April 17th and 18th Severe Weather Forecast.

A three day severe weather event is expected to unfold across the southern Plains, Dixie Alley and the Carolinas spanning April 17th to April 19th. I did not have the time to make an extensive separate forecast for April 17th (today), but will include a brief synopsis and forecast for the 18th and may make a separate forecast for the 19th.

Today (the 17th) will feature a severe risk anywhere from southern Texas up to northern Illinois; with the primary risk areas for tornadoes being located in the warm sector ahead of the dry line across most of Oklahoma, northeastern Texas. There's a second conditional tornado concern across eastern Iowa and northern Illinois along the warm front, ahead of the surface low. All areas have concerns: capping concerns to the south, messy VBV wind profiles in Oklahoma, and uncertainty in models as to both severity issues and whether or not storms will fire exist in the northern target.

The area of largest financial concern is that of the Dallas/Fort Worth metro area that may be subjected to a very large hail and damaging winds. It seems as if this area gets an annual threat of large hail, and today will be another day of risk.

If there was ever a sounding for large hail, this would be it. Near Denton, Tx. 

The Forecast

April 18th- LA/MS/AL

A set up that has some reminders of April 13th will occur tomorrow (April 18th) across Dixie Alley. A high amplitude trough will continue to push eastward and will reach Dixie Alley by mid-day. Low and mid level jets (500 mb 700 mb, and 850 mb), with wind speeds upwards of 50 kts+ in the jets, will be located over eastern Louisiana by 21z. The surface low will also be making its way east and should be over northern LA by 18z tomorrow. The Low Level Jet will continue to intensify throughout the day into western Alabama after 00z. Surface speed and directional shear (SSE winds at the surface) will provide a favorable environment for supercells and tornadoes, but with strong low level forcing likely to amalgamate storms that form, especially later in the day, the isolated supercell/tornado risk may be limited in time and based on prefrontal trough supercells and their advance. Additionally, storms may become severe and present the damaging wind, torrential rain and embedded QLCS tornadoes early on (15z), so those living in Louisiana should be on alert early.

A sounding for eastern Mississippi at 21z. Speed and directional shear look pretty good (especially at the low levels), but CAPE (energy) is a concern; at least for the NAM 3k.

Energy for the storms to utilize is still a bit of an unknown at this point, as there's plenty of model disagreement. With mid 70F temps, and dew points in the high 60s, low 70Fs, such a small dew point depression will lend to very low LCLs, but preexisting storms moving east from today (well, as I write this, storms are firing in central Texas) and cloud cover that they bring may put a damper on total energy availability due to a lack of solar heating. This will sort itself out in the morning.

Projected cloud cover for 18z tomorrow suggests that low level cloud (both for GFS/NAM) will be present over the target states.

In general, the further east you go, and the later you wait, the better the Storm Relative Helicity will be as the LLJ picks up, but in doing so, he higher the chance that the storm threat becomes linear. CAPE also becomes a concern the further east you go.







Storm Concerns/Target Area


1) No guarantee of strong instability. As stated before, there's a ton of model disagreement on CAPE, cloud cover and preexisting convection moving into the target area, and these are unlikely to be clarified until the morning. With clearing, diurnal heating and strong low level flow, CAPE levels may lead to very strong storms. Prefrontal cells could become tornado producers.

2) Quick upscale growth of storms. Similar to April 13th, strong forcing at the surface with the cold front pushing east and a strong LLJ may lead to the quick linear convergence of any discrete storms, which would reduce the tornado risk. This isn't going to rule it out completely (Dixie has a habit of producing QLCS tors, like those after dark last week), so it is still a concern.

3) Some VBV in SKEW-Ts may lead to messy storms, or hinder their growth, early before the strong low level shear and SRH begins to increase. More messy HP storms are possible.

4) A bit nit picky, but I'd like to see a bit more veering (more of a westerly upper level wind component) above the 700 mb mark, but if low level shear, especially below the 3km mark, is as intense as forecast, this shouldn't matter as much.


If I had to choose a time/location to target for a chance at tornadoes, I'd pick somewhere near Jackson, Mississippi. I'll be honest, however: I'm not very confident in this target because I'd be hoping for prefrontal cells firing in an environment with building shear and, hopefully, building instability. Anything west of that target area would likely already be linear in nature. The morning will determine the outcome of supercell and tornado formation based on instability. Even if the tornado risk remains minimal, localized flash flood and damaging winds are still a major risk.