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Sacramento Mosquito Forecast

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

Bite Index · Sunday, November 8

Low

Mosquito activity in Sacramento is low today.

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

About normal for November in Sacramento County (typically around 6).

  • Only 0.1 in. of rain Oct 17–Oct 28, so few new breeding sites; what's biting is coming from irrigation, pools and creeks.
  • At a two-week average of 56°F, larvae develop slowly and the population stays small whatever the rain.
  • It's the edge of the season; warm weeks like this one keep it going past the calendar.

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
    3Low72°/45°dry
  2. Mon
    3Low75°/41°dry
  3. Tue
    4Low76°/43°dry
  4. Wed
    4Low74°/57°dry
  5. Thu
    4Low73°/56°dry
  6. Fri
    4Low74°/55°dry
  7. Sat
    5Low75°/55°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 Sacramento's mosquitoes do what they do

Sacramento sits where the American River meets the Sacramento in the flat, irrigated Central Valley, and its mosquitoes come from the rice fields to the north, the river parkway and the flood-irrigated orchards and lawns of a city that gets no summer rain. The Sacramento-Yolo Mosquito and Vector Control District runs one of California's largest programs and the state publishes West Nile activity by county every week.

  • Rice and flood irrigation: the valley's irrigated fields are the main Culex tarsalis nursery; the river parkway adds floodwater species.
  • No summer rain: breeding water is irrigation and river, so the index never shows a rain-driven surge here and dusk conditions carry more weight.
  • West Nile: Sacramento County is reliably among California's most active counties for positive samples and human cases.
  • Real winter: cool, wet winters shut the season from November to April.

A typical year in Sacramento County

What the Bite Index typically reads here each month, from Sacramento 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: 1
  4. AApril: 3
  5. MMay: 13
  6. JJune: 22
  7. JJuly: 27
  8. AAugust: 27
  9. SSeptember: 24
  10. OOctober: 19
  11. NNovember: 6
  12. DDecember: 0

Local surveillance and West Nile virus

The Bite Index measures nuisance biting, not disease. Sacramento 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 (0.1 in. between Oct 17 and Oct 28), 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 0%

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 20%

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 47%

GGround truth

A nudge from local trap counts and reports when we have them. For Sacramento 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 60%

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 Sacramento

When is mosquito season in Sacramento?
June through October, peaking from July through September. The district's surveillance runs April to October; the first cool, wet weeks of November end the season.
Why are there mosquitoes in Sacramento when it never rains in summer?
Irrigation. Rice fields, orchards, lawns and the river itself provide standing water all summer, and Culex tarsalis, the West Nile vector, breeds in exactly that. The index's rain factor stays low here; temperature and dusk conditions do the work.
Is West Nile virus a risk in Sacramento?
Yes, one of the highest in California. The district announces positive samples and spray operations by neighborhood, and the state's weekly county table is authoritative. Follow those over the index.
Where does this forecast's data come from?
Open-Meteo weather for downtown Sacramento, scored by our documented model. Irrigation isn't in the weather data, so the index may run low in irrigated areas; the "typical for this month" baseline is the same model on the county's climate normals.

Also covers Elk Grove, Roseville, Folsom, Davis, Rancho Cordova, Citrus Heights, the American River Parkway, Discovery Park: the forecast point is central Sacramento, and conditions within about 30 miles track it closely. Weather fetched Nov 8, 2:28 PM local time.

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