Sunday Winter Weather: Snow/Mix >>> Rain

Posted on by David

Current Radar 

Updates from this morning in red.

Wintry Mix Sunday Afternoon >>> Rain Sunday Night

Precip will move in Sunday afternoon. We are calling it “precip” because it may take many forms during the day and night, depending on the time, your exact location, and maybe even your elevation. This is life in Middle Tennessee, right on the line of stuff.

There are two main questions.

(1)  What will we see? We look to temperatures at 2,000 feet and 5,000 feet to try and figure out what the precip type will be — rain, sleet, snow, and/or freezing rain.

(2)  Will it stick? Obviously, nothing will “stick” if temps aren’t at or below freezing.

Most days, the coldest temps happen just before sunrise. The temp rises with the sun, we reach a high around mid-afternoon, then when the sun sets, it gets cold again.

Not Sunday. Sunday will be weird. We will be below freezing in the morning and afternoon, then settle just above freezing (around 34°) by mid-afternoon. The weird part is that the temp won’t fall when the sun goes down. It may just stay the same (right around 34°), but there is consensus the temp will increase a degree or two after the sun goes down — keeping us above freezing Sunday night and overnight into the wee hours of Monday morning.

Of course, the models have different ideas how this will all go down.

GFS — snow in Davidson County starts sometime just after noon Sunday. For the southern half of Williamson County, it may be freezing rain because temperatures aloft appear to be just over freezing. By mid afternoon, everyone in both counties gets a mix of rain, snow, and sleet, as warmer temps push in. Around sundown, it’s all rain in Williamson County and the west half of Davidson County, with a snow/mix in the remainder of Davidson County. By 9 PM, it’s rain for everyone.

Update:

NAM4 — just after noon, a mix of snow, sleet, and maybe some freezing rain near south Williamson County arrives from the west. By mid afternoon, it looks like snow along and north of I-40, with a wintry mix south of I-40. As the afternoon wears on, the snow slowly lifts north and out of Davidson County, and is replaced by a wintry mix of sleet, freezing rain, and, gradually, regular rain. However, almost comically, when the sun goes down, snow tries to push its way back down towards I-40, with rain south of I-40. By Sunday night, it all slowly changes over to rain.

Update:

Euro — it all starts some time after noon. The model — which does not distinguish precipitation types — thinks it will be snow during the afternoon. I am not so sure. On close inspection, the Euro has both of our counties right at freezing when the precipitation arrives. Temperatures at 2000 feet along into north of I-40 are below freezing, but at 5,000 feet temperatures in both counties are 1° or 2° above freezing. So it’s hard to say that would be snow south of I-40, but would be snow north of I-40. I imagine snow will mix in among the sleet and rain. By Sunday night, temperatures aloft and at the surface rise either at or above freezing.  This suggests a cold rain.

Update:

The latest run of the Euro delivers colder air aloft — suggesting snow — but temps above freezing at the surface when the precip starts after noon. By sunset, temps will still be increasing, changing snow to rain. This would limit accumulation, or at least keep it from causing widespread problems. Bridges and overpasses would be impacted most.

One thing we will need to account for is the potential for evaporative cooling. When the snow falls into air where the dew point is significantly lower than the temperature, the snow evaporates. When that happens, it brings the temperature down until the dew point and temperature is about the same. The models often fail to account for this process. The amount of evaporative cooling we’ll see is very difficult to determine, but it may result in more snow than was described above. But then there is the Short Range Ensemble Forecast — a blend of several models — which has temps warmer than what’s seen above.

This forecast is like trying to predict the final score of a basketball game.

My gut thinks we will see some snow after noon, changing to a mixed bag of snow and sleet/ice late afternoon, then eventually transitioning to rain later Sunday night. Expect more wintry precip the further north you go.

The official forecast is 1″ along and north of I-40 by Sunday night, before the transition to rain occurs. Those north of 40 should see more, those south, less.

This prompted NWS to issue a Winter Weather Advisory for Davidson County only:

Williamson County is outside the formal Advisory, but is subject to a Special Weather Statement:

For map lovers…

Snow:

And ice:

Impacts to the Monday morning commute should be low/minimal, but temp trends should also be closely monitored through the day Sunday. By 6 AM, temps should be around 36°, warming to 42° by noon. A steady, soaking rain will be ongoing most of Monday.

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