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Denver Mosquito Forecast

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

Bite Index · Sunday, November 8

Low

Mosquito activity in Denver is low today.

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

About normal for November in Denver County (typically around 0).

  • The week has averaged 46°F, below the 50°F where mosquitoes stop flying and breeding. Nothing to forecast until it warms up.

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
    0Low68°/35°dry
  2. Mon
    0Low73°/38°dry
  3. Tue
    0Low69°/36°dry
  4. Wed
    0Low65°/48°dry
  5. Thu
    0Low63°/47°dry
  6. Fri
    0Low67°/49°dry
  7. Sat
    0Low66°/50°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 Denver's mosquitoes do what they do

Denver is high, dry and sunny, and its mosquitoes come from the South Platte, Cherry Creek, irrigation ditches and the lawns they water, with a short but real West Nile season when July heat arrives. Denver Public Health & Environment traps and larvicides through the summer and announced West Nile-positive samples in 2025.

  • South Platte and Cherry Creek: the river corridors are the city's permanent habitat.
  • Irrigation: ditches and watered lawns breed Culex tarsalis in a place that would otherwise be dry.
  • Dry air: afternoon humidity near 20% grounds mosquitoes; dusk is the only biting window.
  • Very short season: July and August; the index is zero from October to May.

A typical year in Denver County

What the Bite Index typically reads here each month, from Denver County's 30-year climate normals run through the same model as today's number. The month you're in is marked; the season map shows the same long-run picture for every county.

  1. JJanuary: 0
  2. FFebruary: 0
  3. MMarch: 0
  4. AApril: 0
  5. MMay: 1
  6. JJune: 21
  7. JJuly: 49
  8. AAugust: 50
  9. SSeptember: 16
  10. OOctober: 1
  11. NNovember: 0
  12. DDecember: 0

Local surveillance and West Nile virus

The Bite Index measures nuisance biting, not disease. Denver 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. Warmth and water set the size of the population, tonight's weather sets how much of it flies, and the season and local reports adjust the result. Every factor and today's value for it is listed here so you can check the number yourself. Warmth and Water are exactly the climate core of our county-level mosquito season map; the map shows that core on 30-year normals for every county, and “typical for this month” is the same model run on this county's normals, so the comparison is like for like.

WWater

Rain over the egg-to-adult window (0.5 in. between Oct 8 and Oct 23), expressed as a monthly rate on the same wetness curve the season map uses. At summer temperatures eggs become biting adults in about a week, so today's mosquitoes are last week's rain. Cool weather stretches the window; hot dry days shrink it. Irrigation, creeks and pools keep a 35% floor even in a drought.

Role scales
Today 18%

TWarmth

The trailing two-week average temperature on the season map's curve: nothing below 50°F, full speed at 78–88°F, a small penalty in extreme heat. Multiplied with Water, so a cool fortnight caps the index however wet it has been.

Role gates
Today 0%

ATonight

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. Scales the index between half and full.

Role 0.5–1×
Today 0%

GGround truth

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

Role ±15%
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.

Role ×
Today 15%

index = 100 × S × [T × (0.35 + 0.65·W)] × (0.5 + 0.5·A) × (1 + 0.15·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 Denver

When is mosquito season in Denver?
July and August, with some activity in June and September. Cool nights end it early; the index is zero most of the year.
Why does Denver have West Nile at all?
Culex tarsalis breeds in irrigation ditches and flooded lawns along the Front Range, and Colorado's dry summers concentrate birds and mosquitoes at the same water. The state records cases every year, mostly in August.
Is West Nile virus a risk in Denver?
Yes, in late summer. Denver detects it in mosquito samples most years and records human cases in many. The department's announcements are authoritative; follow them over the index.
Where does this forecast's data come from?
Open-Meteo weather for downtown Denver, scored by our documented model with the northern breeding lag, and compared with Denver County's season-map climatology. Irrigation isn't in the weather data.

Also covers Aurora, Lakewood, Littleton, Englewood, Arvada, Westminster, Washington Park, the Cherry Creek Trail: the forecast point is central Denver, and conditions within about 30 miles track it closely. Weather fetched Nov 8, 3:28 PM local time.

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