September 21 is generally a great time to visit Seattle. While there is a slight chance of rain (20%), showers are usually brief. Expect comfortable 20°C weather. Seattle is currently in its most popular travel window, meaning vibrant crowds and premium pricing.
View the full calendar
Essential Travel Info for Seattle
Currency
USD
$ United States dollar
Language
English
Driving
Right side
Connect
The area code for the City of Seattle is...
Safety
Moderate Safety
GPI: 2.44
Seattle Weather Forecast for September 21
"Mild & Comfortable. Expect average highs of 19.5°C. 20% chance of rain."
What to Wear in Seattle in September
Based on historical data for September 21, Seattle typically experiences temperatures between 11.3°C and 19.5°C. We recommend packing light jacket, long pants to stay comfortable. Trends suggest it will stay mostly dry (20% rain probability), but checking the forecast before heading out is always wise.
Complete Packing Guide
Weekly breakdown & essential items for Seattle
Smart Suitcase
- Light Jacket
- Long Pants
Average High
19.5°C
Daytime Peak
Average Low
11.3°C
Nighttime Low
Feels Like
19.5°C
🌤️ Feels accurate
Rain Chance
20%
Avg 1.2mm if it rains
Cloud Cover
53%
Overcast
Pressure
1016.8 hPa
Medium volatility
Seattle Weather Conditions & UV Index
Weather Confidence
30-year variance analysis
Based on 10 years of data
🎯 This date historically delivers consistent weather 99% of the time. Safe to plan outdoor activities with confidence.
Sun Exposure
UV Index & Safety Guide
Protection Needed
SPF 15 for extended outdoor time
Type I-II
🔴 31 min to burn
Type III-IV
🟡 47 min to burn
Type V-VI
🟢 78 min to burn
UV estimate based on latitude, season & cloud cover. Actual UV may vary.
Outdoor Comfort
How pleasant is it outside?
Dew Point
15.4°C
Pleasant
Heat Stress
Low
No heat warning
😊 Best outdoor time:
Any time during daylight hours
Travel & Logistics Insights
Local Calendar
Business Day
Standard hours in United States
Flight Traffic
High
91/100 pressure index
Health Advisory
Healthy
Routine only • View Details →
Landing Slots
~675
Daily arrival options (inc. codeshare)
Sunrise, Sunset & Wind in Seattle
Daylight & Sun
Solar Elevation Cycle
🌅 Sunrise
05:56
☀️ Noon
12:03
🌇 Sunset
18:11
⏱️ Daylight
12h 15m
Wind Speed
11.9 km/h
Light breeze
Sunshine
9.1 hrs
Direct Sun (Cloud-free)
Humidity
77%
Humid / Damp
Travel Intelligence
Exclusive flight analytics & health data for Seattle
Smart Flight Estimator
Airport-to-Airport Logistics
unlock smart flight estimates
Add your home location in settings to see airport details.
Travel Health & Safety in Seattle
Travel Comfort Index
Real-time tourism conditions
Weather Quality
☀️ Excellent
Tourist Crowds
🟡 Busy
Price Level
💰💰💰 Expensive
Health & Wellbeing
Meteo-Health Forecast
Moderate fluctuations. Some sensitivity possible.
Weather unlikely to affect joint pain.
Moderate fish activity expected.
Air quality is excellent. Perfect for outdoor activities.
Regional Safety Profile
Seismic Activity
Rare minor tremors. Buildings are earthquake-resistant.
Volcanic Activity
Nearest: Mount Rainier
94.3km away
"According to 30YearWeather's analysis of 30 years of NASA satellite data, Seattle on September 21 averages 19.5°C with a 20% precipitation probability. Wedding/Outdoor Event Score: 97/100."
— Source: 30YearWeather.com|Data: NASA POWER (1991-2021)|View Methodology
Seattle Weather History (2010–2024)
Temperature History
Hall of Fame
Records from last 10 years
Better Alternatives Nearby
Consider these dates within ±7 days
September 14
21.2°C • 17% rain
Score
99
September 22
19.5°C • 16% rain
Score
99
September 15
20.7°C • 20% rain
Score
98
💡 Moving your event by a few days could significantly improve weather conditions
Compare Other Cities
Same date (September 21) in other destinations
Prague
Czech Republic
Paris
France
Rome
Italy
London
United Kingdom
Amsterdam
Netherlands
🌍 Explore the same date in different destinations around the world
📍 Is Seattle Worth Visiting on September 21?
Planning a trip to Seattle on September 21? Our historical weather analysis based on 30 years of NASA satellite data shows that this time of year is characterized by pleasant mild weather averaging 19.5°C with a 20% historical precipitation risk.
✈️ Travel Logistics: Approximately 675 landing slots available daily during this period, indicating peak tourist activity.
📊 Historical Extremes: The hottest September 21 recorded in the last 30 years saw temperatures hit 23.6°C in 2022, while the coldest dropped to 7.9°C in 2016.