Forecast Experience Research Articles

A comparison between subjective and objective thunderstorm forecasts

We made a comparison between man-made 24-h thunderstorm forecasts (subjective forecasts) and 24-h pure index thunderstorm forecasts (objective forecasts). The forecasts concern the plane of Friuli–Venezia Giulia region (NE Italy) during summer. Subjective forecasts are based on forecaster interpretation of numerical models outputs and forecaster's experience, while objective forecasts use models outputs and the thunderstorm climatology of our region to issue two independent indexes. Both these indexes are calculated by a linear combination of meteorological variables extracted from the European Centre for Medium Range Forecast (ECMWF) model. In one case, the coefficients of the combination were obtained by a linear multiregression over 8 years of data, while in the other, the biserial correlation coefficient was maximized on a subset of 2 years. To estimate the quality of forecasts, we used the data collected during the summer of 1998 and 1999; thunderstorm days are defined by lightning data and ground stations observations. The joint probability density functions of forecasts and observations have been used to study the forecast quality. We also studied the reliability diagrams of subjective forecasts. We found that subjective forecast skill and objective forecast skill are comparable although there were some differences as well. Subjective forecasts have a low false alarm rate (FAR) and they showed good facility in forecasting days without thunderstorms, but they score low in probability of detection (POD). Objective forecasts, in contrast, showed lower FAR and facility in forecasting stormy days but detected thunderstorms better than subjective forecasts. There are significant fluctuations in forecasts' performance from 1998 to 1999 summer seasons. This is likely due to variation in thunderstorm climatology and thunderstorm type frequencies. Furthermore, feedback of past performances could have played an important role in subjective forecasts.

An Assessment of Flood Forecasting in Bangladesh: The Experience of the 1998 Flood

Floods are among the most costly natural disasters interms of human sufferings and economic losses inBangladesh. Approximately 20% of the countryexperiences normal annual flooding while all thehistorical catastrophic floods inundated more thanfifty percent of the country's total area. The recentflood in 1998 has been found to be more severe thanall previous floods. During the flooding season of1998, the Flood Forecasting and Warning Center (FFWC)of the Bangladesh Water Development Board (BWDB)prepared daily flood bulletin and incorporatedinformation about rainfall, rise/fall of riverwater, flood forecasting for 24 and 48 hr inadvance and warning messages (if any). The FFWCattempted to provide adequate services to the localand national level decision-making process. Theforecasting procedure adapted by the FFWC was based onhydrological information, forecaster's experience, andmodel simulation. This paper primarily examines therole and activities of the FFWC, especially in floodforecasting and warning. Identification of the causesand consequences of 1998 flood is the other pertinentarea of discussion of the paper. Findings of this research revealed that the flood of1998 was caused by heavy downpour in the upstream thatwas drained out through the major rivers inBangladesh. Three major rivers' peak was synchronizedand characterized it as the most prolonged flood inthe history of Bangladesh. It also revealed that,despite various limitations, the flood forecasts ofthe FFWC were reasonably adequate to meet nationaldemand during the crises of 1998.

Fog Forecasting Objectively in the California Coastal Area Using LIBS

Abstract Weather forecasts and warnings are the most important services provided by the meteorological profession. As one aspect of these services, a year-round fog forecasting objective is selected for California coastal sites and their surrounding areas, both alongshore and offshore. The effort was to provide a simple, straightforward forecasting method using primarily data from the coastal site, which might be supplemented and improved using the forecaster's experience, any new technology, and new results of research. The meteorological objective is to forecast light, moderate, and dense fog, the probability of each occurring, and the expected times of beginning and ending of each period of reduced visibility, both for terminals and for limited nearby areas. Forecasts of five days' duration are sought using a frequently observed sequence of synoptic events. Methods in use were reviewed. The synoptic approach, emphasizing sea–air relationships, has produced the most promising results at this time. This ...

Forecasting the Dissipation of Fog and Stratus Clouds

F the time at which fog or stratus clouds will break has become a particularly important problem in recent years with the development of commercial and military aviation and the improved methods of flying which have resulted. Many times the completion of an airline schedule is dependent upon the dissipation of a fog or low cloud deck within a given time. Even the rate at which the ceiling is rising becomes important, for may flights could be dispatched toward fog-bound terminals without long delays if a definite indication of sufficient improvement were available. At present, forecasts involving these elements are largely a function of the forecaster's experience, and are not always extremely accurate or reliable. Various empirical methods employing such elements as the thickness of the cloud, the height of the cloud base, and the fact that terrestrial radiation is completely absorbed by the lower levels of the clouds, appear to have formed the principal aids to forecasting the breaking of fog and stratus systems. The absorption of terrestrial radiation has been generally credited as the source of heat providing for the dissipation of a cloud, since the process normally takes place upward from the cloud base. A moment's thought will convince one that this factor cannot be of very great importance. Brunt has shown that the radiative transfer of heat in the atmosphere in regions of the spectrum where water vapor is the effective absorbing agent, is negligible. This means that any net gain of heat at a cloud base by radiation must originate in the transparent regions of the spectrum, a series of bands accounting for only about 25% of the total. This gain is further reduced by the fact that a cloud acts as a black body, and therefore the sky radiation on a cloudy day cannot differ greatly from that originating at the earth's surface, especially if the cloud is low and its temperature nearly the same as that of the ground beneath. Hence, it seems illogical to assume that cloud decks several thousand feet in thickness may be dissipated within a few hours by heat gained from the absorption of terrestrial radiation.