Rockfish Rick

Okay, instead of spreading climate doomer doctrine, I will take a moment to explain the meteorology behind the floods in North Carolina [and Tennessee]. The precipitation analysis for the last two-days [from 12z today] reveals that over a foot of rain has fallen in the Appalachians. This amount of rain in such a short amount of time, regardless of land topography, results in widespread [and often devastating] inundation. The reason you see 14-18 inches of rain (purple / pink contours) along the eastern slope of the Blue Ridge is because of strong mountain-enhanced orographic lift. As moist parcels of air in the bands of Helene rotated counterclockwise through the region, they were then dynamically lifted to supersaturation on sloped terrain causing excess vapor to condense and precipitate out. On the leeward side in Tennessee, there was a bit of a rain shadow effect due to downslope flow, so rainfall totals were lower. This is what happens on the Hawaiian Islands on just about a daily basis. Now, you will frequently hear the claim that because the atmosphere is warming due to our greenhouse gas (GHG) emissions, the atmospheric carrying capacity for water vapor has increased and as a result, rainfalls are heavier. However, there is a fundamental issue with this claim. Yes, it is true that for every 1°C rise in air temperature, the amount of water that can evaporate into a parcel of air increases by a little over 7%. This means that the vapor pressure, 𝑒, at which vapor is in thermodynamic equilibrium with its condensed state (i.e., there is as much water vaporizing as there is vapor condensing), increases by ~7% for every +1°C increase in the parcel temperature. This is the saturation vapor pressure and is denoted as 𝑒ₛ This is given by the Clausius-Clapeyron relation, 𝑒ₛ = 𝑒₀ 𝑒𝑥𝑝 [𝐿ᵥ / 𝑅ᵥ ((1 / 𝑇₀) - (1 / 𝑇))], where 𝑒₀ = 611 Pa, 𝐿ᵥ = 2.5×10⁶ J kg⁻¹ is the latent heat of vaporization, 𝑅ᵥ = 461.5 J kg⁻¹ K⁻¹ and 𝑇₀ = 273.15K. To make this simple, what this equation tells us is that at higher temperatures, more water can evaporate into the air without reaching saturation. But, just because the atmosphere can contain more water vapor at higher temperatures does 𝒏𝒐𝒕 mean that it will in any given scenario. And, even if it does, it does not guarantee all of that water will precipitate out of the cloud over one area. To review, the Clausius-Clapeyron relation tells us that warmer air 𝒄𝒂𝒏 wick up more moisture. But, it does 𝒏𝒐𝒕 tell us that it will. And, it does 𝒏𝒐𝒕 tell us how exactly how much rain will precipitate down over a given area. Scientists, including meteorologists, inappropriately use this equation to fearmonger anytime there is flash flooding somewhere. Go back to your undergraduate Cloud Physics course and re-learn the physical concepts.