What is the meaning of "u'Varad b'Redes ha'Ya'ar"?
Rashi: You are forced to say that this "Barad" is not a noun, for it has a Kamatz and Patach. It is a verb (to send hail), like Rachatz, Yashav and Amad. The Beis in Beredes is part of the root, like Ateres and Akeres. Hashem will rain on the officers of the Resha'im, which are now built and fulfill of cities like a forest. Yonason translates like this - hail will descend and kill from camps of the nations.
Radak: Hashem will send hail in the forest, lest it damage crops and fruit trees, like hail [is prone to damage them].
Malbim: The Ya'ar (forest) and Shefelah (valley) were both in Yehudah. Sancheriv destroyed both of them - Lachish and Livnah (37:8) in the valley, and the forest (37:24). Va'Rad is when it descends and is destroyed.
What is the meaning of "uva'Shiflah Tishpal ha'Ir"?
Rashi: Yisrael were lowly until now. They will lower the great city of Paras 1 .
Radak #1: No lowliness will be mentioned in the city of Yerushalayim, unless they mention the low land, i.e. the valley 2 , but the level of Yisrael will not be lowered.
Radak #2: They will dwell in open cities in the valley. They will not need to build strong towers in the mountain
Malbim: All cities in the valley descended from their level when everything was destroyed.
