Plaincast

What has changed

The risk of an MCS moving through the region is trending downward (details below) with a limited severe risk for Tuesday. Added a quick highlight to note the rest of the week pattern that is light on impacts. Aviation discussion updated for 12z airport forecasts issuance.

Messages

Varying degrees of severe threats and thunderstorm timing this afternoon into Tuesday morning.

Lowering severe threat for Tuesday but non-zero probability remains favor far southeastern zones.

Generally dry and seasonable temperature favored into the start of the weekend.

Discussion

KEY MESSAGE 1. The broad overview notes that embedded shortwaves within northwest flow between the Central Plains ridge and eastern seaboard trough will travel SE through Tuesday morning and offer increased thunderstorm chances starting this afternoon.

Through the early evening, hi-res model guidance has seemingly narrowed the area of focus to northwest PA where the uncapped environment (nearing 2000 J/kg) receives enough lake-breeze lift to trigger initiation whereas everywhere else lacks such forcing mechanism to get storms started. Enough shear (around 30kts) plus DCAPE around 800 J/kg would allow any developed storm to create a downburst damaging wind threat; resultant outflows would then be the source of storm coverage expansion S/W/E through 00z. The lack of strong forcing aloft and greater organization methods alludes to only isolated instances of damaging wind (and small hail) that would translate to a "Marginal Risk" outlook from SPC.

The "greater threat" leading into today was MCS potential after 00z/8pm that has garnered the "Slight Risk" SPC outlook. Latest model trends more favor this threat to remain west of the region as it dives west into the more unstable airmass, leaving this area in the convective cirrus shield less conducive to severe potential. The approaching mid- level shortwave may still initiate overnight thunderstorms but these are likely to be more elevated (thus greatly reducing the damaging wind risk) and less organized. The MCS also could produce an outflow wind that reaches the area that could aide convective initiation, but it would still remain tough to see this reaching higher severe risk levels beyond an isolated instance of damaging wind.

This is a lot to just say that afternoon storms are mainly favored in NW PA with the more widespread threat being determined by the path a MCS takes after forming in the western Great Lakes region.

KEY MESSAGE 2. Timing will be key in severe weather potential Tuesday as forcing/lift sees a significant uptick with the passing cold front and stronger shortwave trough either in the morning hours (lesser severe threat) or early afternoon hours (greater severe threat for southeast zones). And even this simplified projection doesn't account for convective evolution late tonight into early Tuesday morning.

In the latter scenario, the general lack of overnight convection plus afternoon frontal arrival allows for greater buoyancy buildup (approx. 1500 J/kg SBCAPE) to better manifest a straight-line damaging wind threat as storm clusters form along the advancing front. Severe probabilities would be lowest from NW PA to NE OH but slowly trend up progressing southeast toward the mid-Atlantic.