Past IEM Features tagged: cape
Very hot air arrived in Iowa on Wednesday and muggy conditions were not far behind. With surface dew points near 80 degrees in some locations, the lower atmosphere is loaded with moisture. This moisture represents a potential energy source for thunderstorms as condensing water within clouds releases heat which promotes upward motion. The featured map presents a measure of this energy named convective available potential energy (CAPE). CAPE values above 2,000 are often considered significant for severe storms, so do the values shown above 6,000 in Southern Iowa on Thursday evening mean the world is coming to an end? The key word in the CAPE acronym is potential. Just as a watermelon sitting on the top of a sky scraper has a high potential to accelerate if pushed off the edge, the initial push needs to occur. For the atmosphere, weak winds and little convergence of air often means these profiles are left unrealized (nothing to give them a push).
The featured chart presents the highest forecasted surface based convective available potential energy (CAPE) from the GFS model for a grid point near Ames since the first of the year. This variable gives an indication to the degree of instability in the atmosphere that helps to facilitate thunderstorms. Values above 1,000 typically are when the stronger storms are possible. While values shown are well below 1,000 for surface based air parcels, CAPE computed for elevated parcels is expected to be larger and will allow for some hail producing storms in the coming week.
The featured map displays the RUC computer weather model forecast of surface based CAPE for yesterday afternoon. CAPE is a measure of the vertical accelerations possible if air is forced upward initially. Typically, values of around 3,000 indicate severe storms are possible. Due to the extreme surface temperatures and dew points yesterday, values were over 10,000 J/Kg. Values this large are extremely rare.
Tags: cape model