The first observations of large-amplitude GPS L1 amplitude scintillations in the midlatitude ionosphere were made on 25–26 September 2001. The scintillations, which were intense at times (≥20dB, S4 ≈ 0.8), occurred during the main phase of a moderate geomagnetic storm (Dst = −100 nT) when a storm-enhanced density (SED) event coupled with a broad and structured ionospheric trough occurred over the eastern United States. The preliminary results, using a modified single-frequency GPS receiver and a dual-frequency (L1/L2) GPS receiver and measuring total electron content at Cornell University (53.2°N magnetic latitude), have been published previously. In this effort we examine the scintillation autocorrelation functions using data from the fast sampling (50 Hz) single-frequency GPS receiver. The intention is to derive a measure of the scintillation pattern velocity which can be estimated from the Fresnel radius and the width of the autocorrelation function. We show that a minimum in the autocorrelation function width, corresponding to a maximum in the velocity, is located near the equatorward boundary of a broad trough. We also demonstrate that estimates of the scintillation pattern velocity are consistent with other measures of ionospheric drift in midlatitude storm time convection. In at least two of the GPS signals the inferred velocity has narrow peaks of several hundred meters per second, characteristic of subauroral ionospheric drifts (SAID) and perhaps driven by plasma pressure gradients in the inner magnetosphere. This presentation, along with the paper by Ledvina et al. , strongly demonstrate that the SED and associated SAID can cause debilitating scintillation levels for GPS receivers.