John F. Weaver

Mesoscale and Radar Observations of the Fort Collins Flash Flood of 28 July 1997

Abstract On the evening of 28 July 1997 the city of Fort Collins, Colorado, experienced a devastating flash flood that caused five fatalities and over 200 million dollars in damage. Maximum accumulations of rainfall in the western part of the city exceeded 10 in. in a 6-h period. This study presents a multiscale meteorological overview of the event utilizing a wide variety of instrument platforms and data including rain gauge, CSU—CHILL multiparameter radar, Next Generation Radar, National Lightning Detection Network, surface and Aircraft Communication Addressing and Reporting System observations, satellite observations, and synoptic analyses. Many of the meteorological features associated with the Fort Collins flash flood typify those of similar events in the western United States. Prominent features in the Fort Collins case included the presence of a 500-hPa ridge axis over northeastern Colorado; a weak shortwave trough on the western side of the ridge; postfrontal easterly upslope flow at low levels; w...

The Impact of Crop Areas in Northeast Colorado on Midsummer Mesoscale Thermal Circulations

Abstract The present study provides a preliminary evaluation of mesoscale circulations forced by surface gradients of heating arising from irrigated areas adjacent to dry land, utilizing a combination of satellite, observational, and modeling approaches. The irrigated crop areas of northeast Colorado were chosen for the study. For the cases studied satellite surface infrared temperature data indicated a typical temperature contrast of approximately 10 K at noon, between the irrigated area and the adjacent dry land. Surface observations and aircraft measurements within the lower region of the atmospheric boundary layer indicated, in general, a significant temperature contrast and moisture difference, thereby implying a potential thermally driven circulation. The anticipated thermally induced flows, however, were reflected in the measurements only by modest changes in the wind speed and wind direction across the contrast location. It is suggested that the daytime, elevated, terrain-forced flow in the area, ...

Influence of Cold Pools Downstream of Mountain Barriers on Downslope Winds and Flushing

Abstract The influence of cold pools downstream of mesoscale mountain barriers on downslope winds and flushing is investigated in this study by means of a numerical mesoscale model. The model is compared with existing analytical and numerical solutions. It is found that the numerical model produces phases and amplitudes of nonlinear mountain waves reasonably well. The solutions show that the structure of mountain waves can be modified greatly by the presence of the cold pool. When a cold pool is present downstream of the mountain, the development of a large amplitude mountain wave is inhibited. In the absence of surface heating, downslope winds associated with a mountain wave can be prevented from penetrating the cold pool to reach the surface and flush out the very stable cold air, particularly when the synoptic pressure gradient is oriented so as to continuously replenish the cold air. Results also suggest that shear-induced turbulent mixing at the top of the cold air has little effect on flushing. Base...

Convective Cloud Climatologies Constructed from Satellite Imagery

Abstract Composites of satellite imagery are constructed for various hours and various summer months on Colorado State Universitys interactive processing system. Simple averages of visible wavelength imagery are considered as well as averages of bispectrally classified data. The classified images use both visible wavelength and infrared wavelength data to identify probable deep convection. Results reveal the diurnal convective cycle over the Rocky Mountains and high plains in greater detail than has been previously possible. The convective frequency composites are compared with precipitation averages and differences between “normal” versus severe weather patterns are discussed. Practical forecasting applications for the composited data are suggested and discussed.

Two Floods in Fort Collins, Colorado: Learning from a Natural Disaster

A flash flood in Fort Collins, Colorado, on 28 July 1997 resulted in 5 deaths, 62 injuries, and more than

Fire Detection Using GOES Rapid Scan Imagery

250 million in property damage. Following the 1997 flood, a great many changes were made in the citys preparedness infrastructure. On 30 April 1999, a combination of heavy rain and melting snow caused a second, less serious flood event. This article reports on the changes implemented following the first flood and their effectiveness during the second.

An Interesting Mesoscale Storm-Environment Interaction Observed Just Prior to Changes in Severe Storm Behavior

Abstract This paper demonstrates the proper use of geostationary satellite imagery in wildland fire detection. The roles of both the visible and the 3.9-μm channels are emphasized. Case studies from June 2002 are presented to illustrate techniques that can be utilized in both the detection and short-range forecasting processes. The examples demonstrate that, when utilized correctly, the sensitivity of the shortwave infrared channel to subpixel heat sources can often result in detections that match the timelines of human observations. Finally, a derived satellite product that increases the detection rate of wildland fires from space is described.

An Unusual Summertime Downslope Wind Event in Fort Collins, Colorado, on 3 July 1993

Abstract Satellite images are presented to illustrate an interesting interaction that occurred between a severe thunderstorm and a mesoscale feature that originated in its nearby environment. Immediately following that interaction, a series of tornadoes began, starting with a long-lived F5 tornado that produced major damage in Hesston, Kansas. Some speculation is presented regarding the physical processes that may have contributed to the observed changes in thunderstorm behavior.

Observing Forest Fires with the GOES-8, 3.9-µm Imaging Channel

Abstract An unseasonal, severe downslope windstorm along the eastern foothills of the Colorado Rocky Mountains is described. The storm, which occurred on 3 July 1993, produced wind guts in Fort Collins, Colorado, over 40 m s−1 and resulted in extensive tree and roof damage. The synoptic pattern preceding the wind event resembled a pattern typical of that for a Front Range late fall or wintertime wind storm, including a strong south–southwest-oriented height gradient at 700 mb and a strong west to east sea level pressure gradient across the Front Range. A particularly interesting facet of the event was that one small geographical area in and near Fort Collins experienced wind gusts nearly 40% stronger than any other location involved in the event. The mesoscale forecast version of the Regional Atmospheric Modeling System (RAMS) with 16-km grid spacing over Colorado was run for the storm. Consistent severe winds were not predicted by the model in this configuration. Increasing resolution in postanalysis to ...

Some Mesoscale Aspects of the 6 June 1990 Limon, Colorado, Tornado Case

Abstract Blackbody radiation at 3.9 µm increases rapidly with temperature. This fact suggests a potential application by fire weather meteorologists of the 3.9-µm imagery data provided by the new GOES-8 weather satellite. The ability of the 3.9-µm channel to sense forest fires is briefly discussed and an example case study is presented. A simple radiation model is used to estimate the minimum detectable fire size for several types of wildfires.