Give this a try.

; Usage $weatherm(ZIPCODE,F) or $weatherm(ZIPCODE,W)
alias weatherm {
  if ($2 == W) { set %usweatherWF W }
  if ($2 == F) { set %usweatherWF F }
  set %usweatherzip $1 | set %usweatherchan $chan
  sockopen usweather www.localweather.net 80
}