California calyptra.

It rained today; heavy at times, but mostly a miserable, stubborn drizzle that lasted all-the-grey-day-long.  And the temperature climbed to a whopping 65° F.   Hmmph!  Not exactly optimum conditions for bloom in the grapevines - which seems to be going quite slowly this year. Come back, California-sunshine.
The number of days that the grapevines are in bloom is dependent upon the weather.  When conditions are favourable bloom can take about 8-10 days, (but it can take as long as two weeks under cool conditions like the Napa Valley is experiencing now).  And, of course, the flower clusters on a grapevine will not all bloom at exactly the same time, so that spreads bloom out a bit.  Still, I think bloom is more protracted this year.  I have also noticed that there are more clusters per shoot and that the clusters seem to be a bit bigger than the past two years.
Do not be alarmed by this picture of detached grape flowers from one cluster of Cabernet Sauvignon at TWWIAGE (and hand-modeled by a coworker).  Although it looks like a lot of dead material; anthers, calyptra, stamens, etc., it is quite normal.  As few as 20 to 30 percent of the flowers on a cluster will develop into berries, most will fall from the cluster during the initial stages of berry growth.  Furthermore, a significant number of flowers will drop from the cluster about 8 to 12 days after full bloom: a stage commonly known as shatter.  Fruit set is not complete until after shatter.  Phew!  It's a jungle out there.  Fruit, set, now!