As to the original post: It’s not that simple. Palms when affected by the cold have a range of appearances. From burned leaf edges, to entire exposed leaves dead (but the apical bud alive) to the dead bud. You could have that line of palms from Florida to Missouri, and you would see them all looking differently (I would bet some would be dead to the south of a few who would be barely alive, depending on what palm you plant).
Microclimates DO effect how well a tree is going to survive the cold. Palms are a good example. There are palms as far north as portland Oregon, i hear (they are windmill palms, Trachycarpus fortunei).
I believe the most cold hardy palm is the Needle palm (Rhapidophyllum hystrix, a native of south carolina forests IIRC). It can stand at it’s lowest, temps of -20 F.
Looking at a map of the USDA hardiness zones (which are not all nice and even, look here, a Needle palm could grow as far north as mid New York state, or in southern Michigan.
Anyway, it’s all about microclimates that will affect the outcome of a line of palms being alive in one spot and dead in another.
Oh and there is a needle palm in Washington DC at the National arboretum in the “Asian Valley” section of the garden, as well as one near the corner of 15th & P, NW, in Washington, DC.