Temperatures reached 90 degrees yesterday at the Nation's Capital, but with dewpoint temperatures in the low to mid 70s, the 90 degree heat became an oppressive 94 or 95 degree oven. Today, high temperatures will once again touch, if not eclipse, the 90 degree mark. High moisture levels will create heat index values in the mid to upper 90s. A few showers and strong to severe thunderstorms are possible later today as a weak upper level disturbance passes through the region.

The high temperatures and elevated dewpoints will contribute to large levels of instability this afternoon (see image at right). Convective Available Potential Energy (CAPE) is a measure of the relative amount of instability present in the atmosphere and is a good indication of the likelihood of severe weather. Values at or above 2000 Joules per kilogram are normally sufficient to sustain severe storms. The NAM model depicts CAPE values at or above 3000 J/kg later this afternoon. So, any storms that manage to develop today will have the potential to become severe. Working against the potential for severe storms are a lack of upper level wind shear, relatively little forcing to initiate convection, and dry mid-level air.

The Rest of the Week:
The main story for the rest of the week remains the heat. Temperatures are forecast to hit 97 or 98 degrees tomorrow and Wednesday. Heat Index values will soar into the triple digits due to high humidity and atmospheric moisture content.


A chance of thunderstorms will develop during the afternoon hours through the rest of the work-week, but the best chance of thunderstorms looks to come on Wednesday as a cold front sweeps through the area--probability of precip on Wednesday: 70%.