5/11/15: Ohio Severe Thoughts

For the record, here is a discussion I privately posted for other Neoweather.com forecasters to read and comment to as we came up with a severe forecast for Monday. This was posted at about 5PM Sunday afternoon however I haven’t had much reason to significantly change this since then:

A lot of conflicting signals here however after putting everything together I feel there is enough of a threat to go with a low risk across the state (more like enough of a chance of some stuff happening that I don’t feel comfortable going with no risk area).

Observed 12z soundings from today from ILN, BNA, BMX and FFC reveal that the low level moisture is not very rich. This gives me concern that with any heating and mixing on Monday that our surface dew points would lower and limit instability moreso than the models indicate. This is especially a concern since much of the state has been dry lately. Observed 12z soundings also show meager mid-level lapse rates until you get to the Plains. Additional convection between here and the Plains between now and tomorrow should insure that lapse rates are weak over OH tomorrow. The combination of limited low level moisture and poor lapse rates makes me skeptical that MLCAPE can climb much above 1000 J/KG even with daytime heating.

A weak vort max may be moving over OH in the morning per the NAM. This may be enough of a spark for storms rather early (noon give or take) to fire over the eastern half or third of the state. Shear and instability really don’t support severe weather this early, and any early storms may limit instability even more for later in the day if too many fire.

Better forcing arrives from the west starting late afternoon as the surface cold front moves in. Convergence along the front isn’t particularly strong, however some upper level divergence thanks to the entrance region of a jet streak spreading in late in the day combined with a low to mid level jet along with the cold front should be enough for storms to fire along the front by late afternoon and into the evening. Most convection allowing models agree with this scenario.

Shear increases by late afternoon and evening as the jet streak provides 30-40 knots of effective bulk shear…with the low level jet causing respectable low level shear (sfc to 850mb shear of roughly 25 knots and sfc to 700mb shear of roughly 30-35 knots) by late afternoon and evening.

Putting it all together, with linear forcing along the front and some large scale ascent, combined with shear vectors somewhat parallel to the front I think any storms would organize into a more linear mode. With less than stellar instability/mid-level lapse rates and a likely linear storm mode, I don’t think the hail threat is impressive…perhaps a couple of marginally severe stones with the strongest cells but not much more. With a mainly linear storm mode, potentially higher than optimal LCLs due to a rather large T-Td spread and weak low level turning resulting in very marginal helicity values I don’t think there will be a very high tornado threat. With a reasonably strong low level flow and linear storm mode there could be some damaging wind gusts. I expect fairly decent storm coverage in the late afternoon/early evening especially in western OH along the front and with some stronger wind gusts possible feel like a low risk is justified.

This really doesn’t look like much to write home about for me, but a few wind reports across the state (especially farther west) are a enough of a possibility to justify the low risk.

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