G3 (Strong) Geomagnetic Storm of April 16, 1999
A G3 (strong) geomagnetic storm began April 16, 1999, reached Kp 7+ and lasted 15 hours, with a daily Ap as high as 47; at that Kp the aurora may be seen from about 48 degrees geomagnetic latitude.
The Kp through the storm
Began Apr 16, 21:00 UTC, peaked in the block from Apr 17, 03:00 UTC, ended Apr 17, 12:00 UTC.
The storm
- Peak Kp
- 7+ (7.33)
- NOAA level
- G3 Strong
- Duration
- 15 hours
- Highest daily Ap
- 47
- Highest 3-hour ap
- 154
- Rank since 1932
- 378 of 4,139
How far south the aurora could have been seen
At Kp 7+ NOAA’s scales put the aurora’s reach at about 48 degrees geomagnetic latitude: 40°N in the central US, 39°N in the eastern US, 43°N in the western US and 49°N in northern Europe.
NOAA says a G3 storm’s aurora has been seen as low as Illinois and Oregon.
US cities within reach, the southernmost first
- Baltimore, MD 48.5° geomag.
- Indianapolis, IN 48.7° geomag.
- Columbus, OH 49.0° geomag.
- Lincoln, NE 49.1° geomag.
- Philadelphia, PA 49.1° geomag.
- Eugene, OR 49.4° geomag.
- Pittsburgh, PA 49.6° geomag.
- Omaha, NE 49.6° geomag.
- Staten Island, NY 49.8° geomag.
- Brooklyn, NY 49.8° geomag.
53 US cities of 150,000 people or more sit at or poleward of that line. Low on the northern horizon under a dark, clear sky; not a record of sightings.
Day by day
The daily Ap is the mean of the day’s eight three-hour ap values.
| UT day | Daily Ap | Highest Kp that day |
|---|---|---|
| April 16, 1999 | 18 | 5 |
| April 17, 1999 | 47 | 7+ |
Flares before it
GOES recorded no M or X class flare from four days before this storm to its end. Storms without a large flare are commonly driven by fast solar wind from a coronal hole, or by an eruption no flare announced.
More
The storm before, Mar 29The storm after, May 13Every storm of 1999The strongest on recordThis day in weather historyTonight’s aurora forecast
Sources
Kp and ap from GFZ Helmholtz Centre for Geosciences, Geomagnetic Observatory Niemegk, licensed CC BY 4.0: Matzka, J., Stolle, C., Yamazaki, Y., Bronkalla, O. and Morschhauser, A., 2021, The geomagnetic Kp index and derived indices of geomagnetic activity, Space Weather, doi:10.1029/2020SW002641; data doi:10.5880/Kp.0001. Storms, levels and aurora reach worked out by WeatherOverTime from those values and NOAA’s space weather scales. Definitive through August 31, 2026; later values are preliminary. Rebuilt 2026-09-24. Flares from NOAA NCEI’s GOES X-ray flare reports (1975 to 2017) and SWPC’s edited solar event lists, public domain; classes as NOAA recorded them at the time.