Exodus 9:18

כעת מחר means AT THIS TIME (כָּעֵת not כְּעֵת) TO-MORROW — He scratched a mark on the wall, saying, “To-morrow when the sun-rays reach here the hail will descend” (Exodus Rabbah 12:2; cf. Rashi on Genesis 21:2). הוסדה means [FROM THE DAY] WHEN IT WAS FOUNDED (נתיסדה). Every word (verb) the first root-letter of which is י, as יסד and ילד and ידע and יסר, when it is used in the Hithpael sense (Rashi means our Niphal which often is the reflexive of the Kal), a ו replaces this י; e. g., הוסדה here, from יסד; (Hosea 2:5) “her being born (הִוָלְדָּהּ)”; (Esther 2:22) “and it was known (ויודע)”; (Genesis 66:20) “and there was born (ויולד) to Joseph”; (Proverbs 29:19) A servant will not be corrected (יוסר) by words”.

הִנְנִ֤י מַמְטִיר֙ כָּעֵ֣ת מָחָ֔ר בָּרָ֖ד כָּבֵ֣ד מְאֹ֑ד אֲשֶׁ֨ר לֹא־הָיָ֤ה כָמֹ֙הוּ֙ בְּמִצְרַ֔יִם לְמִן־הַיּ֥וֹם הִוָּסְדָ֖הֿ וְעַד־עָֽתָּה׃

behold, tomorrow about this time I will cause it to rain a very grievous hail, such as has not been in Miżrayim since its foundation until now.