The Storm Prediction Center updated the risk for severe weather to “Moderate” for much of Middle TN, with the “Slight” risk still over the rest of the mid-state. A Moderate risk means they are more confident in severe weather possibilities.
Although the greatest risk is for damaging winds (SPC says 10% chance 75 mph winds, 45% chance 58 mph winds), there is still a chance for isolated tornadoes to develop.
The rough ETA for Nashville is 6-9pm, 4-7 near Tennessee River, and 8-11pm for eastern counties.
Our dew points have crept back up into the mid to upper 50s, and should reach the 60s later today and this evening ahead of the storms.
This is one good indicator of returning moisture.
Another factor will be how warm our temps get today. Temps rose through the 60s overnight, and should be in the 70s today. Sunshine will be a factor, how many breaks we get in the clouds today will contribute to warmth.
We have been saying that this is a “Low CAPE”/High Shear” situation, but the CAPE (indicator of instability) is creeping upwards in the model forecasts with 750-1000 j/kg forecasted by GFS and NAM. We might want to call it a “Moderate CAPE/High Shear” situation now, which increases severe storm chances.
The 3 km Helicity is forecasted to be 350-400 m2/s2 by the NAM and 350-500 m2/s2 by the GFS.
500 mb bulk shear, 55-65 kts by the NAM and 55 kts GFS.
Our friends at our local National Weather Service like to see Helicity values greater than 150-200 m2/s2 and 500 mb Bulk Shear greater than 35 kts. We have both.
Although we're on the edge with CAPE, they feel that any CAPE in the cool season (when jets are very active) is important.