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Washington Mosquito Forecast

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

Bite Index · Sunday, November 8

Low

Mosquito activity in Washington is low today.

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

About normal for November in District of Columbia.

  • 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.
  • 0.7 in. of rain Oct 15–Oct 28 is now producing adults (cool weather slowed the hatch, so the window is longer than usual).

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
    5Low63°/44°dry
  2. Mon
    4Low60°/43°dry
  3. Tue
    6Low67°/41°dry
  4. Wed
    5Low67°/51°dry
  5. Thu
    4Low59°/52°0.1 in
  6. Fri
    3Low53°/41°1.4 in
  7. Sat
    3Low46°/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 Washington's mosquitoes do what they do

Washington was built on tidal flats where the Anacostia meets the Potomac, and its summers are famously humid. DC Health runs a trap network across the District and announces the season's first West Nile-positive mosquitoes each summer; Rock Creek, the Anacostia and the city's many alleys and catch basins keep the Culex supplied.

  • Tidal rivers: the Anacostia and Potomac flats and Kingman Island are permanent floodwater habitat.
  • Humidity: July and August dusk humidity is among the highest of any major US city.
  • Alleys and catch basins: the dense rowhouse neighborhoods breed Culex in drains and the Asian tiger mosquito in yards.
  • Short, intense season: June through September; cold springs and falls keep the gate shut outside it.

A typical year in District of Columbia

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: 18
  5. MMay: 45
  6. JJune: 69
  7. JJuly: 86
  8. AAugust: 89
  9. SSeptember: 76
  10. OOctober: 46
  11. NNovember: 0
  12. DDecember: 0

Local surveillance and West Nile virus

The Bite Index measures nuisance biting, not disease. District of Columbia'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 (0.7 in. between Oct 15 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 38%

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 29%

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 10%

GGround truth

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

When is mosquito season in Washington, DC?
June through September, peaking in July and August. DC Health's trapping runs roughly May to October; the first frost ends the season.
Why is DC so bad for mosquitoes in August?
Heat, humidity and the tidal rivers. The Asian tiger mosquito bites through the day in shaded yards and alleys; Culex come out at dusk from catch basins. August combines peak heat with the summer's accumulated rain.
Is West Nile virus a risk in DC?
Yes. DC Health detects West Nile in mosquitoes and birds every summer and records human cases in most years. Its announcements are authoritative; follow them over the index.
Where does this forecast's data come from?
Open-Meteo weather for downtown Washington, scored by our documented model and compared with the District's season-map climatology. Not a trap count.

Also covers Arlington, Alexandria, Bethesda, Silver Spring, Georgetown, Capitol Hill, Rock Creek Park, the National Mall: the forecast point is central Washington, and conditions within about 30 miles track it closely. Weather fetched Nov 8, 1:38 PM local time.

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