A line of storms will move through the Midstate late this evening and tonight, with an outside chance of damaging straight-line winds. The HRRR model’s radar simulation shows the approaching storms this evening:
The exact path of that storm complex is tough to pin down, but I think the best chance of stronger storms will take shape south of I-40. The strongest storms will move quickly west-to-east into the wee hours of Friday morning:
The Storm Prediction Center has included us in a “Marginal Risk” (level 1 of 5) for severe thunderstorms tonight:
That seems right to me, although I wouldn’t be surprised if the “Slight Risk” (level 2 of 5) is extended into Middle Tennessee south of I-40 and west of I-65. In general, that means a possibility of damaging winds, but no higher than that since the humidity doesn’t look like it will be high enough to support a significant threat. That said, the SPC’s short-range ensemble forecast model is showing a 70+% chance of storms with severe characteristics around midnight (and the HRRR model has been trending in that direction too), so we’ll keep a close eye on things:
Cold this weekend, with a rain/snow mix on Saturday changing to snow Saturday evening and overnight. Before we get into an early attempt at pinning down some details, let me shout a disclaimer: THE FORECAST WILL CHANGE. With winter storms (especially those in late winter/early spring), the dividing line between “all rain” and “oh my, that’s a lot of snow” is razor-thin. My current thinking is that we’ll start the day dry on Saturday (it will be cold, but bundle up and get your outdoor stuff done). Cold rains showers move in Saturday afternoon, mixing with and changing over to snow from north to south throughout the late afternoon and evening on Saturday. The latest run of the RPM model is in pretty good agreement with that scenario:
That doesn’t mean it’s locked-in, though…the European model shows the heaviest snow in a stripe along I-40:
And the American GFS model shows a more-widespread area of measurable snow:
The way things look to me, the best chance of seeing measurable accumulation will be along and north of I-40 — for an early estimate of some numbers, let’s go back to the RPM model, which estimates less than 1″ along I-40, but up to 4″ in the far northern reaches of our counties in southern Kentucky:
But the amounts are still really up in the air…for an example of that, let’s get nerdy. This is the SREF “plume” of total snow accumulation for Nashville:
That chart shows the total snowfall amounts of the 26 different members of the short-range ensemble forecast (SREF) model. A “high-confidence” forecast would show all or most of the lines in close proximity to one another…instead we have some members (6 or 7, in fact) of the ensemble saying Nashville will get NO snow, one that says Nashville will get 6″, and the others scattered in between. The average of all 26 is around 2″ for Nashville, but if you take the average of trash you still get trash. An additional complication: the ground will be wet and relatively warm, two factors that decrease potential snow accumulations. I think the SREF map has good handle on the pattern, even if the amounts are still fluctuating:
For now, I’m more-or-less buying what the RPM is selling above…but we’ve still got 48 hours to go before this system starts doing its thing around here, and that’s an eternity in the forecasting world. That’s why I’m keeping the Panic-ometer on a modest setting:
The trickiest travel time would likely be from sunset Saturday through early Sunday morning. If you choose to disregard all of what you’ve just read, keep these two things in mind: 1) Stay plugged into the forecast over the next 48 hours as things evolve, and 2) The weekend looks gross. Not ideal for golfing, a trip to the lake, soccer/baseball practice, or finally taking down your Christmas decorations.
Temperatures Sunday should be warm enough to melt whatever manages to add up, but a re-freeze Sunday night will bear watching:
Yet another rain/snow mix chance late Monday into early Tuesday, but let’s get through the weekend before we worry about that.
No nerd-links today or tomorrow — I’m doing double-duty on weather and traffic…as if the weather pattern isn’t active enough to keep me busy!