Lake Hennessey — Current Water Temperature, Annual Trends and Forecast
This page provides information on the current water temperature in Lake Hennessey, trends, monthly ranges and the forecast for the coming days.
Current Water Temperature
Water Temperature in Lake Hennessey today is 20.8°C.
Water Temperature: Daily Trends, Historical Records, and Forecast
✅ As of today, the minimum water temperature ranges from 68°F (20°C) to 74°F (23°C). The water is warm and pleasant for swimming, and in this range, one can stay in the water for extended periods without the risk of hypothermia. This is the ideal temperature for relaxation, swimming, and water sports. Families with children, as well as outdoor enthusiasts, will enjoy swimming and time spent in the water. For most people, this is the optimal temperature for a comfortable beach experience. It is important to note that these swimming tips and recommendations apply only in good weather. When the air temperature is below 68°F (20°C), and sometimes even below 77°F (25°C), swimming can be unpleasant even with relatively high water temperatures.
Water Temperature Chart Over the Last 30 Days
Table of Daily Sea Temperature Data
| Date | Current | Average | Forecast |
|---|---|---|---|
| OCT 8 | 22.3°C | 21.7°C | |
| OCT 9 | 22.1°C | 21.7°C | |
| OCT 10 | 22.0°C | 21.7°C | |
| OCT 11 | 21.4°C | 21.7°C | |
| OCT 12 | 21.3°C | 21.7°C | |
| OCT 13 | 21.9°C | 21.7°C | |
| OCT 14 | 21.1°C | 21.7°C | |
| OCT 15 | 20.8°C | 21.7°C | |
| OCT 16 | 21.7°C | 20.8°C | |
| OCT 17 | 21.7°C | 20.9°C | |
| OCT 18 | 21.7°C | 20.9°C | |
| OCT 19 | 21.7°C | 21.0°C | |
| OCT 20 | 21.7°C | 21.0°C | |
| OCT 21 | 21.7°C | 21.1°C | |
| OCT 22 | 21.7°C | 21.1°C |
* Current - Actual water temperature value
** Average - Average water temperature on this day in recent years
*** Forecast - Our water temperature forecast
Actual nearshore water temperatures may fluctuate by several degrees from the reported values, especially following heavy rainfall or extended periods of strong winds. Certain wind patterns can cause colder, deeper waters to rise and replace the sun-warmed surface water, leading to noticeable variations.
Our forecast is based on a proprietary mathematical model that considers real-time water temperature changes, historical trends, key weather patterns, wind strength and direction, and air temperature specific to each region. Additionally, we incorporate data from other resorts to enhance accuracy.
Monthly Water Temperature Range (Min, Max & Average)
| Month | Min | Avg | Max |
|---|---|---|---|
| January | 8°C | 10°C | 13°C |
| February | 6°C | 10°C | 14°C |
| March | 9°C | 13°C | 18°C |
| April | 13°C | 16°C | 19°C |
| May | 16°C | 21°C | 25°C |
| June | 21°C | 24°C | 26°C |
| July | 23°C | 27°C | 31°C |
| August | 23°C | 27°C | 32°C |
| September | 20°C | 24°C | 28°C |
| October | 19°C | 22°C | 24°C |
| November | 13°C | 17°C | 20°C |
| December | 11°C | 13°C | 16°C |
Lake Hennessey: Water Temperature by Location
Lake Hennessey: Weather Forecast
October 15
October 16
October 17
October 18
October 19
October 20
Location & Local Information
Lake Hennessey, a reservoir in Napa County, typically shows marked seasonal variation in surface water temperature. In winter months the surface often stays cool, roughly 45–55°F (7–13°C). Spring brings gradual warming into the 50s and low 60s°F (10–17°C) as runoff and air temperatures rise. By summer the shallow, sun-exposed areas can warm to about 65–75°F (18–24°C), sometimes a bit higher in calm, late‑summer conditions; fall temperatures generally decline back into the 50s–60s°F (10–18°C) as air temperatures cool and inflow increases. Thermal stratification can occur in deeper sections, so bottom waters remain colder than the surface throughout the year.
Swimming opportunities at Lake Hennessey are limited and generally discouraged because the reservoir serves as part of the local water supply and has restricted public access and safety considerations. Municipal reservoirs often prohibit or restrict swimming to protect water quality and reduce contamination risk, and to avoid hazards such as cold water shock, steep drop-offs, and fluctuating water levels. Recreational use that is allowed—such as regulated fishing or boating—varies by season and permit rules, so prospective visitors should confirm current access and rules with local water authorities before planning any activities.