Or HaChaim on Exodus 9:14/אור החיים על שמות ט׳:י״ד

1 כי בפעם הזאת…את כל מגפותי, "for this time… all My plagues." G'd informed Pharaoh of the powerful impact of the impending plague of hail. This is why the warning was as severe as the death that would follow if it were not observed. This was in accordance with the rule that if someone guilty of a sin carrying the death penalty by stoning was warned of a lesser death penalty such as strangulation, the court would not be able to convict.

2כל מגפותי, "all My plagues, etc." The obvious question here is that we observe that this was not the last plague but was followed by three more plagues; what did G'd mean then when He described hail as "all My plagues?" One would have been tempted to say that the severity of that plague made it qualify for the description "all My plagues," were it not for Rashi who said that the plague of the dying of the firstborn was equal in severity to all the other plagues combined. Perhaps we have to understand this plague as being equal in severity "to all the preceding plagues together." If we accept such an interpretation the plague of the dying of the firstborn would then be equivalent to 14 plagues, something that does not seem to make too much sense.

3I therefore feel that G'd meant to tell Pharaoh that the impact of this particular plague would change his attitude to all the plagues G'd had brought upon him thus far. Up until now Pharaoh had not really attributed the plagues to G'd's superior power, but had seen in them a display of powerful magic, the work of ghosts and demons. One of his reasons was that his own magicians appeared able to duplicate those plagues. Although his magicians had told him that they could not duplicate (or remove) the plague of כנם, insects, and this was a "finger of G'd," Pharaoh chose to interpret this as an admission that they felt outclassed by Moses and Aaron in their craft. Pharaoh was still convinced that somewhere there were magicians who could do what Moses and Aaron had demonstrated. G'd wanted to disabuse Pharaoh of such ideas and that is why He said "this time," etc., meaning that even Pharaoh would be forced to admit that no human being was capable of producing the kind of hail he was about to experience. G'd added that He would bring this to the attention of Pharaoh's heart, לבך. Up until that moment Pharaoh may have reacted only with his eyes or with his mind. G'd stressed the word מגפותי, "My plagues," to emphasise that these phenomena emanated directly from G'd. No demon or sorcerer could possibly wield such power.