Where did Achav go?
Radak #1: He ascended the chariot to go to Yizre'el to eat and drink. When he saw Eliyahu ascend Har ha'Karmel to pray, he waited to see what will happen before going.
Radak #2: He ascended from the valley to go to his tent to eat and drink.
What is the meaning of "va'Yighar"?
Rashi citing Targum Yonasan: He bowed to (prostrated on) the ground [to pray for rain]. Radak - he prayed that it come while Achav is still there.
Radak: He did not [prostrate on the ground and] spread his hands and face. Rather, he bowed towards the ground, and put his head between his knees.
Hashem already promised rain (verse 1). Why did Eliyahu need to pray?
Radak (23): He prayed so Yisrael will know that he is a true Navi, and Hashem answers his prayer. Without Tefilah, they could say that [the rain] was chance.
Malbim: Hashem promised rain via nature. Eliyahu prayed that it come immediately, miraculously (Radak - while Achav is still there).
QUESTIONS ON RASHI
Rashi writes that he prostrated on the ground. The end of the verse says that he put his head between his knees!
Radak #1: First he prostrated to the ground. Afterwards, he put his head between his knees.
Radak #2: Refer to 18:42:2:2.
