Lake Forest weather in September

In September, Lake Forest sees average daytime highs of 71°F and overnight lows near 63°F, with 3.3 in of rain across 9.5 wet days and about 12.3 hours of daylight. It is the 4th-warmest and 7th-wettest month of the year here.

Highs ease over the month, from about 75°F in the first days to 66°F by the end. The sky is clear or mostly clear about 45% of the time in September, and the air most often feels dry.

Daylight stretches from about 13 hours at the start of September to 11 h 36 min by month's end. Set against the rest of the year, September is Lake Forest's 1st-clearest and 8th-windiest month. For the whole year in context, see Lake Forest's year-round climate.

Daytime high
71°
4th warmest
Overnight low
63°
9th coldest
Rainfall
3.3 in
9.5 wet days
Daylight
12 h 18 min
5:37 AM – 5:54 PM
Cloud cover
49%
1st clearest
Wind
12 mph
8th windiest
Solar energy
4.44 kWh/m²
Travel score
6.8
out of 10

Lake Forest is city in Lake County, Illinois, United States.

Density
435/km²
people per km²
Founded
1861

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 75°F in the first days to 66°F by the end.

A typical September day in Lake Forest reaches about 71°F in the afternoon and slips back to 63°F overnight — a 7°F spread between the day's warmth and the night's chill. On most days the high lands between 62°F and 78°F. Set against its neighbours, September runs about 6°F cooler than August and about 12°F warmer than October.

40°50°60°70°80°151015202530
Avg high (°F)Avg low (°F)
17568
27568
37568
47467
57467
67467
77466
87366
97366
107365
117265
127265
137264
147164
157164
167163
177063
187062
196962
206962
216961
226861
236860
246760
256759
266759
276659
286658
296558
306557

How September really feels in Lake Forest

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.

Lake Forest weather by month

How September compares with the rest of the year at a glance. September is the 4th-warmest and 7th-wettest month here — the current month is highlighted below.

MonthHigh / Low (°F)Rainfall (inches)Wet daysSnow daysDaylight (hours)Travel score
Jan30° / 21°2.99.879.30.0
Feb31° / 21°2.47.75.110.30.0
Mar37° / 29°3.09.83.211.60.0
Apr46° / 39°4.8121.413.11.3
May58° / 50°4.111.7014.44.1
Jun71° / 64°3.510.50156.5
Jul78° / 70°3.110.5014.86.8
Aug77° / 70°3.810.7013.77.0
Sep71° / 63°3.39.5012.36.8
Oct58° / 51°4.010.50.110.84.4
Nov46° / 39°3.4101.39.61.2
Dec36° / 28°3.1104.990.0

Cloud cover in September

ClearMostly clearPartly cloudyMostly cloudyOvercast

The sky is clear or mostly clear about 45% of the time in September, the clearest month of the year.

Beyond those clear spells, partly cloudy skies cover about 13% of September, with mostly cloudy or overcast conditions the remaining 42%. Skies tend to cloud over as the month goes on.

0%25%50%75%100%151015202530
Clearer (%)Cloudier (%)
16238
26238
36238
46238
56337
66337
76238
86238
96238
106139
116139
126040
136040
145941
155842
165842
175743
185743
195644
205644
215644
225644
235545
245545
255446
265446
275347
285347
295347
305446

Chance of precipitation in September

Any precipitation

Lake Forest gets around 3.3 in of rain over 9.5 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 32% and peaks near 34% on the wettest days.

0%10%20%30%40%151015202530
Rain chance (%)
129
229
329
429
529
629
730
830
930
1031
1131
1232
1332
1432
1533
1633
1733
1834
1934
2034
2134
2234
2334
2434
2534
2634
2734
2834
2934
3033

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 37% of days. Sticky, muggy-or-worse air turns up on roughly 19% of them.

0%25%50%75%100%151015202530
Dry (%)Comfortable (%)Humid (%)Muggy (%)Oppressive (%)
11420272811
21521262710
31622262610
4182226269
5192226258
6212326247
7222326236
8242326225
9252326214
10272326204
11292325203
12302325193
13322325182
14332325172
15352325162
16362324151
17382323141
18402322131
19422321121
20442420111
21472419100
22492318100
2351231790
2453231680
2555231570
2657221470
2759221360
2861211260
2963211150
3065201050

Wind in September

Winds average 12 mph in September.

Across September, daily wind ranges from about 11 mph on the calmest days to 13 mph at its breeziest. It stays fairly steady from day to day.

051015151015202530
Wind speed (mph)Feels
111light breeze
211light breeze
311light breeze
411light breeze
511light breeze
611light breeze
711light breeze
811light breeze
911light breeze
1011light breeze
1111light breeze
1211light breeze
1312light breeze
1412light breeze
1512light breeze
1612light breeze
1712light breeze
1812light breeze
1912light breeze
2012light breeze
2112light breeze
2212light breeze
2312light breeze
2412light breeze
2512breezy
2613breezy
2713breezy
2813breezy
2913breezy
3013breezy

Sunrise & sunset in September

DaylightNight

Daylight runs from about 13 hours at the start of September to 11 h 36 min by month's end. The lit band spans sunrise to sunset each day.

Days shrink by about 84 minutes over September. Sunrise moves from 5:21 AM to 5:53 AM, and sunset from 6:21 PM to 5:29 PM.

12 AM4 AM8 AM12 PM4 PM8 PM12 AM151015202530
Daylight (h)Sunrise (AM)Sunset (PM)
113.05:216:21
213.05:226:19
312.95:236:18
412.95:246:16
512.85:256:14
612.85:266:12
712.75:276:11
812.75:296:09
912.65:306:07
1012.65:316:05
1112.55:326:03
1212.55:336:01
1312.45:346:00
1412.45:355:58
1512.35:365:56
1612.35:375:54
1712.25:385:52
1812.25:395:50
1912.15:415:49
2012.15:425:47
2112.05:435:45
2212.05:445:43
2311.95:455:41
2411.95:465:39
2511.85:475:38
2611.85:485:36
2711.75:495:34
2811.75:515:32
2911.65:525:30
3011.65:535:29

Solar energy in September

Daily solar energy (kWh/m²)

Average shortwave solar energy reaching the ground each day in September, about 4.44 kWh/m².

For context, solar energy here swings from a low near 1.48 kWh/m² to a high of 6.5 kWh/m² in June across the year — September's 4.44 kWh/m² falls in the middle.

0246151015202530
Solar energy (kWh/m²)Level
15.1Bright
25.0Bright
35.0Bright
44.9Bright
54.9Bright
64.8Bright
74.8Bright
84.8Bright
94.7Bright
104.7Bright
114.6Bright
124.6Bright
134.5Bright
144.5Bright
154.4Bright
164.4Bright
174.3Bright
184.3Bright
194.3Bright
204.2Bright
214.2Bright
224.1Bright
234.1Bright
244.0Bright
254.0Bright
264.0Moderate
273.9Moderate
283.9Moderate
293.8Moderate
303.8Moderate
Planning a trip in September

Is September a good time to visit Lake Forest?

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

6.8
Sightseeing
2nd-best month of the year
4.3
Beach & pool
3rd-best month of the year

September is a reasonable time to visit Lake Forest for sightseeing.

What to pack for Lake Forest in September

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

In September, Lake Forest sees daytime highs around 71°F and nights near 63°F. Worth packing: rain gear. About 10 days see rain.

rain gear

Sea temperature near Lake Forest in September

The nearest coast averages about 67°F in September — the other half of the beach question. The full-year curve and swim-comfort zones are below.

The sea at the nearest coast (about 8 mi away) is warmest in August at about 71°F and coldest in March near 36°F.

It's comfortable for a long swim from Jul 18 to Sep 8 — about 1.7 months when the water holds above 68°F.

30°40°50°60°70°80°JanFebMarAprMayJunJulAugSepOctNovDecbracingcoolrefreshingwarmcomfortable to swim
Sea temperature (°F)Comfort
Jan38Bracing
Feb36Bracing
Mar36Bracing
Apr38Bracing
May43Bracing
Jun55Bracing
Jul68Refreshing
Aug71Refreshing
Sep67Cool
Oct57Bracing
Nov47Bracing
Dec42Bracing

UV index in Lake Forest in September

LowModerateHighVery highExtreme

Clear-sky midday UV in September peaks around 6 — enough to warrant sun protection. The full-year curve, shaded by WHO exposure level, is below.

Under clear skies, the midday UV index in Lake Forest peaks around June 22 at about 9 (very high) — sunscreen, a hat and midday shade are essential. It bottoms out near January 1 at about 1 (low).

The index reaches 3 or more — the level where the WHO recommends sun protection — in 8 months of the year.

025710JanFebMarAprMayJunJulAugSepOctNovDecLowModerateHighVery highHighModerateLowJun 22UV 9Jan 1UV 1
UV index (midday)Level
Jan1Low
Feb2Moderate
Mar4Moderate
Apr6High
May7Very high
Jun9Very high
Jul9Very high
Aug8Very high
Sep6High
Oct3Moderate
Nov2Moderate
Dec1Low

Air quality in Lake Forest 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 Lake Forest is worst in July — around 21 µg/m³ PM2.5 (moderate), about 2.2× the cleanest month (October, 10 µg/m³).

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

0102030JanFebMarAprMayJunJulAugSepOctNovDecModerateGoodJul 921 µg/m³Oct 2410 µg/m³
PM2.5 (µg/m³)Level
Jan12Moderate
Feb15Moderate
Mar18Moderate
Apr19Moderate
May20Moderate
Jun21Moderate
Jul21Moderate
Aug18Moderate
Sep15Moderate
Oct10Good
Nov10Good
Dec11Good
Location & terrain

Where is Lake Forest?

Lake Forest sits at 42.3°N, 87.8°W, 689 ft above sea level, in United States. The nearest listed city is North Chicago, 4.3 mi away.

Topography around Lake Forest

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

Within 2 miles, the terrain around Lake Forest has only modest variations in elevation — a maximum elevation change of 200 ft around an average of 666 ft above sea level; within 10 miles, only modest variations in elevation — a maximum elevation change of 318 ft; within 50 miles, significant variations in elevation — a maximum elevation change of 892 ft.

The land around Lake Forest is covered by trees (54%) and water (23%) within 2 miles, water (39%) and trees (29%) within 10 miles, and water (43%) within 50 miles.

2 miles
200 ft
10 miles
318 ft
50 miles
892 ft
treesgrasslandcroplandartificial surfaceswater
September weather nearby: North Chicago 4.3 mi · Highland Park 5.6 mi · Libertyville 6.2 mi · Deerfield 6.2 mi · Waukegan 7.5 mi · Vernon Hills 7.5 mi · Gurnee 8.1 mi · Mundelein 8.1 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 Lake Forest (42.2586°, -87.8406°, elevation 689 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. Sea temperatures are the NOAA OISST v2.1 1991–2020 monthly climatology (public domain), sampled at the nearest ocean point. 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).