A strong positive storm phase was observed by both the Millstone Hill and Arecibo incoherent scatter radars during a moderate geomagnetic storm on 10 September 2005. The positive storm phase featured an interesting UT–altitude profile of the F region electron density enhancement that closely resembles the Greek letter L. The radar measurements showed that the uplift of the electron density peak height corresponded to a strong upward ion drift, whereas the subsequent falling of the peak height coincided with a downward ion drift. Using realistic, time-dependent ionospheric convection and auroral precipitation as input, the thermosphere–ionosphere electrodynamics general circulation model (TIEGCM) is able to reproduce the same L-like structure in the electron density profile, along with many large-scale features in electron temperature and vertical ion drift as observed by the radars. Over the 3-day period of 8–10 September, our simulation results show an error of 1%–4% for hmF2, electron, and ion temperatures at both radar locations. The estimated error for NmF2 is about 9% at Millstone Hill and 19% at Arecibo. However, the simulated vertical ion drifts are less accurate, with the normalized root-mean-square errors of 72% at Millstone Hill and 52% at Arecibo, due largely to model’s inability to capture the large temporal fluctuations measured by the radars. However, it reproduces reasonably well the overall large-scale variations during the 3-day period, including the storm-time-enhanced upward ion drift that is responsible for the interesting F region density profile. The model is also able to reproduce the temporal and spatial total electron content variations as shown in the global GPS maps. The comparison with the GUVI O/N2 is less satisfactory, although there is a general agreement in terms of relative O/N2 changes during the storm in the longitudinal sector between 60°W and 80°W where the radars are located. The detailed data–model comparison carried out in this study is helpful not only to validate the model but also to interpret the complex observations. The TIEGCM simulations reveal that it is the enhanced meridional neutral wind, not the penetration electric field, that is the primary cause of the L structure of the F region electron density profile.