Locally, amongst fishing buddies, the phrase is used when when one of us observes the first sighting on our local waters of the Drunella Flavilinea; the Little and/or Lesser Western Green Drake or simply the "Flav". A first, reliable report of their appearance was Saturday, August 25th. The...sparse...hatch will last 3-4 weeks.
Green Drake (Drunella Grandis or Doddsi); I prefer a size 12 regardless of nymph, emerger, dun or spinner pattern. The adults emerge late afternoons during late August thru September. If "head-hunting", 4:30-6:30 PM is the optimum time to be on the water. They are "crawlers" that emerge in the slow-margins of a riffle and the trout generally will set-up in the spill-in of riffles into quieter water or smoother flows. With clear skies and warm weather they can/do lift-off quickly...floating awhile during overcast and/or drizzly weather.
An Excerpt from an authored Sierra Fisherman magazine article:
"...Green Drakes are largest, in sizes #6-#8. The duration of the hatch is short-lived; at most three weeks**. The daily emergences are unpredictable; sometimes 10:30-12 noon and other times 1-3PM. There are few adults; observing 2-3 dozen is considered a good number. Nevertheless, trout are aware that this high-caloric food-item is around and they are on the look-out. The Little Western Green* Drake ( “Flav” ), in #10-#12 sizes, hatches for 4-5 weeks. It also has sparse numbers. Time-wise, they are more predictable than their big brother; late afternoons from 4-6 PM, at most…then they’re gone..."
* image from Hatch Guide For Western Streams
**unbelievably this season they were in the immediate Truckee area for a bit over 7 weeks!! Adults on the BT first seen (confirmed) June 8th...to June 29; the LT June 26-August 1st!