As we head into the last few days before Christmas how is the Christmas forecast looking? We recently wrote a post about Christmas snow probabilities, and an official White Christmas is very rare in the Valley. But it's not as rare to see the occasional flurries or wintry mix. (See previous post on Christmas probabilities for more details. )
That's all based on history, as our forecast becomes more fine tuned the closer we get towards Christmas let's take a look at what could actually happen.
For about two weeks now it's been looking like a cold Christmas Eve and Christmas day, but a dry one. But recently, and I mean the last day or two, some forecast model data has been suggesting a weak low pressure system developing to our south that could drop a quick batch of precip on or close to the area. This is why forecast are so unreliable two weeks out because things can change!
At this point let's say it's not impossible! The cold air hasn't changed but will it be cold enough for snow? This is the newest run of the GFS model, which is one of two of the most dependable forecast models. For our area and the last several storms this model has done extremely well and out preformed the European model. Although it is the 18z run which is kind of an "in between run", and it's not given as much credibility as the main runs of the day. (0z and 12z)
The GFS suggests enough cold air could be in place especially in the northern Valley to see some rain changing into snow Christmas Eve night.
This image is for 7pm Christmas Eve.
The blue line is the freezing level. Anything above this would roughly suggest an area for possible snow.
All of this will heavily depend on cold air aloft, or in the upper levels of the atmosphere. If the cold air isn't in place, then we could see that rainy/sleety mix instead with temperatures border line close to freezing.
Pretty much every thing else suggests a very cold rain. Not only do we have to factor in temperatures cooling, but we also have to factor in rain as it cools when it falls. That's called Evaporative Cooling. As precipitation falls, some of it evaporates in the air and this is a cooling process. We have to take into account how much cooling will possibly take place. Will it be enough cooling to cool the air closer to freezing quickly?
Here is another look at a forecast model probability. On the left is probability of rain.
The right side is probability of snow. This also doesn't take into account evaporative cooling. Which with our temperatures Christmas Eve, it's going to be close. This is for about 1am in the morning so it's also slightly later than the previous model showed. I have our area circled. On the left it shows a better chance of the precip falling as rain. But, as shown on the right it's not ruling out snow with a 20-30% chance, and even slightly better in the Highlands.
So, the moral of this blog is, I wouldn't count on a true White Christmas, BUT, it's not completely out of the question to see some form of a wintry mix Christmas Eve, which is personally my favorite part of Christmas anyways, that's when Santa comes!
I think there is also a decent chance of seeing some snow if you live in the Highlands or even into the northern Valley. We will be tracking this developing system. And as with any winter system, a degree or two makes a huge difference and can add to the difficulty of winter weather forecasting.
Merry Christmas and Happy Holidays! Please be safe in all your travels no matter the weather.