Derechos and MCSs by Stephen Corfidi
NWS Storm Prediction Center / NWS Warning Decision Training Division PowerPoint introduction to d... more NWS Storm Prediction Center / NWS Warning Decision Training Division PowerPoint introduction to derechos and derecho-producing convective systems
NWS Storm Prediction Center / NWS Warning Decision Training Division Powerpoint presentation on t... more NWS Storm Prediction Center / NWS Warning Decision Training Division Powerpoint presentation on the determining the motion and predominant hazards (damaging wind vs. flash-flooding) posed by mesoscale convective systems (MCSs)
Weather and Forecasting, 2017
The goal of this study is to document differences in the convective structure and motion of long-... more The goal of this study is to document differences in the convective structure and motion of long-track, severe-wind-producing MCSs from short-track severe-wind-producing MCSs in relation to the mean wind. An ancillary goal is to determine if these differences are large enough that some criterion for MCS motion relative to the mean wind could be used in future definitions of ''derechos.'' Results confirm past investigations that well-organized MCSs, including those that produce derechos, tend to move faster than the mean wind, exhibiting a significantly larger degree of propagation (component of MCS motion in addition to the component contributed by the mean flow). Furthermore, well-organized systems that produce shorter-track swaths of damaging winds likewise tend to move faster than the mean wind with a significant propagation component along the mean wind. Therefore, propagation in the direction of the mean wind is not necessarily a characteristic that can be used to distinguish derechos from nonderechos. However, there is some indication that long-track damaging wind events that occur without large-scale or persistent bow echoes and mesoscale convective vortices (MCVs) require a strong propagation component along the mean wind direction to become long lived. Overall, however, there does not appear to be enough separation in the motion characteristics among the MCS types to warrant the inclusion of a mean-wind criterion into the definition of a derecho at this time.
Bulletin of the American Meteorological Society, 2016
A revised, more physically based definition of " derecho " is proposed that is more specific to t... more A revised, more physically based definition of " derecho " is proposed that is more specific to the type of intense convective windstorm that first inspired the term in the late-nineteenth century.
Monthly Weather Review, 2012
This work presents an analysis of the vertical wind shear during the early stages of the remarkab... more This work presents an analysis of the vertical wind shear during the early stages of the remarkable 8 May 2009 central U.S. derecho-producing convective system. Comments on applying Rotunno-Klemp-Weisman (RKW) theory to mesoscale convective systems (MCSs) of this type also are provided. During the formative stages of the MCS, the near-surface-based shear vectors ahead of the leading convective line varied with time, location, and depth, but the line-normal component of the shear in any layer below 3 km ahead of where the strong bow echo developed was relatively small (6-9 m s 21). Concurrently, the midlevel (3-6 km) line-normal shear component had magnitudes mostly .10 m s 21 throughout. In a previous companion paper, it was hypothesized that an unusually strong and expansive low-level jet led to dramatic changes in instability, shear, and forced ascent over mesoscale areas. These mesoscale effects may have overwhelmed the interactions between the cold pool and low-level shear that modulate system structure in less complex environments. If cold pool-shear interactions were critical to producing such a strong system, then the extension of the line-normal shear above 3 km also appeared to be critical. It is suggested that RKW theory be applied with much caution, and that examining the shear above 3 km is important, if one wishes to explain the formation and maintenance of intense long-lived convective systems, particularly complex nocturnal systems like the one that occurred on 8 May 2009.
Weather and Forecasting, 2007
The problem of forecasting the maintenance of mesoscale convective systems (MCSs) is investigated... more The problem of forecasting the maintenance of mesoscale convective systems (MCSs) is investigated through an examination of observed proximity soundings. Furthermore, environmental variables that are statistically different between mature and weakening MCSs are input into a ...
Weather and Forecasting - WEATHER FORECAST, 2007
The prediction of the strength of mesoscale convective systems (MCSs) is a major concern to opera... more The prediction of the strength of mesoscale convective systems (MCSs) is a major concern to operational meteorologists and the public. To address this forecast problem, this study examines meteorological variables derived from sounding observations taken in the environment of quasi-linear MCSs. A set of 186 soundings that sampled the beginning and mature stages of the MCSs are categorized by their production of severe surface winds into weak, severe, and derecho-producing MCSs. Differences in the variables among these three MCS categories are identified and discussed. Mean low- to upper-level wind speeds and deep-layer vertical wind shear, especially the component perpendicular to the convective line, are excellent discriminators among all three categories. Low-level inflow relative to the system is found to be an excellent discriminator, largely because of the strong relationship of system severity to system speed. Examination of the mean wind and shear vectors relative to MCS motion suggests that cell propagation along the direction of cell advection is a trait that separates severe, long-lived MCSs from the slower-moving, nonsevere variety and that this is favored when both the deep-layer shear vector and the mean deep-layer wind are large and nearly parallel. Midlevel environmental lapse rates are found to be very good discriminators among all three MCS categories, while vertical differences in equivalent potential temperature and CAPE only discriminate well between weak and severe/derecho MCS environments. Knowledge of these variables and their distribution among the different categories of MCS intensity can be used to improve forecasts and convective watches for organized convective wind events.
Weather and Forecasting, 2006
An examination of severe wind-producing mesoscale convective systems that occur in environments o... more An examination of severe wind-producing mesoscale convective systems that occur in environments of very limited moisture is presented. Such systems, herein referred to as low-dewpoint derechos (LDDs), are difficult to forecast as they form in regions where the level of convective instability is well below that normally associated with severe convective weather. Using a dataset consisting of 12 LDDs that affected various parts of the continental United States, composite surface and upper-level analyses are constructed. These are used to identify factors that appear to be associated with LDD initiation and sustenance. It is shown that LDDs occur in mean kinematic and thermodynamic patterns notably different from those associated with most derechos. LDDs typically form along or just ahead of cold fronts, in the exit region of strong, upper-level jet streaks. Based on the juxtaposition of features in the composite analysis, it appears that linear forcing for ascent provided by the front, and/or ageostrophic circulations associated with the jet streak, induce the initial convective development where the lower levels are relatively dry, but lapse rates are steep. This convection subsequently grows upscale as storm downdrafts merge. The data further suggest that downstream cell propagation follows in the form of sequential, downwind-directed microbursts. Largely unidirectional wind profiles promote additional downwind-directed storm development and system sustenance until the LDD ultimately moves beyond the region supportive of forced convective initiation.
Weather and Forecasting, 2003
The primary factors that affect the direction of propagation and overall movement of surface-base... more The primary factors that affect the direction of propagation and overall movement of surface-based mesoscale convective systems (MCSs) are discussed. It is shown that although propagation is indeed related to the strength and direction of the low-level jet as previous studies have shown, it is more specifically dependent upon the degree of cold-pool-relative flow and to the distribution of conditional instability present along a system's gust front. An updated technique that may be used to forecast the short-term (3–6 h) motion of MCS centroids based on these concepts is introduced. The procedure builds on the long-established observation that MCS motion is a function of 1) the advection of existing cells by the mean wind and 2) the propagation of new convection relative to existing storms. Observed wind and thermodynamic data, in conjunction with anticipated cold-pool motion and orientation, are used to assess the speed and direction of cell propagation, that is, whether propagation will be upwind, downwind, or some combination of the two. The technique ultimately yields an estimate of overall system movement and has application regardless of scale, season, or synoptic regime.
Weather and Forecasting, 1996
A procedure for operationally predicting the movement of the mesobeta-scale convective elements r... more A procedure for operationally predicting the movement of the mesobeta-scale convective elements responsible for the heavy rain in mesocale convective complexes is presented. The procedure is based on the well-known concepts that the motion of convective systems can be considered the sum of an advective component, given by the mean motion of the cells composing the system, and a propagation component, defined by the rate and location of new cell formation relative to existing cells. These concepts and the forecast procedure are examined using 103 mesoscale convective systems, 99 of which are mesoscale convective complexes.
It is found that the advective component of the convective systems is well correlated to the mean flow in the cloud layer. Similarly, the propagation component is shown to be directly proportional (but opposite in sign) and well correlated to the speed and direction of the low-level jet. Correlation coefficients between forecast and observed values for the speed and direction of the mesobeta-scale convective elements are 0.80 and 0.78, respectively. Mean absolute errors of the speed and direction are 2.0 m s−1 and 17°. These errors are sufficiently small so that the forecast path of the centroid of the mesobeta-scale elements would be well within the heavy rain swath of the typical mesoscale convective complex.
Elevated convection by Stephen Corfidi
Weather and Forecasting, 2008
The term elevated convection is used to describe convection where the constituent air parcels ori... more The term elevated convection is used to describe convection where the constituent air parcels originate from a layer above the planetary boundary layer. Because elevated convection can produce severe hail, damaging surface wind, and excessive rainfall in places well removed from strong surface-based instability, situations with elevated storms can be challenging for forecasters. Furthermore, determining the source of air parcels in a given convective cloud using a proximity sounding to ascertain whether the cloud is elevated or surface based would appear to be trivial. In practice, however, this is often not the case. Compounding the challenges in understanding elevated convection is that some meteorologists refer to a cloud formation known as castellanus synonymously as a form of elevated convection. Two different definitions of castellanus exist in the literature—one is morphologically based (cloud formations that develop turreted or cumuliform shapes on their upper surfaces) and the other is physically based (inferring the turrets result from the release of conditional instability). The terms elevated convection and castellanus are not synonymous, because castellanus can arise from surface-based convection and elevated convection exists that does not feature castellanus cloud formations. Therefore, the purpose of this paper is to clarify the definitions of elevated convection and castellanus, fostering a better understanding of the relevant physical processes. Specifically, the present paper advocates the physically based definition of castellanus and recommends eliminating the synonymity between the terms castellanus and elevated convection.
Weather and Forecasting, 2007
A 5-yr climatology of elevated severe convective storms was constructed for 1983–87 east of the R... more A 5-yr climatology of elevated severe convective storms was constructed for 1983–87 east of the Rocky Mountains. Potential cases were selected by finding severe storm reports on the cold side of surface fronts. Of the 1826 days during the 5-yr period, 1689 (91%) had surface fronts east of the Rockies. Of the 1689 days with surface fronts, 129 (8%) were associated with elevated severe storm cases. Of the 1066 severe storm reports associated with the 129 elevated severe storm cases, 624 (59%) were hail reports, 396 (37%) were wind reports, and 46 (4%) were tornado reports. A maximum of elevated severe storm cases occurred in May with a secondary maximum in September. Elevated severe storm cases vary geographically throughout the year, with a maximum over the south-central United States in winter to a central and eastern U.S. maximum in spring and summer. A diurnal maximum of elevated severe storm cases occurred at 2100 UTC, which coincided with the diurnal maximum of hail reports. The wind reports had a broad maximum during the daytime. Because the forecasting of hail from elevated storms typically does not pose as significant a forecast challenge as severe wind for forecasters and tornadoes from elevated storms are relatively uncommon, this study focuses on the occurrence of severe wind from elevated storms. Elevated severe storm cases that produce only severe wind reports occurred roughly 5 times a year. To examine the environments associated with cases that produced severe winds only, five cases were examined in more detail. Common elements among the five cases included elevated convective available potential energy, weak surface easterlies, and shallow near-surface stable layers (less than 100 hPa thick).
Case studies by Stephen Corfidi
Environmental overview of the 3-4 April 1974 tornado outbreak, 2024
The 1974 Super Outbreak of tornadoes, affecting a broad region from the Southeast northward acros... more The 1974 Super Outbreak of tornadoes, affecting a broad region from the Southeast northward across the Tennessee and Ohio Valleys to the lower Great Lakes, remains the most widespread and intense tornado outbreak in recorded history. This presentation addresses the unique meteorological environment and antecedent conditions responsible for the event.
Weather and Forecasting, 2016
A significant, convectively induced windstorm known as a derecho occurred over parts of Utah, Wyo... more A significant, convectively induced windstorm known as a derecho occurred over parts of Utah, Wyoming, Idaho, and Colorado on 31 May 1994. The event was unusual in that it occurred not only in an environment of relatively limited moisture, but also one with a thermodynamic profile favorable for dry microbursts in the presence of moderate midtropospheric flow. The development and evolution of the severe wind-producing convective system is described, with emphasis on the synoptic and mesoscale features that may have contributed to its strength and maintenance. A very similar derecho that affected much the same region on 1 June 2002 is more briefly introduced. Questions are raised regarding the unique nature of these events and their potential utility in achieving an increased understanding of the mechanics of derecho-producing convective systems in more moisture-rich environments.
Monthly Weather Review, 2011
This study documents the complex environment and early evolution of the remarkable derecho that t... more This study documents the complex environment and early evolution of the remarkable derecho that traversed portions of the central United States on 8 May 2009. Central to this study is the comparison of the 8 May 2009 derecho environment to that of other mesoscale convective systems (MCSs) that occurred in the central United States during a similar time of year. Synoptic-scale forcing was weak and thermodynamic instability was limited during the development of the initial convection, but several mesoscale features of the environment appeared to contribute to initiation and upscale growth, including a mountain wave, a midlevel jet streak, a weak midlevel vorticity maximum, a ''Denver cyclone,'' and a region of upper-tropospheric inertial instability. The subsequent MCS developed in an environment with an unusually strong and deep low-level jet (LLJ), which transported exceptionally high amounts of low-level moisture northward very rapidly, destabilized the lower troposphere, and enhanced frontogenetical circulations that appeared to aid convective development. The thermodynamic environment ahead of the developing MCS contained unusually high precipitable water (PW) and very large midtropospheric lapse rates, compared to other central plains MCSs. Values of down-draft convective available potential energy (DCAPE), mean winds, and 0–6-km vertical wind shear were not as anomalously large as the PW, lapse rates, and LLJ. In fact, the DCAPE values were lower than the mean values in the comparison dataset. These results suggest that the factors contributing to updraft strength over a relatively confined area played a significant role in generating the strong outflow winds at the surface, by providing a large volume of hydrometeors to drive the downdrafts.
Weather and Forecasting, 2010
The Super Outbreak of tornadoes over the central and eastern United States on 3-4 April 1974 rema... more The Super Outbreak of tornadoes over the central and eastern United States on 3-4 April 1974 remains the most outstanding severe convective weather episode on record in the continental United States. The outbreak far surpassed previous and succeeding events in severity, longevity, and extent. In this paper, surface, upper-air, radar, and satellite data are used to provide an updated synoptic and subsynoptic overview of the event. Emphasis is placed on identifying the major factors that contributed to the development of the three main convective bands associated with the outbreak, and on identifying the conditions that may have contributed to the outstanding number of intense and long-lasting tornadoes. Selected output from a 29-km, 50-layer version of the Eta forecast model, a version similar to that available operationally in the mid-1990s, also is presented to help depict the evolution of thermodynamic stability during the event.
Weather and Forecasting, 2002
Forecasters at the Storm Prediction Center (SPC) were faced with many challenges during the 3 May... more Forecasters at the Storm Prediction Center (SPC) were faced with many challenges during the 3 May 1999 tornado outbreak. Operational numerical forecast models valid during the outbreak gave inaccurate, inconsistent, and/or ambiguous guidance to forecasters, most notably with varying convective precipitation forecasts and underforecast wind speeds in the middle and upper troposphere, which led forecasters (in the early convective outlooks) to expect a substantially reduced tornado threat as compared with what was observed. That, combined with relatively weak forecast and observed low-level convergence along a dryline, contributed to much uncertainty regarding timing and location of convective initiation. As a consequence, as the event approached, observational diagnosis and analysis became more important and were critical in identification of the evolution of the outbreak. Tornadic supercells ultimately developed earlier, were more numerous, and produced more significant tornadoes than anticipated. As forecasters addressed the meteorological facets of the tornadic storms on the evening of 3 May 1999, there were other areas of simultaneous severe-storm development, and one of the tornadoes posed a threat to the facility and family members of the forecast staff. These uncertainties and challenges are discussed in the context of SPC convective outlooks and watches for this outbreak. Recommendations are made for continued research aimed at improving forecasts of convective initiation and mode. 1 The current (at the time of writing) SPC product suite is described online at http://www.spc.noaa.gov.
Historical by Stephen Corfidi
The NOAA Storm Prediction Center (SPC), based in Norman, Oklahoma, specializes in forecasts of to... more The NOAA Storm Prediction Center (SPC), based in Norman, Oklahoma, specializes in forecasts of tornadoes, other severe convective storms, fire-weather potential, and mesoscale winter-weather hazards across the conterminous United States. From a small start as a tornado-prediction unit in the 1950s, the SPC has evolved to a staff of 22 full-time forecasters, a science-support branch, and a small management staff. In addition to the forecast roles, the SPC performs operationally relevant scientific research, offers various online forecasting tools to the meteorological community at large, and provides informative public outreach via its website and social media. An overview of the past and present SPC is presented, along with visions for the center’s future.
The Timetable - A quarterly publication of the Maryland and Pennsylvania Railroad Historical Society, 2016
A discussion of the segment of the American Telephone and Telegraph Company's first New York-to-W... more A discussion of the segment of the American Telephone and Telegraph Company's first New York-to-Washington long-distance telephone line that paralleled the Maryland and Pennsylvania Railroad between Forest Hill and Baltimore, Maryland. The paper also discusses the first successful use of multiplex communication --- accomplished along the line at the company's long-distance test facility in Baltimore.
Bulletin of the American Meteorological Society, 2013
The following essay is excerpted from conversations with Edwin Kessler, founding director of NOAA... more The following essay is excerpted from conversations with Edwin Kessler, founding director of NOAA's National Severe Storms Laboratory in Norman, Oklahoma, conducted by Ariel Cohen and Stephen and Sarah Corfidi of the National Weather Service's Storm Prediction Center. Cohen and the Corfidis based the discussion in part on Kessler's speech at the 50th anniversary of the School of Meteorology at the University of Oklahoma in October 2010, and Cohen converted this discussion to a Q&A format. The conversations range over many subjects, including accomplishments of NSSL, Kessler's role as its manager, and the many people who were crucial to the lab's growth. The entire conversation is published by BAMS online, but here we've selected Kessler's words regarding the move to Norman, the community that evolved there, and how NSSL thrived there because of—and despite—the bureaucratic changes that federal agencies underwent at that time.—The Editors
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Derechos and MCSs by Stephen Corfidi
It is found that the advective component of the convective systems is well correlated to the mean flow in the cloud layer. Similarly, the propagation component is shown to be directly proportional (but opposite in sign) and well correlated to the speed and direction of the low-level jet. Correlation coefficients between forecast and observed values for the speed and direction of the mesobeta-scale convective elements are 0.80 and 0.78, respectively. Mean absolute errors of the speed and direction are 2.0 m s−1 and 17°. These errors are sufficiently small so that the forecast path of the centroid of the mesobeta-scale elements would be well within the heavy rain swath of the typical mesoscale convective complex.
Elevated convection by Stephen Corfidi
Case studies by Stephen Corfidi
Historical by Stephen Corfidi
It is found that the advective component of the convective systems is well correlated to the mean flow in the cloud layer. Similarly, the propagation component is shown to be directly proportional (but opposite in sign) and well correlated to the speed and direction of the low-level jet. Correlation coefficients between forecast and observed values for the speed and direction of the mesobeta-scale convective elements are 0.80 and 0.78, respectively. Mean absolute errors of the speed and direction are 2.0 m s−1 and 17°. These errors are sufficiently small so that the forecast path of the centroid of the mesobeta-scale elements would be well within the heavy rain swath of the typical mesoscale convective complex.
nearby Tinker Air Force Base hosted a three-day celebration
to commemorate the 50th anniversary of the
first successful tornado forecast and the advances that
have occurred in tornado forecasting and warning in
the past half-century. The event was a cooperative effort
of the National Oceanic and Atmospheric Administration
(NOAA), the United States Air Force
(USAF), and the University of Oklahoma. Participation
also included the Norman Chamber of Commerce, as
well as the chambers of commerce from surrounding
municipalities in central Oklahoma.
and eastern United States is discussed. Summarizing the results
of various studies dating back to the mid I970s, the microphysical
basis of haze is described, with emphasis on the fact that
haze appears to be largely anthropogenic (man-made). The
synoptic meteorology of haze in the United States is then presented.
It is shown that haze forms primarily in modified polar
air, not tropical air, as is commonly believed, and that its
movement may be estimated by trajectories at the 850-mb level.
A brief discussion of haze-free tropical surges along the East
Coast follows, using synoptic charts to illustrate some recent
surge events.
part of the central United States on 8 May 2009.
Modern severe weather forecasting began in the 1940s, primarily employing the pattern recognition approach throughout the 1950s and 1960s. Substantial changes in forecast approaches did not come until much later, however, beginning in the 1980s. By the start of the new millennium, significant advances in the understanding of the physical mechanisms responsible for severe weather enabled forecasts of greater spatial and temporal detail. At the same time, technological advances made available model thermodynamic and wind profiles that supported probabilistic forecasts of severe weather threats.
This article provides an updated overview of operational severe local storm forecasting, with emphasis on present-day understanding of the mesoscale processes responsible for severe convective storms, and the application of recent technological developments that have revolutionized some aspects of severe weather forecasting. The presentation, nevertheless, notes that increased understanding and enhanced computer sophistication are not a substitute for careful diagnosis of the current meteorological environment and an ingredients-based approach to anticipating changes in that environment; these techniques remain foundational to successful forecasts of tornadoes, large hail, damaging wind, and flash flooding.