There is increasing confidence in a chance of severe thunderstorms and tornadoes over the Mississippi River Valley on Friday and a much more significant tornado threat over the Southeast on Saturday. A deep upper-level trough and associated surface low pressure system will pass through the southern tier of the Central/Eastern US Friday and Saturday, resulting in strong southerly winds at the surface veering to westerly winds aloft. The combination of wind shear and instability resulting from warm, humid, southerlies originating from the Gulf of Mexico and cold westerlies aloft is a favorable environment for strongly rotating supercell thunderstorms, including the threat of strong, long-track tornadoes. To emphasize, the threat over the Southeast is not going to approach April 27, 2011 in intensity. It looks more similar to the tornado outbreaks that infrequently occur in the winter or early spring, with lots of low-level shear and helicity.

Before discussing the tornado threat, I want to stress how volatile the weather has been this past week! Last Wednesday, we had our first Blizzard Warning of the season, closing classes at Iowa State University. On Monday, we reached 74 with sunny skies and gusty winds, prompting a Red Flag Warning. And now we have a threat of severe thunderstorms on Friday, though the threat will be larger further east. I knew Iowa was famous for its volatile weather, but I couldn't have imagined a Blizzard Warning, a Red Flag Warning, and a severe thunderstorm threat all within 10 days! Such is spring in the Midwest.

Current Conditions

As of Wednesday morning 3/12, we have a deep trough in the Eastern Pacific bringing cool, unsettled weather to the West Coast, a vigorous shortwave trough moving through Southern California/Northern Mexico enhancing precipitation and winds over the Southwest, and expansive but relatively flat ridging over the eastern half of the country.

A look at the current watches/warnings/advisories from the National Weather Service shows the multiple Winter Storm Warnings over California and Red Flag Warnings over New Mexico and the Southwestern Plains due to strong southerlies ahead of the SoCal trough.

Credit: National Weather Service

As the week progresses, the shortwave trough will gradually dissipate over the Southeast but a much stronger and broader trough will move over the entire West Coast, bringing even heavier mountain snow and wind to California. This trough will eventually spawn a severe weather threat over the Mississippi Valley on Friday and a potential tornado outbreak over the Southeast on Saturday.

The below images show the dewpoints, surface-based CAPE, storm-relative helicity (SRH), and energy helicity index (EHI) at 21Z (6pm CDT) Friday 3/14. I've discussed CAPE a few times on this blog, but I haven't dug into SRH or EHI before, as they are parameters that are commonly used for tornado forecasting and we very rarely see tornados in the Pacific Northwest!

Credit: pivotalweather.com

Storm-relative helicity refers to the amount of vorticity (rotation) in an airmass relative to the movement of a storm, meaning that we take the center of the storm as our stationary reference frame. What the heck is a "stationary reference frame," you might ask? A simple example is to imagine yourself on a train - you could either take the ground to be the reference frame and say that the train is traveling 40 mph, or you could take yourself, on the train, to be stationary and state that the ground is moving 40 mph past you. Both are equally valid statements.

A high SRH value means that a lot of air is being "sucked" into the storm in a rotating fashion. The more air is sucked into a storm and the stronger its rotation, the higher the potential for a rotating mesocyclone and a severe thunderstorm. CAPE is an acronym for "convective available potential energy" and is a measure of atmospheric instability, with more CAPE corresponding to stronger updrafts and more "energy" that storms can draw from the environment if they do develop. The EHI is a composite parameter that combines CAPE and SRH to quantify the potential for strong rotation (as you would see with high SRH values) and strong updrafts (as you would see with high CAPE). A strong, rotating updraft is conducive for the formation of a rotating supercell with a mesocyclone (rotating updraft) and - in some cases - a tornado underneath the mesocyclone.

The SRH (and by extension, the EHI) is an integrated value through the atmosphere, and it is usually given as the SRH in the 0-3km layer or the 0-1km layer. The images shown above show the 0-3km layer, but the 0-1km layer is becoming increasingly used, and that layer may be a better indicator of the potential for violent tornadoes. If you are more interested in learning about SRH, check out my atmospheric science co-advisor's notes (link) on the topic at Iowa State.

In the above image, note the high dewpoints over the Mississippi River Valley, CAPE values of 1000-2000 J/kg from Louisiana into Wisconsin, widespread SRH values of 250-350 m²/s² over the Mississippi River Valley, and EHI values of 1.5-3.5 over the Mississippi River Valley. SRH and CAPE by themselves cannot support tornadoes, but the combination of the two creates an environment conducive to their formation, and this is illustrated well by the EHI. Take a look at the Storm Prediction Center's forecast for severe weather potential on Friday and how the peak threat aligns well with the area of highest EHI. EHI values in the 1-5 range are associated with F1-F3 tornadoes, and those above 5 have been associated with violent (F4-to-F5) tornadoes (source).

Credit: Storm Prediction Center

Saturday's Severe Threat

As I've mentioned, Saturday's severe weather threat looks even more significant than Friday's. At 6pm CDT Saturday, dewpoints will be in the mid 60s over the Southeast with CAPE in the 1500-2500 J/kg range. However, take a look at the SRH values! Much of Alabama has values over 300-400 m²/s². The combination of moderate CAPE and extreme SRH values gives EHI values of 4-6 across Alabama, extreme northern Tennessee, Eastern Mississippi, and portions of the Florida panhandle. These EHI values alone would support the possibility for F4 or even F5 tornadoes.

Moreover, my prior experience with tornadic events over the southeast has also shown that CAPE values need not be excessive for violent tornadoes and that SRH plays a bigger role. April 27, 2011 was a perfect example of this, with moderately high CAPE values peaking in the 2000-3000 J/kg range but extreme SRH values near 400 m²/s² in the 0-3km layer and exceptional values of 250 m^2/s^2 or greater on the 0-1km layer (link).

Credit: pivotalweather.com

The Storm Prediction Center gives a 30% threat of severe weather over the Southeast on Saturday. As we get closer to this event, expect them to issue at least a "moderate" risk of severe weather, and a "high risk" is not out of the question.

Credit: Storm Prediction Center

One of my favorite forecasting tools during severe weather outbreaks is to look at the expected weather soundings at a certain point. The below forecast sounding is valid at 21Z Saturday over South-Central Alabama in the area near the highest EHI values. Take a look at how winds rapidly increase with height and "veer" from southerly to westerly with altitude. CAPE values are also moderate at 2000 J/kg.

The box in the lower right indicates the type of threat that this sounding might be associated with based on past observed soundings. "PDS" stands for "Particularly Dangerous Situation," and it is the official NWS terminology used when referring to an especially severe tornado or outbreak of tornadoes. It's always important to take those analogs with a grain of salt, but it indicates that notable historic tornado outbreaks over Alabama have had similar soundings to the one that is forecast for Saturday.

Credit: pivotalweather.com

There are still a few ways that this tornado threat could fail to materialize. The primary threat is if convection occurs early in the day, as this early convection would "use up" all the available CAPE and prevent instability from building up during the day. An ideal tornado sounding has a small "capping inversion" that prohibits convection until this capping inversion finally breaks in the afternoon, releasing all of this instability all at once. The current setup does not have much of a capping inversion so it is possible that we see widespread severe storms and heavy rain earlier in the day rather than seeing fewer but much stronger tornadic storms that peak in the afternoon.

Another threat is if storms form along an organized cold front associated with a surface low moving north of the area, forming a well-defined squall line, instead of each storm developing as a discrete supercell. Squall lines pose more of a high wind threat than a tornado threat because each storm "robs" a little bit of energy from the adjacent storm, preventing it from growing violent enough to spin up a tornado.

To summarize, here are the key points:

• A strong, multi-hazard winter storm will impact California Thursday evening. This storm will move into the Plains/Midwest Friday and severe weather is possible in the region of strong southerly winds ahead of the storm's cold front.
• A much more significant severe weather threat is possible over the Southeast (esp. Alabama) on Saturday as a surface low forms over the Midwest, bringing a combo of strong southerly winds from the Gulf of Mexico, ample low-level moisture, very strong low-level wind shear and helicity, and moderate CAPE.
• This is a common winter/early spring tornado outbreak pattern for the Southeast but will NOT be anything remotely close to April 27, 2011 or April 3, 1974.
• There are two main ways the tornado outbreak could fail to come to fruition: (1) convection develops earlier than expected and (2) storms focus along a squall line instead of developing as discrete supercells
• Despite those 2 above forecast risks, the primary indicators of CAPE, low-level wind shear, and helicity look favorable for a few tornadoes over the Mississippi River Valley on Friday and a more widespread outbreak over the Southeast on Saturday.

Thanks for reading and we'll keep you posted as this event approaches.