Paper in Weather & Forecasting August 2009

FWIW, a "Reply" paper (in full PDF form here) I wrote in response to comments by Roger Edwards and Rich Thompson was published in the latest issue of Weather & Forecasting journal (Aug 2009). The case study concerns a violent tornado (see photos above) in North Dakota on July 18, 2004, with the original paper published here in 2007.

The discussion focuses on mixed-layer LCL heights that appeared "relatively high" for a violent tornado, near 1500 m AGL, compared to means and medians that were 900-1000 m AGL from a database of RUC soundings 2001-2008 that I put together associated with violent tornadoes (F4-F5 or EF4-EF5). What's "relatively high" and what isn't is pretty subjective, but the North Dakota case did fall at the far upper end of MLLCL height distributions I've seen for violent tornadoes, and I felt the case was worth documenting. Forecast-wise, given CAPE-shear combinations that appear supportive of supercell tornadoes over a fairly large area, it's sometimes easy to get focused on areas where the MLLCL heights are lower, say less than 1000-1200 m. The North Dakota case examined and others, such as 7/11/08 in west central Minnesota, are a reminder to keep and eye on areas farther west and southwest where MLLCL heights on the SPC mesoanalysis (see graphic above) appear to push values at the upper end of published tornadic database distributions (e.g., near 1600-1800 m AGL).

Researcher Dr. Paul Markowski recently suggested to me by e-mail that such MLLCL heights aren't really that "high", which may be true. Certainly, when MLLCL heights get up above 2000 m AGL, that appears prohibitive for strong or violent tornadoes due to subcloud mixing and the potential for evaporative cooling and cold pooling in low levels. Going back to Dr. Markowski's 2002 paper (here) about rear-flank downdraft observations, he suggests that, statistically, the spread between surface temperature and dewpoint may be a more reliable limiting factor than LCL height when assessing potential for supercell tornadoes. This is because we have denser and more reliable surface observations, while model-based mixed-layer parcel lifted computations (e.g., from the RUC, which includes "ML" products from the SPC mesoanalysis) are highly affected by model forecast moisture depth and accuracy issues.

Although not in the August 2009 paper, observed surface temperature and dew point spreads from my 2001-2008 tornado environment database suggest that spreads of 5-15 degrees F appear most optimum for strong or violent tornadoes, which agrees with Dr. Markowski's observational field work from VORTEX1. When those spreads get near 20 degrees F and higher, that's when support for stronger supercell tornadoes seems to really break down. But please don't use that as a hard and fast "threshold", because nature doesn't recognize "thresholds"! FYI, the North Dakota violent tornado case in my "Reply" paper had temperature to dew point spreads that were around 18 degrees F.

- Jon Davies 9/20/09

Surprise tornado in Minneapolis on Wednesday 8/19/09!

People close to me know that I love studying "surprise" situations that are very difficult as forecasts. Wednesday's tornado near downtown Minneapolis around 2 p.m. (see photos above) was indeed a surprise, as the area was socked in by clouds, and the small cell that produced the tornado was embedded in a larger complex of rain (see 2nd graphic above) where total CAPE appeared small. There were no watches or warnings prior to the tornado. At first glance, it appears that the small tornadic supercell was an odd random incident. But, though difficult if not impossible to forecast, a closer look reveals some clues that point to why the tornado occurred in the area where it did. These might help to heighten short-term situational awareness in future situations of a similar nature.

The 3rd graphic above shows the midday surface map, with a small low and convergence focus visible in the wind flow fields near Minneapolis. The low appeared to be right at the intersection of some subtle wind shift boundaries, organized not unlike the surface pattern in some cold core events, although this setup was not associated directly with a 500 mb cold core low. Instead, a sharp negatively-tilted 500 and 700 mb trough (see SPC mesoanalysis, 4th graphic above) was moving northeast across southern Minnesota, with a comma cloud and vorticity center visible with the wave moving northeast in satellite imagery (see the inset enhanced IR image on the surface map graphic). This vorticity center was trackable from near Omaha after daybreak to the Minneapolis area at early afternoon, with the comma cloud expanding and becoming better defined in the enhanced satellite imagery. The surface low and boundaries over Minnesota were organizing ahead of and in response to this upper feature and forcing, even within the large complex of rain and embedded thunderstorms.

Going back to the radar image (2nd graphic), if one looks closely, there was a pattern to the precipitation echoes, with a flat "S" shape (indicated by heavy white dashed line) formed by the stronger reflectivity returns corresponding to the N-S boundary roughed in on the surface map. The surface low position was likely located near the middle of this "S" (marked by a red "L" in the radar image), and the small tornadic supercell was located immediately east or northeast of this focus where increased shear and convergence would be expected, consistent with the surface analysis (3rd graphic). The SPC mesoanalysis graphics (4th graphic) also suggested increased 0-1 km storm-relative helicity (SRH) pointing into the area from the southeast, with maximized low-level CAPE in the Minneapolis-St. Paul area, even within the large convective complex.

The RUC analysis sounding at MSP (Minneapolis International Airport, southeast of downtown) at about the time of the tornado also suggests that large low-level shear and SRH were present (see hoodograph on last graphic above). The same sounding confirms that large low-level CAPE bunched close to the ground was present in a nearly saturated environment (little or no mixing for near-surface air parcels), even though total CAPE was small (at best, around 400 J/kg). This vertical arrangement of CAPE and SRH co-located in low-levels probably helped to optimize tilting and stretching of horizontal low-level vorticity near the boundaries and surface low focus to generate the tornado.

Thankfully, with the marginal total CAPE, nothing more than a weak tornado (EF-0) could get going, but not without leaving a south to north trail of damage across south Minneapolis to near the downtown. A later tornado east-southeast of Minneapolis near Cottage Grove (not shown) was rated EF-1, and occurred with a cell on the bulging boundary south and southeast of Minneapolis as it "curled" and "wrapped" northeastward during the next 45 minutes in response to the evolving wave and energy passing aloft.

This case suggests that evolving and expanding comma clouds in IR imagery indicative of a strong vorticity center and wave aloft be watched carefully as they move northeastward. Even with rain and only small total CAPE, if there is focused surface congergence (e.g., a mesolow with boundaries) in response to the wave and vorticity aloft, and at least some CAPE (e.g. 300-500 J/kg low in the vertical profile with little or no CIN), look out! A surprise may be in store with that energy focusing into a relatively small area.

Additional information and graphics for this event are on the NWS Minneapolis web site here.

- Jon Davies 8/22/09

Updated material on 700 mb temperatures and estimating the "cap"

I put together a new case study (10 July 2009) showing how 700 mb temperatures can be helpful in estimating the location of an inhibiting capping inversion:
10 July 2009 "cap" case study in the central plains

I've also updated my original material that is an informal reference about using 700 mb temperatures as a first guess estimate of the "cap":
Using 700 mb temperatures as an estimation of the "cap" in the central plains

There are several caveats given, including the fact that the 700 mb temperature values often don't work well in the High Plains and westward due to surface heating over elevated terrain in the warm season, and also upslope situations. Although only a very rough guide, the table and information with this material can be useful in raising awareness about possible "cap busts" when forecasting. Hope some find these useful.

- Jon Davies 8/17/09

A surprise tornado in a very subtle setting - northeast Kansas on July 28, 2009.

Weak tornadoes probably occur more often than we think. If there's no one to see a tornado and it doesn't hit anything over open country, it doesn't get reported. A weak, brief, but interesting tornado was photographed in northeast Kansas near Centralia KS last Tuesday 7/28/09. The Topeka NWS office has a story about it on their web page here.

Veteran storm chaser Doug Nelson of Seneca KS happened to notice the tornado form from a rapidly rotating cloud base under a developing "shower" southwest of his shop at Centralia, and took the photos above around 1:20 pm CDT (1820 UTC). The tornado was brief and probably wouldn't have been noticed if not for Doug's observations.

In and of itself, this tornado isn't important... it was weak and no damage reported. But such tornadoes in subtle settings can help meteorologists study and become aware of ingredients that, when more pronounced, might lead to other more significant tornadoes in atypical settings.

Above, the first of two low-level radar base reflectivity images (about 1830 UTC) showed a subtle boundary as a hard-to-see ragged "fine line" oriented WSW to ENE (indicated by dashed white line). The tornado occurred with a small unimpressive echo (indicated by the white arrow) on this boundary. In the 2nd radar image above, the boundary was more evident as a line of storms fired along it, but this was well after the tornado occurred. Even at 1845 UTC on satellite (3rd graphic above), it was hard to see either the boundary or the "shower" (arrow), although local clearing was evident behind the morning storms over Missouri and extreme northeast Kansas, providing a little heat and air mass recovery.

The boundary was also hard to pick out on the 18 UTC surface map (see heavy dashed line on 4th graphic above), apart from the ESE wind at Manhattan KS and southwest wind at St. Joseph, with winds northerly at stations to the north. But, in fact, the boundary was probably a weak cool front moving slowly southeastward under an unseasonably deep trough at 500 mb (shown in the same graphic), obscured by the morning clusters of storms and outflows well in advance.

The NAM/WRF analysis sounding at Holton KS, located north of Topeka and southeast of Centralia KS, is shown in the last graphic above. This local environment estimate showed not a lot of CAPE (400-500 J/kg), but notice that a well defined "fat" area of CAPE was evident between 700 and 600 mb (about 3.5 km above ground). With this CAPE low to the ground (typical spring/summer thunderstorm soundings have the "fattest" CAPE much higher, around 6-7 km above ground), this suggests potential for rapid acceleration of air parcels in developing local updrafts, resulting in strong stretching. With the boundary, vertical CAPE distribution, and the shower right over the boundary with stretching, those ingredients are what probably came together as a local "mesoscale accident" to spin up a brief surprise tornado. Certainly not an event that could be forecast or even nowcast... a very subtle setting.

Nature never ceases to surprise!

- Jon Davies 8/3/09

Waterspout and separate tornado near Daytona Beach on 7/24/09 - a similar setting to some landspouts setups in the Plains?

Friday 7/24/09 saw a photogenic waterspout in Florida off Ormond Beach (see 1st photo above) just north of Daytona Beach, followed by a tornado inland at Port Orange (see 2nd photo above, south of Daytona Beach) that damaged or destroyed many manufactured homes. The setting for Friday's waterspout and tornado appeared similar in some respects to some landspout tornado settings in the Plains.

The second set of graphics above shows lowest elevation angle radar reflectivity images from around 2100 UTC to 2230 UTC on Friday, with some key features labeled. The white dashed line and arrows in the first 2 images indicate the position of a sea breeze boundary inland over eastern Florida, seen by a fine line on Melbourne's radar. Notice how the sea breeze boundary extended back out over the Atlantic near Daytona Beach (DAB) at 2106 UTC, with thunderstorms having formed in a separate convergence zone to the northwest of DAB. By 2141 UTC, these storms had moved east and encountered/converged with the sea breeze boundary near DAB. At this time the waterspout was in progress off Ormand Beach (OMN) and lasted until around 2155 UTC. It moved slightly onshore at one point and did minor damage as a tornado. By 2230 UTC, the storms had back built along the boundary onshore to the south and southwest of DAB. Around 2220 UTC to 2230 UTC, a tornado developed inland over Port Orange, and did the aformentioned damage to many homes.

The boundary seen on radar (and also satellite, see 3rd graphic above) was of critical importance in generating the waterspout and later tornado. It is common knowledge that most waterspouts in Florida occur along land/sea breeze boundaries. What made this case interesting was that a tornado eventually formed inland from the same boundary, similar to how some non-supercell landspout tornadoes develop southwestward along stationary northeast-southwest boundaries in the Plains. A paper by Caruso and Davies discusses some of these Plains settings.

In addition to the boundary, SPC mesoanalysis graphics (last set of graphics above) from 2100 UTC on 7/24/09 indicated some environment characteristics that were somewhat similar to the Plains settings mentioned above. Low-level lapse rates (0-3 km) were steep and maximized just southwest of DAB (heavy black dot in the graphics), and 0-3 km CAPE was maximized as well. The combination of these ingredients with a stationary or slow-moving boundary over which thunderstorms are developing can increase stretching in updrafts to create non-supercell tornadoes using vertical vorticity along the boundary. Although the setting and orientation of the boundary did have some similarities to Plains landspout settings, it should be emphasized that such storms in the Plains are typically much higher based, and the low-level lapse rates tend to be much steeper (around 9 deg C per km) due to the higher elevation. Florida tornado settings don't require such steep lapse rates, low-level environments are much more moist, and land/sea breeze fronts provide plentiful boundaries.

WInds at 500 mb (see last SPC graphic above) were only around 20-25 kts over northeast Florida on Friday, so while one or two of the storms along the boundary may have had some brief/marginal supercell characteristics, it appears that the primary contributors to the waterspout and separate tornado were non-supercell/non-mesocyclone processes along the boundary.

- Jon Davies 7/26/09

New papers about nighttime tornadoes by Davies & Fischer

Two new peer-reviewed papers about nighttime tornado environments written by Andy Fischer (AWC in Kansas City) and me have just been published online in National Weather Association Electronic Journal of Operation Meteorology (NWA EJOM):

Environmental Characteristics Associated with Nighttime Tornadoes
By: Jonathan M. Davies and Anthony Fischer

Significant Nighttime Tornadoes in the Plains Associated with Relatively Stable Low-Level Conditions
By: Anthony Fischer and Jonathan M. Davies

There's some interesting things we found with nighttime tornado environments, including much larger storm-relative helicity (SRH), and significant differences in CAPE and CIN between the Plains and the southeastern United States. In the second paper, we looked specifically at a couple of significant nighttime tornado events in Kansas in 2008 that were associated with unusually large CIN and low-level stability.

I hope that meteorologists and students find these useful!

- Jon Davies 7-12-09

"Surprise" supercells & tornadoes in Kansas on 6/20/09

Severe weather events the past couple weeks have incuded a highly visible tornado along the Palmer Divide in Colorado, and strong tornadoes near Macksville KS, Aurora NE, and Austin MN. Tornadoes in Kansas yesterday (5-10 reports on Saturday 6/20/09) weren't anywhere near as strong, but were more of a surprise within a subtle and somewhat unusual environment. Pics above from a supercell in Franklin County of eastern Kansas show one of the brief tornadoes, courtesy Rick Schmidt and Randy Cooper.

The surface map (2nd graphic above) at late afternoon confirmed an east-west warm front moving north through Kansas, with easterly winds and dew points in the 70s F. The RUC model at 500mb (3rd graphic above) showed upper energy in the form of a wind max moving across Kansas from the southwest, and CAPE-SRH combinations (EHI) and CAPE in low-levels (also in 3rd graphic above) forecast to be maximized across central and eastern Kansas. For any thunderstorms forming along and north of the front, this suggests that supercells were possible, and maybe some tornadoes. Radar above (4th graphic above) shows storms shortly after 7 p.m. CDT over Kansas, with arrows indicating tornadic storms at that time (the complex near Hutchinson KS became tornadic later).

The RUC analysis sounding over eastern Kansas at Topeka at 7 p.m. CDT (last graphic above) matched the observed Topeka RAOB very well. To my mind, the most interesting feature was the area of fattest CAPE located at 600 mb (about 4 km or 13,000 ft above ground). Compare that with the RUC analysis associated with the Aurora NE tornado (also shown in last graphic above), where wind shear/SRH and total CAPE were much larger, but the fattest area of CAPE was located much higher (near 300 mb, or roughly 9 km/30,000 ft above ground). On the Topeka profile from yesterday, the fattest area of CAPE was at least _3 miles_ lower in the vertical than in Wednesday's Nebraska tornado environment. Although Saturday's storms weren't anywhere near as strong or severe, they produced a number of tornadoes (9 reports on the SPC log). The fat area of CAPE near 600 mb (relatively low in the profile, associated with cold air at that level) was evident on RUC profiles across much of Kansas, and was probably a notable contributor to the number of weak tornado and funnel reports from western to central and eastern Kansas on 6/20/09. What probably happens in such settings is that "fat" CAPE closer to the ground produces more rapid upward accelerations in updrafts, translating to more vertical stretching which can cause tornadoes and funnels, even in a weaker shear environment, not unlike many tornado events associated with 500 mb cold core lows. The lack of large SRH and low-level shear on Saturday (compare the wind profiles on the 2 soundings shown above) probably kept Saturday's tornadoes brief and weak.

Sorry about no posts lately... been busy with non-weather stuff,

- Jon Davies 6/21/09