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John D. Locatelli and Peter V. Hobbs

Abstract

On 22 June 1947, Holt, Missouri, experienced a world-record rainstorm when 304.8 mm (∼1 ft) of rain fell in 42 minutes. In this paper, evidence is presented that this extremely heavy rain may have been produced by cold frontogenesis aloft (CFA). It is shown that what was earlier analyzed as a surface cold front was probably a drytrough, and that CFA was located at 700 hPa east of the drytrough, close to the location of the squall line, that produced the record precipitation rate.

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Peter V. Hobbs, John D. Locatelli, and Jonathan E. Martin

Abstract

Brief descriptions are given of four cases that illustrate the important role that cold fronts aloft (CFA) can play in producing significant weather cast of the Rocky Mountains. In all four cases, the CFA, and its associated short wave, were located ∼200–300 km ahead of a surface trough. Precipitation (and in some cases severe weather) developed along the leading edge of the CFA. The nested grid model generally did a good job of locating the CFA. Analysis of absolute momentum confirms that these features were fronts, in a dynamic, as well as a thermodynamic sense.

A conceptual model for CFA is presented. In the cases examined, this model provides not only a useful picture of the distribution of clouds and precipitation associated with CFA, but also means for locating them. It also helps to define a major class of systems that do not fit the Norwegian cyclone model. Therefore, it should help in the identification of CFA and in improving the forecasting of precipitation and severe weather associated with them.

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John D. Locatelli, Ralph D. Schwartz, Mark T. Stoelinga, and Peter V. Hobbs

Abstract

Conventional data and mesoscale model simulations are used to analyze two cyclones that developed east of the Rocky Mountains in June and November 1998. Both cyclones formed when a Pacific cold front overtook a lee trough/dryline east of the Rockies. In one case the leading edge of the Pacific cold front was on the surface, as depicted in the classic Norwegian model of a cyclone. In the other case, which is referred to as a cold front aloft (CFA) cyclone, the leading edge of the Pacific cold front was aloft and in advance of the lee trough. The lifting and severe weather associated with the Pacific cold front was along the leading edge of this front in both the Norwegian-type and CFA-type cyclones.

To obtain an estimate of how often CFA cyclones, with a coincident CFA rainband, occurred in the central United States during the period 15 September 1994 through 15 September 1995, 70 cyclones that maintained closed surface low pressure centers for at least 24 h, and produced precipitation in the United States east of the Rockies, were analyzed. Analysis of these cyclones revealed that 46% were CFA type, 23% were Norwegian type, and the remaining 31% could not be readily classified into either type. Lee troughs were common features east of the Rocky Mountains during this period. They were present in 62% of the 70 cyclones analyzed.

The 1000–500-hPa thickness field is suggested as a useful tool in locating the leading edge of a Pacific cold front, and in determining whether a cyclone is a Norwegian type or CFA type. The issue of how the frontal structures in CFA-type cyclones should be analyzed on surface weather charts is discussed, and some suggestions offered.

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Stanley F. Rose, Peter V. Hobbs, John D. Locatelli, and Mark T. Stoelinga

Abstract

A forecast of severe weather and the potential for tornadoes associated with a cyclone that developed in the lee of the Rocky Mountains on 19–21 June 2000 is evaluated. The forecasting methods used by the National Weather Service for this case, which focused on the position of a surface trough and the location of favorable quasigeostrophic jet dynamics, poorly predicted the extent and location of the severe weather. Application of a conceptual model for cyclones east of the Rockies, which highlights the importance of cold fronts aloft (CFA), shows that a CFA was an important trigger to convection in the 19–21 June 2000 cyclone. A simple forecasting method is demonstrated that emphasizes the importance of lifting for cases that involve CFA. This method is applied to the 19–21 June 2000 cyclone and is found to improve greatly the determination of where severe weather occurred.

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Stanley F. Rose, Peter V. Hobbs, John D. Locatelli, and Mark T. Stoelinga
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Stanley F. Rose, Peter V. Hobbs, John D. Locatelli, and Mark T. Stoelinga

Abstract

Observations and numerical model simulations associate rising motions below the right-entrance and left-exit quadrants of an upper-level straight jet streak with the development of convection and severe weather. The occurrence of tornadoes in relation to the jet quadrants is investigated for the continental United States for the spring months of 1990–99. Tornadoes occurred primarily within the two exit quadrants, with the left-exit quadrant favored over the right-exit quadrant. While fewer tornadoes were located below the two entrance quadrants, the right-entrance quadrant was favored over the left-entrance quadrant. For those days on which many tornadoes occurred (“outbreak” days), a greater percentage of tornadoes occurred below the left-exit and right-entrance quadrants than for those days on which only a few tornadoes were reported. Composite diagrams are presented to clarify the relationship between the quadrants of a jet streak, severe weather, and synoptic features such as low pressure centers and frontal boundaries.

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