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Paul J. Roebber, James M. Frederick, and Thomas P. DeFelice

Abstract

Persistent low overcast conditions, defined as continuous overcast conditions (100% cloud cover) with ceiling heights at or below 2 km for a minimum of 5 days, are found to occur in the cold season in the U.S. upper Midwest on average slightly more often than once every two years. These occurrences are associated with two primary large-scale circulation patterns. Most commonly, the midlatitude westerlies are split across North America, with downstream confluence of the northwesterly polar and the southwesterly subtropical jet streams. A second, less frequent, pattern features an amplified westerly jet across North America, with a correspondingly rapid progression of weakly developed cyclones through the region. In the case of the split flow pattern, composite surface high pressure is established, occasionally disrupted by the emergence from either stream of relatively weak cyclones. These systems act to moisten the affected region at low levels through horizontal transport of moisture and, to a lesser extent, moisture convergence. Subsidence inversions established following the passage of these systems act to slowly erode the depth of the surface-based moist layer but are insufficient in combination with the weak solar radiative input to dissipate the cloud. The properties of the event structure, from the large scale down to that of the cloud layer itself, are stable. Under such conditions, the mechanism that finally removes the cloud is the passage of a relatively well-developed baroclinic wave and its associated forcing (subsidence, dry air advection, moisture divergence). Correspondingly, the difficult act of forecasting the end of such periods requires an accurate assessment of the sufficiency of that forcing to remove the low-level cloud. It is suggested that a relatively simple one-dimensional boundary layer model employed for the time to be critically tested in conjunction with the standard forecast model guidance (forecast vertical motion, profiles of temperature and moisture, Model Output Statistics cloud cover and ceiling) would provide additional information regarding forecast uncertainty.

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Paul M. James, Bernhard K. Reichert, and Dirk Heizenreder

Abstract

NowCastMIX is the core nowcasting guidance system at the German Weather Service. It automatically monitors several systems to capture rapidly developing high-impact mesoscale convective events, including 3D radar volume scanning, radar-based cell tracking and extrapolation, lightning detection, calibrated precipitation extrapolations, NWP, and live surface station reports. Within the context of the larger warning decision support process AutoWARN, NowCastMIX integrates the input data into a high-resolution analysis, based on a fuzzy logic approach for thunderstorm categorization and extrapolation, to provide an optimized warning solution with a 5-min update cycle for lead times of up to 1 h. Feature tracking is undertaken to optimize the direction of warning polygons, allowing individual, tangentially moving cells or cell clusters to be tracked explicitly. An adaptive ensemble clustering is deployed to reduce the spatial complexity of the resulting warning fields and smooth noisy temporal variations to a manageable level for duty forecasters. Further specialized outputs for civil aviation and for a public mobile phone warning app are generated. Now in its eighth year of operation, a comprehensive and complete set of thunderstorm analyses and nowcasts over Germany has been created, which is of unique value for ongoing research and development efforts for improving the system, as well as for addressing climatological aspects of severe convection. Verification has shown that NowCastMIX has helped to significantly improve the quality of the official warnings for severe convective weather events when used within the AutoWARN process.

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Paul B. Bogner, Gary M. Barnes, and James L. Franklin

Abstract

One hundred and thirty Omega dropwindsondes deployed within 500-km radius of the eye of six North Atlantic hurricanes are used to determine the magnitudes and trends in convective available potential energy, and 10–1500-m and 0–6-km shear of the horizontal wind as a function of radius, quadrant, and hurricane intensity.

The moist convective instability found at large radii (400–500 km) decreases to near neutral stability by 75 km from the eyewall. Vertical shears increase as radius decreases, but maximum shear values are only one-half of those found over land. Scatter for both the conditional instability and the shear is influenced chiefly by hurricane intensity, but proximity to reflectivity features does modulate the pattern. The ratio of the conditional instability to the shear (bulk Richardson number) indicates that supercell formation is favored within 250 km of the circulation center, but helicity values are below the threshold to support strong waterspouts.

The difference between these oceanic observations and those made over land by other researchers is evidence for significant modification of the vertical profile of the horizontal wind in a hurricane at landfall.

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Christopher S. Velden, Christopher M. Hayden, W. Paul Menzel, James L. Franklin, and James S. Lynch

Abstract

While qualitative information from meteorological satellites has long been recognized as critical for monitoring tropical cyclone activity, quantitative data are required to improve the objective analysis and numerical weather prediction of these events. In this paper, results are presented that show that the inclusion of high-density, multispectral, satellite-derived information into the analysis of tropical cyclone environmental wind fields can effectively reduce the error of objective track forecasts. Two independent analysis and barotropic track-forecast systems are utilized in order to examine the consistency of the results. Both systems yield a 10%–23% reduction in middle- to long-range track-forecast errors with the inclusion of the satellite wind observations.

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David M. L. Sills, James W. Wilson, Paul I. Joe, Donald W. Burgess, Robert M. Webb, and Neil I. Fox

Abstract

Several severe thunderstorms, including a tornadic supercell, developed on the afternoon of 3 November 2000, during the Sydney 2000 Forecast Demonstration Project. Severe weather included three tornadoes, damaging wind gusts, hail to 7-cm diameter, and heavy rain causing flash flooding. A unique dataset was collected including data from two Doppler radars, a surface mesonet, enhanced upper-air profiling, storm photography, and a storm damage survey. Synoptic-scale forcing was weak and mesoscale factors were central to the development of severe weather. In particular, low-level boundaries such as gust fronts and the sea-breeze front played critical roles in the initiation and enhancement of storms, the motion of storms, and the generation of rotation at low levels. The complex and often subtle boundary interactions that led to the development of the tornadic supercell in this case highlight the need for advanced detection and prediction tools to improve the warning capacity for such events.

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Neil I. Fox, Rob Webb, John Bally, Michael W. Sleigh, Clive E. Pierce, David M. L. Sills, Paul I. Joe, James Wilson, and Chris G. Collier

Abstract

One of the principal aims of the Sydney 2000 Forecast Demonstration Project was to assess the utility of advanced nowcasting systems to operational severe weather forecasters. This paper describes the application of the products of a variety of systems by forecasters during a severe weather event in Sydney, Australia, on 3 November 2000. During this day a severe storm developed to the south of the metropolitan area and tracked north producing large, damaging hail, heavy rainfall, and at least three tornadoes. A number of severe weather warnings were issued by the Australian Bureau of Meteorology to a variety of customers throughout the day.

This paper investigates how the novel nowcast products were used by the forecasters and the impact they had on the forecast and warning dissemination procedure. The products used are contrasted with those that were available or could have been made available at various stages of the storm development and the efficiency of use of these products is discussed. The severe weather forecasters expressed their satisfaction with the systems and believed that the additional information enhanced the quality and timeliness of the warnings issued during the event.

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