LAST UPDATED: 10:49 AM EDT (1459Z)
Today's forecast is a bit of step down from yesterday. There's still a wind/rain threat later this afternoon, but there's a bit less energy for storms to tap into.
Storms should develop in Western KY and areas further West in response to daytime heating; those storms will likely develop into a system with torrential rains and the potential for strong straight-line winds. Any isolated thunderstorms also pose a threat for large hail, which is one of the SPC's primary concerns for today:
Honestly, I think it's a rather hit-or-miss situation. Any areas that do feel a storm will certainly not like it; the grounds are totally soaked through much of north-central Kentucky, so I don't think anyone will appreciate the immediate flooding any rains will bring. But I don't see a major organized severe weather threat or even one that rivals the potential from yesterday. We'll see how that changes with the next set of model runs before the storm.
Expect storms sometime after 3PM, but before dark.
Update 9 - 8:16 PM EDT (0016Z+1)
Overnight Severe Weather
Round 2 should be impinging on the Louisville area around midnight this evening, although timing could vary by +/- an hour.
Over the next few hours, there's a chance for scattered thunderstorms. A few of these could be severe, especially any that leapfrog ahead of the next developing mesoscale convective system. Such a system is already beginning to brew in the form of discrete, severe thunderstorm cells south of Chicago -
Over the next few hours, some of these storms will congeal into another squall line, which will propagate towards Louisville. The Storm Prediction Center anticipates extending a Tornado Watch to cover much of south-central Indiana through the overnight hours; all severe weather threats are still in play, but any discrete cell that pops up before the main MCS could rotate and drop large hail or a quick tornado. Thus, there is still a small albeit not zero tornado threat for Louisville if any one of these cells persists.
There exists the potential for more damaging straight-line winds capable of knocking over trees and power lines. This is particularly true for those areas which were really thoroughly soaked through by the afternoon event. Hail and an isolated tornado isn't out of the question. However, the persistent danger will be Flash Flooding, which is all the more dangerous when driving after sun down.
I'll probably offer one or two more updates before I leave MIT to catch the last subway this evening.
UPDATE 8 - 3:25 PM EDT (0025Z)
A second round could be beginning for the Western half of Jefferson County as the squall line has back-built somewhat in the presence of a confluence of several boundaries. Flash Flood Warnings are still in effect for the county.
UPDATE 7 - 2:57 PM EDT (1857Z)
NWS has extended the SEVERE THUNDERSTORM WARNING through 3:30 PM EDT. Strongest winds are around Hurstborne and eastern parts of Jefferson County. Should be finished swinging through the area in another 45 minutes.
I'll update with some damage reports later, and then the forecast for this evening's events around 8PM after my softball match.
UPDATE 6 - 2:41 PM EDT (1841Z)
Strong winds moving into Downtown Louisville, Highlands, etc. Still potential for localized wind gusts in the ~70mph range, but haven't seen too many ground-truth reports just yet.
Picture from the dad, looking due North across I-64 from Bluegrass Parkway in industrial park north of Jeffersontown:
UPDATE 5 - 2:22 PM EDT (1822Z)
WAVE3-TV reporting wind gusts of ~55mph in Oldham County; power outages in Prospect.
UPDATE 4 - 2:17 PM EDT (1817Z)
A SEVERE THUNDERSTORM WARNING is in effect for Jefferson County (KY) until 3:00 PM EDT.
UPDATE 3 - 2:03 PM EDT (1803Z)
Lost of storm reports already coming in from central Indiana; Flash Flood Warning in effect for Louisville, east of I-65 as storms move towards the Ohio River:
Worst of storms should be east of I-65.
UPDATE 2 - 1:43 PM EDT (1743Z)
See Forecast Discussion below.
UPDATE 1 - 1:04 PM EDT (1704Z)
Two-three waves of severe weather will impact the Louisville area this afternoon and late in the evening. The first wave is currently impacting Indianapolis, and should arrive in Louisville around 2-3 PM (18-19Z). The second wave would occur much later, after dark - probably not before 11 PM or midnight, and has the potential to be more significant than this afternoon's event. From each wave, the primary threats are straight-line damaging winds (expect localized winds of 70-80 mph) and flash flooding (a 100% given, considering this weekend's weather). There is a small threat for isolated, short-lived tornadoes, either within the squall in itself as it goes through local fluctuations in strength, but especially moreso from any discrete cells that fire up ahead of the complexes and can tap into the strong instability over much of southern Indiana and central Kentucky. A third event (albeit weaker) is expected tomorrow in the early afternoon.
For those of you who keep score, the SPC just re-issued a Moderate categorical outlook for N Central Kentucky.
Afternoon squall line / derecho
Currently, a squall line associated with a much larger-scale circulation (a mesoscale convective vortex) around Lake Michigan is advancing southward through central Indiana. At 16Z it shows up pretty dramatically on visible satellite imagery:
As the squall line advances and threatens the Louisville area, the SPC will shortly issue a Severe Thunderstorm Watch, with the main weather impacts around 2-3 PM EDT (18-19Z). A squall line like this will bring significant rain and winds; where uncertainty lies is in the precise character of the squall line. The [Louisville NWS office offers two scenarios, which I've condensed below:]
The main player for today`s evolution will be current complex... we [could be] looking at a full blown Derecho event as we would have all day to destabilize and it would arrive near peak heating... Widespread damaging winds would be a certainty along with Flash Flooding... [i]solated instances of straight-line winds over 80 mph would be possible along with isolated tornadoes.
The second scenario involves a few complexes/bowing structures developing ahead of the main [squall line]... [i]f this occurs, onset of severe weather/flooding will occur by midday to mid afternoon.. [w]e will still be primed for large bowing segments that will yield damaging winds and flash flooding. Given several rounds of convection, Flash Flooding would be the bigger concern along with the rounds of damaging winds. Isolated tornadoes would be possible.
As of 12:42 PM EDT, severe watches are already in effect for northern parts of the Louisville news broadcast area as the squall line edges towards Bloomington:
This is really a classic midwestern Summer severe weather event, albeit one that could sustain a derecho - a special type of long-lived storm which brings sustained, damaging straight line winds. A similar event struck the Louisville area 11 years ago to the day, bringing widespread reports of wind damage.
There are two key ingredients to today's storm:
1) Widespread instability
It's currently 92F with a dewpoint of 75F at SDF, which is actually a tad lower than some other pockets in the Kentucky. That's the type of environment where you step outside and can simply feel the energy loaded in the atmosphere. Storms are a given on days like today. So what makes today particularly bad for severe weather?
2) "Ring of Fire"
Today's 250mb setup (reproduced from a high resolution model run below) tells the story:
The brightly-colored blob in the northern plains is a jet streak - an area of stronger-than-their-surroundings winds within the jet stream. Under conditions like this, the "bubble" underneath the right generally experiences stagnant, warm conditions. Moisture and momentum tend to accumulate in the exit region of the streak - over the Great Lakes, where the current squall line originated and where a mesoscale convective vortex is setting up. The accumulation of moisture, combined with the "shear" in the environment - the way winds change speed and direction as you go higher in altitude - is a potent combination to whip up strong thunderstorms. Those that grow into large, organized complexes can tap the strong winds aloft and mix them towards the surface, producing dangerous straight line wind damage.
Most importantly, in strongly-sheared environments like this, severe weather tends to "organize." Thunderstorms have life-cycles; as they mature, they produce rain in the regions where they derive their strength, effectively killing them. However, in strongly-sheared environments (like today), the rain that mature thunderstorms produce falls in different regions. The cooling producing by this rain produces boundaries which propagate, interact with the original storm, and ultimately produce a neighboring, nascent storm cell. That's actually happening right now in Southern Indiana as storms "back-build."