  We investigate how a weak coronal mass ejection (CME) launched on 2016
October 8 without obvious signatures in the low corona produced a relatively
intense geomagnetic storm. Remote sensing observations from SDO, STEREO and
SOHO and in situ measurements from WIND are employed to track the CME from the
Sun to the Earth. Using a graduated cylindrical shell (GCS) model, we estimate
the propagation direction and the morphology of the CME near the Sun. CME
kinematics are determined from the wide-angle imaging observations of STEREO A
and are used to predict the CME arrival time and speed at the Earth. We compare
ENLIL MHD simulation results with in situ measurements to illustrate the
background solar wind where the CME was propagating. We also apply a
Grad--Shafranov technique to reconstruct the flux rope structure from in situ
measurements in order to understand the geo-effectiveness associated with the
CME magnetic field structure. Key results are obtained concerning how a weak
CME can generate a relatively intense geomagnetic storm: (1) there were coronal
holes at low latitudes, which could produce high speed streams (HSSs) to
interact with the CME in interplanetary space; (2) the CME was bracketed
between a slow wind ahead and a HSS behind, which enhanced the southward
magnetic field inside the CME and gave rise to the unexpected geomagnetic
storm.
