be a rising threat of tornadoes on Wednesday, April 8th, for portions of the Plains.

Storm Prediction Center
(click all images to enlarge)
Since our post on this particular day's threat yesterday, the Storm Prediction Center has elevated its wording for this day, indicating that the threat has increased. Today, we see a 15% chance of severe weather within 25 miles of any given point over central Oklahoma, a sliver of north-central Texas, much of central and eastern Kansas, the majority of Missouri, and west-central Illinois. Compared to yesterday's risk area, this 15% threat area has expanded over the Plains and a bit into the Midwest, indicating the increasing confidence in a severe weather event.

Let's take a look at what factors will be playing into this severe weather threat.

Weather Prediction Center
This chart shows the forecasted surface map on Wednesday morning. We see an elongated stationary front extending from the northern Rockies into southern Kansas, central Missouri, and all the way to the Mid-Atlantic. A low pressure system is seen in the Texas panhandle, with an associated dry line across western Texas. Another low pressure system is also moving towards the Plains, shown in Utah in this graphic. By the evening hours, it is expected that this stationary front will lift north into the areas outlooked for severe weather.

Now that we see what will ignite this severe weather threat, let's start checking out the forecasted severe weather parameters.

We begin with the forecasted Convective Available Potential Energy (CAPE) values for the evening of Wednesday, April 8th. The presence of CAPE, the buoyancy of air, is critical for any thunderstorm development. CAPE values indicate the ease of which warm air at the surface can rise and create thunderstorms. Higher CAPE values, in excess of 2000 joules per kilogram, can support thunderstorms at a severe level; values over 4000 joules per kilogram are considered uncommon, and can support explosive thunderstorm development.

In this forecast, a narrow band of high buoyancy stretches from the central Oklahoma/Texas border, almost due northward to central Kansas. The highest values in this corridor appear to be around 3000 joules per kilogram, indicating potentially intense thunderstorm development. It's no coincidence that the highest instability values are forecasted within that risk area put out by the Storm Prediction Center, as well as lower, but still significant CAPE values to the east.

We'll now take a look at the upper air flow for the same Wednesday evening timeframe as that CAPE image we just analyzed. We clearly see the strong upper level trough over the Rockies, with a jet streak rounding the base of that trough in Colorado, New Mexico, and Arizona. Of more importance is the merging of the two jet streams over the Plains. We see the Pacific jet stream contributing a bit, with some weaker winds pushing southward to the east of that ridge along the west coast of North America, even though the northern jet is primarily in Canada. The subtropical jet stream then takes control, as it comes in to Baja California and northward into the southern Plains. We see a jet streak (embedded area of higher wind speeds) over western Texas, with the severe weather risk area just to the east of this streak. That's not a coincidence, either; tornadoes tend to form when a jet streak is displaced just west of the given region, meaning tornadoes may be favored in the eastern half of Oklahoma and eastern Kansas, as well as other states also in this SPC outline.

Finally, we'll take a look at the forecasted CWASP parameter for Wednesday evening. The CWASP parameter gives an estimation, in percentage form, of the likelihood of tornado formation. When the forecast exceeds 65%, significant tornadoes become possible. This forecast has values over 85% in eastern Kansas and central Missouri, indicating a very tornadic environment is present. If convection does form (as the Storm Prediction Center has appeared less enthusiastic about storm coverage on this day than on Thursday), we could see multiple tornado-producing storm cells.

To wrap up, this chart shows how tornadic Wednesday could be. This is a composite image of the top 15 analogs with environments similar to the forecasted environment on Wednesday evening. We see an abundance of tornado reports over Oklahoma and Kansas when a similar set-up presented itself, only confirming this threat.

To summarize:

- The risk for severe weather appears to be growing on Wednesday evening, especially over Kansas and Oklahoma.
- If currently forecasted conditions come to fruition, multiple tornadic storm cells could erupt over the aforementioned regions.
- Considerable uncertainty still exists.