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Bartlett C. Hagemeyer

Abstract

A thermodynamic climatology was constructed to investigate source regions of low-level moist airmasses and lower mid-tropospheric dry airmasses that contribute to the production of convective instability, and thunder-storms, over the United States. Mean monthly 1200 UTC values of potential temperature, mixing ratio, and geopotential height were computed at the surface and 750 mb for March through September (1966–1980) for 69 upper-air stations in the continental United States and Mexico. Analyses indicated that the Gulf of Mexico is the most significant source of low-level moisture east of the Continental Divide for all months, but the Atlantic Ocean also becomes a significant source in summer. The Gulf of California was found to be the most important source of low-level moisture west of the Continental Divide in summer. A distinct synoptic scale dryline was found over west Texas from March to June and analyses suggested that the strongest drylines occur in April and May. Northwest Mexico and the desert southwest were confirmed to be the source regions of the dryline. At 750 mb, source regions of dry airmasses were found to be the Baja California region and desert southwest in spring, the tradewind easterly dry layer in summer, and the dry, cool airmasses over the northern Pacific and northern Great Lakes regions in spring and summer. The moist, summer, tropical Pacific airstream was found not only to be deeper than that from the Gulf of Mexico, but also to be the primary source of moisture west of the Continental Divide. In summer, significant mean horizontal advection of warm, moist air from higher terrain into the Missouri Valley was noted.

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Bartlett C. Hagemeyer

Abstract

An analysis of statistics for 1448 tornadoes documented by the National Severe Storms Forecast Center from 1950 through 1994 at, or south of, 30° latitude in Florida was completed to determine the definition of a peninsular Florida tornado outbreak and develop a tornado outbreak climatology. A peninsular Florida tornado outbreak was defined as the occurrence of four or more tornadoes in 4 h or less. Thirty-five outbreak cases were identified. After a review of all available synoptic data for each case, they were categorized into three basic types: 1) those associated with extratropical cyclones (27 cases), 2) those associated with tropical cyclones of tropical storm or hurricane strength (5 cases), and 3) those associated with hybrid cyclones having both tropical and extratropical characteristics (3 cases). A detailed climatology covering spatial and temporal aspects of outbreak characteristics was completed. Mean severe weather indices, thermodynamic soundings, and hodographs were produced for each outbreak type. Case studies of each type of outbreak are presented to complement the climatology and mean environment information.

It was found that tornado outbreaks account for 3.4% of all tornado days but caused 61% of all tornado deaths and 62% of tornado injuries. Most tornado deaths occurred in trailers or mobile homes. Extratropical outbreak tornadoes were most common from midmorning to early afternoon, while tropical outbreak tornadoes occurred mostly in the afternoon and evening. The outbreaks produced a much greater percentage of strong and violent tornadoes compared to the general tornado population. Hybrid outbreaks were the most consistently dangerous of the three types of outbreaks. Fundamental differences and similarities among the three outbreak environments are presented using mean soundings and case studies. The basic ingredients for tornado development in each type of outbreak were found to be the presence of strong low-level winds and shear, and enough instability to support thunderstorm development.

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Bartlett C. Hagemeyer
and
Gary K. Schmocker

Abstract

Climatological analyses indicate that strong morning tornadoes in the dry season, and weak afternoon tornadoes in the wet season, are significant forecast problems in east-central Florida. To address this issue, an analysis of upper-air soundings for Tampa Bay, West Palm Beach, and Cape Canaveral, Florida, released within ±2 hours of tornado touchdowns in the County Warning Area (CWA) of future National Weather Service (NWS) Weather Forecast Office (WFO), Melbourne, Florida, was completed. Mean dry- and wet-season tornado-proximity soundings to 200 mb were produced, and selected mean diagnostic parameters and variance statistics computed.

Both dry- and wet-season tornado environments were associated with deep moist layers overlain by dry air, but no capping inversions. Dry-season cases were characterized by lower-tropospheric o w values well above normal, very low Convective Available Potential Energy (CAPE) and Bulk Richardson Number (BRN), strong speed and directional shear at low levels, a strong midlevel dry intrusion, and a maximum wind at 200 mb. The thermodynamic environment of the wet-season cases under westerly flow was close to mean seasonal values, but U increased steadily above 650 mb to a mean westerly maximum wind at 275 mb. These middle and upper-level winds, greatly exceeding mean seasonal values, allow thunderstorms developing along low-level convergent boundaries to be organized and sustained by the production of strong, persistent, tilted updrafts and continued low-level inflow of high o w air.

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