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Portland Mosquito Forecast

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

Bite Index · Sunday, November 8

Low

Mosquito activity in Portland is low today.

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

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

  • At a two-week average of 50°F, larvae develop slowly and the population stays small whatever the rain.
  • Off-season: even with rain and warmth, populations are a fraction of summer's.
  • Only 0.3 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
    0Low59°/51°dry
  2. Mon
    0Low58°/48°0.4 in
  3. Tue
    0Low59°/50°dry
  4. Wed
    0Low63°/52°dry
  5. Thu
    1Low61°/51°dry
  6. Fri
    1Low62°/51°0.1 in
  7. Sat
    1Low56°/49°0.9 in

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 Portland's mosquitoes do what they do

Portland's mosquitoes are a floodplain story: when the Columbia and Willamette run high in late spring, the sloughs and bottomlands along Sauvie Island, Smith and Bybee Wetlands and the Columbia Slough flood, and a few weeks later the metro gets its brood. Multnomah County Vector Control larvicides those floodplains and traps through the summer; in most years the dry, cool-night Northwest summer keeps numbers modest, but a big snowmelt year is another matter.

  • Floodplain broods: Columbia and Willamette high water in May–June floods the bottomlands; the brood follows two to three weeks later.
  • Dry summers: almost no rain from July to September, so the index stays low unless the rivers did the flooding.
  • Cool nights: dusk temperatures in the 50s and 60s slow both development and biting; the index weights that.
  • Short season: June through September; the index is zero from October to May.

A typical year in Multnomah County

What the Bite Index typically reads here each month, from Multnomah 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: 4
  6. JJune: 24
  7. JJuly: 29
  8. AAugust: 24
  9. SSeptember: 27
  10. OOctober: 5
  11. NNovember: 0
  12. DDecember: 0

Local surveillance and West Nile virus

The Bite Index measures nuisance biting, not disease. Multnomah 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 (0.3 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 3%

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 2%

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 15%

GGround truth

A nudge from local trap counts and reports when we have them. For Portland 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 Portland

When is mosquito season in Portland?
June through September, peaking in late June and July after the spring floods recede. Dry late summers keep numbers down; frost ends the season by October.
Why was 2023 such a bad mosquito year in Portland?
A big snowpack and late melt kept the Columbia and Willamette high into June, flooding far more bottomland than usual. Floodwater mosquitoes hatch from every acre that floods, and the county warned of record numbers that summer.
Is West Nile virus a risk in Portland?
Low but present. Multnomah County detects it in mosquito pools in some years, and the arrival of the invasive Aedes in 2026 is being watched closely. The county's announcements are authoritative; follow them over the index.
Where does this forecast's data come from?
Open-Meteo weather for downtown Portland, scored by our documented model with the northern breeding lag. River levels, which drive Portland's floodplain broods, are not in the weather data, so treat a high-water spring as an extra reason for caution.

Also covers Gresham, Beaverton, Hillsboro, Vancouver WA, Lake Oswego, Sauvie Island, the Columbia Slough, Forest Park: the forecast point is central Portland, and conditions within about 30 miles track it closely. Weather fetched Nov 8, 2:28 PM local time.

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