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Stacy R. Stewart and Steven W. Lyons

Abstract

The Guam WSR-88D (Weather Surveillance Radar-1988 Doppler) Doppler radar collected reflectivity, Doppler radial velocity, and other information for Supertyphoon Ed as it traversed the northern sections of Guam as a minimal tropical storm on 30 September 1993. This was the first-ever recorded passage of a tropical cyclone over a Next Generation Weather Radar WSR-88D Doppler radar site. Reflectivity data provided valuable information about the location of a precipitation-free “eye,” while radial velocity data provided useful information about tropical cyclone wind center location and strength. The velocity data also provided a 3-h lead time to upgrade the tropical cyclone to tropical storm intensity prior to landfall. Forecasters at Andersen Air Force Base used this information to give what turned out to be a very accurate short-range forecast of a brief period of gales with maximum gusts to 26 m s−1. Land-based surface wind observations correlated extremely well with 75%–80% of the 1500-m radial velocity estimates, which is similar to findings made by Powell and Tanner et al. Additional radar signatures of interest include offsets between the reflectivity center and velocity circulation center, detection of tropical storm and typhoon/hurricane force winds, Doppler velocity maxima within the convective rainbands, and mesocyclonic circulations detected by the WSR-88D's mesocyclone algorithm.

Concerning mesocyclones, one was detected very close to the location of the actual wind center when Ed was developing an eye prior to landfall. Within approximately 1 h of initial mesocyclone occurrence, a cyclonic divergent velocity-couplet pattern formed, possibly due to the development of low-level supergradient winds. The observed Doppler velocity patterns were consistent with the lower-tropospheric horizontal wind and vertical motion patterns in and around the eye as described by Malkus, Kuo, and Gray and Shea. After clearing the west coast of Guam, two separate mesocyclones formed just inward of the eyewall and appeared to be ingested into the main eye circulation. Similar findings were obtained from airborne Doppler radar analyses made by Marks and Houze. Shortly thereafter, Ed underwent a period of rapid intensification. In both instances, the mesocyclones appeared to have played a role in eye development and intensification.

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Wallace A. Hogsett and Stacy R. Stewart

Abstract

Deep convective processes play an important role in tropical cyclone (TC) formation and intensification. In this study, the authors investigate the interaction between discrete buoyant updrafts and the vertically sheared azimuthal flow of an idealized TC vortex by adapting the updraft–shear dynamical framework to the TC. The authors argue theoretically that deep updrafts initiating near the TC radius of maximum wind (RMW) may propagate with a component left of the mean tangential flow, or radially inward toward the TC center. Results suggest that these unique TC updrafts, or “left movers” with respect to the mean azimuthal flow, may play an active role in TC intensification.

The notion that updraft-scale convection may propagate with a component transverse to the mean flow is not at all new. Cyclonic midlatitude supercell thunderstorms often deviate from their mean environmental flow, always to the right of the environmental vertical shear vector. The deviant motion arises owing to nonlinear interactions between the incipient updraft and the environmental vertical shear. Although significant differences exist between the idealized TC considered here and real TCs, observational and high-resolution operational modeling evidence suggests that some intense TC updrafts may propagate with a radially inward and right-of-shear component and exhibit structural characteristics consistent with theory.

The authors propose that left movers constitute a unique class of intense TC updrafts that may be favored near the TC RMW where local vertical shear of the TC azimuthal winds may be maximized. To simulate these left movers in a realistic way, mesoscale TC forecasting models must resolve nonlinear interactions between updrafts and vertical shear.

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Lixion A. Avila and Stacy R. Stewart

Abstract

The 2011 Atlantic season was marked by above-average tropical cyclone activity with the formation of 19 tropical storms. Seven of the storms became hurricanes and four became major hurricanes (category 3 or higher on the Saffir–Simpson hurricane wind scale). The numbers of tropical storms and hurricanes were above the long-term averages of 12 named storms, 6 hurricanes, and 3 major hurricanes. Despite the high level of activity, Irene was the only hurricane to hit land in 2011, striking both the Bahamas and the United States. Other storms, however, affected the United States, eastern Canada, Central America, eastern Mexico, and the northeastern Caribbean Sea islands. The death toll from the 2011 Atlantic tropical cyclones is 80. National Hurricane Center mean official track forecast errors in 2011 were smaller than the previous 5-yr means at all forecast times except 120 h. In addition, the official track forecast errors set records for accuracy at the 24-, 36-, 48-, and 72-h forecast times. The mean intensity forecast errors in 2011 ranged from about 6 kt (~3 m s−1) at 12 h to about 17 kt (~9 m s−1) at 72 and 120 h. These errors were below the 5-yr means at all forecast times.

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Stacy R. Stewart and John P. Cangialosi

Abstract

The 2010 eastern North Pacific hurricane season was one of the least active seasons on record. Only seven named storms developed, which is the lowest number observed at least since routine satellite coverage of that basin began in 1966. Furthermore, only three of those storms reached hurricane status, which is also the lowest number of hurricanes ever observed in the satellite-era season. However, two tropical storms made landfall: Agatha in Guatemala and Georgette in Mexico, with Agatha directly causing 190 deaths and moderate to severe property damage as a result of rain-induced floods and mud slides. On average, the National Hurricane Center track forecasts in the eastern North Pacific for 2010 were quite skillful.

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Miles B. Lawrence, Lixion A. Avila, John L. Beven, James L. Franklin, Richard J. Pasch, and Stacy R. Stewart

Abstract

The 2003 Atlantic hurricane season is described. The season was very active, with 16 tropical storms, 7 of which became hurricanes. There were 49 deaths directly attributed to this year’s tropical cyclones.

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James L. Franklin, Lixion A. Avila, Jack L. Beven, Miles B. Lawrence, Richard J. Pasch, and Stacy R. Stewart

Abstract

The 2000 Atlantic hurricane season is summarized and the year's tropical and subtropical cyclones are described. While overall activity was very high compared to climatology, with 15 cyclones attaining tropical (or subtropical) storm intensity, much of this activity occurred outside of the deep Tropics, over open waters north of 25°N. The season's tropical cyclones were responsible for 54 fatalities, with most of these occurring in Central America in association with Hurricanes Gordon and Keith.

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John L. Beven II, Lixion A. Avila, James L. Franklin, Miles B. Lawrence, Richard J. Pasch, and Stacy R. Stewart

Abstract

The tropical cyclone activity for 2003 in the eastern North Pacific hurricane basin is summarized. Activity during 2003 was slightly below normal. Sixteen tropical storms developed, seven of which became hurricanes. However, there were no major hurricanes in the basin for the first time since 1977. The first hurricane did not form until 24 August, the latest observed first hurricane at least since reliable satellite observations began in 1966. Five tropical cyclones made landfall on the Pacific coast of Mexico, resulting in 14 deaths.

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Richard D. Knabb, Lixion A. Avila, John L. Beven, James L. Franklin, Richard J. Pasch, and Stacy R. Stewart

Abstract

The 2005 eastern North Pacific hurricane season is summarized, the individual tropical cyclones are described, and official track and intensity forecasts are verified and evaluated. The season’s overall activity was, by most measures, below average. While a near-average 15 tropical storms formed, many of them were relatively weak and short-lived. Seven of these storms became hurricanes, but only one reached major hurricane status (an intensity of 100 kt or greater on the Saffir–Simpson hurricane scale) in the eastern North Pacific basin. One of the hurricanes, Adrian, approached Central America in May but weakened to a tropical depression prior to landfall. Adrian was the only eastern North Pacific tropical cyclone to make landfall during 2005, and it was directly responsible for one fatality.

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Richard J. Pasch, Miles B. Lawrence, Lixion A. Avila, John L. Beven, James L. Franklin, and Stacy R. Stewart

Abstract

The 2002 Atlantic hurricane season is summarized. Although the season's total of 12 named storms was above normal, many of these were weak and short-lived. Eight of the named cyclones made landfall in the United States, including Lili, the first hurricane to hit the United States in nearly 3 yr.

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John L. Beven II, Stacy R. Stewart, Miles B. Lawrence, Lixion A. Avila, James L. Franklin, and Richard J. Pasch

Abstract

Activity during the 2001 hurricane season was similar to that of the 2000 season. Fifteen tropical storms developed, with nine becoming hurricanes and four major hurricanes. Two tropical depressions failed to become tropical storms. Similarities to the 2000 season include overall activity much above climatological levels and most of the cyclones occurring over the open Atlantic north of 25°N. The overall “lateness” of the season was notable, with 11 named storms, including all the hurricanes, forming after 1 September. There were no hurricane landfalls in the United States for the second year in a row. However, the season's tropical cyclones were responsible for 93 deaths, including 41 from Tropical Storm Allison in the United States, and 48 from Hurricanes Iris and Michelle in the Caribbean.

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