Ground-based visible absorption spectroscopy of zenith-scattered sky light was used to measure changes in the column abundance of NO2 during the passage of a thunderstorm over Boulder, Colorado, on September 12, 2002. The measurements showed a 10-fold rise in slant column NO2 within the thunderstorm cell, consistent with an increase of about 7 ppbv in the lower core updrafts and main precipitation shaft. Mean mixing ratios of about 3 and 1.5 ppbv were inferred for the upper cell and anvil, respectively. These observations, together with measurements from the Denver NEXRAD and the Colorado State University (CSU) CHILL radars, and cloud-to-ground (CG) lightning flash data from the National Lightning Detection Network (NLDN), are used to estimate the flash mean production of NOx by CG lightning. The measurements suggest a mean production rate of (5.8 ¿ 2.9) ¿ 1026 NOx molecules per CG flash, intermediate between the values derived from previous spectroscopic studies and in good agreement with the widely used value of 6.7 ¿ 1026 NOx molecules per CG flash estimated by Price et al. <1997>. This agreement may be fortuitous, however, since the lightning in the present study was about a factor of two less energetic than the mean assumed by Price et al. and the production of (2.3 ¿ 1.4) ¿ 1017 NOx/J inferred here is significantly larger than the value of 1 ¿ 1017 NOx/J assumed in their calculations. These differences can be reduced, however, if the estimates of Price et al. are recalculated using newer peak current data from the NLDN, and more recently published flash energy production rates of Wang et al. <1998>.