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Chicago Mosquito Forecast

Today's mosquito activity in Chicago, the week ahead, and what to do about it. Updated hourly.

Bite Index · Sunday, November 8

Low

Mosquito activity in Chicago is low today.

Few mosquitoes. You may see one or two at dusk near water.

About normal for November in Cook County.

  • Off-season: even with rain and warmth, populations are a fraction of summer's.
  • Tonight: a cool 46°F evening, light wind — poor flying weather, so fewer bites than the population alone would suggest.
  • Only 0.1 in. of rain Oct 8–Oct 23, so few new breeding sites; what's biting is coming from irrigation, pools and creeks.

The week ahead

Each day's Bite Index, from this week's weather and the rain that is still hatching. Forecast days get less certain as the week goes on.

  1. Today
    2Low60°/37°dry
  2. Mon
    0Low57°/36°dry
  3. Tue
    0Low61°/42°dry
  4. Wed
    0Low55°/47°0.3 in
  5. Thu
    0Low48°/41°0.1 in
  6. Fri
    0Low41°/33°0.2 in
  7. Sat
    0Low44°/32°dry

Protect tonight

You can probably skip repellent tonight.

  1. If you're near a creek, pond or heavy shade at dusk, a light application on ankles and arms is plenty.
  2. Good night to tip out any containers holding water; what you empty now won't hatch next week.
Which repellent, and how much: DEET vs picaridinBest natural repellents

Why Chicago's mosquitoes do what they do

Chicago has the best-instrumented mosquitoes of any city we cover: the Department of Public Health runs a trap network across the city all summer and publishes every week's counts and West Nile test results, by trap and ZIP, as open data. That data feeds this page directly. The season is short and intense — the hot, humid weeks of July and August — and the Culex that breed in the city's catch basins carry West Nile most years.

  • Catch basins: Chicago's storm drains are the main Culex nursery; the city larvicides them in waves each summer.
  • Hot, dry Julys are worse, not better: Culex thrive when dry weather concentrates organic water in drains, which is why West Nile years are often drought years.
  • Lake effect: cool lake breezes keep the lakefront a few degrees cooler and a little quieter than inland neighborhoods.
  • Short season: June through September; the index is zero from November to April.

A typical year in Cook County

The long-run mosquito index by month from our season map (NOAA climate normals plus effort-corrected sightings). Today's Bite Index is the live version of this; the month you're in is marked.

  1. JJanuary: 0
  2. FFebruary: 0
  3. MMarch: 0
  4. AApril: 0
  5. MMay: 36
  6. JJune: 63
  7. JJuly: 79
  8. AAugust: 78
  9. SSeptember: 64
  10. OOctober: 37
  11. NNovember: 0
  12. DDecember: 0

Local surveillance and West Nile virus

The Bite Index measures nuisance biting, not disease. Cook County's health department traps and tests mosquitoes through the season and publishes positive West Nile pools by ZIP code. When your ZIP has an active advisory, follow it regardless of today's number.

How the Bite Index is scored

The index is a model of conditions, not a mosquito count. Four weather factors are combined on a 0–100 scale, then scaled by the season. Every factor and today's value for it is listed here so you can check the number yourself. It uses the same temperature curve and rain logic as our county-level mosquito season map, which has been validated against effort-corrected sighting data across 41 states.

BBreeding pressure

Rain 7–14 days ago (0.1 in. between Oct 8 and Oct 23). Eggs need water to hatch, and at summer temperatures the egg-to-biting-adult cycle takes about a week, so today's mosquitoes are last week's rain. Cool weather stretches the window; hot dry days shrink it as shallow water evaporates.

Weight 40%
Today 0%

DDevelopment

The trailing two-week average temperature, on the same curve as our national season map: nothing below 50°F, full speed at 78–88°F, a small penalty in extreme heat.

Weight 25%
Today 13%

ATonight's activity

Whether the population that exists will actually fly and bite this evening: dusk temperature (65–85°F is ideal), wind (they stop flying around 10 mph), humidity, and rain around sunset.

Weight 25%
Today 0%

GGround truth

A nudge from local trap counts and reports when we have them. For Chicago this is neutral today; the health department's surveillance is linked above and will feed this factor as we wire it in.

Weight 10%
Today neutral

SSeason gate

A multiplier from the calendar: fully open in the city's season months, half-open in the shoulder months, nearly closed in deep winter unless the week is warm. Below a 50°F weekly average the index is simply 0.

Weight ×
Today 15%

index = 100 × S × (0.40 × (0.2 + 0.8·B) + 0.25 × D + 0.25 × A + 0.10 × (0.5 + 0.5·G))

Bands: Low 0–20 · Moderate 21–40 · High 41–60 · Very High 61–80 · Severe 81–100. Weather is from Open-Meteo (observations and the 7-day forecast), refreshed hourly. The lag and temperature thresholds follow published work on Culex and Aedes development (Rueda et al. 1990; Cornell NRCC degree-day emergence; JAMCA 2024 on rainfall two to four weeks before trap peaks).

Questions people ask about mosquitoes in Chicago

When is mosquito season in Chicago?
June through September, peaking in July and August. Floodwater mosquitoes come first after spring rain; the West Nile-carrying Culex peak in the hottest weeks. The first frost ends it.
Why are West Nile years in Chicago often dry years?
Culex pipiens, the vector, breeds in the stagnant, nutrient-rich water of catch basins, which concentrates in dry weather rather than being flushed out. Hot, dry Julys produce more infected mosquitoes, which is why the trap data on this page matters more than rain alone.
Is West Nile virus a risk in Chicago right now?
CDPH's test results are read into this page: when pools have tested positive in the last three weeks, the ZIP codes appear in the banner above. CDPH's own reports are authoritative.
Where does this forecast's data come from?
Open-Meteo weather for the Loop, CDPH's weekly trap counts and West Nile test results from the Chicago Data Portal, scored by our documented model. The trap counts are a real measurement and are the strongest input to the ground-truth factor.

Also covers Evanston, Oak Park, Cicero, Skokie, Naperville, Schaumburg, Lincoln Park, the Lakefront Trail: the forecast point is central Chicago, and conditions within about 30 miles track it closely. Weather fetched Nov 8, 12:38 PM local time.

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