Principal English Translation: 

in this direction; back (from a distant time to now)

Orthographic Variants: 
ual, val
Alonso de Molina: 

ual. haziaca.
Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 154v. col. 1. Thanks to Joe Campbell for providing the transcription.

Frances Karttunen: 

HUĀL- directional prefix hither, in this direction / hacia acá (M)
Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 80.

Attestations from sources in English: 

Hual also has a temporal meaning, as in "back into time." In a similar way, the directional on can relate to time, as in thinking forward into the future.
Rebecca Horn's notes from Nahuatl classes with James Lockhart at UCLA. Card file in the possession of Stephanie Wood.