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Full breakdown on the high-impact winter storm affecting our area today

Widespread moderate rain continues to fall across the Tennessee Valley early this Sunday morning (as of the time I am typing this post). However, the rain/snow transition is steadily creeping south and east toward our area. Our own Live StormTrack Doppler Radar here in Lawrenceburg shows some bright banding features in the radar reflectivity data from northern Wayne through Lewis Counties of Tennessee and then over Lauderdale County, Alabama. When looking at the dual-pol correlation coefficient product and seeing that these same areas also have low correlation coefficient values (meaning that the sizes and shapes of the objects in the radar beam vary a good bit), that tells me that either some sleet is mixing with the rain, or even more possible that there may be wet snowflakes melting at the height of the radar beam in those areas (800-2500 ft). This jives pretty well with what layer of the atmosphere above the ground the latest observational data suggests the freezing/melting layer is located. As colder air deepens and the precipitation gets heavier over the next few hours, that colder air aloft will get brought down toward the ground by dynamic cooling, and the rain will mix with and change over to snow.

Current temperatures across our area are in the lower 40s over north Mississippi and north Alabama but are in the upper 30s and falling over southern middle Tennessee. It should be noted that they are also falling a slight touch faster here than several of the models had suggested and are also running a couple to few degrees cooler than several models had suggested for this time. Still, we are all several degrees above freezing as of the 3:00 AM hour, and that means that road conditions across our area are just wet. We do notice that the Baron Road Temperatures product is showing current road temps as low as the low to mid 30s. This may be running a couple of degrees too cool, but it's quite believable that the pavement temperatures may already be running a little cooler than the air temperatures. That's because as these gusty winds blow across the wet pavement, evaporation occurs. Not enough to actually dry the pavement because of the ongoing rain, but water molecules are still being evaporated to some degree. The evaporational process is a cooling one. There is also the fact that this rain is falling from a layer of sub-freezing air just above our heads, and the actual temperature of that rain itself is likely colder than the air temperature here at the ground level, and this would cause the pavement to cool faster. We are seeing verification of these ideas back west toward the Memphis metro area where, less than an hour after the changeover to snow started, that snow started to accumulate on parking lot pavement surfaces, despite air temperatures still 33-35 degrees at the time. It will be highly important to remember all that going forward today. This WILL be a situation where you can and will get accumulating and impactful snow with an air temperature above 32 degrees!

As of the 3:00 AM hour, all of our Tennessee coverage counties and back through north Mississippi remain under a WINTER STORM WARNING, and our north Alabama counties remain under a WINTER WEATHER ADVISORY. However, the latest trends in radar data, reports to the west and northwest, as well as the short-term high-res models that are performing closest to reality, it wouldn't shock me if parts of north Alabama are upgraded to a warning at some point.

Models are NOT the forecast, they are just TOOLS that we as forecasters have in our tool belt to use to help us make the forecast. Think of it like you are building a bookshelf. There are certain tools that you need to use in order to put the bookshelf together, but the bookshelf isn't completed until you intervene and actually do the work using the tools to put it together. It's not the screwdriver that you end up setting your books on when the job is done. It's the actual bookshelf. Models work the same way. However, now that we are close enough in that we can compare some of these high-resolution models to reality to see how they are behaving with their radar trends and temperatures versus observations, there are certain ones that we know are behaving well and we can use as a visual aid to explain the forecast. The HRRR model seems to be behaving the closest to actual observations so far, and it's not a surprise since the HRRR runs every hour and actually ingests satellite, radar, and surface conditions into it in order to help it come up with what it thinks will happen. Above is a slideshow that you can scroll through to see the timing and evolution of what it shows for today...

The HRRR has the changeover to snow across southern middle Tennessee and northwest Alabama happening from northwest to southeast between 4:00 AM and 6:00 AM. From there, it sets up a heavy band of snow across our area that goes through the morning and into the midday hours. It takes until the mid to late afternoon before that band of snow is finally shifting east of our area. In that time, there seems to be a long duration where heavy snow rates look to occur. Those heavier snow rates will help to cool temperatures, and they will also help the snow to fall so fast that it doesn't have time to melt faster than it is accumulating. This is the big deal for why this will be an impactful and accumulating snow even if temperatures are 33-35 degrees! However, we are getting increasing signals here that the heavy snow may actually be able to lower temperatures down to 31-33 degrees, and often times in these heavy snow events like this, the temperatures under that heavy snow band actually end up verifying a few degrees colder than models will show. That will significantly increase the potential for there to be impactful accumulations on surface roadways, not just bridges and overpasses.

In addition to this, the heavy wet snow will be joined by wind gusts that stay over 15-25 mph through much of the day. This may help to increase the potential for isolated, short-lived type power outages across the area because of the snow possibly sticking to tree limbs, especially evergreen trees like pines and others that still have needles or other greenery on them. The extra weight of the heavy wet snow sticking on those combined with those wind gusts may cause some limbs to break and fall onto power lines.

Our thinking for snow accumulation across the area really hasn't changed much at all. With the latest observations and comparing that to high-res model data, we have expanded the 4+" snow area and 3+" snow area deeper across northeast Mississippi where it looks like the heavy snow band is setting up. We maintain a widespread 4+" snow accumulation expectation across the vast majority of southern middle Tennessee, and as you can tell by the Futurecast Snow Totals map also attached, that lines up very well with the area the HRRR model is showing similar totals. We also maintain a 1-2+" snow expectation over north Alabama, and we have actually inched that down closer to Highway 278 based on the latest trends. However, there are increasing signals that there may be some places in north Alabama that exceed these amounts, and that's why we think it is possible that some portion of north Alabama may be upgraded to a Winter Storm Warning at some point today.

Within those areas of the heavier snow accumulations, dangerous travel conditions are likely. Snow will likely fall too fast to be overcome by melting, allowing it to eventually accumulate on bridges and overpasses and eventually surface roads. Also, the heavy snow rates themselves will likely reduce visibility very significantly. It is very possible that some roads may become impassable for a time after this really gets going later this morning. Even over north Alabama, south of where we have the heaviest snow outlined, significant travel impacts are likely from accumulating snow leading to slush and slick places on bridges, overpasses, and eventually surface roads. Then, as temperatures drop below freezing and then into the 20s late today and overnight, whatever is on the roads... whether it is liquid or otherwise... will freeze, causing widespread black ice issues. It is very likely that these travel issues last into at least Monday morning, and if it stays cloudy most of Monday so that we don't get good sunshine on the roadways, those areas that get the heaviest snow may not fully clear until Tuesday when temperatures areawide climb well into the 40s with widespread sunshine.

This is a high-impact winter storm scenario for our area, with the likelihood of significant snow accumulations and widespread travel impacts. Once the winter storm starts to evolve across our area this morning, we will have live updates through the day on all social media platforms we have and our 24/7 digital weather channel with all the latest winter weather information.

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