Sunday, December 28, 2008
It's unusual to get severe thunderstorms in the Kansas City (KC) area around Christmas, but it sure did happen yesterday morning (Saturday 12/27). The setting involved a very strong upper trough that brought unseasonably warm air northward ahead of it (record temps in the 60s F in KC on Friday), and an early morning bow echo type thunderstorm feature that interacted with a quasi-stationary outflow boundary in the south KC area.
The first graphic above shows the deep mid-level trough around midnight going into early morning on 12/27, along with SPC storm reports, and a WRF forecast of low-level CAPE suggesting an unstable surface-based environment extending northward into the KC area overnight. The 2nd graphic is a radar reflectivity image showing an ENE-WSW outflow boundary in place across the south KC metro area at 4:00 am, the result of thunderstorms that had moved across to the north of KC in the 2-3 hours prior. The bow echo thunderstorm feature is also visible approaching KC from the SW. Surface temperatures south of the ENE-WSW boundary were in the low-mid 60s F, while north of this same boundary, temperatures were at least 10-15 F colder as a result of storm outflow.
The 3rd graphic is a RUC analysis profile estimating the environment just south of the ENE-WSW boundary at Olathe KS (OJC) about 40 minutes before the damaging winds hit Johnson County (the SW side of the KC metro area). Although there was not a lot of CAPE (maybe about 200 J/kg), the profile was fairly surface-based with a deep low-level moist layer and around 50 J/kg of CAPE below 3 km. Also notice how strong the southerly low-level winds were at only 2000 ft above the surface (roughly 900 mb), with sustained speeds to 50 kts (nearly 60 mph)! It wouldn't take much storm downdraft to move these winds downward to the surface, particularly in a surface-based setting like that just south of the ENE-WSW boundary. In contrast, the 4th graphic above is a RUC profile at KC International Airport (MCI), roughly 20-25 miles north of the same boundary. Notice that with colder temps north of the boundary, this profile is "elevated", with no CAPE at all from lifted parcels in the lowest 100 mb. Even though the strongest part of the radar bow echo went across northward near the airport, the damaging winds were limited to the area along and just south of the ENE-WSW boundary. That's probably because that's where the warm surface-based environment was, where it was relatively easy to move strong winds downward from not far aloft, whereas farther north the cold near-surface setting probably kept damaging downdrafts from reaching the ground.
The last graphic above is both reflectivity and velocity showing the strongest winds coming into the south KC area around 4:40 am, reaching measured speeds around 80 mph. This was south of the strongest radar echo, but along the advancing outflow southeast of the bow echo feature. Again, that's where the warm surface-based setting was (south of the ENE-WSW boundary), better for allowing damaging winds to reach the surface.
What a weird week in the KC area... sub-zero (F) temperatures early on (Sun-Mon), then temps recovering to record high levels on Friday with thunderstorms, and back to ice and snow during the day on Saturday. On Friday evening, Shawna and I found ourselves driving in a heavy thunderstorm in north KC with sharp CG lighting, then getting up to ice covered trees on Saturday morning while more storms raged to our southeast. If you live in the Plains, just wait a few minutes... the weather will probably change :-).
- Jon Davies 12/28/08
Sunday, November 23, 2008
Looking back at some photos my wife Shawna took on 6-17-08 while we were in northwest Nebraska waiting for storms to develop, I ran into these shots above that we had forgotten about. We've both seen plenty of "horseshoe vortices", but never a twin pair like this, south of Alliance. Shawna has a good eye for picking them out (I would have missed these).
There are probably several ways such vortices can form, but the most common idea is shown in the black and white explanation figure above. As a cumulus updraft builds, it may encounter rapid and strong vertical wind shear (differing wind speeds and/or directions within a short distance of height). This may cause the updraft to develop horizontal spin that, when stretched further by the small updraft, spins faster and is deformed into a "horseshoe" shape. Condensation within the cloud allows us to visually see the vortex. As dry air mixes into the cloud, it evaporates, except for the spinning horseshoe vortex, which resists dry air entrainment for a little while longer. In reality, horseshoe vortex formation is complex (otherwise they would happen all the time), but the diagram above summarizes some basics.
An estimated wind profile for Alliance is also shown above. Note the rapid and sharp wind shift from southeast to northwest at about 3000 ft above ground. This may help explain the horizontal spin that developed and stretched into "horseshoe" shapes with condensation as these cumulus updrafts on 6-17-08 encountered this sudden wind shear with height.
I don't think I've ever seen a photo of twin horseshoe vortices before. Thanks, Shawna, for catching these!
- Jon Davies 11/23/08
Tuesday, November 11, 2008
Lead forecaster at NWS DDC and storm chaser/photographer extraordinaire Mike Umscheid documented a rare Kansas November wedge tornado, in the far southwest corner of the sunflower state yesterday. Wow! Go to: http://www.underthemeso.com/blog/ to see and read more.
The setting was a cold core setup (read paper here), with a 500 mb low attempting to cut-off over eastern Colorado within a strong upper wave lifting northeast (see SPC graphics above). An area of CAPE was over southwest Kansas, just east of the surface low where boundaries intersected (a fairly classic localized setup, see surface map above). The cell Mike chased looked to be pretty close to this key boundary intersection (see radar above), with sunny skies to the south on satellite (not shown), generating a good surface heat axis pointing into the area.
The RUC analysis sounding in the Johnson/Ulysses KS area at 21 UTC (see above), about 20-25 minutes before the tornado, showed good CAPE for a cold core setup (near 700 J/kg), all bunched below roughly 400 mb for a classic small supercell profile. With plenty of really cold air aloft (-10 C at 700 mb!), dew points in the 40s F and temps in the 50s F were all that was needed. Amazing!
A potential cold core setup in South Dakota last week (11/5/08) didn't pan out for tornadoes, as there appeared to be way too many low clouds for needed surface heating and instability near the boundary intersection with that system, unlike the one in southwest Kansas yesterday.
Excellent job, Mike!
- Jon Davies 11/11/08
Tuesday, October 14, 2008
I won't be able to go to the 24th Conference on Severe Local Storms in Savannah, Georgia later this month. But I have submitted a paper for the online preprint, titled "Three Strong Tornadoes in 2008 associated with Boundary Intersections and Narrow Instability Axes near 700-mb Lows". The final version is now online HERE.
The paper focuses on 3 strong tornado events (see photos above) that were difficult to forecast in 2008:
- May 1 in northwest Iowa (F2 tornado near Rock Valley)
- May 22 in Colorado (the F3 Windsor tornado, with 1 death)
- June 6 in north-central Minnesota (F2 and F3 tornadoes near Park Rapids)
These had some features and ingredients that were similar to so-called "cold-core" events, but probably wouldn't be considered as such using a rigid definition. While the paper is not anything "earth-shattering", I hope some people find the case studies useful.
- Jon Davies (updated 10/22/08)
Sunday, October 5, 2008
The presentation focused on the setting and environment with the Beloit-Jewell-Belleville tornadic supercell in northern Kansas on 29 May 2008, and the Salina-Chapman-Manhattan supercell in central/northern Kansas on 11 June 2008. Both storms were associated with unusually large CIN for such intense tornadoes. MLCIN from lowest 100-mb mixed-layer lifted parcels was large for both events, probably between -120 and -170 J/kg, depending on the computer model used. This seems very large for significant tornadoes based on my database study from a 2004 paper in Weather and Forecasting.
What we found was that 0-1 km storm-relative helicity (SRH) was also unusually large (500-800 m2/s2) for these two supercell tornado events, particularly on 29 May. When combined with moderate total CAPE (at least 2000 J/kg) and strong deep shear (at least 55-60 kts), it appears that the environments for these events supported and enhanced intense mesocyclones to the extent that they were able to overcome the stable near-ground layer to generate tornadoes. Andy's presentation suggests that these combined ingredients (unusually large SRH, strong deep shear, at least moderate CAPE) can definitely support tornadoes in large CIN warm sector environments, and should be noted carefully by meteorologists, even when CIN suggests that an environment is not strongly surface-based. I'm hoping to post some addiitonal material on these events in the near future.
- Jon Davies 10/5/08
Sunday, September 14, 2008
A somewhat rare September tornado episode occured on Friday 9/12/08 in the Kansas City area. While visually impressive (see photos above near Eudora & Desoto, Kansas), the tornadoes were weak, with damage only in the EF0-EF1 range (the National Weather Service has officially rated the tornadoes near DeSoto, Kansas and Sedalia, Missouri EF0 in intensity).
Shawna and I had a birthday celebration planned in north KC for her son Zach on Friday evening, so we missed alot of the excitement. When picking up Zach from school in Plattsburg, Missouri at mid-afternoon, Shawna and I noticed SE to NW bands of low dark clouds with rapid motion, suggesting strong low-level wind shear. With rain all day and a cool surface air mass north of a boundary to our south (see surface map above), I didn't give the setting much thought until Shawna saw what looked like a wispy funnel that lasted a few seconds under a cloud band in the distance. I looked at some maps online, and began to wonder a bit what might happen. SPC issued a tornado watch around 4:30 pm with tornado reports coming in to the southwest of Kansas City near the boundary, but we were committed to our celebration. Watching TV later at Shawna's parent's house, we saw how large the tornado near DeSoto was, and heard about reports of damage. Thankfully, the tornado warned cells that went over the KC metro area did not produce any damaging tornadoes.
The DeSoto, Kansas tornado was a classic case of a supercell crossing a boundary where wind shear increased rapidly, with easterly surface winds. The surface map above shows the stationary front in the Topeka-Olathe area, and the SPC maps above show the large increase in low-level wind shear (storm-relative helicity or SRH) along and north of the front, with CAPE near and south of the front. The best combination of CAPE-SRH was right along the front just south and southwest of the KC metro area. Deep-layer shear through 6 km above ground (not shown) was also on the order of 40 kts along and north of the front, suggesting good support for tornadic supercells. On radar images above, notice how the DeSoto cell (indicated by white arrow) crossed the boundary moving northeastward, which is about the time it produced a tornado. As this cell moved deeper into the cool surface air north of the front, it stopped producing tornadoes.
The RUC analysis profile at Olathe (OJC, also shown above) is interesting, showing a large veering wind profile in the lowest 1-2 km, and around 900 J/kg of CAPE. While the instability wasn't super-impressive, notice that it was bunched low in the profile, with the "fattest" CAPE roughly 13,000 ft above ground. Most warm-season tornadic thunderstorms average larger CAPE, with the "fattest" CAPE up around 24,000-30,000 ft above ground. With the CAPE in the Olathe profile "squeezed" down fairly low, that suggests more rapid movement of air accelerating upward, like a hot air balloon encountering cold air aloft and moving upward faster. This could help with upward stretching, and combined with the wind shear, might be another factor to help increase potential for a tornado in a "smaller CAPE" environment.
- Jon Davies 9-14-08
Wednesday, August 27, 2008
With the Democrats tooling up for their convention this past weekend in Denver, weather tried to steal the show for a bit with a long-lived landpout tornado (see picture sequence courtesy of KUSA above) just southeast of the Denver Metro area.
The setting was fairly typical for landspout tornadoes (a form of non-mesocyclone or non-supercell tornado) near Denver, with a boundary NE-SW across the metro area (see surface map above). Storms formed along this boundary just south of Denver between 4:00 pm and 5:00 pm MDT. Then another sharper southeastward-moving boundary moved off the foothills to the west (see radar image above), and collided with the storms over the southern metro area. These colliding boundaries and the storm updrafts stretching the vorticity ("spin") along them produced the long-lived landspout tornado over northeast Elbert County between 5:30 PM and 6:00 pm MDT, as well as a couple other briefer landspouts elsewhere along the line.
A sharp windshift boundary (or colliding boundaries in this case) is the most important component in landspout cases, providing the vertical "spin" to be stretched into a tornado. But the environment contributes as well, with strong surface heating and rapid temperature change (steep lapse rates) in the lowest 2 km or so, helping to accelerate air parcels upward in low-levels to augment stretching by storm updrafts along the boundary or boundaries (see Davies and Caruso 2005 at http://www.nwas.org/ej/cardav/). Such environments are similar to those that encourage dust devils on hot days.
Above, a RUC analysis profile in the southeast Denver metro area shows these very steep low-level lapse rates, and also the deep well-mixed moisture layer, even though cloud bases were high (MLLCL around 2500 m AGL). I used to think that some low-level CAPE (e.g., CAPE below 3 km AGL) was needed in these events, but I've seen several Colorado events in recent years where this just isn't the case. It seems that a deep well-mixed moist layer (to reduce parcel entrainment and drying) with little if any CIN (to let parcels rise rapidly) is what is really needed, along with the steep low-level lapse rates, and of course, the boundary or boundaries. These ingredients thrown together in the same area increase the chance of a mesoscale "accident" that might result in a landspout tornado. With the local topography around the Denver area that can help set up boundaries on days with steep lapse rates, such events are relatively common there.
- Jon Davies 8-26-08
Sunday, August 17, 2008
This isn't directly related to weather, but Shawna and I went to Niagara Falls last weekend for a very pleasant trip. The sky was picturesque with lots of bubbly cumulus clouds against crisp blue, courtesy of an upper trough with cold air aloft moving through. With that as background, the falls were truly beautiful!
Above are some photos. The first is the American side of the falls, with the city of Niagara Falls, New York, visible. But I think the Canadian side of the falls is more magnificent, in the remaining shots. Shawna, as seen in the second image above, agrees. Enjoy!
- Jon Davies 8/17/08
Sunday, August 3, 2008
The Little Sioux campground EF3 tornado was one of the more publicized tornadoes in 2008 because of 4 boy scouts killed when it destroyed a bunk house where they had been drilled to take shelter (see my earlier post in June). Near the end of the 14 mile track (see first map shown above), this tornado struck storm chasers Kelly Martinson (also known as Kory Hartman) and Kenny Allen as it crossed Highway 183 a couple miles southwest of Moorhead, Iowa, yielding the most impressive up-close example I've ever seen of what it is like to encounter a rain-wrapped tornado. Contrary to some speculation by other chasers online, Kelly and Kenny did not intend to get that close, as the tornado was embedded in rain and the Omaha radar went down near the time of their encounter, making it difficult to judge where the tornado was as they waited for the mesocyclone to approach from the southwest. Also above are images of the same rain-wrapped supercell near Moorhead (observed by storm chasers from Iowa State), and later northwest of Dunlap (from the viewpoint of Shawna and myself).
The ground survey by NWS Omaha the day after the tornado suggested that the tornado did not cross highway 183 southwest of Moorhead, but Kelly and Kenny's video and their personal survey information indicated that it did (see the second map above, a Google maps closeup of the path end showing the discrepancy between the 2 survey tracks). To solve this mystery, Shawna and I revisited the area southwest of Moorhead on a trip to Iowa about a month after the tornado. Using a reference copy of Kelly and Kenny's video provided by Doug Kiesling (see video grab insets on second map above), Shawna and I located where the tornado crossed the highway (see third map above, my own survey of the tail end of the tornado track). From fence damage just west of highway 183 and south of 314th street (see photo above), it was clear that the most intense part of the tornado (50-100 yards wide) did indeed cross the highway where Kelly and Kenny's map indicated. We also found the two broken road signs that are seen being snapped off in Kelly and Kenny's video. Without the video the day after the tornado, it would have been easy to miss this damage, as the fence is located below and some distance west of highway 183 in a treeless area, and the broken signs were hidden in tall grass. Tree damage along and north of 314th street (what was shown as the center of the tornado path in the official survey) appears to have been a result of the north edge of the broader tornado circulation, or possibly inflow winds coming into the tornado from the north or northeast (see my close up map above). This example goes to show that surveying damage is difficult, and not always an exact science, particularly in rural areas.
Also, important to note: The intensity of the low-to-the-ground fence damage lined up with highway 183 strongly suggests that, had Kelly and Kenny been located another 50 yards southwest on highway 183, their vehicle would most likely have been rolled off the highway down a steep embankment, possibly resulting in injuries (the swirling condensation marking the most intense part of the tornado can be seen passing just south of their vehicle on the video). So, they are very lucky... as is the occupant of another vehicle (tail lights seen on the video) a few hundred yards farther down the highway (the most intense part of the tornado passed between the two vehicles). This is a graphic illustration of how dangerous it is to chase high-precipitation supercells with rain-wrapped tornadoes, as well as the potential problems of relying entirely on radar information when you can't see very well.
- Jon Davies 8/3/08
Saturday, July 26, 2008
It's been awhile since I've posted... I've been enjoying (and getting adjusted to) married life!
Last Thursday evening (7/24/08), Shawna and I storm chased in northeast Kansas during what appeared to be a fairly promising tornado setup for July. SPC even issued a tornado watch extending southeastward across the Kansas City area (see the insets on surface map graphic above). But, apart from a supercell along the Nebraska/Kansas border that rapidly gusted out and moved southeast into a line (see our photo images above), not much happened, and there were certainly no tornadoes. Why?
One can only speculate. Here's some thoughts...
Thunderstorm outflow from morning storms over Missouri had reinforced a stationary front/boundary during the day over northeast Kansas that was quite evident at late afternoon and early evening (see the surface map above). Even though wind shear was strong with easterly winds north of this boundary, temperatures were 10-15 degrees F colder on the cool side of the boundary in extreme northeast Kansas and southeast Nebraska where the original supercell developed between 5 and 6 pm CDT. Radar images above show this storm at 6 and 7 pm CDT, respectively, and its location is also marked with a circled "S" on the surface map above.
At Falls CIty, Nebraska (FNB), located just east of where the supercell formed, a RUC analysis profile at 6 pm CDT (shown above) indicated a good wind profile for tornadoes (a clockwise looping hodograph) and good CAPE (instability). BUT, the low-levels were rather stable (blue area on the RUC profile) beneath the CAPE area (red area on the RUC profile). Because tornadoes are a surface-based phenomenon involving near-surface air parcels, this low-level stable area with the cooler surface air may have been a negative factor, particularly because the supercell stayed north of the boundary.
I've noticed (as have other researchers) that when supercells form and _stay_ on the cool side of a significant boundary, they are less likely to produce tornadoes than if they _cross_ the boundary from warm to cool, or develop and stay within the warm sector (if there is significant warm sector wind shear, too). It may be notable that a couple storms that produced surprise tornadoes in north-central Missouri on 7/21/08 (no, we weren't there) occurred on the warm side of a prominent boundary. So the location of the supercell relative to the cool side of the boundary may be one clue in this case as to why no tornadoes happened. In addition, the cooler, heavier surface air on the cool side of the boundary may have made it easier for the storm's outflow to gust out, undercutting the mesocyclone and setting up a more linear configuration. But that's also speculation.
Anyway, it made for an interesting storm chase not too far from home (N of Kansas City).
- Jon Davies 7/26/08
Friday, June 20, 2008
Shawna Helt and I were married at the end of a storm chase yesterday (Thursday June 19) in the Konza Prairie near Manhattan, Kansas. We had talked about doing this on a storm chase for a couple months, and hoped we could pull it off on our chase vacation. We did it!
I am so delighted now to have Shawna as my wife!
Thanks to Rick Schmidt, Dick McGowan, and Jim Reed for sharing their pictures above!
Thanks to Aaron Blaser (in blue Hawaiian shirt) for officiating the ceremony and coordinating things!
And thanks to all our chaser friends who could attend on very short notice after driving through several heavy rain cores to be with us:
Aaron and Wendy Blaser
Many thanks, too, to all those who have sent us their congratulations!
- Jon Davies 6/20/08
Monday, June 16, 2008
Everyone has probably heard about the tornado in western Iowa (north-northeast of Omaha) that killed 4 Boy Scouts and injured around 40 others at a campground in a wooded hilly area. It was a national news item last Thursday and Friday, and a very sad story. But the story is also an encouraging one in that, after looking into the details, the Boy Scouts and their supervisors reacted well to the timely NWS warning via weather radio, and did everything they could, given that there was no concrete shelter or below-ground shelter at the campground. I did an informal damage survey of the tornado on Thursday 6/12/08, which is shown above.
Shawna Helt and I were storm chasing on 6/11/08, and intercepted the same supercell that produced the tornado, about a half hour later between the towns of Charter Oak and Dunlap, Iowa (see video grab and radar images above). We had to work hard to stay out of the way of 5 different mesocyclones as we dropped southeastward from the Sioux City area toward Harlan, Iowa in hilly, difficult viewing terrain on a zig-zaggy road network. Each circulation was rain-wrapped (see the video grab above) and, though we were tempted to penetrate one or two of the hidden rotation areas, we knew that would be too dangerous. After later finding out about the unfortunate tornado event at the campground (we didn't know about it while chasing), we are very glad that we did not drive into any of the rain-wrapped areas, and stayed out of the way of these dangerous circulations.
Storm chaser Kelly Martinson and another chaser had a close encounter a few minutes earlier with the tail end of the Little Sioux campground tornado as it was dissipating southwest of Moorhead, Iowa. His video was televised nationally, and he has had some criticism regarding being so close. I think the lack of visibility (HP storms and rain-wrapped circulations), the outage of Omaha radar toward the end of the Little Sioux tornado, and a frustrating hard-to-navigate road network all contributed to their driving into the edge of the tornado circulation. Having been in the same area on the same storm, Shawna and I can understand how that situation might have happened, and are just happy that Kelly and his chasing companion did not get injured. At any rate, the video is about the best example I can think of to show how dangerous rain-wrapped tornadoes are. The tornado was simply not visible until it was right on top of these storm chasers.
Shawna and I talked to one of the Boy Scouts who survived the tornado last Thursday. The tornado warning was issued 10 minutes before the tornado hit the campground. The Boy Scouts had gone through a tornado drill the previous Monday, and were prepared. They and the camp supervisors went to their designated places. Unfortunately, with no true shelter provided at the campground, bunk houses served as the only places to take cover. One of the Scouts timed the tornado passage at between 7 and 8 seconds, and told us the building "exploded" and lifted away, toppling a chimney onto the boys along with other debris. We were most impressed with stories of how the Scouts stayed calm after the event, giving first response treatment to those injured and handling the situation in a truly admirable way. If there is any lesson to come out of this event, it is that all campgrounds and similar outdoor facilities should have a concrete or underground shelter on the premises, as that is what would have saved lives in this case.
Jon Davies - 6/15/08