Climate and Average Weather Year Round in Mill Creek Washington, United States

In Mill Creek, the summers are short, comfortable, and partly cloudy and the winters are very cold, wet, and mostly cloudy. Over the course of the year, the temperature typically varies from 37°F to 76°F and is rarely below 27°F or above 85°F.

Based on the tourism score, the best time of year to visit Mill Creek for warm-weather activities is from mid July to late August.

The warm season lasts for 2.8 months, from June 22 to September 16, with an average daily high temperature above 69°F. The hottest month of the year in Mill Creek is August, with an average high of 75°F and low of 57°F.

The cool season lasts for 3.6 months, from November 13 to March 1, with an average daily high temperature below 51°F. The coldest month of the year in Mill Creek is December, with an average low of 37°F and high of 45°F.

In Mill Creek, the average percentage of the sky covered by clouds experiences significant seasonal variation over the course of the year.

The clearer part of the year in Mill Creek begins around June 16 and lasts for 3.7 months, ending around October 7.

The clearest month of the year in Mill Creek is August, during which on average the sky is clear, mostly clear, or partly cloudy 69% of the time.

The cloudier part of the year begins around October 7 and lasts for 8.3 months, ending around June 16.

The cloudiest month of the year in Mill Creek is December, during which on average the sky is overcast or mostly cloudy 73% of the time.

A wet day is one with at least 0.04 inches of liquid or liquid-equivalent precipitation. The chance of wet days in Mill Creek varies very significantly throughout the year.

The wetter season lasts 6.6 months, from October 10 to April 30, with a greater than 35% chance of a given day being a wet day. The month with the most wet days in Mill Creek is November, with an average of 17.4 days with at least 0.04 inches of precipitation.

The drier season lasts 5.4 months, from April 30 to October 10. The month with the fewest wet days in Mill Creek is August, with an average of 3.8 days with at least 0.04 inches of precipitation.

Among wet days, we distinguish between those that experience rain alone, snow alone, or a mixture of the two. The month with the most days of rain alone in Mill Creek is November, with an average of 17.0 days. Based on this categorization, the most common form of precipitation throughout the year is rain alone, with a peak probability of 58% on November 18.

Rainfall

To show variation within the months and not just the monthly totals, we show the rainfall accumulated over a sliding 31-day period centered around each day of the year. Mill Creek experiences extreme seasonal variation in monthly rainfall.

Rain falls throughout the year in Mill Creek. The month with the most rain in Mill Creek is November, with an average rainfall of 8.4 inches.

The month with the least rain in Mill Creek is July, with an average rainfall of 0.9 inches.

Snowfall

As with rainfall, we consider the snowfall accumulated over a sliding 31-day period centered around each day of the year. Mill Creek experiences some seasonal variation in monthly snowfall.

The snowy period of the year lasts for 1.5 months, from December 7 to January 23, with a sliding 31-day snowfall of at least 1.0 inches. The month with the most snow in Mill Creek is December, with an average snowfall of 1.4 inches.

The snowless period of the year lasts for 10 months, from January 23 to December 7. The least snow falls around July 14, with an average total accumulation of 0.0 inches.

The length of the day in Mill Creek varies extremely over the course of the year. In 2026, the shortest day is December 21, with 8 hours, 23 minutes of daylight; the longest day is June 21, with 16 hours, 2 minutes of daylight.

The earliest sunrise is at 5:09 AM on June 16, and the latest sunrise is 2 hours, 49 minutes later at 7:58 AM on December 31. The earliest sunset is at 4:16 PM on December 11, and the latest sunset is 4 hours, 56 minutes later at 9:11 PM on June 25.

Daylight saving time (DST) is observed in Mill Creek during 2026, starting in the spring on March 8, lasting 7.8 months, and ending in the fall on November 1.

The figure below presents a compact representation of key lunar data for 2026. The horizontal axis is the day, the vertical axis is the hour of the day, and the colored areas indicate when the moon is above the horizon. The vertical gray bars (new Moons) and blue bars (full Moons) indicate key Moon phases.

We base the humidity comfort level on the dew point, as it determines whether perspiration will evaporate from the skin, thereby cooling the body. Lower dew points feel drier and higher dew points feel more humid. Unlike temperature, which typically varies significantly between night and day, dew point tends to change more slowly, so while the temperature may drop at night, a muggy day is typically followed by a muggy night.

The perceived humidity level in Mill Creek, as measured by the percentage of time in which the humidity comfort level is muggy, oppressive, or miserable, does not vary significantly over the course of the year, remaining a virtually constant 0% throughout.

This section discusses the wide-area hourly average wind vector (speed and direction) at 10 meters above the ground. The wind experienced at any given location is highly dependent on local topography and other factors, and instantaneous wind speed and direction vary more widely than hourly averages.

The average hourly wind speed in Mill Creek experiences mild seasonal variation over the course of the year.

The windier part of the year lasts for 6.1 months, from October 17 to April 20, with average wind speeds of more than 3.8 miles per hour. The windiest month of the year in Mill Creek is December, with an average hourly wind speed of 5.0 miles per hour.

The calmer time of year lasts for 5.9 months, from April 20 to October 17. The calmest month of the year in Mill Creek is August, with an average hourly wind speed of 2.5 miles per hour.

The predominant average hourly wind direction in Mill Creek varies throughout the year.

The wind is most often from the west for 1.6 months, from May 13 to July 1, with a peak percentage of 38% on June 22. The wind is most often from the north for 2.7 months, from July 1 to September 21, with a peak percentage of 44% on July 30. The wind is most often from the south for 7.7 months, from September 21 to May 13, with a peak percentage of 45% on January 1.

Mill Creek is located near a large body of water (e.g., ocean, sea, or large lake). This section reports on the wide-area average surface temperature of that water.

The average water temperature experiences some seasonal variation over the course of the year.

The time of year with warmer water lasts for 3.0 months, from June 24 to September 26, with an average temperature above 54°F. The month of the year in Mill Creek with the warmest water is August, with an average temperature of 56°F.

The time of year with cooler water lasts for 4.0 months, from December 9 to April 10, with an average temperature below 48°F. The month of the year in Mill Creek with the coolest water is February, with an average temperature of 46°F.

To characterize how pleasant the weather is in Mill Creek throughout the year, we compute two travel scores.

The tourism score favors clear, rainless days with perceived temperatures between 65°F and 80°F. Based on this score, the best time of year to visit Mill Creek for general outdoor tourist activities is from mid July to late August, with a peak score in the first week of August.

The beach/pool score favors clear, rainless days with perceived temperatures between 75°F and 90°F. Based on this score, the best time of year to visit Mill Creek for hot-weather activities is from late July to mid August, with a peak score in the first week of August.

Methodology

For each hour between 8:00 AM and 9:00 PM of each day in the analysis period (1980 to 2016), independent scores are computed for perceived temperature, cloud cover, and total precipitation. Those scores are combined into a single hourly composite score, which is then aggregated into days, averaged over all the years in the analysis period, and smoothed.

Our cloud cover score is 10 for fully clear skies, falling linearly to 9 for mostly clear skies, and to 1 for fully overcast skies.

Our precipitation score, which is based on the three-hour precipitation centered on the hour in question, is 10 for no precipitation, falling linearly to 9 for trace precipitation, and to 0 for 0.04 inches of precipitation or more.

Our tourism temperature score is 0 for perceived temperatures below 50°F, rising linearly to 9 for 65°F, to 10 for 75°F, falling linearly to 9 for 80°F, and to 1 for 90°F or hotter.

Our beach/pool temperature score is 0 for perceived temperatures below 65°F, rising linearly to 9 for 75°F, to 10 for 82°F, falling linearly to 9 for 90°F, and to 1 for 100°F or hotter.

Definitions of the growing season vary throughout the world, but for the purposes of this report, we define it as the longest continuous period of non-freezing temperatures (≥ 32°F) in the year (the calendar year in the Northern Hemisphere, or from July 1 until June 30 in the Southern Hemisphere).

The growing season in Mill Creek typically lasts for 9.1 months (277 days), from around February 23 to around November 27, rarely starting before January 19 or after March 17, and rarely ending before November 3 or after December 20.

Growing degree days are a measure of yearly heat accumulation used to predict plant and animal development, and defined as the integral of warmth above a base temperature, discarding any excess above a maximum temperature. In this report, we use a base of 50°F and a cap of 86°F.

Based on growing degree days alone, the first spring blooms in Mill Creek should appear around April 17, only rarely appearing before March 22 or after May 10.

This section discusses the total daily incident shortwave solar energy reaching the surface of the ground over a wide area, taking full account of seasonal variations in the length of the day, the elevation of the Sun above the horizon, and absorption by clouds and other atmospheric constituents. Shortwave radiation includes visible light and ultraviolet radiation.

The average daily incident shortwave solar energy experiences extreme seasonal variation over the course of the year.

The brighter period of the year lasts for 3.3 months, from May 13 to August 22, with an average daily incident shortwave energy per square meter above 5.5 kWh. The brightest month of the year in Mill Creek is July, with an average of 6.6 kWh.

The darker period of the year lasts for 3.8 months, from October 25 to February 18, with an average daily incident shortwave energy per square meter below 2.0 kWh. The darkest month of the year in Mill Creek is December, with an average of 0.9 kWh.

For the purposes of this report, the geographical coordinates of Mill Creek are 47.860 deg latitude, -122.204 deg longitude, and 364 ft elevation.

The topography within 2 miles of Mill Creek contains only modest variations in elevation, with a maximum elevation change of 325 feet and an average elevation above sea level of 397 feet. Within 10 miles contains only modest variations in elevation (682 feet). Within 50 miles contains significant variations in elevation (7,720 feet).

The area within 2 miles of Mill Creek is covered by artificial surfaces (92%), within 10 miles by artificial surfaces (50%) and trees (15%), and within 50 miles by trees (52%) and water (17%).