Issaquah weather in September

In September, Issaquah sees average daytime highs of 68°F and overnight lows near 49°F, with 3.4 in of rain across 9 wet days and about 12.4 hours of daylight. It is the 3rd-warmest and 9th-wettest month of the year here.

Highs ease over the month, from about 71°F in the first days to 65°F by the end. The sky is clear or mostly clear about 40% of the time in September, and the air most often feels dry.

Daylight stretches from about 13 h 12 min at the start of September to 11 h 30 min by month's end. Set against the rest of the year, September is Issaquah's 3rd-clearest and 10th-windiest month. For the whole year in context, see Issaquah's year-round climate.

Daytime high
68°
3rd warmest
Overnight low
49°
9th coldest
Rainfall
3.4 in
9 wet days
Daylight
12 h 18 min
5:52 AM – 6:13 PM
Cloud cover
49%
3rd clearest
Wind
4 mph
10th windiest
Solar energy
4.06 kWh/m²
Travel score
6.6
out of 10

Issaquah is city in King County, Washington, United States.

Density
1,222/km²
people per km²
Founded
1892

Air quality: WHO Ambient Air Quality Database. Density & description: Wikidata (CC0), GeoNames.

Temperature in September

Average highAverage lowShaded bands: 10–90% of days

Highs ease over the month, from about 71°F in the first days to 65°F by the end.

A typical September day in Issaquah reaches about 68°F in the afternoon and slips back to 49°F overnight — a 19°F spread between the day's warmth and the night's chill. On most days the high lands between 60°F and 79°F. Set against its neighbours, September runs about 6°F cooler than August and about 10°F warmer than October.

40°60°80°151015202530
Avg high (°F)Avg low (°F)
17252
27252
37152
47151
57151
67151
77151
87051
97051
107051
117050
126950
136950
146950
156950
166849
176849
186849
196749
206749
216748
226748
236648
246648
256648
266547
276547
286547
296547
306446

How September really feels in Issaquah

Once humidity and wind are folded in, September averages 0 days that feel above 90°F and 0 that feel at or below 32°F — the comfort signal a plain temperature reading hides.

Days feeling ≥ 90°F
0
heat index over the day's high
Days feeling ≤ 32°F
0
wind chill below the day's low

September sits comfortably in between — no days that feel dangerously hot or drop to a hard freeze once humidity and wind are folded in.

Issaquah weather by month

How September compares with the rest of the year at a glance. September is the 3rd-warmest and 9th-wettest month here — the current month is highlighted below.

MonthHigh / Low (°F)Rainfall (inches)Wet daysSnow daysDaylight (hours)Travel score
Jan45° / 35°8.318.73.68.70.8
Feb48° / 36°5.815.52.69.91.5
Mar52° / 37°6.518.71.811.62.2
Apr57° / 41°5.416.50.213.33.4
May63° / 46°3.812.8014.95.1
Jun67° / 50°3.211.4015.75.9
Jul74° / 54°1.36.5015.47.7
Aug74° / 54°1.65.7014.17.9
Sep68° / 49°3.49012.46.6
Oct58° / 43°6.615.1010.63.9
Nov49° / 38°9.018.719.11.6
Dec44° / 35°8.118.43.48.30.6

Cloud cover in September

ClearMostly clearPartly cloudyMostly cloudyOvercast

The sky is clear or mostly clear about 40% of the time in September.

Beyond those clear spells, partly cloudy skies cover about 16% of September, with mostly cloudy or overcast conditions the remaining 45%.

0%25%50%75%100%151015202530
Clearer (%)Cloudier (%)
15842
25842
35941
45941
55941
65941
75842
85842
95743
105743
115644
125644
135545
145446
155446
165545
175446
185446
195347
205446
215446
225347
235347
245347
255446
265446
275446
285347
295347
305149

Chance of precipitation in September

Any precipitation

Issaquah gets around 3.4 in of rain over 9 wet days in September. The line is the day-by-day chance of measurable precipitation.

Day to day, the chance of measurable rain averages around 31% and peaks near 34% on the wettest days. The odds climb toward the end of the month.

0%10%20%30%40%151015202530
Rain chance (%)
127
227
327
427
528
628
728
828
929
1029
1130
1230
1331
1431
1531
1631
1731
1832
1932
2032
2132
2233
2333
2433
2533
2633
2733
2833
2933
3034

Humidity in September

DryComfortableHumidMuggyOppressiveMiserable

Most of September feels dry — the stacked bands show each day's mix of comfort levels, read off the dew point.

By dew point, September lands in the dry band about 84% of days. It rarely turns muggy — the humid and sticky bands barely register.

0%25%50%75%100%151015202530
Dry (%)Comfortable (%)Humid (%)
174251
275241
376231
476231
577221
678211
779201
880191
980191
1081181
1182171
1282171
1383161
1483161
1584151
1684151
1785141
1885141
1986131
2086131
2188120
2288120
2389110
2490100
259190
269190
279280
289370
299370
309460

Wind in September

Winds average 4 mph in September.

Across September, daily wind ranges from about 4 mph on the calmest days to 4 mph at its breeziest. It stays fairly steady from day to day.

0246810151015202530
Wind speed (mph)Feels
14calm
24calm
34calm
44calm
54calm
64calm
74calm
84calm
94calm
104calm
114calm
124calm
134calm
144calm
154calm
164calm
174calm
184calm
194calm
204calm
214calm
224calm
234calm
244calm
254calm
264calm
274calm
284calm
294calm
304calm

Sunrise & sunset in September

DaylightNight

Daylight runs from about 13 h 12 min at the start of September to 11 h 30 min by month's end. The lit band spans sunrise to sunset each day.

Days shrink by about 102 minutes over September. Sunrise moves from 5:32 AM to 6:12 AM, and sunset from 6:44 PM to 5:43 PM.

12 AM4 AM8 AM12 PM4 PM8 PM12 AM151015202530
Daylight (h)Sunrise (AM)Sunset (PM)
113.25:326:44
213.25:336:42
313.15:346:40
413.05:366:38
513.05:376:36
612.95:396:34
712.95:406:32
812.85:416:30
912.75:436:27
1012.75:446:25
1112.65:456:23
1212.65:476:21
1312.55:486:19
1412.55:506:17
1512.45:516:15
1612.35:526:13
1712.35:546:10
1812.25:556:08
1912.25:566:06
2012.15:586:04
2112.05:596:02
2212.06:016:00
2311.96:025:58
2411.96:035:56
2511.86:055:53
2611.86:065:51
2711.76:085:49
2811.66:095:47
2911.66:115:45
3011.56:125:43

Solar energy in September

Daily solar energy (kWh/m²)

Average shortwave solar energy reaching the ground each day in September, about 4.06 kWh/m².

For context, solar energy here swings from a low near 0.9 kWh/m² to a high of 6.18 kWh/m² in July across the year — September's 4.06 kWh/m² falls in the middle.

024151015202530
Solar energy (kWh/m²)Level
14.7Bright
24.7Bright
34.7Bright
44.6Bright
54.6Bright
64.5Bright
74.5Bright
84.4Bright
94.4Bright
104.3Bright
114.3Bright
124.2Bright
134.2Bright
144.1Bright
154.0Bright
164.0Moderate
173.9Moderate
183.9Moderate
193.8Moderate
203.8Moderate
213.7Moderate
223.7Moderate
233.6Moderate
243.6Moderate
253.5Moderate
263.5Moderate
273.4Moderate
283.3Moderate
293.3Moderate
303.2Moderate
Planning a trip in September

Hazard calendar for Issaquah

In seasonPeak

None of Issaquah's seasonal hazards are active in September. The full-year calendar below shows when the rainy season, cyclone risk and dust haze do peak.

Issaquah's rainy season runs October–April, when most of the year's rain falls.

Rainy season
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec

Is September a good time to visit Issaquah?

Our two travel scores rate September for general sightseeing and for beach or pool weather — 0 (poor) to 10 (ideal).

6.6
Sightseeing
3rd-best month of the year
3.5
Beach & pool
3rd-best month of the year

September is a reasonable time to visit Issaquah for sightseeing.

What to pack for Issaquah in September

From September's temperature range, rain chance, sun and wind — what's worth putting in the bag.

In September, Issaquah sees daytime highs around 68°F and nights near 49°F. Worth packing: rain gear. About 9 days see rain.

rain gear

UV index in Issaquah in September

LowModerateHighVery highExtreme

Clear-sky midday UV in September peaks around 4 — enough to warrant sun protection. The full-year curve, shaded by WHO exposure level, is below.

Under clear skies, the midday UV index in Issaquah peaks around July 17 at about 8 (very high) — sunscreen and a hat are worth it in the middle of the day. It bottoms out near December 17 at about 1 (low).

The index reaches 3 or more — the level where the WHO recommends sun protection — in 7 months of the year.

025710JanFebMarAprMayJunJulAugSepOctNovDecLowModerateHighVery highHighModerateLowJul 17UV 8Dec 17UV 1
UV index (midday)Level
Jan1Low
Feb1Low
Mar4Moderate
Apr5Moderate
May7High
Jun6High
Jul8Very high
Aug6High
Sep4Moderate
Oct3Moderate
Nov1Low
Dec1Low

Air quality in Issaquah in September

GoodModeratePoorUnhealthyVery unhealthy

Average fine-particle pollution (PM2.5) through the year — so you can see where September falls, not just the annual average.

Air quality in Issaquah is worst in September — around 22 µg/m³ PM2.5 (moderate), about 3.4× the cleanest month (December, 7 µg/m³).

Modeled monthly averages (CAMS reanalysis, 2015–2024).

0102030JanFebMarAprMayJunJulAugSepOctNovDecGoodGoodModerateGoodSep 1322 µg/m³Dec 177 µg/m³
PM2.5 (µg/m³)Level
Jan8Good
Feb9Good
Mar12Moderate
Apr12Good
May11Good
Jun10Good
Jul10Good
Aug18Moderate
Sep22Moderate
Oct12Good
Nov8Good
Dec7Good
Location & terrain

Where is Issaquah?

Issaquah sits at 47.5°N, 122.0°W, 108 ft above sea level, in United States. The nearest listed city is West Lake Sammamish, 4.3 mi away.

Topography around Issaquah

How much the land rises and falls, and what covers it, within 2, 10 and 50 miles of Issaquah.

Within 2 miles, the terrain around Issaquah has large variations in elevation — a maximum elevation change of 1,998 ft around an average of 512 ft above sea level; within 10 miles, large variations in elevation — a maximum elevation change of 3,002 ft; within 50 miles, extreme variations in elevation — a maximum elevation change of 14,413 ft.

The land around Issaquah is covered by trees (78%) within 2 miles, trees (77%) within 10 miles, and trees (76%) within 50 miles.

2 miles
1,998 ft
10 miles
3,002 ft
50 miles
14,413 ft
treesgrasslandartificial surfacesbare terrainwater
September weather nearby: West Lake Sammamish 4.3 mi · City of Sammamish 5.0 mi · Sammamish 8.1 mi · Fairwood 8.1 mi · Renton 9.3 mi · Mercer Island 9.3 mi · Maple Valley 9.3 mi · Bellevue 9.3 mi

United States travel facts

Power plugs
Type A / B
120 V · 60 Hz
Prices
≈ 100% of US
price level, 2025
GDP per capita
$90,027
2025
Happiness
6.8 / 10
World Happiness, 2025
Safe drinking water
98%
of population, 2024
Homicide rate
5.8
per 100k, 2023
Road deaths
12.7
per 100k/yr, 2019
Income group
High
World Bank
Religion
Protestant 47%unaffiliated 23%Roman Catholic 21%Jewish 2%

Sources: World Bank, US State Department, World Happiness Report, WHO/UNICEF, EF EPI, CIA World Factbook.

Data sources

Temperature, precipitation, cloud, humidity, wind and solar figures are a 1991–2020 climatology for Issaquah (47.5301°, -122.0326°, elevation 108 ft), computed from hourly ERA5 reanalysis — contains modified Copernicus Climate Change Service information. Sun figures are astronomical calculations, not observations. City data © GeoNames, CC BY 4.0. Map outlines from Natural Earth (public domain); interactive map © Google. UV index is derived from CAMS (Copernicus Atmosphere Monitoring Service) clear-sky UV data — contains modified Copernicus Atmosphere Monitoring Service information. Monthly air-quality (PM2.5) seasonality is from the CAMS EAC4 reanalysis (Copernicus Atmosphere Monitoring Service), scaled to the WHO annual mean where available. Topography is sampled from the Copernicus DEM GLO-90 elevation model (© ESA/Airbus) and ESA WorldCover 2021 land cover (CC BY 4.0).