If you stepped outside on Tuesday, you probably immediately noticed the high humidity values. It was very muggy with dew points in the middle 70's. The dew point temperature essentially tells you how humid it is. Whenever those values exceed 70°, it is very humid, if not oppressively humid. When those numbers reach 75°, it feels tropical. That is the air mass we had in place today, and it is one that is not too common for the month of September. In fact, the National Weather Service brought up that the amount of moisture in place is nearly at a record high.
One tool we use to measure the amount of moisture in the atmosphere is precipitable water. That is a value that meteorologists can use to determine if heavy rain is expected or not. In other words, high precipitable water values represent air that has an abundance of moisture in it. Therefore, high precipitable water values are nearly perfectly correlated with heavy rainfall. When these values exceed 2", a red flag for very heavy rain is waved at meteorologists. Values above 2" are not uncommon, but seeing them this late in the summer is. According to one forecast model, our precipitable water values may exceed 2.5" Wednesday night as heavy rain looks to move through the region. That is the value the National Weather Service is referring to as a record. This air mass in place is unseasonably moist, and it is one that may support rain totals well over 1" Wednesday night. As a result, flooding is a legitimate concern with our rain and storm activity that moves through late Wednesday afternoon through early Thursday morning.