A model of return stroke detection by the U.S. National Lightning Detection Network (NLDN) magnetic direction finder (MDF) sensors is used to approximate the pulse width criterion modification made to the sensors during the 1994 upgrade. Decreasing the pulse width detection criterion used by the MDF sensors increases their effective detection range, which increases their sensitivity to weak flashes (because of NLDN network geometry, increasing sensitivity has little effect on detection of strong flashes). Consequently, we observe an increase in the weak flash counts. The increased detection of weak flashes accounts, in part, for the decrease in mean peak currents observed in subsequent years to 1994. In addition to decreasing the mean peak current of detected positive and negative flashes, the NLDN upgrade has apparently had the unwanted effect of increasing the contamination of the positive CG flash data with cloud flashes. ¿ 1999 American Geophysical Union