Anthony A. Rockwood Robert A. Maddox

Transcription

1 Anthony A. Rockwood Robert A. Maddox

2 An unusually intense MCS produced large hail and wind damage in northeast Kansas and northern Missouri during the predawn hours of June 7 th, Takes a look at preconvective period and the interactions between mesoscale processes and the synoptic scale environment that lead to convective development Some scenarios as to why this development occurred are proposed and examined.

3 The first storms formed over southwest Nebraska shortly after sunset. Many forms of severe weather were produced by this MCS: kt wind gusts Large hail Funnel clouds At least one tornado At dawn, winds at about 90 kts (~103 mph) came through downtown Kansas City knocking down trees and damaging buildings as well at creating widespread power outages.

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6 This event is a prime example of the interactions between mesoscale and synoptic scale processes Forecasting convective weather can be described to have two phases Identification of large areas (on the order of 100,000 km 2 ) where there is/may develop, a high potential for sever storms Efforts to predict when and where, within these regions, the convection will actually develop and/or move.

7 Over NE Colorado and most of Nebraska a mild, dry air moved SE along a weak high plains trough. Surface moisture persisted through afternoon heating in Kansas helping form a weak dry line between this area and the dry air behind. In the lower troposphere, SSW flow ahead of a cold front brought moist air across eastern Kansas

8 The 0000 UTC sounding for Topeka, Kansas resembles a Loaded Gun sounding, but the inversion over the moist layer was an obstacle to thunderstorm development. If any storms were to develop, the wet bulb potential temperature being about 13 C cooler than the surface would have provided strong winds with the storms. Even though large scale ascent in lower layers could have weakened the inversion by the time the storm moved through Topeka, it would not have been enough to eliminate the cap all together.

9 Very high θ e air ( K) flowed toward the shallow front at lower levels with much lower θ e above. Such a decrease in θ e with height ahead of the front is a key characteristic of high instability. Behind the front, low values (310 k) within the dry High Plains air mass show that there was no support for moist convection. RH can also be seen to be low behind the front, but ahead of the front, values are between 80%-90% showing high most convective potential that is very near saturation. Even with all of these settings, this air would have to be lifted to the depth of the inversion (about ft) to reach the LFC.

10 Due to the amount of thunderstorm potential in this area and the rapidly changing environment, this situation was obviously difficult to forecast. The National Severe Storms Forecast Center/ Severe local Storms Unit (NSSFC/SELS) convective outlook included a moderate risk of severe storms in NE Kansas. By 1500 UTC the threat had been downgraded because the storms in the morning were approaching an area that was still unstable but unexpected rapid movement of the entire synoptic system was to shift the entire system and severe threat more east.

11 The local environment clearly changed between 0000 UTC and 0300 UTC due to large scale processes. A southern Colorado 850 mb low moving NE resulted in increased moisture advection, upward vertical motion, and convective destabilization. This could have caused the replacement of the large scale subsidence at 0000 UTC with upward motion by 0300 UTC. Midlevel moisture was shown through satellite imagery which showed mid level cloudiness developing in the area. This thermodynamic profile could have supported the convective development seen in the actual surface observations.

12 A concept proposed by Doswell et al (1985) could explain the change in local environment. In this concept regional areas of unstable air in the midtroposphere are advected over low level moist air which would create a deep convectively unstable environment. In this specific case of storms forming over the western High Plains, the origin of unstable air is usually the Rocky Mountains. If the large scale setting is right and daytime heating combines with dynamic lift then it can produce unstable air in the midtroposphere which can be carried over the Plains to the east.

13 Composit of some of the key convection parameters including lapse rates in the mb layer. These high lapse rates over central and southeast Colorado are the combined effects of surface heating of elevated terrain, differential temperature advection, and upward vertical motion. This figure also shows the combination of factors that came together to form this strong MCS It has also been suggested that diurnal changes in the low level wind field may have increased the advection of warm air and moisture into SW Nebraska, and that ascent of air could have led to saturation and thunderstorm development. This may not be the case because of such large scale interactions that were probably underway well before sunset.

14 While the large scale was primarily responsible for providing the destabilization to the area, mesoscale processes can be shown to have provided the low level lifting necessary to initiate convection. At 0300 UTC satellite imagery shows the development of clouds along the intersection between the dryline and approaching surface trough. This convergence could have combined with outflows from a larger area of convective clouds moving from NE Colorado into SE Nebraska. This could be seen as the lift that created the initial convection.

15 Another mechanism that could initiate convection can be called underrunning (Carlson et al (1983)) Moist boundary layer air flowing northward ahead of a front is overrun by a southwesterly flow of hot, dry continental tropical air that originated over elevated terrain to the SE. While enhancing convective instability, the dry air creates a kind of lid that suppresses deep convection. The moist low level air flows out from beneath the lid or underruns the lid along its northern boundary. Case studies show that along this lid thunderstorms can form, and as in this case, cold advection aloft would decrease stability.

16 This cross section shows that there is a slight increase in low level southerly flow toward the front, but most of the flow aloft is southwesterly, or parallel to the front. This would make underrunning have a hard time being a significant process in this case. In the region of initial storms the low level moisture was overrun by the cool dry High Plains air rather than by the warm air. The Northern boundary of the lid is extended from SW Kansas to NE Kansas which is pretty far south of the initial storm area. If the convection developed where the moist air was underrunning the lid then the storms would have been more in central Kansas.

17 Satellite imagery shows that the storm system was slow to intensify. The θ e cross section shows that early development started in areas of K air with slight convective instability. Very moist and unstable air was just to the east where large scale ascent was occurring. Rapid expansion of the cloud shield began by 0730 UTC and by 0830 UTC the satellite view of the system met the initiation criteria of an MCC and many reports of sever weather began to come in. This explosive intensification came about when the systems main outflow pushed southeastward through the frontal zone which caused lifting and released the moist potential energy of the prefrontal environment that we saw in the Topeka sounding. Low level air with θ e values of K and RH of over 90% were fed into the system while very low θ e above the inflow gave the downdrafts even more acceleration downward. With all of this intensification, outflow winds of almost 90 kt were reported in parts of NE Kansas as the MCC reached its maximum extent just before dawn.

18 Synoptic scale data over the area of initial storms indicated slight convective potential and quasi-geostrophically forced subsidence. Advection of moist, unstable, ascending air, associated with the movement of an upper level shortwave trough was increasing the potential. In the destabilizing environment, mesoscale convergence was enhanced by outflows from high convective clouds which initiated thunderstorms along the surface dryline. These weak thunderstorms moved eastward into an environment that had much higher potential for intense convection. When the storm was organizing, the system crossed the frontal zone into an area of unstable, ascending air, and it s outflow and inflow were strong enough to overcome the weakening inversion, which lead to the storms dramatic intensification.

19 This storm system provides an example of how synoptic scale processes can effect a local environment and make it more capable of moderate convection. Combining this with low level mesoscale lifting during dirunal changes can create convective initiation. Synoptic scale influences play a larger role in evolving or intensifying convective storms rather than the cause of their initiation. If a synoptic scale setting is unfavorable for thunderstorms, mesoscale influences may still create convection but without the synoptic influence the storms may not grow into a large storm system. In this specific case, storms formed in an environment with a small amount of convection potential and moved into a highly favorable synoptic scale environment.

20 Many thunderstorms can form in areas that are downplayed because of their preliminary evaluation of unfavorable synoptic scale vertical motion and limited storm potential. If calculations are made regarding vertical motion, forecasters can focus on how marginally favorable areas could change based on the effect of the mesoscale processes. If the mesoscale processes can create convection then the forecaster should try to figure out if the storms could move into a more favorable synoptic environment. A mesoanalysis of the preconvective environment could give clues as to what initiation processes could occur. The challenge to operational meteorologists is to recognize these subtle clues and respond correctly to the multiple scenarios that could play out and lead to thunderstorm development.

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22 Rockwood, Anthony A., and Robert A. Maddox. "Mesoscale and Synoptic Scale Interactions Leading to Intense Convection: The Case of 7 June 1982." Weather and Forecasting 3 (1987): Much of the wording in this presentation is from the article itself to describe the figures and overall information from the article above.