The Storm Prediction Center has outlined much of the Tennessee and Ohio Valleys in a slight risk of severe thunderstorms for Friday and Friday night. This is unusually early in the season to see a severe risk in this area, especially the Ohio Valley, so what gives?
The pattern features a highly positive NAO with well below normal heights over Greenland and a negative PNA, with below normal heights on the west coast of the US. This pattern favors storms cutting north through the Plains into the Great Lakes with a southeast ridge in place, making it easier for moisture to surge northward from the Gulf of Mexico.
On Friday, that is exactly what happens, as an upper level trough swings east out of the Rockies. Moisture will surge north into the Tennessee and Ohio Valleys, and a low pressure that develops over the Plains and tracks into the Great Lakes Friday-Saturday will promote a strong low level flow across the warm sector over the TN/OH Valleys.
Wednesday’s 12z Euro was a bit more subdued, with a weaker wave of low pressure tracking into the lower Great Lakes, with instability limited north of the Ohio River. However, the model would support severe weather over the lower OH and TN Valleys.
Wednesday’s 12z run of the GFS was a bit farther southeast than the Euro, and more strung out with the surface low, resulting in a less significant (but still present) severe threat:
The NAM model has consistently been a bit stronger and farther NW with the surface low track, resulting in a more significant severe threat that extends into northern IN, northern OH and perhaps far southern MI. Tonight’s 0z run of the model brought a 988mb surface low through Chicago Friday evening and showed the warm front and instability into extreme lower MI:
The NAM shows a very dynamic situation, with winds at 18,000 feet above the surface of 100kts roaring across the upper Ohio Valley over an unstable airmass:
This kind of jet streak moving over an unstable airmass would result in an extensive and significant severe weather outbreak over a wide area, mainly east of the Mississippi, on Friday.
The most recent SREF’s agree, showing a large area of expected favorable conditions for tornadoes Friday afternoon into the evening:
So, we will need to watch how strong the low pressure is and where it tracks on Friday, as that will determine who sees severe and how extensive any outbreak is. I would expect a moderate risk for severe thunderstorms to be issued by the Storm Prediction Center in subsequent outlooks from western/central TN north into much of KY into southern IN and OH, with damaging winds, large hail and several tornadoes being likely on Friday.
Thoughts and prayers go out to those killed in Wednesday’s storms and the three children murdered in Chardon, OH on Monday, and their families.