Today's Weather Discussion: Nasty New Year's Setup
Wednesday, Dec. 29:
Big changes are underway with our weather pattern today that will lead to a sloppy end to 2010. Today's rain is the beginning of a general southwest flow pattern that will keep damp and dreary conditions locked in for a few days, and will allow temperatures to gradually and steadily climb through Friday...when a cold front will slam into the region just in time for all of those New Year's Eve festivities.
As we get into the setup, let's first take a look at the jet stream and our overall setup. Take a look below at the image below:
There are two features to look at here. First of all is the little "kinks" or "wrinkles" in the jet stream along the dashed black line. That is the shortwave trough that is bringing our rain this afternoon. Shortwaves are what we sometimes call "mid-level disturbances", and are smaller-scale systems that can bring a quick chance of precip, and then quickly move on....as will be the case as this system moves east tonight. The other thing to look at above is the developing longwave trough over the western U.S. This will develop into a large-scale system over the next couple of days....here's what the trough will look like by Friday morning:
When we have a large trough such as this, we tend to see warm, moist flow ahead (to the right) of the trough, and colder, drier air behind the trough. For us, this means a period of warmer and wetter weather until the trough moves east and northeast over the weekend.
With little going on at the surface between now and Friday, we will be lacking a good trigger for any major area of organized rain, so expect things to just be a bit dreary and drizzly tonight through Friday morning, with perhaps a few scattered showers. Today and tomorrow's rainfall totals should remain around 0.1-0.2 inches of rain. Temperatures will gradually climb overnight tonight into the mid 40's, then we'll head toward the lower 50's by tomorrow evening. By Friday, a fairly strong cold front will be taking shape to our west, and out ahead of that front, breezy winds out of the south will aid high temperatures into the lower/mid 60's. Here's a look at the cold front/surface map at 6PM Friday:
As this front comes through the area, it will provide the lift necessary for our best chance of rain as well as a chance for thunderstorms. The latest trends have been to bring this system through during the later afternoon and early evening hours of Friday....during the "peak heating" hours of the day. If that holds true, then the atmosphere may be just unstable enough to spark a line of stronger storms ahead of the front, with the best chance of strong/severe storms in SEMO. As you can see below, the Day 3 Outlook (Friday) from the Storm Prediction Center in Oklahoma includes a slight risk area that comes right up to the southwestern edge of the Local 6 area.
I'm not totally sold on the idea of severe weather at this point, but it is something to keep an eye on. During the summer we talk about severe storms forming in environments with lots of instability, but in the winter, we really don't need very unstable air to get storms going as long as we have sufficient wind shear (turning and increasing of winds with height)....which we should have Friday. During the winter months, a loose threshold for storms is when we see surface dewpoints climbing into the upper 50's and near 60, letting us know that the air is very moist. As seen below, we'll see that 60 degree dewpoint line sneaking up to near the MO Bootheel by Friday evening:
So, bottom line, the overall threat for severe weather is fairly low for our area for Friday, but it is something to be aware of. As you are making plans for New Year's Eve, be ready for rain, and perhaps some fireworks, courtesy of "Mother Nature." Rainfall may come down a bit heavy at times Friday night, with total rainfall between now and Saturday morning between 0.5-1.0".
After this system departs the area, it is looking more and more like we'll have a dry, but cloudy and cold day on New Year's Day, with highs around 40, then a return to sunshine and seasonal temperatures to begin next week.