What Kp is a G5 storm?

Kp = 9, on the NOAA Space Weather Scales. SWPC's oval-edge rule puts the average edge of the oval at 48 degrees geomagnetic at Kp 9.

How many G5 storms happen per year?

SWPC lists 4 per 11-year cycle on 4 days, about 0.4 storm days a year on average.

Which Glow towns does a G5 storm reach?

10 of 10 sit at or poleward of the Kp 9 oval edge: Reykjavik, Yellowknife, Tromsø, Fairbanks, Anchorage, Duluth, Minneapolis, Seattle, Portland, Maine, Buffalo.

Where has G5 aurora been seen?

NOAA's scale says: "aurora has been seen as low as Florida and southern Texas (typically 40° geomagnetic lat.)". Aurora is often seen hundreds of kilometers equatorward of where it actually sits, per SWPC.

How rare is a G5 storm?

SWPC lists 4 per 11-year solar cycle, on 4 days. That averages about one every 2.75 years, though real storms do not arrive on a schedule.

NOAA G-scale · Kp = 9

G5 extreme geomagnetic storm and the aurora

Answer

G5 Extreme is Kp = 9 on NOAA's scale. At this level, SWPC says, "aurora has been seen as low as Florida and southern Texas (typically 40° geomagnetic lat.)". It occurs about 4 times per 11-year cycle.

Extreme, Kp = 9

NOAA Space Weather Scales

What SWPC says about G5

G5 is the extreme end of NOAA's geomagnetic storm scale, Kp 9. It is also the rarest level by far. SWPC's tips page says that at Kp 8 to 9 the aurora may be seen directly overhead from the northern states of the USA.

"aurora has been seen as low as Florida and southern Texas (typically 40° geomagnetic lat.)"
NOAA Space Weather Scales, G5 Extreme

The scale lists G5 aurora as seen as low as Florida and southern Texas, typically 40 degrees geomagnetic. The oval-edge rule puts the Kp 9 edge at 48 degrees. The gap between the two, 8 degrees or about 890 km, matches SWPC's point that aurora can be seen hundreds of kilometers equatorward of where it actually is.

G5 averages 4 storms on 4 days per 11-year cycle, so the chance that one lands on a given clear night at a given town is small. When a G5 watch is issued, prepare the basics early: a site with a dark sky in every direction, the cloud forecast for your dark hours and the moon's timing. SWPC says CMEs typically take several days to reach Earth, and as little as 18 hours in the most intense cases.

"It should be noted that the aurora can often be observed hundreds of kilometers (miles) equatorward of the actual aurora so these figures do not indicate where the aurora may be but rather the point from which it may be observed."
NOAA SWPC, Tips on viewing the aurora

The 10 Glow towns at G5

10 of the 10 Glow towns sit at or poleward of SWPC's typical G5 latitude of 40 degrees. The Kp 9 edge at 48 degrees reaches all 10. Buffalo sits closest to that edge, 449 km north of it.

Each Glow town against the G5 typical latitude and the Kp 9 oval edge (48 degrees)
Town	Dipole	vs typical latitude	Kp 9 edge
Reykjavik	68.8°	+28.8°	Reaches the town
Yellowknife	68.5°	+28.5°	Reaches the town
Tromsø	67.5°	+27.5°	Reaches the town
Fairbanks	65.7°	+25.7°	Reaches the town
Anchorage	61.9°	+21.9°	Reaches the town
Duluth	55.4°	+15.4°	Reaches the town
Minneapolis	53.5°	+13.5°	Reaches the town
Seattle	53.0°	+13.0°	Reaches the town
Portland, Maine	52.9°	+12.9°	Reaches the town
Buffalo	52.0°	+12.0°	Reaches the town

Dipole latitude from IGRF-14. Typical latitude from the NOAA scale; positive means poleward of it. Edge from SWPC's 66 minus 2 per Kp rule, at 111.2 km per degree. All approximate.

How often G5 storms happen

SWPC lists 4 G5 storms per 11-year solar cycle, falling on 4 days. Spread evenly over the cycle, that is about 0.4 storms and 0.4 storm days a year. G4, one level down, has 15 times as many storm days.

NOAA G-scale average frequency, per 11-year cycle and per year
Level	Kp	Storms per cycle	Days per cycle	Days per year
G1 Minor	Kp = 5	1,700	900	82
G2 Moderate	Kp = 6	600	360	33
G3 Strong	Kp = 7	200	130	12
G4 Severe	Kp = 8, including a 9-	100	60	5.5
G5 Extreme	Kp = 9	4	4	0.4

Per-cycle counts from the NOAA scales page, where 1 cycle is 11 years. Per year: days per cycle divided by 11. Real storms are not spread evenly.

On a G5 night

Anyone north of about 40 degrees geomagnetic with a dark, clear sky has a chance. Look in every direction.
At every Glow town the average oval edge sits south of you. The display can fill the sky.
Use the chance tool for your own cell; at this level it matters more than any town page.
Keep checking the southern edge tool through the night; at Kp 9 the edge the rule predicts is 48 degrees geomagnetic.

Questions

What Kp is a G5 storm?: Kp = 9, on the NOAA Space Weather Scales. SWPC's oval-edge rule puts the average edge of the oval at 48 degrees geomagnetic at Kp 9.
How many G5 storms happen per year?: SWPC lists 4 per 11-year cycle on 4 days, about 0.4 storm days a year on average.
Which Glow towns does a G5 storm reach?: 10 of 10 sit at or poleward of the Kp 9 oval edge: Reykjavik, Yellowknife, Tromsø, Fairbanks, Anchorage, Duluth, Minneapolis, Seattle, Portland, Maine, Buffalo.
Where has G5 aurora been seen?: NOAA's scale says: "aurora has been seen as low as Florida and southern Texas (typically 40° geomagnetic lat.)". Aurora is often seen hundreds of kilometers equatorward of where it actually sits, per SWPC.
How rare is a G5 storm?: SWPC lists 4 per 11-year solar cycle, on 4 days. That averages about one every 2.75 years, though real storms do not arrive on a schedule.

Sources and verification

NOAA Space Weather Scaleshttps://www.spaceweather.gov/noaa-scales-explanation
NOAA SWPC, Tips on viewing the aurorahttps://www.spaceweather.gov/content/tips-viewing-aurora
NOAA SWPC, Planetary K-indexhttps://www.swpc.noaa.gov/products/planetary-k-index

Quoted figures last checked against their sources on 2026-09-30; next review 2026-12-31. Computed figures come from SunCalc and the IGRF-14 dipole as described in the methodology. Every quote is listed in the sources ledger.