Today's Weather Discussion: Warm Weekend/Wet Next Week
Friday, Nov. 19:
I'm looking out at more gray skies here at the weather center this afternoon....all indications yesterday were that clouds would be out of here by now. We've seen a nice clearing trend over MO & IL today, but we just can't shake the clouds across much of KY & TN. This is an example of the complexity of the atmosphere, and an opportunity to talk about some weather terminology. Even though high pressure at the surface was thought to be enough to clear us out, a temperature inversion is at least partly responsible for locking in this cloud cover. Take a look at the image below:
On the top image is an example of the normal setup of the atmosphere. At least in the layer of the atmosphere where weather occurs, called the troposphere, the temperature normally decreases at a fairly regular rate with height. This allows for water vapor and pollutants to become mixed through the entire height of the atmosphere. However, at times, there is a layer of warmer air aloft that acts as a lid on the atmosphere. Sometimes, when dealing with severe weather, this is called "the cap". This warm layer aloft also limits the height to which water vapor and pollutants can mix, resulting in poor air quality around urban areas.....and in cases like what we are dealing with today....leading to low-level moisture being trapped and locking in a low-level cloud layer. This inversion tends to persist until a new air mass comes in and stirs things up a bit. As of this afternoon, we're still waiting for things to get stirring here!
Moving on to my forecast thoughts....I am still confident that we'll be seeing blue skies at least for a while on Saturday. We'll also be getting caught up in a fairly strong SW flow pattern, which will bring some warm temps for the weekend. Still looking at 60's for both Saturday and Sunday. Low-level moisture will be increasing as the weekend progresses, which will lead to increasing clouds on Sunday. Models even try to squeeze out a few drops of rain late Sunday and into Monday morning, if enough moisture arrives.
A cold front will develop over the Plains this weekend, and slowly push into the region from the west on Monday. Could be some a shower or two Monday....but rainfall looks to be more certain as we move into Tuesday and Wednesday. The jet stream will be running SW to NE, running parallel with this cold front. This will result in the front being slow to push across the area, and could mean an extended period of wet weather for a couple of days. There are still some subtle differences in the models with this system, but rainfall almost looks like a certainty, and maybe some thunderstorms as well. I think this will be a nice healthy rainfall event too, with perhaps an inch or two of rain. Here's a look at the GFS rainfall estimates for 6PM Mon-6PM Wed:
After the rain ends, the cold stuff returns. Looks like a dip in the jet stream will send thermometers tumbling for Turkey Day & Black Friday. Today's numerical models show highs struggling to get into the lower 40's for Thursday, and lows falling into the mid 20's for Friday morning. A much drier pattern sets up for the holiday weekend though, with no rain (or snow) expected after Wednesday.