The problem with LCD is the crystals will likely have trouble working/twisting at 20 degrees as a guess, and would need a lot of warm up time unless you always heat this porch all winter long. If your CRT has worked without issues in that environment, then plasma will likely work in that environment since it works similarly to CRT in that phosphors are used to light up the screen.
Maybe you could try this once the temps drop and buy then and see if it works without any issues. Here is the "operating" temp for this from the link here:
http://www2.panasonic.com/consumer-e...702#tabsection
"32°F - 104°F (0°C - 40°C)"
That is the operating temp so I would guess that storage temp would be a wider range and should work as long as you turn the heat on for a little while when the temp is below 32 degrees. Plasmas basically are only available in 42" and higher now and you never listed the size wanted or the viewing distance, so I linked to the smallest 1080p model.
Hope this helps!