Jamieson, Fausset and Brown on Jeremiah 17:8

8. (Psa 1:3).

shall not see--that is, feel. Answering to Jr 17:6; whereas the unbelievers "shall not see (even) when good cometh," the believer "shall not see (so as to be overwhelmed by it even) when heat (fiery trial) cometh." Trials shall come upon him as on all, nay, upon him especially (He 12:6); but he shall not sink under them, because the Lord is his secret strength, just as the "roots spread out by a river" (or, "water-course") draw hidden support from it (2Co 4:8-11).

careful--anxious, as one desponding (Lu 12:29; 1Pe 5:7).

drought--literally, "withholding," namely, of rain (Jr 14:1); he here probably alludes to the drought which had prevailed, but makes it the type of all kinds of distress.