Current Activity Status
Initializing…
Accumulated GDD · Base 50°F · Since Mar 1
—
/ 1,050 mGDD peak target
Elapsed
—
days
650
800
950
1,150
<650
Quiet
Quiet
650–800
Early
Early
800–950
Shoulder
Shoulder
950–1150
Peak
Peak
>1150
Declining
Declining
Projected Peak
—
—
GDD Remaining
—
Days to Peak
—
Avg GDD/Day
—
Source
Open-Meteo
🎟 NPS Lottery Event
Official peak window:
May 20–27, 2026
May 20–27, 2026
GDD Accumulation Curve
Historical confirmed · 7-day forecast · Extended via 3-year historical averages
Confirmed
Forecast
Hist. Avg
Peak
Extended GDD Projection
Through projected peak · Updated as forecast data improves
Forecast
— Open-Meteo 7-day weather forecast
Hist. Avg
— Average of 2023 & 2024 temps for same calendar dates; accuracy decreases as dates extend further out
| Date | Source | High | Low | +GDD | Cumulative | Stage |
|---|---|---|---|---|---|---|
| Loading… | ||||||
* Historical averages based on Open-Meteo ERA5 data for 2023–2024 at Elkmont coordinates. Predictions become less reliable beyond the 7-day forecast window. The NPS uses soil temperature monitoring and on-site observation to refine predictions closer to the event.
Manual GDD Override
Active
Override the calculated total with a known value — useful if you have a reading from the USA-NPN Status of Spring maps or a local observer report.
Peak Date Fine-Tune (Days)
ⓘ
Open-Meteo uses terrain elevation correction for ERA5 data. This slider adds a day offset to the projected peak date for local micro-climate adjustments.
0d
Open-Meteo is elevation-aware. Use this only for micro-climate fine-tuning.
Data: Open-Meteo
Historical: archive-api.open-meteo.com
Forecast: api.open-meteo.com
ERA5 reanalysis · terrain DEM elevation correction
Lat: 35.6547 · Lon: -83.5807 · ~2,100 ft
No API key · Free & public
Forecast: api.open-meteo.com
ERA5 reanalysis · terrain DEM elevation correction
Lat: 35.6547 · Lon: -83.5807 · ~2,100 ft
No API key · Free & public
mGDD Model (Modified)
Daily mGDD = (min(Tmax,86) + max(Tmin,50)) / 2 − 50°F
High capped at 86°F · Low floored at 50°F
Cold nights stop development (not subtract) · Mar 1 start
Extended projection uses 2023–2024 avg temps
Formula validated via atechlabs.org
High capped at 86°F · Low floored at 50°F
Cold nights stop development (not subtract) · Mar 1 start
Extended projection uses 2023–2024 avg temps
Formula validated via atechlabs.org
Species · Event
Photinus carolinus · Synchronous Firefly
Peak: 950–1,150 mGDD (base 50°F, Mar 1)
Historical range: mid-May to mid-June
NPS Firefly Info · ATech Labs · Learn the Science →
Peak: 950–1,150 mGDD (base 50°F, Mar 1)
Historical range: mid-May to mid-June
NPS Firefly Info · ATech Labs · Learn the Science →