Wednesday, June 29, 2011

6/29 - 11am - Quiet Weather is Back!

After what seemed like an endless parade of storms and severe weather across Kentucky, Indiana, and Tennessee, calmer weather is in place and we'll enjoy the fruits of that for most of this week. High pressure centered just east of the Cincinnati area will provide nearly cloudless skies and a rain-free forecast until the weekend. Sounds more like the summertime forecast you were looking for, doesn't it?

We'll also see a break from the humidity, with dew points staying between 45-60 degrees until Saturday. A dew point of 60 is considered to be uncomfortable humidity-wise, so things should stay just below that threshold. Dew points and storm chances, more of the summer-like pop-up variety instead of the spring severe outbreak, will be increasing this weekend and into next week. July 4th looks hot with temperatures in the lower 90's and a slight thunderstorm chance in the afternoon throughout much of the region.

Monday, June 27, 2011

6/27 - 11:15am - More Severe Weather for KY and IN

Had enough storms already? We have one more round to go before we clear things out for a while around Kentucky and Indiana. A few non-severe storms have already moved through the Louisville area this morning and the storms that are still well off to the west are weakening and beginning to take a more southerly turn.

The bit of clearing we'll see this afternoon after this morning's storms pass off to the east should help to destabilize the atmosphere in preparation for development of severe storms this afternoon. If this clearing doesn't materialize and we stay more cloudy than sunny across the area, the severe threat will be much lower. These storms should fire up in the mid afternoon across southwest Indiana and Western Kentucky and impact the Louisville area this evening. While most of these will be your run-of-the-mill severe storms with 60mph winds and hail, a few could have supercell structures or bowing segments. The embedded or isolated supercell structures will be powered by the small amount of wind shear (changing wind direction and speed with height - see 5pm NAM model output left) and helicity (turning in the atmosphere) we'll have in the area as storms form. This means a couple isolated tornadoes are possible.

The bowing segments could produce wind damage as a cold pool of air collects behind some of the more linear storms. These storms will likely last well into the evening and overnight hours and be out of the Louisville area by mid morning tomorrow or earlier. Since there is the risk for wind damage and an isolated tornado or two, the Storm Prediction Center has issued a Slight Risk area for most of Kentucky, Southern Indiana, and portions of Middle Tennessee. A 5% tornado risk exists from Chicago and St. Louis to along the Ohio River, while a 2% risk extends further southward.

Saturday, June 25, 2011

6/25 - 2:45pm - Tomorrow's Entertainment: An MCS

An MCS or Mesoscale Convective System is scheduled to move through Kentucky and Indiana tomorrow morning. An MCS is basically a large cluster of storms that normally has a distinct wind-driven line of strong to severe storms on the east side as it pushes along at a decent speed. Tornadoes can sometimes form in little notches or "eddies" that develop in the strong line of storms on the east or leading side of the MCS. Given that there will be some helicity (turning in the atmosphere) from Indianapolis to Bowling Green, KY as the MCS moves through, it would not surprise me to see a Tornado Warning or two issued since there could be a little rotation aloft. This will not be a tornado event though and I expect the probability of tornadoes to remain low. This will not be an event like Wednesday when we had five tornado touchdowns in the area because we will not have supercells developing. The Louisville area can expect storms to start sometime around 7am and possibly last into the mid afternoon hours. While the initial line of storms in the morning could be strong or severe, heating during the day could intensify storms by around noon. The main threats from this complex of storms will be damaging winds (especially in the morning) and hail. The 12z NAM model run (top left) seems to have a good handle on the storms.

These storms will be driven by a mid-level shortwave trough, which is basically just a ripple or wave in the atmosphere, and a warm front rising northward. Once this warm front passes to our north tomorrow, we can expect temperatures to exceed 90 degrees again next week.

We're under a Slight Risk tomorrow from the Storm Prediction Center (left) and on the very southern edge of another one on Monday (right) since another line of storms may try to come through early in the day:

Be sure to scroll down and read my exhaustive post on the Louisville tornadoes that happened on Wednesday.

Thursday, June 23, 2011

6/23 - Expecting the Unexpected: The Louisville Tornadoes

A horse barn at Churchill Downs
Image: NWS WFO Louisville
The city of Louisville is recovering from an outbreak of tornadoes that even the most seasoned weather watchers didn't expect. As of this writing there have been four confirmed tornadoes (two EF-1's and two EF-2's) inside the Jefferson County/Louisville city limit. A fifth tornado in Harrison County, IN has been confirmed of EF-0 strength that started off the event. The iconic Churchill Downs received damage to barns, several power poles near the University of Louisville's Papa John's Cardinal Stadium were brought down, and buildings in Jeffersontown sustained damage along with massive tree damage as a result of three of the four tornado touchdowns in the county. Thankfully there were no reports of injuries or fatalities as these tornadoes ripped through a city containing three quarters of a million people.

Below is a rough map I've put together based on the NWS WFO Louisville storm survey page of the tornado tracks through Louisville . Purple tracks indicate EF-2 rated tornadoes (the indicated tornadoes only reached this strength briefly) and the red tracks indicate EF-1 rated tornadoes. Be sure to click the image for a larger view.

Timeline of Tornadoes
Information used in this section about each tornado can be found on NWS WFO Louisville's storm survey page.

Yesterday started off cloudy and even misting at times across the city as a cloud shield from overnight persisted well into the morning hours. The Storm Prediction Center had a Slight Risk for severe weather out for a good portion of the Mississippi and Ohio Valleys northward to Michigan, but this was a very conditional risk for Kentucky and Indiana because the cloud cover in the morning inhibited destabilization of the atmosphere. A cold front sagging southeastward from Indiana would be the trigger for any afternoon storms should they form as temperatures warmed. The clouds finally moved east of the Louisville area at around 2:30pm yesterday and temperatures topped 80 degrees by 3pm. By 6pm there were showers on the radar to the north of Louisville and a few specks on the radar to the west.

Just after 7:30pm a Tornado Warning was issued for Harrison County, IN and western portions of Jefferson County/Louisville because a tornado was spotted near Elizabeth, IN. The "storm" that triggered this warning was no more than a shower because it had no lightning strikes associated with it. I'll admit that I was skeptical of this tornado report because there was really no significant rotation on radar. I thought the report was an improper identification of moving scud clouds or even a gustnado. The storm seemed to weaken and spotters near the Shively area reported no funnel clouds, so the warning was allowed to expire. About ten minutes later the storm began to strengthen again in West Louisville and another Tornado Warning was issued just after 8pm for Jefferson County. Multiple reports of a tornado and associated damage began coming in from Churchill Downs around that time.
This time the storm had lightning and the visual tornadic hook echo with prominent rotation on radar, so it was clear at this point that this thing meant business. To the right is a radar image with the supercell and hook echo near Churchill Downs and below is the velocity data with the green/red couplet signifying where the rotation was at the time. Click either image to see a larger view:

This tornado strengthened briefly into an EF-2 with 120 mph winds near the intersection of Floyd and Central Avenue at the University of Louisville campus where the power poles came down. Here's a video from YouTube user kehdsh that was posted on the WAVE 3 Weather Facebook page this afternoon. It's a little far away, but it clearly shows the funnel cloud above and debris cloud on the ground near Churchill Downs. Note that a tornado does not need to have a visible funnel snaking all the way to the ground. The debris dust-up and tube near the ground verifies that a tornado is there. Watch the center of this video closely:

Bent light pole in Jeffersontown
Image: NWS WFO Louisville
The next three tornadoes to hit Louisville were actually from the same storm and you could argue that it was the same tornado that lifted and touched down three separate times. From 9:29pm to 9:41pm (a total of twelve minutes with two short breaks in between) these tornadoes pestered the Louisville incorporated city of Jeffersontown, the suburb of over 26,000 people in Jefferson
Tree on dentist office in Jeffersontown
Image: Ryan Hoke
County that I've lived on the southeastern fringe of for nearly ten years. The initial touchdown occurred in the neighborhood just behind (west of) the Meijer store on South Hurstbourne Parkway west of Jeffersontown. This is just due south of the Stonybrook Cinemas. The tornado continued eastward to downtown Jeffersontown and lifted at the St. Edward School and Church, causing EF-1 (95-100 mph winds) damage along the path. It dropped again just east of the school and went on to cause tree damage at Tully Elementary School and structural damage at an apartment complex and the Good Samaritan Center in downtown Jeffersontown. The Good Samaritan Center had EF-2 winds of 115 mph that tossed and flipped cars about 20 yards. The tornado then went on to cause significant damage at the industrial park just east of Jeffersontown before lifting again. It sat down again in the industrial park causing more warehouse damage and continued on toward the Gene Snyder Freeway (I-265) where it lifted just after damaging a bed and breakfast on Tucker Station Road. This storm wasn't as well-defined on radar as the Churchill Downs supercell because of the merger of multiple storms in the county. The embedded supercell structure with rotation was certainly there as you'll see with the red/green boundary of wind velocities north of the Jeffersontown dot in the image below:

Why it Happened
Caution: This section gets a little technical on the meteorology side of things.
I explain why I think tornadoes hit Louisville unexpectedly and in such a small area.

Nobody expected this rash of tornadoes in Louisville at all. This includes the National Weather Service, local media, and me. In my blog post yesterday morning I even stated: "The tornado risk for [Southern Indiana, Kentucky, and Tennessee] will be quite low." The Storm Prediction Center issued a Slight Risk with a 2% tornado risk area in the morning with mention of wind-driven linear storms possible across the Lower Ohio Valley. Later on in the day they issued a Mesoscale Discussion that indicated they may issue a Severe Thunderstorm Watch for a broad area from Cincinnati to Memphis if storms developed more, but they never did. Some folks are asking why they didn't issued a Tornado Watch as soon as the storms unexpectedly began producing a spotter-confirmed tornado in Central Louisville, especially when the tornado threat dragged on for roughly an additional two and a half hours. My guess is that the very small lead time they would give with the issued watch and the unknown end time for this already unexpected tornado outbreak led them to scrub any plans of issuing one. Not to mention that the watch would be extremely small given that tornadoes occurred across two counties. The National Weather Service office in Louisville did a very good job of handling all the reports and translating that information into warnings for folks in the path of these storms.

These storms formed in a highly speed-sheared environment meaning winds were very fast aloft, but much slower at the surface. The direction of the winds at the surface were forecast to be from the southwest according to most of the computer forecast models. Aloft at 18,000 ft the winds were forecast to be from about the same direction, but slightly more west. This means that there was little directional shear forecast, meaning the chances of storms rotating were slim. The setup was more favorable for damaging straight-line winds. Here's where things get interesting. At 2:43pm, surface observations showed southwest winds in Louisville as forecast. All is well, but watch that southerly wind vector (down-pointing flag) east of Evansville, IN in the top picture on the left. Just before 5pm this southerly surface wind vector is in Louisville as a south-southeast wind. This increased directional shear between the surface and 18,000 ft quite a bit. The timing also lines up with the development of small showers just to our west and north. The surface winds were from the south in Jefferson County between 5pm and just before 10pm. In between these times we had the tornado pandemonium in Louisville. The timing and location is just too perfect for these southerly winds to not be a contributing factor to this event. Low-level wind shear (the kind that produces tornadoes) would have been much higher in Jefferson County than the surrounding area with the surface wind out of the south given the winds aloft being from the southwest.
It's also interesting to note that the bulk shear output on SPC's mesoscale analysis had a 50 kt maximum right over Central Kentucky and Southern Indiana. I make no claim of being a research meteorologist, but this surface wind reasoning comes from my four years of chasing storms in the Plains and the experience I've gained from books on severe weather.

The RUC and NAM models yesterday morning did pick up on the fact that storms would indeed form in the afternoon near Louisville after the clouds cleared. The 12z RUC yesterday in particular seemed to pick out the linear nature of the storms that were to form less than twelve hours later. It was nowhere near perfect, but the timing and shape of the storms were in the ballpark. Check out the comparison of a live radar capture late last night and the aforementioned RUC reflectivity (simulated radar) output:

Finally, I wanted to point out that this all happened with less than 1500 J/kg of CAPE or instability. Usually you need a little more instability than that to get severe storms going, but I think the directional wind shear compensated for that. None of the other storms outside of the localized southerly winds around Louisville were severe. That plus the fact that we had a tornado touch down in Harrison County, IN (just west of Louisville) with little to no lightning present in the storm makes this a once-in-a-lifetime event that could have been much worse if people did not heed warnings. I'm extremely pleased that there were no reports of injuries or fatalities with these five tornadoes yesterday. Hopefully this event will be the last major population center to be hit by a tornado this year, which has seen tornadoes hit cities including Tuscaloosa, Birmingham, Joplin, MO, Oklahoma City, Springfield, MA, and now Louisville, KY.

Check out NWS WFO Louisville's storm survey page for more information on this tornado event.

Wednesday, June 22, 2011

6/22 - 10:30am - Damaging Winds and Hail Possible Today

Storms will likely develop this afternoon ahead of a slow-moving cold front in Southwestern Indiana and Western Kentucky before moving eastward toward Louisville. With a lack of heating (we're only in the lower 70's across Kentucky and Tennessee this morning) and cloud cover present over much of the area, storms will likely take their time to form.

These factors will also limit the strength of the storms this afternoon. Some storms will be severe with damaging winds, but this won't be a knock-out drag-out derecho or giant bow echo. Instead, bowing segments with winds over severe criteria may be present in the line of storms that may form. Speaking of forming, the NAM and RUC computer models are in general agreement that storms will form after 3pm. The RUC, below, even has a linear shape to the storms much like the early morning NAM run had.

Again, this won't be a big severe weather outbreak but the potential for wind damage and possibly some hail exists in Southern Indiana and Kentucky near the Ohio River today. Further south in Tennessee the risk for wind damage won't be as great due to lesser wind shear, but scattered storms will develop with some becoming severe this afternoon. The tornado risk for all of these areas will be quite low. The SPC has issued a Slight Risk for severe weather today from Michigan to Louisiana, so storms could affect a pretty large area.

Tuesday, June 21, 2011

6/21 - 12pm - First Day of Summer, Rounds of Severe Weather

A few storms are moving northeastward through Kentucky and Tennessee right now. While not severe, these storms are moving through a very unstable environment (you can see the 2500 J/kg CAPE values on the right) and could reform or strengthen as the afternoon wears on. There's a Slight Risk for severe weather today across Kentucky, Indiana, Tennessee, and more because these strengthening storms could produce wind damage and hail later on today.

Today at 1:16pm EDT summer will officially begin. This marks the summer solstice, or the day when the sun shines at its northernmost point on the earth due to the planet's tilt. Appropriately, today will feature summer-like highs across the Southeast. Here are some of the highs this afternoon for selected cities:

Louisville: 90  Lexington: 89
Cincinnati: 88  Jackson, TN: 90
Nashville: 88  Memphis: 93 
Knoxville: 93  Atlanta: 93
Birmingham: 93  Jackson, MS: 90

The heat will be breaking soon though because a cold front it scheduled to move through Kentucky and Tennessee late on Thursday. Above and behind this front is a upper-level low and trough that will trigger widespread severe storms across the Southeast on Wednesday. These storms could be supercells, which carry a tornado risk (not that big I think), and/or clusters with hail and high wind. The fast upper-level winds with the trough coupled with pretty high instability across the Lower Ohio Valley will provide a healthy environment for these storms, so be on the lookout tomorrow if you're in the Slight Risk area below:

Saturday, June 18, 2011

6/18 - 1:30am - More Storms Today in KY and IN, Some Severe

Storms are already forming very early this morning over Illinois near a stationary warm front and are forecast to move southeastward across Indiana and Kentucky later this morning and into the afternoon hours. Storms that pass through the Louisville area before lunchtime will have the potential to cause wind damage as they begin to feed off of some morning heating. This initial round of storms may stay to the west of Louisville and Lexington and instead focus more on the Evansville, IN area if the latest short-range models are correct. These precipitation outputs below are very general and rarely depict a perfectly precise forecast for where storms will set up, so take these with a big grain of salt:

NAM, RUC, and WRF precipitation forecasts for 11, 8, and 11am respectively

Computer models are also showing a blossoming of these storms in Kentucky and maybe some new ones in the afternoon as we add on more daytime heating. With wind shear coming into play (which is something that these storms today near the Ohio River didn't have to work with), we could be looking at an increased risk for severe weather both in the morning and afternoon hours. Since these storms will be linear and/or bunched together, tornadoes and severe hail should stay out of the picture for the most part. Wind will be the main threat with these storms, so you may want to secure that garbage can or any patio furniture that you may have sitting loose outside before tomorrow mid-morning if you live in Southern Indiana or Kentucky. Given the wind risk from these storms tomorrow, the Storm Prediction Center has placed most of Kentucky and Southern Indiana under a 30% Slight Risk for severe weather. Tennessee, save for extreme northeastern portions, should outside of the severe weather threat tomorrow. I'll be up bright and early tomorrow, so be sure to follow me on Twitter or Facebook for the latest updates as these storms approach the Ohio River!

A similar situation could unfold again tomorrow with severe weather (a Slight Risk has already been issued), but this time wind shear might be a bit more favorable for stronger storms that are more organized. The main risk again will be wind damage and possibly small hail from some of the stronger storms. Seems like late Spring is keeping things busy around the Lower Ohio Valley!

Thursday, June 16, 2011

6/16 - 11:30pm - Severe Risk Tomorrow in the Ohio Valley

An area of storms is expected to restrengthen south of a warm front in Illinois tomorrow morning as it moves southeastward. By the afternoon these storms should cross into the Lower Ohio Valley and affect the Louisville, Lexington, and Cincinnati areas. Some of these storms will have the capability of producing large hail and high winds. They also should remain linear or bunched together throughout the day due to a lack of wind shear, so the tornado threat will accordingly be low. Not everyone in the region will see storms though as coverage may remain low. The Storm Prediction Center's Slight Risk for severe weather stays north of the Tennessee border (which looks about right given what short range models are showing), so Indiana and Kentucky will be the target for this event. This won't be a big severe weather risk, but it's certainly something to keep an eye on tomorrow afternoon.

Storms (possibly severe) will be possible on Saturday and Sunday as well, so be ready for more storms if you're in Kentucky or Tennessee.

Wednesday, June 15, 2011

6/15 - 6:45pm - KY/IN/TN Severe Weather Update

Severe weather has fired up across Kentucky, Indiana, and Tennessee this evening. A new Severe Thunderstorm Watch has been issued that includes Louisville and Bowling Green, KY as storms that have fired to the west of these cities have gone severe. Storms north of Evansville, IN could hit the Louisville area this evening, but they will have to fight the loss of heat that they'll encounter once the sun goes down. Severe thunderstorm warnings are being issued for the big complex of storms in Middle and Southwest Tennessee, but nothing greatly impressive is happening down that way.

The Storm Prediction Center has extended today's Slight Risk area northward as the warm sector between the warm front and cold front where these storms are setting up has moved further north. I don't think we'll be dealing with a tornado problem from these storms, but some brief and benign upper-level rotation is possible. Hail, high wind, and heavy rain should be the main threats as they develop and move through Southern Indiana and Kentucky. The storm threat should subside before midnight in these areas.

6/15 - 11:15am - Heavy Rain and Severe Weather

A line of heavy rain and lightning moved through the Louisville area this morning and another short burst or two of that could come through again as we approach the noon hour. None of this has been or is expected to be severe, but there's quite a bit of cloud-to-ground lightning in these storms.

The clouds associated with these storms moving through this morning will help to limit instability for severe storm development this afternoon in Louisville and Northern Kentucky. Storms will likely still form this afternoon and evening and move through much of the Lower Ohio Valley and Tennessee, but Southern Indiana, Louisville, and Lexington will probably miss a good chunk of the severe weather from these storms. Further south near Bowling Green, Nashville and points eastward will see a better risk for severe weather this afternoon as they already have some sunlight hitting the ground to build instability.

The SPC has issued a Slight Risk of severe storms for much of Kentucky, Tennessee, parts of Mississippi, Alabama, and Georgia today. With that entails a 5% risk for tornadoes in South Central Kentucky and Middle and East Tennessee.

Monday, June 13, 2011

6/13 - 10pm - Back from Bonnaroo, Storms This Week?

I'm back from a very hot (temperature and music-wise) weekend at the Bonnaroo Music and Arts Festival in Manchester, TN. It only rained briefly on Saturday evening, but the rest of the event was hot and dry. The heat created a couple of notable dust devils at the festival, one of which shook up the crowd for a few seconds as it ripped through one of the main arteries of the event area. It picked up nothing more than small debris (paper, cardboard, etc) but it was very well-formed and unfortunately didn't last long enough to get pictures. Even though the air quality deteriorated greatly throughout the weekend as the dusty air thickened, the festival was fantastic and the weather cooperated fairly well given the time of year.

Storms are in the forecast for Kentucky and Tennessee this week as an area of low pressure moves through the Plains and Midwest. An accompanying cold front will help to provide lift for severe thunderstorm development on Tuesday for much of Tennessee and Western Kentucky, with that threat shifting slightly eastward and northward to include the Louisville area on Wednesday. A warm front extending to the east of this low pressure area will serve as the northward extent of this severe weather threat on both days. On Wednesday it should be on or near the Ohio River, so areas south of that boundary (possibly Louisville) will have to watch out for damaging winds, hail, and maybe even an isolated tornado or two. My gut tells me this will be a linear/straight-line wind event on Wednesday for Louisville due to the west-northwest winds aloft coupled with east-southeast winds at the surface, but supercells certainly aren't out of the question in places that get a more westerly wind vector aloft. Also, temperatures aloft will likely be too cold to support much of an isolated supercell threat in Kentucky, so my thinking is that the tornado threat for this whole event will remain fairly low in the northern portion of the storm area. A supercell or two would be a little more likely in Tennessee, but still remains a small risk. In any case, the Storm Prediction Center has highlighted Slight Risk areas for severe weather on Tuesday and Wednesday (pictured left). The placement of the warm front on Wednesday will be a key factor in determining exactly where severe storms will form on Wednesday, so it's something to definitely keep an eye on!

Wednesday, June 8, 2011

6/8 - 8am - Bonnaroo!

The Bonnaroo Music Festival begins Thursday in Manchester, Tennessee and I'll be there! This year's lineup features Arcade Fire, My Morning Jacket, Primus, and many more world-class bands and performers. Going to Bonnaroo? I have the latest forecast! As usual it look likes this year's festival will be hot and humid, but even more so this year because a bit of ridging in the atmosphere will build up heat to above average levels. There will be daily rain chances from summertime pop-up thunderstorms that will form during the afternoon, so be ready to get wet. These storms will be slow movers, so there could be some mud issues at the festival site should one of these develop and sit over Manchester. Here's a detailed forecast:

Thursday91 / 69         Sunny           20% T-Storms
Friday:       92 / 67         Sunny           30% T-Storms
Saturday:   91 / 68     Mostly Sunny    20% T-Storms
Sunday:     91 / 69     Mostly Sunny    30% T-Storms

The good news is that this heat may give just a little bit next week as the ridge we're under right now across the Southeast begins to break-down and move off. I think most locations in Kentucky and Tennessee will at least get below 90 for highs next week. Daily heat-induced thunderstorm chances will stick around, which is normal for this time of year. Summer may still be a couple weeks away, but the summer weather pattern is already in place!