Watertown NY weather at a glance
Watertown weather swings from deep winter cold to warm, comfortable summer days. July is the warmest month, with an average daily high of 79°F (26.2°C). January is the coldest, with an average low of 10°F (-12.2°C). Some precipitation falls in every month, and October is the wettest month on average, with about 4.2 inches (107 mm).
The figures below are monthly averages for Watertown International Airport, based on the 1991 to 2020 climate normals. They describe what is typical, not what will happen on your dates, and they are not a forecast. Your trip also depends on the season calendars of the places you want to see. If boats and castles are the goal, read our guide to Thousand Islands boat tours next to this page.
Our pick: an evening cruise
Boat trips are a plan for early summer to early autumn, and on the water you want the light layer from the packing list. This 2-hour evening cruise from Clayton fits that season.
Hand-picked tours, booked through Viator. Prices are indicative starting prices in US dollars (as of October 2026). Tap a tour for live availability and the current price in your currency.
Boat tours and day trips for every season
Cruises, castle trips and day tours around the Thousand Islands, with live prices and availability.
Watertown weather by month
Average daily highs and lows, and average monthly precipitation as water equivalent (snow melted to water). Not snow depth, and not a forecast.
| Month | High | Low | Precipitation |
|---|---|---|---|
| January | 30°F (-1.2°C) | 10°F (-12.2°C) | 2.9 in (72.6 mm) |
| February | 31°F (-0.3°C) | 11°F (-11.8°C) | 2.2 in (55.4 mm) |
| March | 40°F (4.4°C) | 21°F (-6.3°C) | 2.4 in (60.2 mm) |
| April | 54°F (12.1°C) | 33°F (0.3°C) | 3.1 in (78.0 mm) |
| May | 66°F (19.0°C) | 44°F (6.5°C) | 3.0 in (75.4 mm) |
| June | 74°F (23.6°C) | 53°F (11.7°C) | 3.1 in (78.5 mm) |
| July | 79°F (26.2°C) | 59°F (14.9°C) | 3.0 in (76.5 mm) |
| August | 78°F (25.7°C) | 57°F (13.7°C) | 3.2 in (81.3 mm) |
| September | 71°F (21.8°C) | 49°F (9.3°C) | 3.6 in (90.4 mm) |
| October | 58°F (14.7°C) | 39°F (4.0°C) | 4.2 in (107.2 mm) |
| November | 47°F (8.1°C) | 29°F (-1.4°C) | 3.4 in (86.4 mm) |
| December | 35°F (1.9°C) | 19°F (-7.2°C) | 3.1 in (79.8 mm) |
What each season means for a trip
Winter, December to February
Highs hover around freezing and lows fall to 10°F (-12°C) in January. Lake-effect snow bands from the Great Lakes, Lake Ontario included, are narrow, so snowfall can differ a lot from one place to the next. Around Fort Drum, snow is possible from October to April and winters are very cold. The Wellesley Island trails stay open every day from sunrise to sunset all year.
Spring, March to May
March still feels like winter, with highs of 40°F (4.4°C). By May the average high reaches 66°F (19.0°C). Boldt Castle reopens on May 8, 2027. In 2026, camping at Wellesley Island State Park started on May 1, and Fort Henry's main season started on May 16.
Summer, June to August
The warmest stretch, with highs of 74 to 79°F (about 24 to 26°C) and lows of 53 to 59°F. Swimming at Wellesley Island State Park ran from June 29 to September 7 in 2026, from 11 am to 7 pm, and only at designated areas with lifeguards.
Autumn, September to November
September is mild, with a high of 71°F (21.8°C). October is cooler and the wettest month. In 2026 Boldt Castle closed for the season on October 12 and Singer Castle on October 18, and camping at Wellesley ended on October 11. By November the average high is 47°F (8.1°C).
Best time to visit Watertown and the Thousand Islands
The best time depends on what you want to do. There is no single rain-free month, and precipitation shows up in the averages in every month. For castles and boat tours, plan between early summer and early autumn, and check each calendar for the year you travel. Both Boldt Castle and Singer Castle are seasonal, and neither is open all year.
For most visitors who want boats, castles and swimming together, July and August are the safest choice: the warmest weather and the fullest schedules. September is a quieter option with highs around 71°F. If you come in winter, the castles are closed, but Thompson Park, the downtown and the wineries stay on the list. See things to do in Watertown for what works in cold weather. We do not give foliage dates, because they vary every year.
Packing and planning by temperature
- On the water: bring a light layer for cruises, even in summer.
- Winter: expect freezing highs and lows near 10°F in January and February. Wear layers, and look at the forecast before driving after snow.
- Shoulder seasons: April and October are changeable. Pack rain gear.
- Rain: precipitation is spread across the year, with 2.2 to 4.2 inches (55 to 107 mm) per month.
Watertown NY weather: frequently asked questions
When is the best time to visit Watertown?
For castles and boats, plan between early summer and early autumn. July and August are warmest, with average highs of 79°F (26.2°C) and 78°F (25.7°C). Check each season calendar for your year.
How cold does Watertown get in winter?
On average, January highs are 30°F (-1.2°C) and lows are 10°F (-12.2°C). February is similar, with a high of 31°F and a low of 11°F.
What is lake-effect snow?
It is snow that forms in narrow bands downwind of the Great Lakes, including Lake Ontario. Local snowfall can vary strongly over short distances.
Which month is the wettest?
October, with about 4.2 inches (107.2 mm) of precipitation on average. The driest month is February, with about 2.2 inches (55.4 mm).
Are Boldt Castle and Singer Castle open in winter?
No. In 2026 Boldt Castle closed on October 12 and Singer Castle on October 18. Boldt Castle reopens on May 8, 2027.
Is this the official weather or tourism website?
No. This is an independent travel guide, not run by the City of Watertown, any weather service or tourism body.