The heat and humidity will both increase on Friday.
High temperatures will top out in the mid 80s Friday afternoon:
Dew points will climb into the 60s, which isn’t where we want to be on the Muggy Meter:
The humidity will be the fuel for the potential of strong to severe thunderstorms along and north of I-40 Friday evening and overnight. This is the part of the forecast that’s going to drive me insane for the next 36 hours, because there are so many uncertainties.
WHAT WE KNOW: The atmosphere will be very unstable, and there will be enough wind energy overhead to make all types of severe weather possible. Any storms that develop are likely to become severe.
WHAT WE DON’T KNOW: Will storms actually develop? We won’t get severe storms if there aren’t any storms in the first place!
So let’s unpack that a bit further…unfortunately, the forecast model data is all over the place — some keep us dry, some develop widespread thunderstorms. The most-useful tool in a situation like this is an ensemble model: a model “collection” that represents the output of multiple individual models. The Storm Prediction Center’s Short-Range Ensemble Forecast (SREF) is exactly that, and it’s showing a 90% chance that the atmosphere will be “ready to go” if any storms develop (technically, this is the probability of the Supercell Composite Parameter being over 6 Friday evening):
But here’s the good news…the SREF is only showing a 20-30% chance of storms firing up to take advantage of that unstable environment:
So we’ll have all the ingredients for severe weather in place, but without much of a trigger to get it started. The SPC has outlined an “Enhanced Risk” (level 3 of 5) of severe weather that includes our northwestern counties, roughly coinciding with the area where storms are more likely. The “Slight Risk” around Nashville indicates the conditional nature of the threat (i.e. lots of “ifs”):
You should still plan on staying weather-aware tomorrow evening — I’m certainly hoping the lid stays on the atmosphere overhead, but it’s a volatile-enough setup to warrant your attention.
The rain should be gone by Saturday morning and the start of the marathon in Nashville, but that’s yet another set of ifs and buts and questions and headaches. Some of the evening/overnight storms could hold together into early Saturday morning — most of the computer models don’t think so right now, but I’m nervous about it. If you’re running or spectating, plan on some rain to at least be in the area to start the race…hopefully we can drop that from the forecast as things come into better focus:
This is the RPM model’s radar simulation for Friday evening through Saturday morning, but keep in mind that this is ONE version of ONE model. The pattern shows why I’m nervous both about our severe weather potential to the northwest of Nashville, AND about the marathon forecast in Nashville itself:
At this point, that’s literally the only model showing rain lingering for the start of the marathon. There’s a LOT more data we’ll be sifting through over the next few days, so stay tuned for updates.
What I’m more confident about for Saturday is that it will be very warm and steamy throughout the day, with highs reaching the upper 80s:
More strong to severe storms will be possible late Sunday. The level of that threat will depend on the timing — afternoon and early evening are bad, late evening and overnight would better…not great, but better. We’ll get a better look at some data for that storm system beginning tomorrow.
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