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Philadelphia Mosquito Forecast

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

Bite Index · Sunday, November 8

Low

Mosquito activity in Philadelphia is low today.

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

About normal for November in Philadelphia County.

  • 1.1 in. of rain Oct 14–Oct 28 is now producing adults (cool weather slowed the hatch, so the window is longer than usual).
  • Off-season: even with rain and warmth, populations are a fraction of summer's.
  • Tonight: a cool 51°F evening, light wind — poor flying weather, so fewer bites than the population alone would suggest.

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
    7Low61°/46°dry
  2. Mon
    6Low57°/44°dry
  3. Tue
    7Low64°/41°dry
  4. Wed
    6Low63°/49°dry
  5. Thu
    5Low63°/51°0.1 in
  6. Fri
    3Low57°/49°0.8 in
  7. Sat
    3Low49°/40°0.2 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 Philadelphia's mosquitoes do what they do

Philadelphia lies between two rivers in a humid summer climate, with big park systems — Fairmount, Pennypack, FDR — whose wetlands and lagoons the health department sprays each summer. Vector Control traps across the city, publishes a weekly West Nile update, and reported the year's first human cases in September 2026.

  • Park wetlands: FDR Park's lagoons, Pennypack Creek and the Wissahickon are the city's Culex nurseries and its regular spray targets.
  • Rowhouse alleys and drains: the Asian tiger mosquito breeds in yards; Culex in catch basins.
  • Humid summers: July and August dusk conditions are ideal for flight.
  • Short season: June through September; cold springs keep the gate shut until June.

A typical year in Philadelphia 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: 14
  5. MMay: 43
  6. JJune: 70
  7. JJuly: 87
  8. AAugust: 88
  9. SSeptember: 75
  10. OOctober: 45
  11. NNovember: 0
  12. DDecember: 0

Local surveillance and West Nile virus

The Bite Index measures nuisance biting, not disease. Philadelphia 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 (1.1 in. between Oct 14 and Oct 28). 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 72%

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 25%

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 7%

GGround truth

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

When is mosquito season in Philadelphia?
June through September, peaking in July and August. The first frost, usually late October, ends it.
Why does the city spray FDR Park so often?
Its lagoons and low ground are prime Culex habitat and the department's traps there test positive for West Nile most summers. Spray notices name the park and the night.
Is West Nile virus a risk in Philadelphia?
Yes. The department finds positive pools every summer and reported human cases in 2026. Its weekly update and spray notices are authoritative; follow them over the index.
Where does this forecast's data come from?
Open-Meteo weather for Center City, scored by our documented model and compared with Philadelphia County's season-map climatology. Not a trap count.

Also covers Camden, Cherry Hill, Upper Darby, Norristown, Conshohocken, Bensalem, Fairmount Park, FDR Park: the forecast point is central Philadelphia, and conditions within about 30 miles track it closely. Weather fetched Nov 8, 12:28 PM local time.

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