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John F. Weaver

Abstract

Data presented indicate that severe thunderstorm areas may become anchored to intense, boundary- layer convergence zones. In such cases. the mean cloud-layer wind vector has little relation to the movement of regions of large hail, wall clouds and tornadoes. Suggestions for improving severe thunderstorm warnings are offered based on this possibility.

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John A. Knaff
and
John F. Weaver

Abstract

A large, low-level thunderstorm outflow boundary was observed as it exited from beneath the cirrus canopy of Hurricane Luis following a period of intense convection in the storm’s eyewall. A description of the feature and a short summary of its behavior are presented.

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John F. Weaver
and
Dan Lindsey

Abstract

Several examples of Geostationary Operational Environmental Satellite (GOES) visible satellite images depicting cloud features often associated with the transition to, or intensification of, supercell thunderstorms are presented. The accompanying discussion describes what is known about these features, and what is left to learn. The examples are presented to increase awareness among meteorologists of these potentially significant storm features.

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John F. Weaver
and
Stephan P. Nelson

Abstract

An investigation of severe storms that occurred on 23 May 1974 in central Oklahoma reveals interesting information on the multifaceted role of thunderstorm-produced gust fronts. The data reveal that the interaction of outflow boundaries with a cold front initiate the storms studied in this report. Then, the gust fronts produced by these storms have important effects on their own structure and life histories. Detailed Doppler analysis show that these storms propagate discretely along their outflow boundaries despite their supercell structure. Additionally, one storm is affected by its interaction with the boundary produced by a previous storm. This interaction appears responsible for tornadogenesis.

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John F. Weaver
and
James J. Toth

Abstract

Conditions leading up to an outbreak of severe hailstorms in northeast Colorado are examined using satellite and surface data. A persistent mesoscale ridge of surface-high pressure, caused by outflow from a mesoscale- convective system, is seen to coincide with the occurrence of large hail which was confined to a narrow band extending nearly parallel to the Front Range of the Rocky Mountains. The development of the mesoscale ridge during its most intense stages is documented using a procedure that yields an approximate streamfunction for the surface geostrophic wind. Unlike alternatives over sloping terrain, this method is quick and can be adjusted to minimize the error over a limited portion of the analysis area.

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John F. Weaver
and
Nolan J. Doesken

Abstract

More than a decade ago, a study was published that identified a short list of precursor conditions for severe thunderstorms on the High Plains of the United States. The present study utilizes data from the summer months of ten convective seasons to estimate how well the criteria fare as a method of forecasting severe weather days in that region.

Results indicate that the technique produces a relatively high success rate in terms of detecting severe weather days for most years studied. False alarms are a bit high in an absolute sense (36% overall), but fall well within acceptable limits in the real world, where the philosophy of “better to overwarn, than underforecast” prevails.

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John F. Weaver
and
John M. Brown

On 15 October 1980, a weather system that had been to the west of Colorado was forecast to move into the state, and to bring with it light to moderate snow in the Rockies, and generally light rain and thundershower activity over the plains to the east. In most regions this forecast was adequate. However, substantially heavier activity (including a small tornado, large hail, heavy rain, and snow) also occurred in some areas. In this paper we show how all relevant real-time data, when properly merged, could have enabled formulation of a useful short-term forecast. In addition we point out how mesonet surface data gathered after the fact could have helped narrow down the forecast area of severe weather and heavy precipitation.

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John F. Weaver
and
James F. W. Purdom

Abstract

Satellite images are presented to illustrate an interesting interaction that occurred between a severe thunderstorm and a mesoscale feature that originated in its nearby environment. Immediately following that interaction, a series of tornadoes began, starting with a long-lived F5 tornado that produced major damage in Hesston, Kansas. Some speculation is presented regarding the physical processes that may have contributed to the observed changes in thunderstorm behavior.

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John F. Dostalek
,
John F. Weaver
, and
G. Loren Phillips

Abstract

A severe left-moving thunderstorm occurred on 25 May 1999 between the cities of Lubbock and Amarillo, Texas. Over its 3.5-h lifetime, the storm was responsible for flash flooding, reports of hail of up to 7 cm in diameter, and two weak tornadoes. Satellite imagery reveals that it was traveling along the northward-moving outflow boundary of the storm from which it formed. The left mover displayed anticyclonic rotation, as was seen in storm-relative radial velocity imagery from the Weather Surveillance Radar-1988 Doppler (WSR-88D) located at Lubbock. The tornadoes developed west of Canyon, Texas, near the intersection of the left mover and a southward-moving boundary. The occurrence of tornadoes with a left mover is a particularly noteworthy event; to the authors' knowledge, only four other tornadic left movers have been reported in the meteorological literature.

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Pete Browning
,
John F. Weaver
, and
Bernadette Connell

Abstract

Tornadic storms that occurred over northeastern Kansas and northern Missouri on 4 July 1995 are examined by combining the latest in National Weather Service technology with more routine datasets. The analysis provides an insightful description of the meteorological setting and evolution that led to the severe weather on this day. Strong thunderstorms first formed where an outflow boundary intersected a cold front, then new activity was triggered along the outflow boundary itself. It was found that small-scale outflow interactions may have played an important role in changing the nature of the convection already under way and were associated with the two most damaging tornadoes of the day. The case is also used to showcase how several new “modernization” datasets can be used together in a quickly accessible manner to provide a valuable and precise overview of a rapidly evolving meteorological event.

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