ArkLaTex Severe Weather/Tornado Forecast. For April 13th, 2019.

The Day Before The Event

A severe weather event, scheduled to unfold later today (it is 1:30 am EST as I type this), certainly has dangerous ramifications associated with a few states across the southern US on Saturday. At the very least, the ArkLaTex region (and likely western Mississippi/Alabama) will be subjected to torrential rain, with the threat of localized flooding, damaging winds, and perhaps several strong, long tracked tornadoes. The SPC and medium/long range models knew of this possibility at least 5 days ahead, and there has been no indication that this system has lost any steam along the way. As of 2:20 am EST, a Moderate risk has been issued for much of the ArkLaTex region and western Mississippi. This may get raised to a High risk tomorrow depending on the evolution of the low's progression and how conditions in the warm sector unfold.

If you live in these states, and neighboring states, be very cautious of the weather today. This includes all areas of the outlook and not just those in the Moderate concern. Create a plan that stresses safety for you and your family: An area to take shelter, or somewhere to drive away from the storm (if living in a building that does not have suitable cover, or a mobile home). Have an idea of what you're going to do if tornadoes become a strong possibility in your region. Storms will be fast moving and intense. You must be ready to act immediately.

The Forecast 

Normally I would go into greater depth, but it's close to 2 am, so I'm going to do a basic rundown. I'll expand further for future events. 

A powerful trough digs its way down nearly into the Gulf of Mexico, with a nearly 100 kt jet streak over eastern Texas as of 21z Saturday (shown below). On the Skew-T (shown below) for Northern Louisiana, you can see both strong speed and directional shear; there isn't an area on the wind field that has less than a 50 kt wind barb until you reach the lower levels of the atmosphere (around 950 mb). You also see veering; especially from the surface winds up to 850 mb. This deep layer shear profile will be very favorable for supercells.

With SRH values exceeding 400 m2/s2 from 0-1 km, and values exceeding 500 m2/s2 from 0-3 km, strong tornadoes are likely on supercells that can remain discrete and prevent being amalgamated by ongoing linear convection, or collisions into other supercells. QLCS tornadoes (supercells that are embedded as part of linear convection) are still a strong possibility here even with linear convection. 

Both the LCL (the base of the cloud formation level) and the LFC (the level at which a saturated parcel of air will continue to rise moist adiabatically) are very low in the sounding, which indicates a low dew point depression and increased chance of tornadogenesis (among other factors). These will be very low in the sky clouds, which presents a visibility risk in terms of seeing the tornado. You may not see it until it is very close as a result.

The 500 mb trough dipping south. Note the jet streak associated with it entering the area with the severe weather outlook.

In terms of energy, the warm sector tomorrow may be the determining key on just how significant this outbreak ends up being:

-If prominent cloud remains in the morning, or pre-frontal storms ignite early, overall energy (CAPE) will not be maximized by diurnal heating. In all honesty, however, this is likely spitting hairs. With dew point temperatures likely to reach the mid 60s and even past 70F in some areas, and models hinting at the warm front eliminating the little cap in place early, there should still be plenty of CAPE for supercell development.

-IMO, with lots of energy, shear, and incredible helicity indices, the largest mitigating factor to tornadogenesis may be the uncapped warm sector. With strong surface forcing at and ahead of the cold front, storms may form "en masse", or as a group, instead of separately. If a mass of storms erupts and combines quickly, the tornado threat will likely be reduced.

The 21z Skew-T for the area ~30 miles SE of Shreveport, LA. 

Storm Chasing Concerns And Target Area

I will not be chasing this event, as I'll be at home over 1,200 miles away, but I'll discuss the concerns that I would have chasing this event if I was there. To round out the blog, I will pick a target area (that I may refine in the morning).

Concerns:

1) Storm motions will be very fast; 50+ kts. This means you will not be able to keep up with, or outrun them; even if your road network was perfect. You must get ahead and try to clear out of the RFD or any other dangerous aspect of the storm that you will be in close proximity to. The Red River cuts through western Louisiana from NW to SE, so if you're west of the river, there are very few crossings to go east. You may end up being trapped. The same goes with the Mississippi River in the east of the state.

2) Northern Louisiana has extremely high tree density, and as a result, limited visibility to see the storms approaching you. Very low LCLs mean you won't see a tall rope tornado, you'll likely see a wall of cloud. The trees will obstruct this view. Not to mention, these trees become projectiles if you're near the tornado or areas with high straight line winds. Here's an example of what I'm talking about.

3) With the potential of storms moving through in the morning, and "training" (supercells moving over the same area repeatedly), localized flooding now becomes a risk. We've already discussed low visibility being an issue in a few different ways above, but now couple that with a shrinking road network (that wasn't prominent to begin with), and your overall level of risk skyrockets. Escape routes become limited, and you won't outrun these fast moving storms.

Target Area

Overall, I'm not a big fan of grungy, non-photogenic, fast moving, "close proximity to the Gulf" storms tend to be, but the area around city of Winnfield would be my target area. Not to mention, the risks are much higher with this system as I covered in the "Concerns" section.

Saturday presents a very real danger. To those in the risk area: best of luck, and be weather ready tomorrow.