A geomagnetic storm warning has been issused due to a coronal mass ejection (CME) occuring on September 10, 2014. The brunt of the the effects are expected to arrive at the earth's atmosphere on Saturday September 13, 2014. While many media outlets love to portray the doom and gloom scenario whenever the sun becomes active, this particular CME has caused the Space Weather Prediction Center (SWPC) to issue a G3 classification alert.

NOAA/NWS's space weather classifications

The G3 classification implies a "strong" storm which may cause intermittant satellite navigation outages, impact spacecraft operations and may cause voltage alarms in high latitude power systems.

Geomagnetic activity is often classified using the planetrary Kp index. This index can be used by amatuer astrnomoers to know when the potential exists to see the aurora borealis. Currently, we see the index has just reached a value of 4, according to the SWPC, and is expected to increase over the next 24 hours.
Planetary Kp Index

The planetary Kp Index is computed by using a weighted average of magnetometer measurements made around the world. One of the draw backs of the Kp Index computed using this approach is that a user could see a low Kp Index but still be experienceing high geomagnetic activity in thier region or vis-versa.

Some of the effects that can occur during periods of high geomagnetic activity include scintillation and large ionosphere anomalies.

Ionosphere scintillation is normally restricted to high latitudes and equatorial latitudes but during these events scintillation effects can even move into mid-latitude regions. This can cause GNSS users to lose lock on the satellite signals, particularly on the GPS L2 frequency due to the semi-codeless tacking techniques. If many satellites are in veiw the overall impact on positioning may be low as it is unlikely for all satellites to be affected at once.

In addition to scintillation, some geomagnetic storms are associated with large gradients in the ionosphere delay. One particularly bad event occured March 31st, 2001. During this event, large gradients in total electron content (TEC) were seen through the US and Canada causing 40+ meters horizontal errors for DGPS users.
Storm enhanced density event, March 31, 2001

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Predicting the exact outcome of these events is extremely difficult and a particular interest for scientists. So we may have to wait until next week to know the full impact of the storm.