Wednesday, June 26, 2013

Severe Thunderstorms This Afternoon/Evening... High Wind Threat

As mentioned in this week's earlier forecast, there is the risk for severe thunderstorms this afternoon and into the evening hours. The Storm Prediction Center has continued to outline south-central Kentucky under a "Slight Risk" for severe thunderstorms this afternoon/evening. In fact the SPC increased the probability of severe storms in our area from 15% to 30% for most of the state of Kentucky. The area was also extended further south so as to include the entire region in the "Slight Risk" area. Specifically the SPC has outlined Bowling Green and Warren County with a 30% chance of severe wind, a 15% chance of large hail, and a 2% chance of a tornado (note: the tornado threat is outlined as a 5% chance just to our NW over much of western Kentucky.

Discussion: An upper-level ridge which has been in control of Kentucky's weather, bringing us scattered storms from time to time, is breaking down. What this means is that the storm track which has been over the lower Great Lakes region will be allowed to dip further south into the Ohio Valley. A mid-level shortwave or disturbance will be set to ride along this storm track right into the Ohio Valley by this afternoon. This mid-level disturbance moving through the area will be the focal point for thunderstorms today. Storms popping up in recent days, including with the disturbance on Monday, have lacked upper-level support in order for the storms to become severe. This afternoon's storms however will be in an environment with adequate upper-level winds to organize the thunderstorms and thus allowing for the storms to become strong to severe. Meanwhile instability or the potential energy available for these storms to grow will be more than sufficient, exceeding threshold values by 2000-3000 units. This will all correspond with high moisture levels today. Dewpoints will be in the low 70s making it feel very uncomfortable outside ahead of the storms. There will also be some low-level directional wind shear, but certainty not high amounts. This creates a tornado potential but keeps the potential low.

With today's thunderstorm setup (with reference from convective forecast models) we start out looking at the  ongoing convection from overnight that is in progress in the making its way across north-central Kentucky. This could provide an outflow boundary that will create the potential for some scattered thunderstorms across south-central Kentucky as early as mid-morning. Furthermore another area of storms is located near St. Louis that is also moving toward western Kentucky; this complex could also move in and spark additional thunderstorms in the mid-morning to mid-afternoon time frame. A few storms could develop in parts of western Kentucky by around the 2-5 pm time frame that may be supercellular. If these storms do develop, they will pose the highest severe threat with high winds, large hail and even tornadoes. These storms will likely begin to merge together into a line as they move toward south-central Kentucky lowering the tornado threat but increasing the high wind threat. I think the threat of supercells today remains a low risk compared to the threat of high winds associated with a squall line/bow echo later in the evening.

The data suggests that another round of storms will fire in the Illinois/Indiana region in response to the mid-level disturbance and quickly move southeast into Kentucky. The storms will likely merge into a line and thus could pose a high wind threat. At this time this seems likely with high damaging winds associated with such a line of thunderstorms. The most likely timing for this line is between 5 pm and 9 pm in Warren County. With high amounts of instability, large hail is also a possibility but morning convection may limit some instability keeping this threat lesser so as compared to the high wind threat. The tornado threat, while not zero, should remain minimal as a line or bowing segment with high winds is a more likely storm mode than tornadic supercells that are possible mainly further west earlier in the day. The risk of an squall line tornado also cannot be ruled out given low-level wind shear today. Finally with the high amounts of moisture present both at the surface and throughout the atmosphere areas will likely experience heavy downpours and some flash flooding could be a possibility. This will be especially true if thunderstorms move over some of the same areas for an extended period of time. In general there most areas can expect 0.5" - 1.0" of rainfall through the day today though with the convective nature some areas will likely receive less while other areas may pick up 1.0" +.

Final Summary:
  • A "Slight Risk" of severe thunderstorms has been outlined for our area by the Storm Prediction Center.
  • Scattered thunderstorms could impact our area even during the morning to midday hours... these should pose less of a severe threat.
  • One or more squall line(s)/bowing segments of thunderstorms are expected to move across south-central Kentucky between 5 and 9 pm.
  • The line(s) of thunderstorms will likely contain strong damaging wind gusts; some large hail is also a possibility.
  • The tornado threat is low for this afternoon but an isolated tornado cannot be ruled out especially with any supercells that manage to develop before storms merge into a squall line. The supercell potential is highest just west of our area in western Kentucky and exists earlier in the day,,, 2-5 pm.
  • Heavy rains will accompany these storms and flash flooding could occur in some locations... especially in locations where storms may move over the same areas.
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Forecaster: Ryan Difani

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