Over the past week, a subtropical dome of high pressure has been developing across eastern Mexico and the northern Gulf Coast. Pushing into the Southern Plains and Lower Mississippi Valley, it is both shunting Pacific storm systems to the north and fueling the Southwest Monsoon.
Along the outer edge of this atmospheric dome, moisture flows across Mexico (from east to west) and then up through the Four Corner States; this flow is augmented by low pressure over the Desert Southwest, which draws in moisture from the Sea of Cortez. Just north of the dome, the moisture combines with inflow from the Pacific and moves eastward along a stationary front; pulses of low pressure move along the front, igniting thunderstorms and torrential rain. "Training" across the same path (from eastern Kansas to southern Ohio), the storms will drop up to 5 inches of rain in some areas; northern Missouri is forecast to receive the brunt of the deluge.
Here in Columbia, light drizzle developed at 6:30 AM; steady rain has since moved in and will likely continue (off and on) for the next two days. While the rains will provide welcome relief from our ongoing drought, heavy downpours on sun-baked soil will rapidly drain into creeks and rivers, setting the stage for flash floods.