Contemporary writers like Philips sometimes use "whelm" to denote a middle stage between "underwhelm" and "overwhelm." But that’s not how "whelm" has traditionally been used. "Whelm" and "overwhelm" have been with us since Middle English (when they were "whelmen" and "overwhelmen"), and throughout the years their meanings have largely overlapped. Both words early on meant "to overturn," for example, and both have also come to mean "to overpower in thought or feeling." Around 1950, however, folks started using a third word, "underwhelmed," for "unimpressed," and lately "whelmed" has been popping up with the meaning "moderately impressed."