It's the primary reason we don't do severe well here. Folks blame alot on the marine influence but IMO that is not the biggest reason. It certainly doesn't help, but when you have the proper ingredients/setup the marine influence is nada. Obviously early in the season it's going to be a big factor. IMO, these are the top reasons why we don't do well with severe here:
1) Poor mid-level lapse rates: There is a difference between low-level or surfaced-based CAPE and mid-level or mixed-layer CAPE. You can have robust surface CAPE but weak mixed-layer CAPE. What happens here is the strong surfaced-based CAPE will accelerate your parcels to the LCL and LFC and boom that's when you see rapid storm development...but at a certain point in the storm development the strengthening stops. With weak mid-level lapse rates the temperature difference between the parcels and environment lessens which results in less buoyancy and slower acceleration. When you have steep mid-level lapse rates (driven by EML or cold mid-level temperatures) the temperature difference remains great and parcels can continue to accelerate rapidly.
2) Weaker forcing: It is possible the curvature of the Earth plays a role in this, but often times as shortwave troughs advance into southeastern Canada we'll see the strongest shortwave forcing remain north of the International border and subsequently the stronger dynamics will also remain north. In these scenarios as the cold front drives east along with the thunderstorms, the thunderstorms become removed and displaced from this stronger forcing/dynamics. This becomes critical because this stronger forcing can aid significantly in the acceleration of parcels through the troposphere so convection can remain deep.
We've seen setups before with severe occurring down to the coast and even some significant severe. If the marine influence was as strong as some say, we probably wouldn't see that happen. Don't get me wrong, marine air does play an influence but it holds much weight than what some think. The biggest reasons why severe chances weaken as you move south and east are really more tied into my #2 above. The situations where we do have favorable ingredients and the stronger forcing is south of the border...we see severe persist closer to the coast.