Snow-way! Snow chances back for a 3rd time!

HOUSTON - "It's a small chance but we could see a dusting of snow on Sunday night into Monday." Many in Houston thought that to be the joke of the day -- until it happened. The day was December 7th.

"It's hard to believe but the chance of snow is looking more likely by Tuesday." The roaring guffaws across a skeptical Houston were silenced by hundreds of reports of dancing flurries that night. That was Tuesday, January 2nd.

"For the first time in 45 years and only the second time on record, we could see a third snowfall in Houston in the same season." Do you believe me, now?

It really is hard to believe. After all, we're a city known for stifling humidity and suffocating heat. For it to snow once in Houston is a miracle. For it to happen twice in the same season prompts laughs and eyerolls. For it to happen three times in the same season is historical. 

We might make history on Tuesday.

There are many more questions than answers currently and whatever happens at the surface is governed by whatever happens above our heads in the atmosphere.

For the last few days, the models have been painting a wintry scenario across Houston with the greatest chance for areas well north of downtown. As with most winter storms in Houston, they almost  always offer up a surprise or three. Therefore, it's worth watching.

MODELS

The models as of Saturday morning have been trending down with the amount of frozen precipitation we could see around the area. On Friday, the gold standard European model was painting quite a vigorous winter storm around here with heavy snow and ice (as seen in the top image with a large swath of dark blue indicating heavy snow). Saturday morning's European model run in the second picture shows significantly less with some light pink showing up indicating some sleet. 

The GFS (American model) has consistently shown a cold rain for Houston with most of the frozen precipitation north of metro Houston. You can see the last two GFS model runs below. Green is rain, pink and orange are sleet and ice and blue is snow.

For what it's worth, the Canadian model paints the rain transitioning to all snow by Tuesday evening here in Houston but the Canadian model is the class clown of the models -- the drunk everybody wishes would just go away. So there's that.

Models will continue to waffle back and forth over the next few days. It's important to not focus on the specifics of the models but rather the overall broad picture. I can say 4 days out that there's a chance of frozen precipitation on Tuesday but I can't tell you much more than that. There's an almost equal chance nothing happens at all but a cold rain.

The Atmosphere Above Us

In order for it to snow, the temperature does NOT have to be below freezing at the surface. Cold, yes, but not freezing. However the air just above your head has to be. 

In the image above is what we call a Skew-T, Log-P diagram (probably best seen on a desk top. The picture may be cut off on the mobile version of KHOU.com). It's a 3D picture of the atmosphere as you ascend. The red line is temperature with height. The green line is the dew point with height. That diagonal yellow line is the freeze line. 

What's important here is the placement of the green and red line in relation to the yellow line (the freeze line). In order for it to snow, you would want to see the red and green lines completely to the left of the yellow line, never to cross it. If you cross it, that means the temperature goes above freezing and the snowflakes melt.

What you see in the picture above is the temperature and dew point criss-crossing the yellow line not once but twice. Therefore as the snowflakes fall above our heads, they would encounter an area of above freezing temperatures that would melt the dendrites (snowflakes). However at around 5,000 feet, the air drops below freezing again and the partially melted snowflakes refreeze to become sleet. Then just before it hits the ground, the temperatures go above freezing again (goes right of the yellow line) before finally hitting the ground.

So what's all that mean? It means that it may be difficult to get an all-snow event here in Houston but a mix is certainly possible. 

Normally, I would look for an opportunity to cool the atmosphere through a process called wet-bulbing -- where rain falling through a drier layer of air would evaporate therefore cooling the atmosphere (think of getting out of a swimming pool on a hot day. The water evaporates off your skin so you get cold). Unfortunately in order to wet-bulb, you'd want the red and green line to be separated towards the bottom of the chart. As you can see, they're almost on top of one another which means the air is saturated and almost no chance of evaporative cooling.

YEAH, YEAH -- IS IT GOING TO SNOW OR NOT?

I suspect for areas well north of Houston, maybe even well north of Conroe, I'd say there's a pretty good chance of snow -- and not just snow but maybe some accumulations as well.

Here in the city of Houston, we're borderline. I can't say yes or no right now because the Skew-T diagram above (the cross section of the atmosphere) is based completely off model data which itself can be flawed four days out.

Keep a close watch of the forecast. It's happened twice this season and it just might happen again on Tuesday. There will be many changes to the forecast regarding the type of precipitation and how much we could get over the next few days.

Believe me, the minute I think it's definitely going to snow, I'll be shouting it from every roof top. I'm not screaming yet but I do have my ladder out.

 

 

 

 

 

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