* AFFECTED AREA...In South Dakota, Marshall, Roberts, Day and Grant. * WINDS...Through the late morning and midday, winds will be shifting from southerly to southwesterly at 10 to 20 mph with gusts up to 35 mph. Then, late in the afternoon into the evening another shift around to northwesterly and then northerly behind a cold front. Wind gusts increase up to 35 to 40 mph behind the front. * RELATIVE HUMIDITY...As low as 20 percent. * IMPACTS...Any fires that ignite will spread rapidly and become difficult to control or suppress.