19 07 2013 Geomagnetic Storm HD

 
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A faint CME from July 16th is delivering a glancing blow to our geomagnetic field late on July 18th/19. This in itself is not expected to cause storming, however when you factor in the coronal hole solar wind stream expected to arrive at nearly the same time, this could help generate minor geomagnetic storming at high latitudes.
The solar wind carries with it the magnetic field of the Sun. This magnetic field or the IMF (interplanetary magnetic field) has a particular orientation - southward or northward. If the IMF of the solar wind is southward and the solar wind crosses the Earth for long periods of time, geomagnetic storms can be expected.

The southward IMF causes magnetic and particle energy to be injected into the Earth's magnetosphere creating storms.