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San Diego Mosquito Forecast

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

Bite Index · Sunday, November 8

Moderate

Moderate mosquito activity in San Diego today and climbing toward high by Nov 14: expect some bites at dusk.

Some bites at dusk and dawn, mostly near creeks, gardens and shade.

Worse than a typical November in San Diego County (usually around 6).

  • 0.5 in. of rain Oct 21–Oct 30 is now producing adults (cool weather slowed the hatch, so the window is longer than usual).
  • It's the edge of the season; warm weeks like this one keep it going past the calendar.
  • A two-week average of 68°F keeps populations growing, just more slowly than midsummer.

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
    21Mod.87°/59°dry
  2. Mon
    29Mod.87°/65°dry
  3. Tue
    32Mod.80°/64°dry
  4. Wed
    33Mod.81°/71°dry
  5. Thu
    46High79°/70°dry
  6. Fri
    42High78°/70°dry
  7. Sat
    48High73°/69°dry

Protect tonight

Worth repellent if you'll be outside at dusk or dawn.

  1. A botanical or 10% picaridin lotion covers a patio evening; reapply after about two hours.
  2. A fan on the patio does real work: mosquitoes are weak fliers and avoid moving air.
Which repellent, and how much: DEET vs picaridinBest natural repellents

Why San Diego's mosquitoes do what they do

San Diego's mild, dry climate keeps mosquito numbers lower than most big cities, but the San Diego River, Mission Bay's marshes, the lagoons up the coast and the irrigated canyons and yards are enough to sustain Culex, and the invasive Aedes aegypti has spread through the county's neighborhoods. The county's Vector Control Program traps and tests and posts West Nile detections by community.

  • Coastal lagoons and the San Diego River: the county's permanent freshwater and brackish habitat.
  • Aedes aegypti: the daytime ankle-biter is established across the county's neighborhoods.
  • Irrigation: with no summer rain, yards and canyons' runoff are the breeding water.
  • Mild nights: dusk temperatures sit in the ideal 60s–70s range most of the summer, so what mosquitoes exist do fly.

A typical year in San Diego County

What the Bite Index typically reads here each month, from San Diego 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: 2
  5. MMay: 8
  6. JJune: 19
  7. JJuly: 25
  8. AAugust: 27
  9. SSeptember: 24
  10. OOctober: 17
  11. NNovember: 6
  12. DDecember: 1

Local surveillance and West Nile virus

The Bite Index measures nuisance biting, not disease. San Diego 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 21 and Oct 30), 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 36%

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 65%

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 83%

GGround truth

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

When is mosquito season in San Diego?
June through October, peaking in late summer. Numbers stay modest by national standards, but Aedes aegypti can bite in any warm month.
Why are there suddenly daytime mosquitoes in San Diego?
Aedes aegypti arrived in the county in the 2010s and has spread through most neighborhoods. It bites by day, low on the legs, and breeds in the small containers around homes. The county's program tracks it community by community.
Is West Nile virus a risk in San Diego?
Lower than in the Central Valley or LA, but real: the county detects it in mosquitoes most summers and records occasional human cases. The county's page is authoritative; follow it over the index.
Where does this forecast's data come from?
Open-Meteo weather for downtown San Diego, scored by our documented model and compared with the county's climate normals run through the same model. Irrigation isn't in the weather data.

Also covers La Jolla, Chula Vista, El Cajon, Escondido, Oceanside, Coronado, Mission Bay, Balboa Park: the forecast point is central San Diego, and conditions within about 30 miles track it closely. Weather fetched Nov 8, 2:28 PM local time.

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