G3 (Strong) Geomagnetic Storm of May 15, 1948

May 15 to 16, 1948 · 27 hours

A G3 (strong) geomagnetic storm began May 15, 1948, reached Kp 7 and lasted 27 hours, with a daily Ap as high as 72; at that Kp the aurora may be seen from about 50 degrees geomagnetic latitude.

The Kp through the storm

Began May 15, 21:00 UTC, peaked in the block from May 16, 00:00 UTC, ended May 17, 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 (7.00)
NOAA level: G3 Strong
Duration: 27 hours
Highest daily Ap: 72
Highest 3-hour ap: 132
Rank since 1932: 439 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 50 degrees geomagnetic latitude: 42°N in the central US, 41°N in the eastern US, 44°N in the western US and 51°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

Washington Heights, NY 50.0° geomag.
The Bronx, NY 50.0° geomag.
Fort Wayne, IN 50.1° geomag.
Des Moines, IA 50.1° geomag.
Yonkers, NY 50.1° geomag.
Akron, OH 50.2° geomag.
Salem, OR 50.3° geomag.
Aurora, IL 50.6° geomag.
Cleveland, OH 50.6° geomag.
Toledo, OH 50.7° geomag.

32 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 15, 1948	47	6+
May 16, 1948	72	7

The storm before, May 15 The storm after, May 21 Every storm of 1948 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.