PM Wed. June 1 - AM Thu. June 2 Cirrus Blow-off

  A weak cold front passed through Indiana early Wednesday morning and we were under the influence of high pressure that had lowered the heat and humidity. Skies started out very sunny on Wednesday. Forecasts for the night ahead made me think that I was going to have one of those "pristine" nights with great sky transparency and no dew issues. Unfortunately, skies were overcast with high clouds by Wednesday evening and conditions were fair to poor overnight.

  The reason for the unexpected cloudiness was a common spring and summer phenomenon called (probably informally) cirrus blow-off. When a region has humid, warm, and unstable air, thunderstorms often appear as the air warms and rises. If these storm clouds reach high enough into the atmosphere the ice-crystal cirrus clouds that form at the highest altitudes can spread hundreds of miles ahead of the storm centers. On Wednesday a mesoscale convective complex set up in Kansas, Nebraska, Western Missouri, and Western Iowa. These storms were vigorous and long-lasting enough to send cirrus clouds into Illinois and Indiana. So, even though my area was under stable high pressure, we still had the blown-off high clouds from two or three states away!

  I read once that narrowing down where summer thunderstorms will appear is incredibly difficult to do. One meteorologist compared it to trying to predict where bubbles will pop up when water is starting to boil! That means that predicting which clear nights will be ruined by cirrus blow-off from thunderstorms is also nearly impossible.