If temperature and pressure are inversely related, how are both high at the plains?

According to the ideal gas law:

PV = nRT

where P is the pressure, V is the volume, n is the number of moles, R is the universal gas constant and T is the temperature. From this equation, we can see that pressure and temperature are directly proportional to each other and not inversely proportional. 

According to Gay-Lussac's Law, pressure and temperature are directly proportional, which means P/T = constant, provided that the volume is held constant. 

Thus, it is entirely feasible to have high pressure and temperature at the plains, as long as the gas volume is held constant. At high temperatures, the gas molecules attain higher kinetic energy and thus exert more pressure at the surfaces, as long as the volume is fixed. This causes an increase in the pressure exerted by the gas and hence high pressure and high temperatures exist together.

Hope this helps.