Lake Washington Water Temperature Today
Lake Washington in Washington, United States reaches comfortable swimming temperatures in July and August, and drops close to single digits in winter.
In August Lake Washington averages about 23°C at the surface, peaking near 23°C in July, August, September and dropping to roughly 7°C at its coldest.
Lake surface temperatures are modelled from long-term monthly averages and refined with live air temperature, wind and UV. Sea temperatures elsewhere on SwimTemperature come from measured marine model data.
Lake Washington water temperature by month
| Month | Average water temperature | Swimming |
|---|---|---|
| January | 8.1°C | cold |
| February | 7.0°C | cold |
| March | 8.1°C | cold |
| April | 11.0°C | cool |
| May | 15.0°C | mild |
| June | 19.0°C | mild |
| July | 21.9°C | warm |
| August | 23.0°C | warm |
| September | 21.9°C | warm |
| October | 19.0°C | mild |
| November | 15.0°C | mild |
| December | 11.0°C | cool |
Lake Washington swim conditions FAQ
Best time to swim, water safety and the temperature thresholds that matter for open-water swimming in Lake Washington.
Best time to swim
The warmest water in Lake Washington is in July, August, September, around 23.0°C.
The comfortable swim season — 18°C or warmer — covers June, July, August, September, October. Right now (August) expect about 23.0°C.
Within a day, the surface is warmest in the late afternoon, while early morning is calmest and quietest for boat traffic.
Water safety tips
- Enter Lake Washington slowly and let your breathing settle before swimming out — cold shock is the biggest single risk in fresh water.
- Swim parallel to the shore rather than out and back, so you are never far from a safe exit.
- Lakes have no lifeguard on most shorelines: swim with a partner and use a bright tow float so boats can see you.
- Check for boat traffic, sudden drop-offs, weed beds and submerged obstacles before your first swim of the season.
- Deep lakes stay cold below the surface even in a heatwave — expect a sharp thermocline a metre or two down.
- After heavy rain or during a warm calm spell, avoid swallowing water: runoff and algal blooms can cause illness.
- Get warm and dressed quickly after cold swims — afterdrop peaks 10–30 minutes out of the water.
Temperature thresholds
| Water | Feels | What to do |
|---|---|---|
| 24°C and above | Warm | Comfortable for long swims in swimwear. Watch UV and stay hydrated. |
| 20–23°C | Pleasant | Ideal open-water conditions for most swimmers; no neoprene needed. |
| 18–19°C | Fresh | Fine for 30–60 minutes; a shorty wetsuit extends comfort. |
| 14–17°C | Cool | Cold-water rules apply. A 3/2mm full suit is sensible. |
| 10–13°C | Cold | Limit to short swims, 4/3mm suit, gloves and boots; never swim alone. |
| Below 10°C | Very cold | Cold shock and swim failure risk. Acclimatised swimmers only, minutes not hours, with supervision. |
At today's estimate of 23.0°C, Lake Washington rates as warm — Excellent.
What is the water temperature of Lake Washington right now?
In August the surface of Lake Washington averages about 23°C. The reading at the top of this page refines that monthly average with today's live air temperature, wind and UV.
Can you swim in Lake Washington?
Yes. Lake Washington reaches 18°C or warmer in June, July, August, September, October, which most swimmers find comfortable without neoprene. Outside those months treat it as cold-water swimming.
When is the best time to swim in Lake Washington?
July, August, September offer the warmest water, around 23°C. Within the day, late afternoon is usually warmest at the surface, while early morning is calmest with the least boat traffic — the usable swim season runs across June, July, August, September, October.
How cold does Lake Washington get in winter?
The coldest month averages roughly 7°C at the surface. At that temperature cold shock is a genuine hazard for unacclimatised swimmers.
What water temperature is safe to swim in?
Above 20°C is comfortable for most people, 18–20°C is fresh but fine for shorter swims, 14–17°C calls for a wetsuit, and below 10°C is experienced cold-water swimmers only — minutes rather than hours, always with someone watching.
Do I need a wetsuit in Lake Washington?
Based on the monthly averages: no wetsuit needed at 22°C or above, a shorty from about 18°C, a 3/2mm full suit from 14°C, and 4/3mm with gloves and boots below 14°C. Lake Washington peaks near 23°C and bottoms out near 7°C.
Is Lake Washington safe to swim in?
Most of the year, yes, provided you pick a known entry point, swim parallel to the shore with a partner, and check local signage. Avoid swimming after heavy rain or during visible algal blooms, and remember lake shorelines are rarely lifeguarded.
Compare lake temperatures
Pick up to 4 lakes to compare today's water temperature and the next seven days.
| Lake | Today | Next 7 days |
|---|---|---|
| Lake WashingtonUnited States | 23.0°Cwarm | Loading forecast… |
| Lake UnionUnited States | 23.0°Cwarm | Loading forecast… |
| Lake SammamishUnited States | 24.0°Chot | Loading forecast… |
Water temperatures are modelled from long-term monthly averages for each lake and refined daily with forecast air temperature, so the seven-day column shows the expected trend rather than a measured buoy reading.
Open the full lake comparison tool · See Lake Washington on the live lake temperature map
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