Severe Weather Outbreak Research Articles

Role of stratospheric air in a severe weather event: Analysis of potential vorticity and total ozone

The role of dry stratospheric air descending to low and middle tropospheric levels in a severe weather outbreak in the midwestern United States is examined using NCEP Eta model output, Rapid Update Cycle (RUC) analyses, and Earth probe Total Ozone Mapping Spectrometer (EP/TOMS) total ozone data. While stratospheric air was not found to play a direct role in the convection, backward trajectories show stratospheric air descended to 800 hPa just west of the convection. Damaging surface winds not associated with thunderstorms also occurred in the region of greatest stratospheric descent. Small‐scale features in the high‐resolution total ozone data compare favorably with geopotential heights and potential vorticity fields, supporting the notion that stratospheric air descended to near the surface. A detailed vertical structure in the potential vorticity appears to be captured by small‐scale total ozone variations. The capability of the total ozone to identify mesoscale features assists model verification. The total ozone data suggest biases in the RUC analysis and Eta forecast of this event. The total ozone is also useful in determining whether potential vorticity is of stratospheric origin or is diabatically generated in the troposphere.

Analysis of Meteorological and Terrain Features Leading to the Izmir Flash Flood, 3–4 November 1995

Flash floods associated with heavy precipitation has become a hazardous phenomenon along the Mediterranean coasts of Turkey in recent years. During 3 and 4 November 1995 heavy and intense rainstorm activity over the Aegean coast led to flash flooding in the city of Izmir. Damage exceeded $50 million and 61 people died as a result of the flood. The Karsiyaka district suffered the most severe damage. This study presents an analysis of the meteorological settings that led to the development of these intense storms and describes the role of the terrain features involved in the Izmir flood. The important mesoscale features which initiated the severe weather outbreak included pronounced low-level advection, positive vorticity and strong upper level divergence. A surface low centered over the Aegean Sea enhanced the advection of warm and moist unstable air masses coming from the southwest over the Mediterranean Sea along a southwesterly low level jet (LLJ). A squall line oriented NE-SW over the Aegean Sea also contributed to the storm development, and intensity of the storms was further enhanced by the orographic effect. The presence of a frontal system, the stability indices associated with the event, and other meteorological features are all reminiscent of the synoptic type flash floods identified by Maddox. While the pressure and moisture patterns were favorable for severe storm activity, nonmeteorological factors including the topography, geomporphology, and and land-use contributed to the flooding to a great extent. Settling in the flood-prone zone, insufficient floodwater structures, and the lack of channel improvements in the creeks enhanced flood damage to the city. Many of the deaths occurred in the settlements located in the flood-prone zone of the Ilica and Dallik creeks. Language: en

Characteristics of Cloud-to-Ground Lightning Associated with Violent Tornadoes

Abstract Cloud-to-ground (CG) lightning patterns were analyzed in 42 violent tornado-producing (F4, F5) supercells that occurred between January 1989 and November 1992. The purpose of this analysis was to identify potential correlations between CG lightning patterns and tornadogenesis. Lightning characteristics were analyzed for a 30-min period before, during, and 30 min after each tornado’s lifetime. Thirty-one of the storms were characterized by a peak in CG flash rate preceding tornado formation; 20 storms displayed a decrease in CG flash activity coincident with tornado touchdown. Six of the 42 storms exhibited a polarity reversal, from positive to negative, in the sign of the charge lowered to ground. Storms exhibiting a majority of positive flashes were generally associated with long-track tornadoes, F5 damage ratings, or severe weather outbreak conditions. The total number of flashes associated with each storm had no correlation with tornadogenesis (total number of flashes ranged from 16 to 3394). ...

The Importance of Understanding Mesoscale Model Parameterization Schemes for Weather Forecasting

Abstract A severe weather outbreak that occurred on 21–23 November 1992 in the southern United States is used to illustrate how an understanding of model parameterization schemes can help in the evaluation and utilization of mesoscale model output. Results from a mesoscale model simulation show that although the model accurately simulated many of the observed mesoscale features, there are several aspects of the model simulation that are not perfect. Through an understanding of the model parameterization schemes, these model imperfections are analyzed and found to have little effect on the overall skill of the model forecast in this case. Mesoscale model output also is used to provide guidance to evaluate the severe weather threat. By using the model output to produce hourly calculations of convective available potential energy (CAPE) and storm relative environmental helicity (SREH), it is found that regions with positive CAPE, SREH greater than 150 m2 s−2, and model-produced convective rainfall correspond...

The Influence of the Rocky Mountain on the 13–14 April 1986 Severe Weather Outbreak. Part I: Mesoscale Lee Cyclogenesis and Its Relationship to Severe Weather and Dust Storms

Abstract In this first of a two paper series, a sequence of dynamical processes involving the evolution of a mesoscale Ice cyclone and its subsequent interaction with a mesoscale tropopause fold downstream of the Rocky Mountains is investigated. These scale-interactive phenomena, which resulted from the jet streak interaction with the topography, were examined in detail using the observational data obtained from the Program for Regional Observing and Forecasting Services' mesonetwork and wind profilers, as well as conventional surface and rawin-sonde data and Total Ozone Mapping Spectrometer satellite data over the Colorado region for the severe weather event that occurred during 13–14 April 1986. Large-scale analysis indicated that as a baroclinic low pressure system approached the Rockies with its attendant upper-level jet streak, a typical prestorm environment over western Kansas formed in the early morning hours of 13 April. Hourly mesonet data analysis revealed the formation and eastward progression ...

The Influence of the Rocky Mountains on the 13–14 April 1986 Severe Weather Outbreak. Part II: Evolution of a Prefrontal Bore and Its Role in Triggering a Squall Line

Abstract In this paper, Part II of a series, the evolution of a prefrontal bore on the leeside of the Rockies and its subsequent propagation and initiation of convection farther downstream over eastern Colorado and western Nebraska are investigated. The observational evidence for this sequence of events was obtained from combined analyses of high-resolution GOES satellite imagery and Program for Regional Observing and Forecasting Services mesonetwork data over the Colorado region for the severe weather event that occurred during 13–14 April 1986. A 2D nonhydrostatic numerical model is used to further understand the initiation of the bore and its ability to propagate farther downstream and trigger convection. Analysis of satellite imagery and mesonet data indicated that an internal bore (ahead of a cold front), a moderate downslope windstorm, and a quasi-stationary hydraulic jump were generated within a few hours along the Iceslope as a Pacific cold front and its attendant upper-level jet streak advanced o...

Autumnal Return of Tropical Air to the Gulf of Mexico's Coastal Plain

Abstract The return of tropical air from the Gulf of Mexico is examined in the autumnal cool season. Results from the thermodynamic equilibrium model of Betts and Ridgway are used to calculate the equilibrium equivalent potential temperature (θe) over the gulf and the northwestern Caribbean Sea. With a climatological study as a backdrop, a case of severe weather outbreak in mid-November 1988 is analyzed with emphasis on the analysis of low-level θe that flowed into the storm region from the Gulf of Mexico. The primary results of the study are the following: The climatological distribution of equilibrium θe over the gulf and the Caribbean in November serves as a useful tool for the analysis of the 1988 case study. Between 5 and 15 November 1988, equilibrium in the marine layer was established over the gulf due to the absence of any deep cold-air penetrations during this period. The high-valued θe that streamed into the severe storm region on 15 November 1988 tracked from the Yucatan straits and the northwe...

Operations of the National Severe Storms Forecast Center

The National Severe Storms Forecast Center in Kansas City, Missouri, is composed of several operational forecasting units, all national in scope. It includes the Severe Local Storms Unit (SELS), the National Aviation Weather Advisory Unit (NAWAU), and a Techniques Development Unit (TDU). Within SELS there is a synoptic interpretation message (SIM) section and a mesoscale discussion (MD) section. The SELS Unit has been in Kansas City since 1954 and is responsible for providing a wide array of products related to severe weather: convective outlooks, public weather statements, mesoscale discussions, status reports, and tornado and severe thunderstorm watches for the 48 conterminous states. SELS is also responsible for preparing a national weather summary. NAWAU provides alerts, nationally, of hazardous weather, known as AIRMETs and SIGMETs, and convective SIGMETs to aviation interests. NAWAU also issues aviation area forecasts. The SIM section prepares a diagnostic interpretative message that incorporates satellite and other information and relates them to model trends. The MD section supports the SELS program by providing short-range updates on developing mesoscale systems. The Techniques Development Unit has been responsible for working hand in hand with SELS to reap the benefits of applying interactive computer-processing technology, progressively giving the forecaster more effective means to provide improved services. Illustrating these improvements is the fact that severe weather watches have significantly increased in accuracy, especially in handling violent death-dealing tornado situations. An illustrative example of SELS products is shown for the severe weather outbreak of 15–17 June 1992. The example shows how skillfully this episode, producing 172 tornadoes and 462 other reported severe weather events, was forecast. Emphasis is made of the need for consistent and well-coordinated watches and other advisories when dealing with widespread, rapidly moving, severe weather-producing systems.

Wave disturbances associated with the red river valley severe weather outbreak of 10?11 April 1979

An analysis of mesoscale gravity wave events during the severe weather outbreak in the Red River Valley on 10–11 April 1979 is presented utilizing surface pressure data and the 3 h rawinsonde data from the AVE-SESAMEI special network. The unique data set provided by the SESAME field experiment makes it possible to relate the wavelike characteristics observed at the surface to the variability of the temperature, humidity, and wind fields over a deep tropospheric layer that act to initiate and sustain the waves over long distances and time periods.

Rapid evolution of a jet streak circulation in a pre-convective environment

An analysis of upper- and lower-tropospheric jet streaks and their associated vertical circulations prior to and during the Red River Valley severe weather outbreak of 10 April 1979 is presented utilizing the three-hourly SESAME data sets. The paper emphasizes the transformation of a transverse indirect circulation within the exit region of an upper-level jet streak (ULJ) in which the cross-contour components of the upper-level ageostrophic flow diminish in magnitude and reverse direction as the circulation's ascending branch shifts from the cyclonic-shear side to a position along the axis of the ULJ. The change in character of the vertical circulation over a 3- to 6h period appears to be related to the increasing cyclonic curvature associated with the propagation and amplification of a short-wavelength trough embedded within cyclonic flow.

Diagnosis of a Jet Streak in the Vicinity of a Severe Weather Outbreak in the Texas Panhandle

This is a case study of the synoptic and mesoscale aspects of a severe-weather outbreak in the Texas Panhandle. We offer circumstantial evidence that the rising branch of a thermally indirect circulation in the exit region of an unusually intense upper-level jet streak played a role in storm formation and sustenance. The jet streak's vertical circulation could not be accounted for by straight dynamics alone; curvature was important, especially along the right side of the exit region. The geostrophic momentum approximation leads to a reasonable qualitative explanation of the ageostrophic circulation, while quasi-geostrophic theory does not.

A Synoptic Climatology of Northwest-Flow Severe Weather Outbreaks. Part II: Meteorological Parameters and Synoptic Patterns

Abstract A climatology of meteorological parameters and synoptic patterns associated with severe weather outbreaks occurring in arm where the mid-tropospheric flow has a north of west component is presented. This climatology utilizes data and criteria previously described by Johns. A comparison of the northwest flow parameters and those associated with general severe weather is given. The importance of conditional instability and low-level warm advection in northwest flow situations is discussed. An explanation is offered for the location of the axes of highest frequency of northwest flow outbreaks. Furthermore, the varying nature of wind shear associated with severe weather is discussed and the importance of the directional contribution of wind shear to northwest flow severe weather is demonstrated.

Mesoscale Analysis of Surface Variables During the Severe Storm Outbreak of 10–11 April 1979

A mesoscale analysis of surface variables is presented for an AYE-SESAME case, 10–11 April 1979, when 89 severe weather events were reported, including the Red River Valley tornado outbreak. Two severe weather outbreaks, the first along the Red River Valley and the other in north-central Texas, were found to be associated with two sub-synoptic low pressure centers. The first low formed between 1900 and 2000 GMT over the Texas Panhandle, moved outward, and was located near Wichita Falls at 0000 GMT 11 April, the time of the deadly tornado there. It then moved slowly northeastward, having a lifetime of 7–8 hours. Barograph traces showed that a second low formed quickly at about 0120 GMT near Abilene causing additional severe storms to occur throughout the night in central Texas. After the formation of each sub-synoptic low, moisture convergence increased in the area, creating a local environment conducive to severe storm formation. In addition, the second surge of severe weather in north-central Texas was accompanied by an eastward protrusion of the dry line. Tegtmeier's idealized sub-synoptic low (SSL) dryline wave (DLW) model agreed well with the present study. Severe thunderstorm formation occurred northeast of the SSL and also along the DLW in accordance with his conceptual model.

A Synoptic Climatology of Northwest Flow Severe Weather Outbreaks. Part I: Nature and Significance

Abstract A climatology of severe weather outbreaks occurring in areas of the contiguous United States where the mid-troposphere flow has a north of west component has been developed for the period 1962–77. During the 16 years, 163 outbreaks of severe weather have been identified that fit a specific set of criteria for “northwest flow”. Analyses of this data set reveal the diurnal, seasonal and geographical frequencies and characteristics of this phenomenon. The nature of “northwest flow” outbreaks is examined in relation to the effect on life and property.

On Upper Tropospheric Kinematics and Severe Weather Occurrence

Abstract Upper tropospheric wind maxima and their associated divergence fields are examined in terms of severe weather occurrence. A two-part project is described. Part I involves examination of severe weather occurrences and divergence areas qualitatively evaluated in terms of upper tropospheric wind maxima quadrants and the curvature of the flow. Part II calculates synoptic-scale divergence directly from radiosonde wind data. These quantitative divergence estimates are compared with the location of upper tropospheric wind maxima, wave troughs and severe weather occurrence. It is concluded that regions of upper tropospheric divergence are present with the occurrence of severe weather and, in particular, with the occurrence of severe weather outbreaks. Several examples are presented to support the ideas discussed.