A significant swath of Tennessee — including Nashville and Knoxville and the Cumberland Plateau in between — has an “enhanced” risk for severe weather on Saturday, according to the National Weather Service’s Storm Prediction Center.
That map essentially means that there is a 30% chance of severe weather within 25 miles of any particular point within that risk zone on Saturday.
Not much has changed in the overall thinking in the last 24 hours. It still appears that large hail and damaging winds are the primary threats, but there’s also a tornado threat that will be present, as well.
Just how high are those threats? The National Weather Service’s Nashville forecast office produced this handy little graphic:
As you can see, the risk for damaging winds is certainly there, according to NWS-Nashville, but the hail risk is almost off the chart. With steep lapse rates showing up on virtually every major weather model, there’s a very good bet that some folks who wind up under the storm cells are going to see hail tomorrow. With any luck, it’ll be the pea-sized hail that makes a racket but not much noise. But golfball-sized or larger hail — the kind that is an insurance adjustor’s nightmare — is certainly a possibility.
Here is the scenario that is expected to unfold across the region on Saturday: Rain with perhaps a few embedded thunderstorms is expected to overtake much of Tennessee as we get into tomorrow morning. Meteorologists aren’t ruling out the threat of severe weather with this morning activity, but the chances are relatively low comparative to later in the day. As the day progresses, most models show the precipitation tapering off and even some sunshine. Don’t be fooled. The weather might seem nice, but a break in the rain, sunshine and warming temperatures are actually a bad thing if you dislike severe weather. With the warming of the day, the atmosphere will become destabilized as the cold front begins to move closer and wind profiles begin to become more favorable.
By tomorrow afternoon, it is anticipated that supercell thunderstorm structures will be developing across Middle Tennessee and pushing towards East Tennessee. It is with those storms that the threat of large hail, damaging winds and tornadoes will be highest. Later in the evening, as the cold front gets closer, a more congealed line of thunderstorms will approach. By that time, there will still be a threat of damaging winds and even large hail, but the tornado threat will be somewhat diminished.
Here’s what the Storm Prediction Center has to say about the setup:
Supercells will likely be the preferred storm mode and pose a risk for large to very large hail and damaging wind gusts. Relatively backed near-surface winds along the warm front will enlarge hodographs and potentially lead to a heightened threat for supercell tornadoes — a few of which could be strong — with any robust storm tracking invof the boundary.
The potential exists for strong and severe thunderstorms to develop Saturday afternoon and evening. Primary threats look to be large hail, damaging winds, and isolated tornadoes. The timeframe for severe potential looks to be from around noon until 8 p.m. or so.
Exact evolution of storm development is still uncertain, but current thinking is isolated-scattered supercell development along the warm front early afternoon across the plateau, then moving into the central/southern valley. Isolated tornadoes are possible.
Models continue to show a MCS developing across Kentucky, then moving east-southeast into the region following the warm front. There could be a squall line associated with the MCS. Forecast soundings show strong indications of strong damaging high winds. Good mid-level dry air enhancing downbursts strength and low-level jet of 40-50 KTS, produces derecho composite parameters in the very favorable range of 2 to 4.
Just a couple of notes about NWS-Morristown’s discussion: An MCS is a complex of thunderstorms; a derecho is essentially an MCS on steroids, and very often carries a high risk of severe damaging winds.
So is severe weather a guarantee tomorrow? Hardly. There are several potential flies in the ointment, though there’s no way for meteorologists to know what role they’ll play until we actually get into the day tomorrow. If we see more convective-driven rain tomorrow ahead of the warm front, it could help work the atmosphere over and keep things stabilized. Persistent thunderstorms along the Gulf Coast could help rob the moisture inflow. And most models show a capped atmosphere — that is, a thin layer of cool air aloft that helps suppress thunderstorm development. However, that cap is relatively minor and can be eroded, especially if the sun pops out and surface temperatures soar.
The best bet is to keep an eye (or ear) to your favorite weather forecast source on Saturday.