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Edward J. Szoke
and
Raymond H. Brady

Abstract

The case of a tornadic thunderstorm on 26 July 1985 in northeastern Colorado is described from the synoptic to the thunderstorm scale utilizing a number of datasets some of which will become operational in the 1990s. The available data included profilers, Doppler radar, surface mesonet, satellite, and special soundings. Although the synoptic environment did not favor tornadic thunderstorms, strong thunderstorms formed in localized area during a 2-h period in the afternoon and produced an 18-min tornado. A number of events took place to produce the stronger then anticipated development, including interaction among mesoscale outflow and stationary boundaries. Of particular importance was the change in the local environment along a stationary boundary known as the Denver Convergence-Vorticity Zone. Special soundings taken near the stationary boundary revealed a deepening moist layer over time in association with the convergent wind field. Additional forcing from the collision of this boundary with other outflow boundaries was required, to release the increasing convective potential. Of further importance was the coincidence of sunshine over a portion of the boundary where the eventual collision would occur. In association with experiments under way at the time, three groups having various access to the datasets issued probability forecasts in real time. The short-range forecasts and warnings of thunderstorm initiation and severe weather provide a useful evaluation of the problems of predicting the events of this day. Two major problems encountered on this day are commonly faced by forecasters of summertime convection: evaluating the importance and timing of an approaching weak upper-1evel feature, and monitoring low-level mesoscale boundaries. Use of the datasets in real time to diagnose these problems is emphasized. The forecasters using the new datasets demonstrated the ability to focus on the most likely area of thunderstorm initiation, although the timing and intensity of the development still presented forecast problems. Analyses of the special soundings revealed the above-surface temperature and moisture changes resulting from the two features and their importance for this case. Means of addressing such changes operationally with the new datasets are discussed.

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Raymond H. Brady
and
Edward J. Szoke

Abstract

The evolution of the 26 July 1985 Erie, Colorado tornado is described using data from NCAR's CP-2 Doppler radar. This tornado develops within 20 km of the radar site under weakly forced synoptic conditions and weak tropospheric flow, and is not accompanied by a mesocyclone. The initial circulation forms near the surface at the intersection of two mesoscale boundaries and develops vertically, intensifying into an Fl tornado when it becomes collocated with the intense updrafts of a rapidly developing cumulonimbus.

This tornado appears to be the land equivalent of a waterspout, and comparisons between the two vortices are made. It is speculated that Florida and portions of the western High Plains may be prone to nonmesocyclone tornado development, and that vortex-intensification processes associated with nonmesoscyclone tornadoes may be important in mesocyclone tornadogenesis. Suggestions on how to better forecast these tornadoes are also presented.

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Raymond H. Brady
and
Jeff S. Waldstreicher

Abstract

WSR-88D depictions of two mountain wave–induced precipitation shadows observed near the Wyoming Valley of northeast Pennsylvania are presented. These mountain waves developed in similar synoptic environments that featured a strong south to southeast low-level jet, a stable layer situated near mountaintop level, and cross-barrier flow that decreased with height. One event was associated with a well-defined, singular precipitation shadow, while the second event displayed multiple precipitation shadows. Subtle differences in the vertical distribution of temperature and wind shear between the two cases appeared to be instrumental in defining what type of structure the mountain wave and their associated precipitation shadows displayed. This is supported by calculations of Froude number, Brunt–Väisälä frequency, Scorer parameter, and horizontal wavelength for the two events.

These mountain waves appear to have a significant effect on the local precipitation distribution in and near the heavily populated Wyoming Valley, with amounts reduced within and up to 15 km downstream of the valley. These effects are evident in the radar-estimated storm total precipitation products for both cases, and implied by rain gauge data for one of the events. Precipitation drift appears to play a role in the actual surface location of these precipitation minima with respect to the radar-indicated position, in cases of strong low-level flow.

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Andrew D. Stern
,
Raymond H. Brady III
,
Patrick D. Moore
, and
Gary M. Carter

The National Weather Service Eastern Region is carrying out a national risk-reduction exercise at the Baltimore–Washington Forecast Office in Sterling, Virginia. The primary objective of this project is to integrate information from remote sensor technologies to produce comprehensive state-of-the-atmosphere reports that promote aviation safety. Techniques have been developed and tested to identify aviation-oriented hazardous weather based on data from conventional radars, a national lightning detection network, and collateral observations from new Automated Surface Observing System (ASOS) sites that are being deployed throughout the nation. From July through September 1993, an experimental observational product to identify convective activity within 30 n mi of six airports from southern Virginia to Delaware was transmitted three times each hour to personnel at Weather Service Offices and Center Weather Service Units and to the meteorologists and flight dispatchers of five major air carriers. This user-oriented evaluation and the associated statistical analysis has provided important feedback to assess the utility of the product as a supplement to ASOS. Integration of information from several products generated by the new Doppler radar at Sterling with lightning network data is being pursued for the second phase of the project. The National Weather Service will determine the viability of this approach to generate products to routinely supplement the information provided by ASOS on either a national or a local basis.

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Harold H. Opitz
,
Solomon G. Summer
,
David A. Wert
,
Warren R. Snyder
,
Richard J. Kane
,
Raymond H. Brady
,
Paul M. Stokols
,
Stephan C. Kuhl
, and
Gary M. Carter

Abstract

Over the years, as the recognition and understanding of the structure and climatic frequency of heavy-rain events has expanded, there has been a corresponding improvement in the available forecast guidance on both the national and local level. Numerous operational procedures, forecast applications, and objective techniques have been developed at National Weather Service (NWS) field offices to assess the potential for heavy precipitation and flooding. The use of simple models and operational checklists, as well as the identification of precipitation enhancements due to the effects of terrain and local climatology, provide forecasters with useful tools that help interpret and improve upon the central guidance products. In addition, the NWS Eastern Region has devised and implemented an aggressive and comprehensive program to support the daily formulation of quantitative precipitation estimates appropriate for the production of more timely and accurate river forecasts. Finally, access to high-resolution information from new remote sensor technologies such as Doppler radar, vertical wind profilers, lightning detection networks, and the next generation of geostationary satellites presents the possibility of a substantial improvement in the prediction of heavy precipitation.

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