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John E. Hales Jr.

Even though numerical models have made great strides forward in recent years, their accuracy is limited by initialization. The forecaster has available high quality satellite pictures which enable him to qualitatively evaluate circulation systems. Using satellite imagery makes it frequently possible to recognize regions where there is a model initialization problem. When such an area is identified, the numerical guidance can be modified before being incorporated in the public forecast. A detailed analysis of the development of the damaging storm that struck Southern California on 10 February 1978 is presented. With this storm there were serious initialization problems.

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John E. Hales Jr.

Historically, tornadoes have not been a meteorological concern to California. There has never been a tornado-related fatality recorded in the state, and those tornadoes that do occur are believed to be of the weak variety.

Upon closer examination of the tornado data it is found that there is a small region in the highly populated Los Angeles area that has a tornado incidence disproportionately higher than the rest of California. This coastal zone is about 75 km long by 35 km wide. It is hypothesized that this region is a favorable location for tornadoes because the shape of the coastline and the inland location of the mountains enchance the low-level convergence field.

The tornado occurrences all seem to have very similar synoptic patterns. Low centers extending from the surface to 500 mb are located off the coast of California northwest of Los Angeles. In all cases, a very strong upper-tropospheric jet is located along the California/Mexico border. Many of the tornadoes occurred several hours after the passage of a cold front when the lower troposphere was destabilized and the low-level moisture increased as a result of the long overwater trajectory of the air mass. Several other tornado cases occurred at approximately the same time that cold fronts passed the area. A case study of the 9 November 1982 tornado outbreak is presented. During this particular episode, there were a record-setting seven tornadoes, all of which occurred in or near the tornado-prone zone.

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John E. Hales Jr.

Abstract

The diurnal variability of thunderstorm occurrence in Arizona is complex and related to terrain elevations. Generally, thunderstorms occur in the higher mountains during the afternoon with activity primarily of the nocturnal nature in the adjacent desert valleys, most noticeably the Phoenix area.

Aircraft temperature probes have found cooling in cumulus areas. This cooling can frequently advect with the steering wind to destabilize the desert air mass and increase nighttime thunderstorms. The same destabilization process could contribute to nocturnal thunderstorms in the High Plains.

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John E. Hales Jr.

On 12 September 1977, severe flash flooding in Kansas City, Mo., resulted in widespread damage and 25 fatalities. It was the worst flash flood of modern times in this area in terms of loss of life and damage. Meteorological analysis of the flood situation indicates the key ingredients for the unprecedented flooding were: 1) two heavy convective rain events occurring in essentially the same location within a 20 h period, and 2) the formation by the first storm system of a small-scale surface boundary wave that concentrated the low-level convergence over the Kansas City area for an extended period of time preceding the second event. Total rainfall amounts for the 24 h storm period were >16 inches (40 cm). Rainfall rates were as high as 8 inches (20 cm) in 4 h during the flooding on the evening of the 12th. Besides the conventional surface and upper air observational network, radar and satellite input were of significant value in analyzing the storm evolution.

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John E. Hales Jr.

Abstract

Intense thunderstorms, which frequent the desert Southwest of the United States in the summer months, have been known by several different names: chubasco, haboob, and Sonora storm. Prior to the advent of satellites and radar, the sparsity of observations in the desert Southwest precluded any determination of where these storms developed, as well as information on their areal coverage and life cycle.

A particularly severe and long-lasting thunderstorm occurred 19–20 August 1973. This storm was followed through its life cycle by means of radar, satellite, and surface observations. This particular storm was noteworthy for its very strong winds, locally heavy rain, and the magnitude of the pressure jump associated with it. The converging of two separate mesohighs is believed to be the cause of the very intense storm that moved westward across the Imperial Valley of Southern California.

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JOHN E. HALES JR.

Abstract

Several surges of maritime tropical air into the deserts of Arizona and California are investigated. These surges are related to the large cloud masses of tropical origin located over the Gulf of California. A natural channel exists for this surge transport; it is about 200 mi in width, the western boundary being the rugged ridge line extending the length of Baja California and the eastern boundary being the broad Sierra Madre Range. The surge process resembles that of a very large-scale sea breeze with the greatest energy transport occurring near the surface and disappearing in the middle troposphere. Four case studies are analyzed in detail.

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John E. Hales Jr.

Abstract

Moist air from the tropics moves into northwest Mexico and the southwestern portion of the United States during the early part of the summer season. This is analogous to the monsoon circulation in other sections of the globe. The annual change in circulation has been related to the northward and westward development of the large subtropical high pressure system over the southern United States. With this change the moist tropical air over the Gulf of Mexico is believed to be carried across Mexico and into the southwestern United States. The mountains of Mexico provide a formidable barrier for the movement of the Gulf of Mexico moisture westward. It has been shown that half of the precipitable water vapor in the atmosphere over the Arizona deserts in the summer months is below the 800-mb level. This is inconsistent with the idea of the Gulf of Mexico supplying the greatest proportion of the moisture.

This paper presents support for the greatest percentage of tropical moisture in the southwest United States and northwest Mexico coming from the Pacific Ocean by way of the Gulf of California rather than from the Gulf of Mexico.

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Edward W. Ferguson, Frederick P. Ostby, Preston W. Leftwich Jr., and John E. Hales Jr.

Abstract

Tornado events of 1984 are reviewed. Significant and interesting aspects of the 907 reported tornadoes are noted. Synoptic patterns associated with four noteworthy tornado days are examined.

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Katherine L. Horgan, David M. Schultz, John E. Hales Jr., Stephen F. Corfidi, and Robert H. Johns

Abstract

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).

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