Chances are building today for some severe thunderstorms, with heavy rain and large hail. But they get even stronger tomorrow.

The National Weather Service forecasters out in Sterling are looking at a patch of low pressure in the upper atmosphere that they expect will kick off showers and thunderstorms across the region this afternoon and evening. As the morning sunshine heats up the air at the surface, it will begin to rise into much colder layers of air aloft. That's the definition of atmospheric "instability," and it leads to the formation of tall cumulous clouds and thunderheads. And that means thunderstorms.

There's plenty of moisture in the air to feed the storms. We're on the west side of a big high-pressure center over the western Atlantic. Highs circulate clockwise, so this one is dragging lots of warm, humid air up from the Gulf of Mexico.

Add daytime heating and we're off to the races. Forecasters say some storms could reach "severe" levels today. Cold air aloft means hail - perhaps large hail - is a possibility. (Not this big!)

These slow-moving storms could cause a good deal of rain to fall in localized areas, leading to a potential for flooding, too.

By tomorrow, forecasters say we'll be moving into another regime as a cold front approaches from the north and west. Shove a colder, dense air mass into the warm, humid air that's here now, and you force the warm air upward. Add daytime solar heating and you set off more thunderstorms. And forecasters expect tomorrow's will be stronger than today's.

Plus, the cold front is expected to slow or stall here, causing the storms to linger and concentrate their rainfall in localized areas.

"Should be numerous thunderstorms by afternoon," forecasters say in their discussion this morning. "Once again, dual concerns severe weather and local flooding ... Strong, damaging wind gusts might prevail .... Flooding will be an increasing threat, especially since coldfront appears to stall in metro corridor Wednesday evening, making training storms more possible."

Once the storms move out, high pressure builds, offering us sunny skies and seasonably high temperatures in the mid-80s to near 90 degrees right through the weekend.

We're moving into the warmest weeks of the year, statistically. Two weeks from now the average daily highs begin to decline as days grow shorter and we head toward fall and winter.

NOAA photo