Ohio Valley Severe Threat: 4/3/18
Forecast schematic for Tuesday afternoon, courtesy Pivotal Weather for model graphic.
Three regimes for potentially severe convection on Tuesday are as follows:
- Warm front from central Indiana to central Ohio.
- Pre-frontal trough from central/southern Indiana into western/central Kentucky and southwestern Ohio.
- Cold front from west to east across the Ohio/Tennessee valleys.
Summary: Severe thunderstorms, including all hazards of hail, wind and tornado, will be possible from midday Tuesday into early Tuesday evening across the Ohio Valley, Tennessee Valley and adjacent portions of the Mississippi Valley. Given the complex nature of the setup, severe thunderstorms could start as early as midday over Indiana/Ohio and continue into the early evening as a cold front sweeps from west to east across the area.
Pros: Shear, particularly in the low levels, instability (given climo), mid-level lapse rates; multiple boundaries for storm initiation.
Cons: Directional shear is not ideal (especially near the cold front), storm mode a question in at least two of the regimes, storm longevity unclear near the warm front and pre-frontal trough; low-level CAPE profiles are relatively skinny, particularly across eastern portions of the threat area.
Placement of the warm front is a bit of a question mark, although models seem to be converging on a solution that would place it roughly in the vicinity of central Indiana to central Ohio by Midday Tuesday, before lifting north somewhat during the afternoon. It does look like storm modes may be mixed/messy in this regime, but given the degree of wind shear, it won’t take much instability for storms to rotate. Anytime there is 0-1km shear in excess of 40 knots with at least modest instability, watch out. The question marks will probably boil down to how discrete storms can stay and if any updrafts are able to remain sustained. Given the lack of low level instability, these storms may produce more sporadic severe than anything else, although all severe hazards (hail, wind, tornado) will be possible.
A pre-frontal trough type feature extending south from the warm front and well ahead of the cold front may be the most intriguing aspect of this event. By early and especially mid-afternoon, the parameter space in vicinity of the trough will become increasing supportive of severe thunderstorms. Mid-level warmth is not as big of an issue as it may be farther west, but the biggest question here is how long of a window do updrafts have to be maintained. With eastward extent, the area becomes more and more displaced from stronger large scale forcing, as models suggest that convection may struggle to remain sustained, if develop much at all, south of the OH/KY border. Simulated radar data seems to suggest that although discrete/semi-discrete storm modes will be favored, storms may only have a 2-4 hour window before they weaken. Even if the window may be somewhat limited, the wind fields could support a relatively long-lived supercell or two (in excess of an hour) with an isolated strong tornado threat. If the warm sector up in southeastern Indiana to southern Ohio remains relatively free of debris through the morning, there may be a narrow area to support a longer-lived severe threat, but mesoscale details will need to be assessed before having any sort of high confidence in such a scenario. Either way, steep mid-level lapse rates would also support large to possibly very large hail with any robust convection in this vicinity, but lapse rates become less steep with eastward extent as well, narrowing the threat zone a bit,
The cold front is another interesting case. Wind fields aren’t totally unidirectional and there is enough low to mid-level curvature in the hodographs to support embedded supercell structures, if not just a broken line of semi-discrete supercells. This is where mid-level warmth actually becomes a net positive, in terms of severe. In the wake of the pre-frontal trough, some warming in the mid-levels will cap the atmosphere (for the most part) until stronger forcing arrives with the cold front. This means that areas such as southern to central Indiana and north-western/north-central Kentucky may see some convection around midday or early afternoon, but the area is able to support yet another round of potentially intense convection by late afternoon. Immediately ahead of the cold front, we’ll have a scenario where 700-500mb lapse rates should easily reach 8 C/km, if not 8.5 C/km or steeper, which is moving into uncharted territory, given climatology. There is actually consensus with the HRRR/HRRRX/3km NAM that at least some area should see these lapse rates reach 9 C/km for a time. That’s remarkable. If any supercells near the cold front do remain isolated, very large hail would be possible, as well as a few tornadoes, from the Ohio Valley into western portions of the Tennessee Valley. Severe thunderstorms may also develop farther southwest along the cold front into the lower/mid-Mississippi Valley region (southern Arkansas/Louisiana/Mississippi/East Texas), but deep layer shear will tend to be parallel to the initiating boundary, and there are some veer-back-veer signatures in forecast soundings, suggesting complex (trending toward messy) storm modes.
Overall, it’s a complex setup that holds a lot of potential given the parameter space. Looking at the details, there are multiple areas to watch and it’s conceivable that some places could see two rounds of severe convection. Considering parts of the area just had snow yesterday and it’s only early April, that would be pretty impressive. There is a fair amount of conditionality involved as well, so I don’t think we’re going to see a widespread severe weather outbreak. With that said, I think there’s reason to believe there will be at least a few significant severe storms (very large hail in excess of 2 inches in diameter and/or strong tornadoes, EF-2 or greater in strength).
