EF-1 tornado in Alabama on 1/10/09: a subtle small CAPE setting

A weak tornado occurred shortly after 2300 UTC (5 pm CST) on Saturday, Jan. 10, about 30 miles north of Mobile in southwest Alabama. It wasn't very impressive (1-2 mile path, EF-1 winds around 100 mph), and thankfully no one was hurt, but it did do some damage (see NWS photos above, also click here). Given the subtle setting, it was kind of interesting from an environment standpoint.

Radar reflectivity at 2304 UTC from Mobile above shows the tornadic cell, and NWS Mobile did a good job with tornado warnings that were issued across southern Washington county for more than 45 minutes before the tornado struck the town of McIntosh.

The RUC analysis profile above for Mobile (MOB) at 2300 UTC is somewhat unusual, with small total CAPE (only around 400 J/kg!), and most of the CAPE below 600 mb, very low to the ground. Settings like this can enhance stretching near the ground, and with decent SRH (> 200 m2/s2 in the lowest 1 km), they can occasionally support tornadoes, even with small total CAPE.

The SPC graphics above show a large positive tilt 500 mb trough was heading southeastward, with a cold front (not shown) ahead over central Tennessee into central Mississippi. The Alabama tornadic cell occurred well in advance of this front in a moisture axis of 60s F dew points. In the parameter graphics, the SPC total MLCAPE field over southwest Alabama wasn't impressive (> 250 J/kg), but low-level CAPE was large (near 100 J/kg below 3 km) along with low-level SRH (200-300 m2/s2) overlapping the southwest Alabama moisture axis. These ingredients, combined with the low-to-the-ground vertical CAPE distribution in the RUC analysis above, were just enough to help generate a tornado with the cell north of Mobile.

This case is a reminder that storm environments with CAPE "squeezed" low to the ground certainly aren't limited to cold-core events (for example, see here).

- Jon Davies 1/12/09

Damaging thunderstorm winds in south Kansas City on 12/27/08

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

Twin Horseshoe Vortices! (6-17-08 in Nebraska)

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

Southwest Kansas cold core setup on 11/10/08 - Mike Umscheid photographs large tornado

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

New Severe Storms Conference paper online

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)

High Plains presentation now online

Andy Fischer of NOAA/AWC Kansas City and I submitted a paper to the 12th Annual High Plains Conference (Hays, Kansas Sept. 4-5), "Significant Nighttime Tornadoes in 2008 Associated with Relatively Stable Low-level Conditions". I could not attend, so Andy presented the paper. Andy's presentation is now online in PDF format here.

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

Tornadoes on 9/12/08 near Kansas City

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