There is a significant threat for severe thunderstorms later today and tonight from the Ohio Valley southward to the central Gulf Coast. A strong mid-level trough will become negatively tilted this evening over the Ozark Plateau and will then eject northeastward over night across the Tennessee and Ohio Vallies.
Severe weather outlook from the Storm Prediction Center (SPC).
An associated surface cyclone will move across eastern Kansas this evening and should be near the Chicago area by early Monday morning. The attendant cold front will trail to the southwest and will move eastward through the Mississippi Valley, while a warm front will move northward across the mid-Mississippi Valley and eventually into the lower Ohio Valley by late tonight/early Monday morning. Dewpoints in the 50s will surge as far north as southern Illinois and southern Indiana, while dewpoints in the 60s will be widespread across the Mississippi and lower Tennessee Vallies.
There will also be a very strong low level jet of 50-70 knots, increasingly strengthening as the night wears on. Jet streaks of 100+ knots at 500 mb and 250 mb will provide essential upper-level divergence across the warm sector, and instability at the surface will approach 500-1000 J/Kg as far north as the lower Ohio Valley and as high as 1500 J/Kg across the lower Mississippi Valley.
Damaging wind outlook from the SPC.
The Total Totals Index (TTI) will be around 54-56 for much of the warm sector. The TTI is a combination of the Vertical Totals (VT) and Cross Totals (CT). The VT is the temperature difference between 850 and 500 mb while the CT is 850 mb dewpoint minus the 500 mb temperature. A value of 54-56 means widely scattered severe storms are possible.
The SWEAT (Severe Weather Threat Index) values will range from 300-500. Any value over 400 suggests tornadic supercells are possible.
SRH (Storm Relative Helicity) values will range from 250-450 m**2/s**2. SRH “is a measure of the potential for cyclonic updraft rotation in right-moving supercells, and is calculated for the lowest 1-km and 3-km layers above ground level. There is no clear threshold value for SRH when forecasting supercells, since the formation of supercells appears to be related more strongly to the deeper layer vertical shear. Larger values of 0-3-km SRH (greater than 250 m**2/s**2) and 0-1-km SRH (greater than 100 m**2/s**2), however, do suggest an increased threat of tornadoes with supercells. For SRH, larger values are generally better, but there are no clear ‘boundaries’ between non-tornadic and significant tornadic supercells (2012, SPC).”
Tornado outlook from the SPC. Large and long tracked tornadoes are possible in the 15% hatched area!
Everything appears to be coming together for a significant severe weather outbreak over the areas mentioned later today and into tonight. Below is a list of the threats you can expect…
- Damaging winds in excess of 80 mph will be widespread
- Tornadoes, some large and long-track tornadoes possible, especially across western Tennessee and upper Mississippi Vallies
- Hail to 2″ in diameter
If you live anywhere within the outlooked areas shown above, please pay attention to every warning issued by your local NWS office and take them seriously. If you are under a tornado warning take shelter immediately as you will not be able to see the tornado coming after dark. If you know of anyone living in the above areas, give them a call and let them know what to expect.
