Happy Thursday NBC Action Weather bloggers,
Chance of a white Christmas is going UP! A second storm has snow in your backyard on Christmas Day. This second storm is still very suspect but the latest models do have it snowing and accumulating on Christmas. We will have our updated chance of a white Christmas on the air tonight at 5, 6, and 10 on NBC Action News.
A significant winter storm is possible on Saturday. The latest model trends continue to have a stronger southern branch storm system. I will give a few of you bloggers out there credit for your forecasting ideas yesterday, as it appears you made a good early prediction. Snow is now likely late Saturday and there are a few questions that we still do not have answered. How much snow will fall? Where will the heaviest snow accumulate? Will there be enough wind to blow the snow around Saturday night? When will it begin? Could it begin as rain or a mixture? Will there be a rain/snow/mix line in the viewing area? These are the first questions that come to mind. Let's look at the storm on the latest GFS model.
O.K. bloggers, let me take you through the possibilities, and what these maps mean. The above map is valid Saturday morning at 6 AM. The below map is valid at noon Saturday, just 6 hours later. The main feature to watch is the upper level storm. On the map above, do you see the big X near the southwest Oklahoma/Texas border? This is the "vort max" as we meteorologists like to call them. This X is the center of highest vorticity, or really the center of an upper level disturbance rotating around the main upper low. The upper low is really not well defined and this makes the vort max that much more important for our chance of snow.
Now, track this X in the next 6 hours. Look where it is forecast to be around noon on Saturday. It is crossing the Kansas/Oklahoma border, just southwest of Chanute. Do you see the yellow, orange, dark orange, then red colors? These are varying levels of vorticity. When voricity is increasing along the flow, then rising motion is induced. This is what we call PVA, or positive vorticity advection. PVA is one form of lift. Warm advection is another. We will have PVA comined with a little warm advection above the surface during the afternoon Saturday. And, along this axis will be some very strong rising motion. Precipitation will be generated and it could be quite heavy. But, will it be snow?
The vort max, and upper low crossing into Kansas Saturday afternoon will rapidly phase with the upper low near the US/Canada border. This will cause this disturbance to weaken and quickly exit the region on Saturday evening, thus shutting off the precipitation.
Below is the surface map at noon Saturday. A surface low is moving northeast along the advancing frontal system over Missouri. The dashed lines are 1000mb to 500 mb thickness lines. Thickness is a measure of temperature in a layer. The first blue dashed line is the 540 thickness line. 540 means 5,400 meters. The distance between 1,000 mb and 500 mb is 5,400 meters along that blue dashed line. The first red dashed line to the southeast is the 546 thickness line, or 5,460 meters. This distance is larger indicating that the layer is warmer. When the thickness is 540 or lower then we consider it cold enough to snow in most situations, especially in the winter months.
So, as we get to Saturday afternoon the GFS is crashing our thickness from around 546 to below 540 as the precipitation increases. Then about 1/2 inch liquid is generated and someone will likely see heavy snow. If these maps are close to being correct, then the precipitation would start as rain, or a mixture of precipitation before changing to all snow.
This is targeting our viewing area. A lot of things can go wrong. Just imagine if that X, the vort max, is off by 100 miles. Then everything gets thrown off. A 100 mile change is nothing when you look at the big picture, but it is one of the reasons why snow forecasting is so hard. A very slight change can shift all of the snow north or south by 100 miles. Other things can go wrong. What if the system is just slightly weaker, faster, or stretched a bit differently. This is another thing we will continue to monitor as this system approaches.
The bottom line........It is only 48 hours away. The models have been consistent. And, I believe it is going to snow on Saturday afternoon. Probably more than 1 or 2 inches. The most likely scenario is for about 8 hours of precipitation from noon to 8 PM on Saturday. Heavy precipitation from 2 PM to 6 PM Saturday. This is a small window, and as we talked about yesterday, it is like threading a needle. Apparantly Mother Nature has some pretty good fingers working right now because it looks like we will thread that needle. We will have our forecast on the weathercasts tonight at 5, 6, and 10 PM.
Then, onto Christmas. The latest GFS model does have a few inches of snow forecast during the day on Christmas. Just in case you are dreaming of a white Christmas. So, we have two chances to have snow on the ground on Christmas. One, this first storm, and two, the Christmas storm. Keep dreaming. Our dreams may just come through this year.
I am on vacation beginning Friday. Jeremy Nelson and Brett Anthony will keep you updated on these developments. We, as a weather team, will stay very consistent and in contact with each other as these storm systems come through. I may take a break from the blog, as I have been consumed by it in recent weeks.
Have a great day! Watch our newscasts tonight for Kansas City's most accurate forecast!
Gary
