SEVERE WEATHER -HOMEWORK NUMBER 2
3) What group is most vulnerable to injury and death from derechos? Why is that the case?
18) What role did the developing LLJ play in enhancing storm development? 19) At 09Z, a bow echo formed and accelerated to the ESE. Why did this occur? 20) Describe the observed values for Precipitable Water and mid-level lapse rates in the vicinity of the MCS. What did they contribute to the developing derecho? How did they affect the approach of the SPC? 21) What is DCAPE and how did it compare to typical values associated with MCS environments? What caused this? 22) While the derecho is based on downdrafts and downbursts, why are updrafts considered just as important in their development? The MetEd quiz will count for 33% of your 2nd quarter homework grade. You will take an exam on Parts 2 and 3 of the homework. All are due by the Second Quarter due date listed in the Syllabus. Annotations
Click on each of the images shown in the “Radar Imagery†section. — Three distinct radar-based signatures are mentioned in the text. List and describe each one and indicate where it was found relative to Vivian.
=0.5 Degree Reflectivity Loop of Event 4 Panel Radar Reflectivity valid at 2258 UTC (0.5, 0.9, 1.3, 1.8 degrees) 4 Panel Storm Relative Velocity valid at 2258 UTC (0.5, 0.9, 1.3, 1.8 degrees) ~ The storm scale structure of the storm was very impressive. In the images above, three body scatter spikes (TBSSs) are noted at several elevation angles at 2258 UTC. In addition, a very pronouced bounded weak echo region (BWER), and very intense mesocyclone are also indicative of a strongly rotating updraft. Initial estimates indicate that the updraft strength in the Vivian hail storm likely ranged from 160-180 mph!
1) What were the dimensions of the hail stone which struck Vivian? How does that compare to theAurora, NE and Coffeyville, KS hail stones?
A record setting hailstone was ultimately discovered in Vivian, measuring 8.0 inches in diameter, 18.625 inches in circumference, and weighing in at an amazing 1.9375 pounds!! - This hailstone broke the previous United States hail size record for diameter (7.0 inches - 22 June 2003 in Aurora, NE) and weight (1.67 pounds - 3 September 1970 in Coffeyvile, KS). The Aurora, Nebraska hailstone will retain the record for circumference (18.75 inches).
4) (Under the heading "Casualty and Damage Risks, click on the link to the July 4-5, 1980 derecho). Whatwas associated with the greatest number of injuries? Deaths?
Boats overturned...............................................2 injuries and 4 deathsVacation campers overturned............................6 injuriesMobile homes overturned................................20 injuriesTrees fell on people in campgrounds.................5 injuries and 1 deathTrees and limbs fell on people...........................4 injuriesTrees fell on mobile homes................................4 injuriesTrees fell on a vehicle........................................6 injuries and 1 deathUtility pole fell on a vehicle.................................1 injuryWindows broken and flying glass.......................5 injuriesHome heavily damaged......................................4 injuries ————- https://www.spc.ncep.noaa.gov/misc/AbtDerechos/casepages/jul4-51980page.htm#casualtylist.
14) What does this map illustrate? How would this factor aid in the development and maintenance ofsupercells?
Bulk Shear - The bulk wind difference over a layer, calculated by vector subtraction. Bulk shear through 0-6 km AGL layer discriminates strongly between supercell and nonsupercell thunderstorm environments. The transition from nonsupercell to supercell thunderstorms occurs as the 0-6 km bulk wind difference increases from roughly 25 kt to 40 kt, with larger values favoring supercells. Research also suggests that increasing values in the 0-6km layer correlate with increasing tornado potential.
1) What are the criteria for derecho classification? (be sure to refer to the lectures to complete the criteria)
Cloud shield must meet the following criteria: ○ Size A: Continuous cold cloud shield with IR temperatures ≤ -32°C with an area ≥ 100,000 km2 ○ Size B: Interior cold cloud region with IR temperatures ≤ -52°C with an area ≥ 50,000 km2 ○ Initiate: Size A and B are met ○ Duration: Size A and B must be met for ≥ 6 hours ○ Maximum extent: Continuous cloud shield (≤ -32°C) reaches maximum size ○ Shape: Major axis/minor axis ≥ 0.7 at maximum extent ○ Terminate: Size A and B are no longer satisfied
4= What was the speed of the updraft which produced the hail stone
Initial estimates indicate that the updraft strength in the Vivian hail storm likely ranged from 160-180 mph!
7) Describe the "low dew point" derecho in terms of environment and temporal distribution
Low-dewpoint derechos Most derecho-producing thunderstorm systems originate in and/or move through areas of seasonably rich low-level moisture. For example, nearly all the cases included in Noteworthy Events occured with surface dewpoints at or above 70° Fahrenheit. In contrast, bands of widespread wind-producing storms sometimes form in environments of very limited moisture, for example, with dewpoints in the 40s or low 50s (°F). Such systems are known as low-dewpoint derechos. Low-dewpoint derechos most often occur between late fall and early spring in association with strong low pressure systems, and are a form of hybrid derecho. On occasion they appear during the warmer half of the year, especially in drier regions. The May 31, 1994 Utah-Wyoming derecho shown in the animated satellite loop in Derecho Development (above) is an example of a low-dewpoint event. This system produced a 105 mph wind gust at Provo, Utah, where sixteen people were injured, and severely damaged the Saltair Pavilion on the Great Salt Lake. A wind gust of 140 mph was measured on Camelback Mountain in nearby Dugway Proving Ground. Surface dewpoints were in the mid-40s °F along the path of the storm. Low-dewpoint derechos are essentially organized bands of successive, dry downbursts or microbursts. Some low-dewpoint systems appear to be ordinary serial-type events that happen to form in drier-than-average environments, while others more closely resemble progressive derechos, producing serial, convectively-driven downdrafts. More information on derechos in environments of limited moisture is available here.
Click on the Lowest 100mb MLCAPE map and answer the following questions. (NOTE: the contours are MLCAPE and the hatched, shaded area represents MLCIN)
https://pdfs.semanticscholar.org/ec84/7b5430f03e0852edab1f57e42f5f6f27889d.pdf
20) Describe the observed values for Precipitable Water and mid-level lapse rates in the vicinity of the MCS. What did they contribute to the developing derecho? How did they affect the approach of the SPC?
le. Evolution of the thermodynamic environment after MCS development Along with the strong surge of low-level southerly flow, the environment feeding into the developing MCS by 0700 UTC became very moist. - In fact, a sizeable region of precipitable water (PW) over 5 cm was analyzed over southern Kansas (Fig. 13a), in which as > 3 in some areas (Fig. 13d). -The standardized anomalies were large in the region where the MCS developed bow echo characteristics and accelerated to the east-southeast (Fig. 3e). Very high low-level moisture content is a common characteristic of severe, long-lived MCSs in both the warm and cool seasons (Johns and Hirt 1987; Johns 1993; Coniglio et al. 2004; Burke and Schultz 2004). Also in contrast to the environment for the initial convective development was a region of very steep mid-level lapse rates (3 - 6 km lapse rates > 8.5 K km-1) ahead of the maturing MCS (Fig. 14a). In fact, the values of as > 4 for the lapse rates in far north central Oklahoma (Fig. 14d) were the largest standardized anomalies found for any variable at any time during the MCS development. Consequently, the very steep lapse rates and the high moisture content in the lowest 2 km AGL (Fig. 8) produced unusually large values of CAPE along the ensuing MCS path, particularly in the few km above the level of free convection. One of the motivations to study this event arose from the ability of the NOAA Storm Prediction Center to anticipate an unusually strong convective wind event. The recognition of sufficient (although not unusually large) downdraft CAPE (DCAPE) to help organize convective outflows early in the convective event and sufficient mean flow/deep shear to organize the convective updrafts played a role in the forecast process. But the recognition of very high PW values and lapse rates in the downstream environment played a decisive role in the decision to issue a "Particularly Dangerous Situation" (PDS) Severe Thunderstorm Watch for this event, along with the recognition that a strong LLJ impinging on this area was likely to lead to an abundance of strong thunderstorms in a relatively confined region. Operational experience suggests that a high spatial concentration of thunderstorms commonly precedes the development of derechos.
10) Describe the factors that determine the overall motion.
the same time, the background wind field around a derecho rarely remains static. Because the overall motion of a derecho-producing convective system is a combination of (1) advection (the movement of individual storm cells by the environmental wind outside the storms), and (2) propagation (the development of new storm cells relative to older ones), spatial and/or temporal changes in the environmental wind add further complexity to the forecasting of derecho tracks.
Estimate the temperature, dew point and pressure at 22Z in Vivian.
with dew points of 21 to 22°C
15) Where did the "Super Derecho" originate?
After 0000 UTC on 8 May, weak convection continued to form over north central Colorado. A southwest-moving outflow boundary originating from this convection interacted with the Denver cyclone to help initiate convection over northeastern Colorado. Precipitation falling through the deeply-mixed boundary layer with very steep low-level lapse rates (Fig. 4) likely fostered the development of evaporatively cooled low-level air that expanded rapidly over northeastern Colorado through 0300 UTC (Fig. 3c). b. Evolution of the environmenT
19) At 09Z, a bow echo formed and accelerated to the ESE. Why did this occur?
Along with the strong surge of low-level southerly flow, the environment feeding into the developing MCS by 0700 UTC became very moist. In fact, a sizeable region of precipitable water (PW) over 5 cm was analyzed over southern Kansas (Fig. 13a), in which as > 3 in some areas (Fig. 13d). The standardized anomalies were large in the region where the MCS developed bow echo characteristics and accelerated to the east-southeast (Fig. 3e). Very high low-level moisture content is a common characteristic of severe, long-lived MCSs in both the warm and cool seasons (Johns and Hirt 1987; Johns 1993; Coniglio et al. 2004; Burke and Schultz 2004). Also in contrast to the environment for the initial convective development was a region of very steep mid-level lapse rates (3 - 6 km lapse rates > 8.5 K km-1) ahead of the maturing MCS (Fig. 14a). In fact, the values of as > 4 for the lapse rates in far north central Oklahoma (Fig. 14d) were the largest standardized anomalies found for any variable at any time during the MCS development. Consequently, the very steep lapse rates and the high moisture content in the lowest 2 km AGL (Fig. 8) produced unusually large values of CAPE along the ensuing MCS path, particularly in the few km above the level of free convection.
6- Define and differentiate between the terms "downburst", "downburst cluster", "microburst", and"burst swath". How do they form?
DOWNBURST: This is because the swaths of stronger winds within the general path of a derecho are produced by what are called downbursts, and downbursts often occur in irregularly-arranged clusters, downburst is a concentrated area of strong wind produced by a convective downdraft. Downbursts have horizontal dimensions of about 4 to 6 miles (8 to 10 kilometers), and may last for several minutes. The convective downdrafts that comprise downbursts form when air is cooled by the evaporation, melting, and/or sublimation (the direct change to vapor phase) of precipitation in thunderstorms or other convective clouds. Because the chilled air is denser than its surroundings, it becomes negatively buoyant and accelerates down toward the ground. DOWNBURST CLUSTER: downburst clusters" that arise in such situations may attain overall lengths of up to 50 or 60 miles (80 to 100 kilometers), and persist for several tens of minuteS MICROBURSTS: Within individual downbursts there sometimes exist smaller pockets of intense winds called microbursts. Microbursts occur on scales (approximately 2 1/2 miles or 4 km) that are very hazardous to aircraft; several notable airline mishaps in recent decades resulted from unfortunate encounters with microbursts. Still smaller areas of extreme wind within microbursts are called burst swaths. Burst swaths range from about 50 to 150 yards (45 to 140 meters) in length. The damage they produce may resemble that caused by a tornado.
16) List and describe the 5 factors which contributed to the initial development of the derecho.
However, consideration of those environmental parameters that were unusual has important implications for predicting the strength of downdraft outflow, by virtue of what these parameters say about updrafts. For example, anomalously strong fields in this event included low-level storm inflow, PW, conditional instability, and inertial instability aloft. The combination of these multiple factors suggests that intense upward mass fluxes were strongly favored in this event - the potential energy supply was high, the relative humidity was high over deep layers (favoring minimal dilution of updrafts by entrainment), and an upper tropospheric environment was present that could accommodate massive detrainment of mass. Why are these factors important for downdrafts? In a given thermodynamic environmONT
9) What factors help determine the location and rate of new storm development relative to a derecho?
Once a derecho has formed, the forecast task reduces to determining where the parent convective system will move, and how long will it last. As noted in Derecho Development, a derecho will persist as long as the environment on the downwind side of its elongating cold pool is favorable for the formation of new storms. But both the location and rate of new storm development in the downwind direction typically vary over space and time, with factors such as the distribution of low-level moisture, atmospheric instability, and "capping" (warm layers aloft that hinder storm formation) --- amongst others --- typically involved. Such variables complicate the forecast process. At the same time, the background wind field around a derecho rarely remains static. Because the overall motion of a derecho-producing convective system is a combination of (1) advection (the movement of individual storm cells by the environmental wind outside the storms), and (2) propagation (the development of new storm cells relative to older ones), spatial and/or temporal changes in the environmental wind add further complexity to the forecasting of derecho tracks. Various parameters and techniques have been developed in recent years to help forecast derecho formation, movement, and longevity (see References). These tools will require modification as observational and modelling studies provide further understanding of derecho mechanics.
12) What is the SHIP used to forecast? What is the threshold value for this index?
Significant Hail Parameter (SHiP)The Sig. Hail Parameter (SHIP) was developed using a large database of surface-modified, observed severe hail proximity soundings. It is based on 5 parameters, and is meant to delineate between SIG (>=2" diameter) and NON-SIG (<2" diameter) hail environments.SHIP = [(MUCAPE j/kg) * (Mixing Ratio of MU PARCEL g/kg) * (700-500mb LAPSE RATE c/km) * (-500mb TEMP C) * (0-6km Shear m/s) ] / 44,000,000It is important to note that SHIP is NOT a forecast hail size.Developed in the same vein as the STP and SCP parameters, values of SHIP greater than 1.00 indicate a favorable environment for SIG hail. Values greater than 4 are considered very high. In practice, maximum contour values of 1.5-2.0 or higher will typically be present when SIG hail is going to be reported. —-\ \—- Significant Hail ParameterThe Sig. Hail Parameter (SHIP) was developed using a large database of surface-modified, observed severe hail proximity soundings. It is based on 5 parameters, and is meant to delineate between SIG (>=2" diameter) and NON-SIG (<2" diameter) hail environments. It is important to note that SHIP is NOT a forecast hail size. Since SHIP is based on the RUC depiction of MUCAPE, unrepresentative MUCAPE "bullseyes" may cause a similar increase in SHIP values. This typically occurs when bad surface observations get into the RUC model. Developed in the same vein as the STP and SCP parameters, values of SHIP greater than 1.00 indicate a favorable environment for SIG hail. Values greater than 4 are considered very high. In practice, maximum contour values of 1.5-2.0 or higher will typically be present when SIG hail is going to be reported.
Why would a bad surface observation lead to an incorrect value for SHIP?
Since SHIP is based on the RAP depiction of MUCAPE - unrepresentative MUCAPE "bullseyes" may cause a similar increase in SHIP values. This typically occurs when bad surface observations get into the RAP model. Developed in the same vein as the STP and SCP parameters, values of SHIP greater than 1.00 indicate a favorable environment for SIG hail. Values greater than 4 are considered very high. In practice, maximum contour values of 1.5-2.0 or higher will typically be present when SIG hail is going to be reported. Figure 1 - Distribution of SHIP for two data sets - Ping-Pong or smaller hail (left), and Tennis Ball and larger (right). All 1.75"-2.00" reports are filtered out in order to magnify separation based on SHIP. The 10th, 25th, 75th, and 90th percentiles are shown for each subset.
Estimate the MLCAPE value at 22Z in Vivian.
The MLCAPE and the 03-km SRH were greater than the 90t h percentiles of Rasmussen and Blanchard (1998) for storms that produced > 5.07 cm (2.0 in) diameter hail, and the boundary layer to 6-km shear was near the 75th percentile. The freezing level was 4 km (13,100 ft) above ground level (AGL), and the wet bulb zero height was at 3.2 km (10,500 ft) AGL. Although significant melting of hail can occur with these freezing levels (e.g., Johns and Doswell 1992), the height of the freezing level proved insignificant in this case. Donavon and Jungbluth (2007) noted that storms with a MLCAPE (red) contoured every 500 J kg-1 and MLCIN (fill and blue dashed lines, J kg-1) valid for 2200 UTC 23 July 2010. The black star represents the approximate location of Vivian, SD. Image adapted from SPC. Click image for larger version.
18) What role did the developing LLJ play in enhancing storm development?
The atmosphere was undergoing rapid changes by 0600 UTC in response to the development of a strong south-southwesterly LLJ over the Southern Plains. The jet tapped a reservoir of very moist air over western Oklahoma and the Texas panhandle. Low-level horizontal convergence (not shown) increased in the region ahead of the LLJ and south of the outflow boundary that extended west from the original convection (Figs. 3c and 3d). This enhanced low-level convergence likely aided the development of thunderstorms along the southwardsurging outflow boundary in west central Kansas at 0600 UTC (Fig. 3d). After 0600 UTC, the convective outflow surged 20 - 30 km ahead of the weakening original convection. Explosive convection subsequently developed along
11) What is "echo training"? How does it relate to a derecho?
The persistent source of low-level uplift provided by the stalled gust front can then serve as the seat of repetitive thunderstorm development in the upstream direction. As individual storms grow and mature, they move parallel to the boundary, causing multiple episodes of heavy rain at locations along the line. Such convective evolution is known as echo training. Prolonged echo-training in a moisture-rich environment nearly always results in excessive rainfall or flash flooding. Smaller scale or more intermittent episodes of echo training frequently occur on the rear flanks of derechos, and may cause localized flooding in the wake of a derecho's high winds. In addition, back-building lines of storms sometimes form atop the cold pool left in the wake of a derecho; the combined derecho-heavy rain producing storm structure is then known as a bow-arrow convective system. More information on the relationship between derechos and flash-flood producing convective systems is available here.
5) What feature is found to the SW of Vivian (NW Nebraska)?
The images below depict the synoptic and mesoscale environment at 22:00 UTC (6PM CDT) - about one hour before the record hailstone fell in Vivian. Note that the kinematic and thermodynamic environment was very supportive of supercell thunderstorms due to the high shear and high CAPE environment.
14) By what mechanism can tornadoes form in an environment with unidirectional shear?
The occurrence of tornadoes with derecho-producing convective systems reflects the fact that both tornadoes and strong convective wind gusts share, to some extent, common origins in the background atmospheric environment. In short, the great degree of thermodynamic instability; i.e., buoyancy, that gives rise to strong updrafts and, ultimately, the thunderstorms that spawn tornadoes also promotes the formation of storm downdrafts. In addition, both tornado and derecho environments are characterized by the presence of substantial vertical wind shear; i.e., large changes in wind speed and/or direction with height. While derecho-producing convective systems tend to be most favored when the vertical wind profile is unidirectional, a unidirectional wind profile may still contain appreciable shear. At the same time, in even a modestly sheared environment, small-scale stretching and tilting motions often present along storm gust fronts in a squall line may yield low-level circulations that, on occasion, can "tighten up" into a tornado.
12) What is an "EML"? How does it relate to both heat waves and derechos? Which types of derechosare common in a heat wave and where do are they most likely to develop?
The primary link between heat waves and derechos is the presence of an elevated mixed layer, or EML. An EML is a layer of mid-tropospheric air that originates over the arid, elevated terrain. Because of their origin, EMLs exhibit sharp decreases in temperature with height. The large vertical temperature differentials (or "steep" lapse rates) in EMLs are analogous to those observed over black-topped roofs and parking lots on sunny days. Such thermal stratification encourages the formation of strong updrafts that can lead to the development of thunderstorms. In fact, the frequent presence of an EML on days othwerwise favorable for thunderstorm formation to a large extent accounts for the intensity of the storms commonly encountered over the Great Plains. During a typical heat wave over the central and eastern United States, a large, stationary upper-level high pressure area usually is present over the south-central states. Persistent westerly winds on the poleward side of the high allow EMLs generated over the Rocky Mountains to extend eastward into the Ohio Valley and Northeast, well beyond their usual range over the Plains. Warm air aloft associated with the base of the EML acts as a "cap" or "lid" that prohibits thunderstorm development along much of the extent of the EML, southward into the heat wave-associated "high." But on the northern fringe of the EML, where low-level uplift frequently is focused along a stationary front marking the northern edge of the heat wave, updrafts that form in the strongly heated air near the ground may breach the cap, resulting in an explosive release of instability. If other conditions are favorable (e.g., low-level moisture is abundant along the front, winds are largely unidirectional, parallel to it, and increase with height), additional storms may erupt in concentrated fashion along the boundary, yielding a band of downstream-developing storms and, on occasion, a full-blown derecho.
21) What is DCAPE and how did it compare to typical values associated with MCS environments? What caused this?
The recognition of sufficient (although not unusually large) downdraft CAPE (DCAPE) to help organize convective outflows early in the convective event and sufficient mean flow/deep shear to organize the convective updrafts played a role in the forecast process. But the recognition of very high PW values and lapse rates in the downstream environment played a decisive role in the decision to issue a "Particularly Dangerous Situation" (PDS) Severe Thunderstorm Watch for this event, along with the recognition that a strong LLJ impinging on this area was likely to lead to an abundance of strong thunderstorms in a relatively confined region. Operational experience suggests that a high spatial concentration of thunderstorms commonly precedes the development of derechos.
UPDRAFTS
The recognition of sufficient (although not unusually large) downdraft CAPE (DCAPE) to help organize convective outflows early in the convective event and sufficient mean flow/deep shear to organize the convective updrafts played a role in the forecast process. But the recognition of very high PW values and lapse rates in the downstream environment played a decisive role in the decision to issue a "Particularly Dangerous Situation" (PDS) Severe Thunderstorm Watch for this event, along with the recognition that a strong LLJ impinging on this area was likely to lead to an abundance of strong thunderstorms in a relatively confined region. Operational experience suggests that a high spatial concentration of thunderstorms commonly precedes the development of derechos.
2) The stone was not found immediately and may have shrunk. By what two processes was the stonereduced?
The stone did shrink considerably (melting and sublimation) between impact and when it was first measured by NWS personnel due in part to a power outgage at the residence of the individual who found the stone. —-It should also be noted that many other stones with diameters exceeding 6 inches were also noted during the storm survey!
2) Why do the wind speeds associated with a derecho show variability?
The winds associated with derechos are not constant and may vary considerably along the derecho path, sometimes being below severe limits (57 mph or less), and sometimes being very strong (from 75 mph to greater than 100 mph). This is because the swaths of stronger winds within the general path of a derecho are produced by what are called downbursts, and downbursts often occur in irregularly-arranged clusters, along with embedded microbursts and burst swaths. Derechos might be said to be made up of families of downburst clusters that extend, by definition, continuously or nearly continuously for at least 250 miles (about 400 km). The derecho of July 4-5, 1980 is a good example of an event that exhibited wide variation in observed wind speeds due to embedded microbursts, downbursts, and downburst clusters. More on microbursts, downbursts, and downburst clusters may be found in Derecho-producing storms.
3) What group is most vulnerable to injury and death from derechos? Why is that the case?
Those most at risk from derechos Because derechos are most common in the warm season, those involved in outdoor activities are especially at risk. Campers or hikers in forested areas are vulnerable to being injured or killed by falling trees, and those at sea risk injury or drowning from storm winds and high waves that can overturn boats. Occupants of cars and trucks also are vulnerable to falling trees and utility poles. Further, high profile vehicles such as semi-trailer trucks, buses, and sport utility vehicles may be blown over. At outside events such as fairs and festivals, people may be killed or injured by collapsing tents and flying debris. Even those indoors may be at risk for death or injury during derechos. Mobile homes, in particular, may be overturned or destroyed, while barns and similar buildings can collapse. People inside homes, businesses, and schools are sometimes victims of falling trees and branches that crash through walls and roofs; they also may be injured by flying glass from broken windows. Finally, structural damage to the building itself (for example, removal of a roof) can pose danger to those within. Another reason that those outdoors are especially vulnerable to derechos is the rapid movement of the parent convective system. Typically, derecho-producing storm systems move at speeds of 50 mph or greater, and a few have been clocked at 70 mph. For someone caught outside, such rapid movement means that darkening skies and other visual cues that serve to alert one to the impending danger (e.g., gust front shelf clouds --- see photos elsewhere on this page) appear on very short notice. In summary, the advance notice given by a derecho often is not sufficient for one to take protective action. The following links provide personal stories of those who have survived derechos while outdoors: - A camper's close brush with death during the July 4-5, 1999 derecho in Maine. - A boater's encounter with the May 17, 1986 derecho on Lake Livingston, Texas. - A dramatic account of a boat overturned by intense straight-line winds during a July 1943 wind storm near Goshen, Indiana. For a more comprehensive view of derecho hazards, the "Noteworthy Event" page for the July 4-5, 1980 derecho lists the cause of death or injury for the 73 casualties of that event, providing a typical example of the risks to humans posed by derechos.
13) Which type of derecho is most conducive to tornado development? Where are these tornadoesfound relative to the system?
Tornadoes Derechos and tornadoes can occur with the same convective system. This is particularly so with serial derechos associated with strong, migratory low pressure systems. The tornadoes may occur with isolated supercells (rotating thunderstorms) ahead of the derecho producing squall line, or they may develop from storms within the squall line itself. An example of a serial derecho that produced both extremely damaging straight-line winds and significant tornadoes from supercells embedded in the derecho-producing squall line is that which affected Florida during the early stages of the so-called "Storm of the Century" of March 12-13, 1993. Although not as common, tornadoes sometimes occur with progressive derechos. When they do, the tornadoes typically form within the bow echo storm system itself, and only rarely are associated with isolated supercells ahead of the bow. The occurrence of tornadoes with derecho-producing convective systems reflects the fact that both tornadoes and strong convective wind gusts share, to some extent, common origins in the background atmospheric environment. In short, the great degree of thermodynamic instability; i.e., buoyancy, that gives rise to strong updrafts and, ultimately, the thunderstorms that spawn tornadoes also promotes the formation of storm downdrafts. In addition, both tornado and derecho environments are characterized by the presence of substantial vertical wind shear; i.e., large changes in wind speed and/or direction with height. While derecho-producing convective systems tend to be most favored when the vertical wind profile is unidirectional, a unidirectional wind profile may still contain appreciable shear. At the same time, in even a modestly sheared environment, small-scale stretching and tilting motions often present along storm gust fronts in a squall line may yield low-level circulations that, on occasion, can "tighten up" into a tornado.
11) What two factors are used to calculate the parameter? Why would these parameters be useful indetermining severe potential?
Traditionally, composite parameters designed to im- prove the forecasting of severe hazards—particularly tornadoes—have used CAPE as a constituent ingredient; examples include STP (Thompson et al. 2012), the su- percell composite parameter [SCP; effective-layer version from Thompson et al. (2004)], the vorticity generation parameter10 (VGP; Rasmussen and Blanchard 1998), the energy helicity index10 (EHI; Rasmussen and Blanchard 1998), and the Craven-Brooks significant severe param- eter (Craven and Brooks 2004). As a result of this reliance on CAPE (and because these parameters were developed using datasets consisting of a wider range of environmental conditions—e.g., the inclusion of a large number of springtime plains tornado events—leading to relatively high normalization values for CAPE components), oper- ational meteorologists have noted a general un- derestimation of risk in HSLC environments by composit https://journals.ametsoc.org/doi/pdf/10.1175/WAF-D-13-00041.1 https://www.spc.ncep.noaa.gov/exper/mesoanalysis/help/begin.html
5) Discuss why large metropolitan areas are particularly vulnerable to derechos. In your answer, explainwhy the annual pattern of derecho occurrence exacerbates the problem.
Whether in an urban or rural area, those out-of-doors are at greatest risk of being killed or injured in a derecho. But of particular significance in urban areas is the vulnerability of electrical lines to high winds and falling trees. In addition to posing a direct hazard to anyone caught below the falling lines, derecho damage to overhead electric lines sometimes results in massive, long-lasting power outages . Hundreds of thousands of people may be affected; in the worst events, power may not be restored for many days. It is the complex and dense concentration of overhead distribution feeders in urban areas --- and their frequent proximity to large trees --- that make cities especially vulnerable to electrical outages following wind storms . The density and mileage of overhead electric distribution lines in urban areas far exceeds that of any rural or exurban area. Pole lines often carry multiple circuits and voltages, as well as lines for street lighting and customer service connections that further add to their vulnerability. Because of this, and because urban electrical feeders typically serve smaller territories relative to their rural counterparts, significantly greater manpower is necessary to restore service after major storms. In addition, unlike the localized damage produced by a tornado, derecho damage may be widespread. As a result, repairs often require greater effort, with additional delays related to shortages in supplies. Cities in which derechos have resulted in prolonged power outages include Baltimore (June 29, 1980), Kansas City (June 7, 1982), and Memphis (July 22, 2003). More recently, the Ohio Valley / Mid-Atlantic derecho of June 29, 2012 caused protracted, widespread power outages in cities from Cincinnati, Columbus, and Dayton to Atlantic City, Baltimore, and Washington, D.C. There also is evidence to suggest that the impact posed by derechos has increased in recent years due, in part, to the maturation of shade trees planted in suburban areas in the 1950s and 1960s. The vast tracks of post-war suburbs, with their overhead utility lines and older trees, are especially vulnerable to damage from high winds of any source.
8) Why are derechos difficult to forecast? Which type is easiest to forecast? Why is that the case? Howcould derecho forecasting be improved?
some ways, serial derechos are easier to forecast than progressive events because the primary forcing mechanisms responsible for their development (e.g., strong cold fronts or jet stream disturbances) are fairly well sampled by today's observing network. On the other hand, unlike many other significant weather events of any type, progressive derechos may arise in relatively benign large-scale environments, with little or no identifiable atmospheric forcing. Many progressive derechos, in fact, arise in a manner that might best be described as a form of "atmospheric boot-strapping," wherein seemingly random interactions between individual thunderstorm cells --- or a loosely organized group of cells --- sometimes lead to a much larger, more strongly-organized system. One factor that appears important in realizing such development is the release of very strong latent heating in a relatively confined region. This may occur through the simultaneous development of many vigorous individual thunderstorm cells near a weak front or other local lifting mechanism. Such latent heating involves the lofting of abundant rain, snow, and hail particles that can lead to the rapid formation of strong storm downdrafts and an expanding cold pool. The "Super Derecho" of May 8, 2009 is an example of a storm that exhibited this type of development.
