Sorry for the long-winded tale... it's how I remember it.
This set of circumstances has only happened to me once when I was on a trip through WY, fishing the N Fork Shoshone. Clouds were moving in from over the ridge to the west above the river and building above me. I was not getting or seeing any fish. It seemed sudden; the wind kicked up and the sky let forth a driving rain. My new waterproof(ed) insulated softshell was not keeping me dry. I took shelter under the lee and conveniently overhanging side of a BIG streamside boulder where I could see the water. After 10+ minutes the rain stopped, and a substantial dark winged Green Rock Caddis hatch began. 20 ft upstream in a cleft between two boulders that I couldn't reach from downstream a BIG fish began sipping them in. I tied on a #12 green Deer Hair Caddis and I carefully went further downstream, crossed the river, walked upstream near the other bank, and worked my way back down. I got to practice my drift on another large fish a ways upstream but lost it in the current. The BIG guy was still sipping as I worked into position for a quartering downstream cast. On the first cast the line got caught in the hydraulic in front of the outside boulder forming the cleft which took my fly out into the current. I put a big upstream mend in the line on the 2nd cast so the presentation was fly first with at least 10 feet of line straight upstream just inches away from the boulders lining the bank. The fly went into the cleft, up rose the fish to sip it in. I set the hook. As it darted out into the current I did a 2nd firm but controlled hookset (that really helps for me). This time I worked my way down below the fish and fought him upstream to use the current against him. I wasn't using Measure Nets and didn't have a tape or markings on my rod but I netted a fat (conservatively) 18"-20" fish. Also, as the wind was kicking up, my buddy had worked his way a few hundred yards downstream to a sheltered bend and landed a bunch of smaller fish during and after the rain in the slack water seam at the bend.
The moral to this story: If you're on or near the water as a brief storm cell passes through, watch the water for insect activity as it lets up.