Harvest Bulletin: New Enologix Harvest Date Model

New Model Predicts 135 Days Bloom to Harvest for Cabernet Sauvignon in Oakville

September 1, 2016–Grape Harvest Date (GHD) was predicted for October 3rd, 2016 if September temperatures equal the 10-year average, according a brand new model developed by Enologix scientists for Cabernet Sauvignon. However, GHD will be October 12 if temperatures are as cool as they were in August. We are proud to make these predictions using the Enologix New Model which has a standard error of prediction of 6-days. Grapegrowers will begin harvesting the first grapes September 27th, largely from vineyards that are either first through fourth leaf or on the crowns of hills and mountains. Adjust these numbers based on the grape maturation order of petit verdot, cabernet, merlot and cabernet franc.1

Huglin Index heat summations 1989-2016 in Oakville, CA. 2016 is now the 3rd coldest growing season in the past decade through August 31st.

Enologix has been a primary actor in building models to predict grape harvest date (GHD) since 2001. We have used the Winkler Index (WI) out of deference to Albert Winkler (1) and University of California, Davis viticulturists. We used weather station data supplied by California Irrigation Management Information System (CIMIS) online at the UC IPM website to download the raw data from several weather stations to calculate growing degree days (GDD) using the Winkler Index (Equations 1 and 2 below).

OUR HARVEST MODELS failed to predict the 2015 GHD, and why? Two reasons: It has been generally accepted that Cabernet Sauvignon grapes reach optimum maturity at 120 Days Bloom to Harvest (DBH). Unfortunately, actual DBH has increased every decade since Napa Valley became a benchmark by which cabernet wines are judged — climbing from 120 days the year of the Judgement of Paris 1976 to 130 in 2000 and now over 140 DBH in 2015.

This model began to unravel in the mid-2000’s at which time we began adjusting the DBH upward or downward using Growing Degree Days (GDD) calculated from the Winkler Index. However, we soon found that there were never enough degree-days to “catch up”, and had a hunch that both the DBH model and the Winkler method of calculating heat summations were flawed for calculating GHD in 21st century coastal California wine regions.

OUR NEW MODEL, is introduced for the first time in this Growing Bulletin, September 1st, 2016. Our primary modification was to use the Huglin Heliothermal Index (HI) to calculate heat summations, which are then used to determine an adjusted DBH calculation. Huglincomputed the growing season from April 1 to September 30, as the clock ticks we will determine whether to use the October data which currently seems unnecessary(2).


The Huglin Index calculates heat summations as:

Where kis factor used to normalize Napa Valley latitude to Bordeaux. Huglin Degree Days (HDD) give more weight to daily maximum temperatures calculated as follows:

Example Calculations

EXAMPLE CALCULATIONS for Napa, California during the 12-year period 2004 to 2015 allowed Enologix to predict the GHD with a margin of error of 6-days for the period.

Issues and Solutions

Several new coefficients of correction were introduced to account for the impact of climate change on winegrowing. One was for latitude, k, which enables Enologix to standardize Napa Valley to Bordeaux in terms of the sum of the temperatures required for vine development and luxury cabernet grape ripening.

Moving forward, we will continue to refine the model. In 2017, we will introduce Enologix Cold Night Coefficient (CNC) and Drought Index (DI). Huglin Index reduces the error of prediction when year to date heat summations through August 31 are used with ten-year average September heat summations. In the hottest growing season, 2015, Huglin Index corrected the failure of the Winkler Index withthe calculation for actual heat summations from bloom to August 31, 2015 plus 10-year average September heat summations to predict GHD.

We must agree that for multiple reasons, the conventional heat summation models used to this point are flawed for California wine industry. Using over 12 years of climate data and phenological dates collected from customers across multiple regions, we built a new model to predict harvest date and quality using a modification of the Huglin Index combined with our own process based phenology models. We invite you to consider using this model for your unique region and variety. Further, we encourage your participation by sharingyour phenological and temperature data with us to make the model more robust for more regions and varieties (Crowdsourcing is all the rage, folks!).

Early Observations for Vintage 2016

Our sense is that grape color will track higher than 2015. We have beenseeing fermenter samples of early ripening varieties, and it appears that extraction is higher than normal, especially for Pinot Noir. Your Standard Fermentation Plan (SFP) should hedge toward moderating tannin extraction at this point. It is always a good strategy to make a light wine from first few tanks than over-extract. We highly recommend that you sample first fermenters at 18, 12, 6 and 0 °Brix to confirm extraction and fine-tune your SFP.

Weather Summary for August

Relatively cool ocean temperatures just offshorehave resulted in a robust marine layer, and the traditional “June Gloom” associated with California’s marine stratus has persisted into August in most areas inCalifornia's North Coast region.

Meanwhile, across all of Southern California and even more inland parts of Northern California, this summer has been yet another hot one–and several heatwaves have occurred since June. Unusually warm ocean conditions have persisted along the Southern California coastline, keeping the marine influence in check and allowing for very hot conditions at times just a few miles inland from the immediate coastline.

Weather Forecast for Week of September 8 - 12

Temperatures will start to cool Thursday as the onshore flow returns. Wednesday's warmup yielded temperatures in the70s to lower 80s near the coast andwidespread 80s and 90s inland with a few of the warmest inland areasreaching to around 100 degrees. Conditions are forecast to cool byThursday as the ridge aloft weakens and shifts further away from the coast and onshore flow increases. The marine layer will deepenover the region. Thus, more widespread clouds are forecast to develop by Thursdaymorning along with the onset of cooler conditions spreading backinland. Temperatures on both Thursday and Friday should be closer toseasonal averages with increased likelihood of late night andmorning low clouds spreading inland.

Further Cooling in Forecast for Next Week

Dry weather conditions will also persist into the weekend with onlyminor changes in day-to-day temperatures as weak high pressureremains over the region. Conditions cool further early next week asthe latest forecast models show a mid/upper level trough droppingsouthward out of the Pacific Northwest down through California.Depending on the exact track, could see a deepening of the marinelayer and potentialfor coastal drizzle to return during the latenight and morning hours. Otherwise, mostly dry weather conditionsare forecast to persist with any chances of convection staying wellinland over the Sierra.

1For Cabernet Sauvignon Grapes in Oakville California which was 50% Bloom on May 15th, 2016. Harvest dates will vary for other locations and bloom dates. Contact Enologix for more information on determining harvest dates for your specific vineyard.


(1)Amerine, M.A., et A.J. Winkler. 1944. Composition and quality of musts and wines of California grapes. Hilgardia. 15(6): 493-673.

(2) Huglin, P. 1978. Nouveau mode d'évaluation des possibilités héliothermiques d'un milieu viticole. Comptes Rendus de l'Académie de l'Agriculture de France. 64: 1117- 1126

(3) Tonietto, J., et A. Carbonneau. 2004. A multicriteria climatic classification system for grape-growing regions worldwide. Agricultural and Forest Meteorology. 124(1/2): 81-97.

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