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I’ve been holding off on blogging about this, but since I’m heading out of town tomorrow, I feel I must mention it today. There’s actually a decent chance of some measurable snowfall on Christmas day this year not only in Nashville, but in most of TN, and even some states south of TN. Here are the official statistics on a white Christmas in Nashville, from the National Weather Service:

THERE HAVE BEEN ONLY NINE CHRISTMASES WITH MEASURABLE AMOUNTS OF SNOW SINCE SNOWFALL RECORD KEEPING BEGAN IN NASHVILLE BACK IN THE WINTER OF 1884 AND 1885. THE LAST TIME MEASURABLE SNOW FELL ON CHRISTMAS DAY IN NASHVILLE WAS IN 1993 WHEN THREE TENTHS OF AN INCH WAS MEASURED. TRACE AMOUNTS OF SNOW FELL ON CHRISTMAS IN 2002. STATISTICALLY THERE IS ONLY A 7 PERCENT CHANCE OF MEASURABLE SNOW ON ANY GIVEN CHRISTMAS. THE MOST SNOW EVER TO FALL ON CHRISTMAS IN NASHVILLE IS 2.7 INCHES IN 1969.

The storm system that has been contributing to the unusual rainfall in southern California will be sliding across the southwestern states today and tomorrow, and joining with an upper level disturbance from the north. The upper level disturbance will be providing the cold, while the low pressure system to the south will be providing the moisture.  As the two combine, moisture will be spread north of the low pressure system over the gulf coast on southerly winds, which will bring enough warm air to change the precip to rain on Christmas eve day, but that night the cold air from the upper disturbance will take over and switch it back to snow, which should continue into at least Christmas morning. From what I’ve read, the models are in fair agreement on the timing of the whole thing, but not on the amount of moisture (read: depth of snow) involved. So we’ll have to wait until probably this time tomorrow, or even tomorrow night to really get a good grip on snow amounts. Even then, models can only go so far. I wouldn’t get my hopes up for any “deep” snow though, I’d be very surprised if Nashville got more than 2 inches. I will be with my family in northeast TN, so this will all happen about 6 hours later than Nashville, but naturally there will probably be a little more snow there, and I have every intention of taking a trip to the mountains where there’ll be a lot more snow. Maybe even go skiing.

I may find something to blog about over the weekend, but this will probably be my last post until next week. So Happy Holidays!

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Winter weather in the south

December 12, 2008

So I’m sure if you’re from the Nashville area you know how ridiculous people around here can be when even a flurry of snow is mentioned in the weather forecast. If you’ve been paying much attention in the past couple of days, you’ll also know that Mississippi (yes, Mississippi!) got a pretty significant snowfall recently…. even 2-4 inches in some areas across central Miss. That’s absolutely unheard of in the south! If you know me, you’ll also know that I’m a weather nerd- I follow the forecasts and even chase storms when the opportunity arises. Often times I hear people talking about how crazy winter weather is in Nashville, and how we never get any snow (some people like it that way, some don’t), and how the forecast is never right. I thought this would be a good time to do a little post explaining a few things about why winter weather can be such a roller-coaster sometimes, and why it’s so difficult to accurately forecast winter weather in the south.

Basically, the location of TN, especially its latitude, causes it to always be influenced by air masses created in other areas. During the winter, the jet stream is essentially split into two branches, a southern branch that resides over northern Mexico and the Gulf, and the northern branch that resides over the northern US states and Canada. (Sometimes it can actually split into 3 branches, but for simplicity’s sake let’s leave it at 2.) TN’s latitude causes it to be right in the zone where those two branches can sometimes come together, and with them comes air masses created in their respective areas. The southern branch can sometimes bring northward a warm, moist airmass from the Gulf, and the northern branch can sometimes bring southward cold, dry airmasses created over Cananda. In order to have snow you have to have A) cold air- freezing or below- and B) moisture/clouds. Because of our location relative to the track of winter storms, we rarely ever see moisture and cold air at the same time. Most often we see the moisture in the form of rain as the system/front approaches, and then the cold air comes in after the system has passed, and taken the moisture away with it. I’ve made some diagrams to help illustrate what I’m talking about. To understand these you need to know a few basic things about weather- high pressure (blue H)=calm, clear weather and has clockwise circulation around it, and low pressure (red L)=cloudy, rainy/snowy weather and counter-clockwise circulation. The blue line with teeth is a cold front, which means that to the west (left) of it is a cold airmass advancing eastward. The red line with round humps is a warm front, which means that to the south of it is warm air advancing northward. In this first diagram I’ve made, you will see the typical scenario for a winter storm in the south. You can see that the rain is in the moist sector to the south and east of the low pressure system. The cold doesn’t make it to those areas because it’s being pulled down from the west of the system, behind the cold front. Because that cold air mass originated over land (Canada), it’s fairly dry, and if you’re to the south of the low, once the cold front passes the moisture is gone. It’s only in that northwest quadrant of the system that moisture gets pulled around and mixed with the cold air, creating snow. Most of these storm systems track a little too far north or south (this diagram has it going too north) and there either isn’t enough cold air in place over the northern plains, or the warm moist air simply rides up over the cold air (we’ll talk about that in a minute). Click on the image to show the full size diagram.
winterweathermap-usualstorm

So what happens when the warm air is pulled up and then rides over the cold air? Ice. Normally this happens right along the warm front, in the northeast quadrant of the storm. The warm air is lighter and rises (we all learned that in elementary school) and creates a wedge. The precipitation starts as snow way up in the clouds, then melts when it gets to that layer of warmer air, then the rain drops re-freeze when they get closer to the ground where the colder air sits. This is called sleet. If the layer of colder air is really shallow, the rain may not freeze until it touches a surface. It will then freeze and form a glaze of ice on everything. This is called freezing rain. Another diagram to illustrate: (click to enlarge)
icestorm

Finally, this last diagram will show you what the “ideal” scenario for a heavy snowfall event in Nashville/middle TN would look like. Unfortunately if you’re a snow-lover like me, this doesn’t happen very often because the low has to track in JUST the right place for it to happen. There also has to be a very strong, very cold airmass parked over the northern plains, waiting to plunge down to the southeast. The southern branch of the jetstream also must be very active and the low pressure must be very strong- strong enough that you have full 360 degree circulation (called a “closed low”). The exact track of the low makes all the difference. It has to track just to the southeast of us so that we are in that “sweet spot” of heavy snow just to the north and west of the low. The perfect Nashville snowstorm: (click to enlarge)
winterweathermap-idealstorm

Essentially this is the great “Blizzard of ’93,” except its track is shifted slightly to the west of that storm. Ok, now that we’ve all had a full nerd-gasm, hopefully you now have a better understanding of why Nashville’s winter weather can be rather fickle and difficult to forecast. Back to the normal posting tomorrow.

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