G3 (Strong) Geomagnetic Storm of May 31, 1991

Name: G3 (Strong) Geomagnetic Storm of May 31, 1991
Start: 1991-05-31T09:00:00.000Z
End: 1991-06-03T00:00:00.000Z
Location: Earth

May 31 to June 2, 1991 · 63 hours

A G3 (strong) geomagnetic storm began May 31, 1991, reached Kp 7+ and lasted 63 hours, with a daily Ap as high as 74; at that Kp the aurora may be seen from about 48 degrees geomagnetic latitude.

The Kp through the storm

Began May 31, 09:00 UTC, peaked in the block from Jun 1, 15:00 UTC, ended Jun 3, 00:00 UTC.

From 00 UT the day before to the end of the day after. Blocks in the storm are colored by level; the rest are gray.

The storm

Peak Kp: 7+ (7.33)
NOAA level: G3 Strong
Duration: 63 hours
Highest daily Ap: 74
Highest 3-hour ap: 154
Rank since 1932: 292 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.

Tonight’s aurora forecast

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
May 31, 1991	52	7-
June 1, 1991	74	7+
June 2, 1991	60	7

Flares before it

Peak (UTC)	Class	Region	Before the storm began
Jun 1, 15:29 UTC	X12.0	NOAA 6659	during it
May 29, 23:43 UTC	X1.0	NOAA 6654	33 hours
May 30, 09:44 UTC	M8.2	NOAA 6654	23 hours
May 28, 10:23 UTC	M5.3	NOAA 6654	71 hours
Jun 2, 07:46 UTC	M5.2		during it
May 29, 12:10 UTC	M4.3	NOAA 6654	45 hours

The strongest M and X class flares GOES recorded from four days before the storm began to its end, the window in which a coronal mass ejection launched with a flare usually arrives. A flare in this list is context, not proof of the cause: many storms come from coronal holes, and many flares launch nothing toward Earth.

The storm before, May 28 The storm after, Jun 4 Every storm of 1991 The strongest on record This day in weather history Tonight’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.