The above chart shows the precipitable water at BHM as a function of time, starting with this morning's balloon launch and then going out through next Tuesday morning at 7 am, using computer model prediction. I'll explain precip water below, but the bottom line is that we will begin to see a lot more showers and thunderstorms this weekend than during this week. Moisture is increasing as an upper level storm moves from near the Texas Gulf Coast toward Alabama, brining in lots of Gulf moisture.
"Precipitable water" (PW) indicates how much water vapor, or humidity is in the entire atmosphere above a point (in this case Birmingham). If a storm could wring out every drop of water vapor, turn it into rain, and drop it on the ground, that is how much rain would fall. Of course, most storms don't wring out all the PW, so most would not produce 2" of rain on Saturday or Sunday.
However, some storms sit still for a while, raining out the PW in that area and then pulling in more from adjacent areas, so a few storms could produce isolated amounts larger than 2" this weekend. Actually, it is possible that between Saturday and Tuesday, a few spots might get over 4" of rain, while most of us will see 1-2".
Not only do storms in a moist atmosphere produce more rain, but storm coverage is usually bigger too, since clouds that might normally stop at being a towering, puffy cumulus cloud in an atmosphere with PW 1.5", will grow this weekend into showers.
Dr. Tim Coleman
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