When I was a student at Penn State, I came back to visit during the 1988 Arts Festival and was in town when State College last reached 102 degrees. Being a weather fanatic who knew a lot about the local weather, I knew that I was in town for what may be the only 100 degree reading during my lifetime. The reason why is it takes a perfect scenario to actually reach 100 degrees in our area.
Last week it looked like we may get that scenario and this week it looks like we had a fair chance. Even into the middle of this week we thought it may happen, but it is so hard, we still had to go with the odds and kept the forecast for the upper 90s and mention the century mark. Why? Because so many things could interfere with reaching the mark. History proves the point as we really have only reached there last than a handful of times.
What created this perfect storm to create the heat?
First let's talk about the start of the heat wave itself:
- The South Central states have been in an extreme drought. A dry ground allows for the maximum heating from the sun and helps temperatures to reach as high as they can. Most major heatwaves need these air masses to build in the middle of the nation during such dry stretches.
- A ridge of high pressure in the upper levels of the atmosphere developed over the middle of the nation allowing for a hot air mass to build there and then spread northward and eventually eastward.
But not every major heatwave will send our temperatures past the lower 90s. We needed a lot locally, specifically these things:
- We have been extremely dry. It's hard to even reach 90 in Central PA with a lot of moisture in the ground. The reason why is evaporation takes a lot of energy to happen. When there is evaporation from moisture in the ground, it prevents us from getting as hot as possible. Our vegetation and trees also sap moisture from the ground and put it out through their leaves. Therefore, we would need abnormal dryness to slow this effect and we did have this from late June through now.
- Lastly on top of all of the things mentioned above, we need two more things to help us get to 100. Sunshine and a westerly flow. This westerly flow downslopes from the highlands and this as air descends the increase of pressure allows the air to compress and warm. We had both the sunshine and this westerly flow for the past couple of days.
Of course, the increased radiative forcing from greenhouse gases -- and the tremendous buildup of heat stored in the upper oceans over the last several decades -- makes the extreme heat and drought more likely.
ReplyDeleteBy the midcentury, 100+ readings in State College will be commonplace -- somewhere between a week or a month of 100+ weather, depending on the amount of carbon pollution we add in the next decades.
ReplyDeletehttp://www.climatechoices.org/assets/documents/climatechoices/pennsylvania_necia.pdf
Brad, it is completely irresponsible to make a short term event and use it as an example for the long term. Do keep in mind that these records are just matching that something that happened in the 30s. Even the most dire model predictions would not have 100 degree plus readings for a week or a month. There's a lot more to reaching that number than you are taking into account.
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