Grapes have no internal timer, they just respond to the temperature.
The Winkler scale, expressed in growing degree days, was developed in the 1940's by the university of Davis as a method to quantify heat available for vine development during the growing season and identify 5 climate zones for suitable grape varietals.
Temecula, with 3,000–3,500 degree days is in Region III, similar to the Rhône in France.
Napa and Paso Robles have an average between 2,500–3,000 degree days. They are Region II, similar to Bordeaux.
The Winkler Scale is calculated as as the sum of degree days over 10°C (50 °F) from April 1 until October 31.
= Σmax [(avg. daily temp. – 10), 0] (ref.: Arnold J. Bloom, Dept. of Plant Sciences, UC Davis)
However, if the growing season starts earlier due to climate change, the formal calculation start date of April 1 is out of pace with the actual grape vine growth. It leads to a lower growing degree count till harvest (100-120 days after flowering), as grapes don't care about the date.
It's time to reconsider the Winkler calculation method and factor in the actual growing cycle of the grapes.
The Winkler method assumes that grapes start growing at average day temperatures higher than 10°C/50 °F.
A good adjustment to deal with the new reality of climate change is defining the day where an daily temperature of 10°C/50 °F is reached as the new Winkler growing degree start date.
That would make the Winkler scale future proof.