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I I I I I I I I I I I I I I I I I I I These storms remained just out of the District but caught the attention of the HMS afternoon meteorologist who anticipated rainfall of 0.75"/30 and 1.50"/60 minutes from a storm. He noted storm movement was to the south-southeast at 20 mph due to northwesterly cloud steering winds and expected the steering winds to decrease overnight as shower activity increased with the approach of the upper level disturbance. One final indication of the atmosphere's volatility preceded the Virginia Canyon storm. About two hours later another thunderstorm flared up over southeastern Lakewood and western Littleton. This storm had a duration of about 30 minutes. It formed as surface temperatures hovered around 800F and dew points held near 520f in an area of briefly converging low level winds. The storm produced a moderate rain which radar estimated at 0.60-0.75" in 20-25 minutes and very active cloud-to-ground lightning. The storm quickly died as its rain-cooled air dropped surface temperatures below 750f choking off the supply of warm, unstable air needed to fuel the storm's updraft. The storm's movement was to the southeast at 5-10 mph or much slower than the earlier storms over Douglas County. The cloud shield produced by this storm shaded most of the Denver metro area from any further solar heating and hastened a drop in local temperatures into the low 70's. As surface temperatures over the plains dropped, a low level inversion formed which capped any further storm development from occuning over the plains. The stage had been set for storm development activity to shift to the west over the mountains. A plot of the low level temperatures, dew points and winds is shown in Figure 6 at 0010GMT or 1810L for the Denver metro area. The 770f isotherm represents the temperature needed to support thunderstorm development is dashed in place. Note that the Littleton storm had already dropped Lakewood (LAK) and Littleton (L TN) temperatures into the low 70's and the temperatures in the 60's to the south of the metro area caused by earlier storm cooling. Temperature across all of the metro area cooled to less than 760F in the next 30 minutes in response to the effects of the Jefferson County storm's cloud shield and rain-cooled air mass. It is important to note that temperatures in the Jefferson County foothills were still in the 60's and were in the 50's in the Boulder County foothills where winds were converging westward towards higher terrain. The importance of these factors becomes apparent when the vertical temperature and moisture structure of the atmosphere is considered. Figure 7 is a plot of the atmosphere's vertical structure using two information sources on a standard Skew T, Log P thermodynamic diagram. First, the NWS upper air sounding released at OOOOGMT (1800L) is plotted for elevations above 650mb or 12,000 feet MSL. Below 12,000 feet the surface temperature and dew point from the Denver (DEN), Lakewood (LAK), Evergreen (EVG), Blue Mountain (BLU), Rollinsville (ROL), Ward (WRD) and Squaw Mountain (ISG) mesonet weather stations are plotted. The use of these two sources allows a more realistic approximation ofthe atmospheric structure which supported the Virginia Canyon storm. A solid black line connects the temperature points and a dashed line the dew points. 9