NOAA National Weather Service Eastern Region

Moist air from the tropics moves into northwest Mexico and the southwestern portion of the United States during the early part of the summer season. This is analogous to the monsoon circulation in other sections of the globe. The annual change in circulation has been related to the northward and westward development of the large subtropical high pressure system over the southern United States. With this change the moist tropical air over the Gulf of Mexico is believed to be carried across Mexico and into the southwestern United States. The mountains of Mexico provide a formidable barrier for the movement of the Gulf of Mexico moisture westward. It has been shown that half of the precipitable water vapor in the atmosphere over the Arizona deserts in the summer months is below the 800-mb level. This is inconsistent with the idea of the Gulf of Mexico supplying the greatest proportion of the moisture. This paper presents support for the greatest percentage of tropical moisture in the southwest United States and northwest Mexico coming from the Pacific Ocean by way of the Gulf of California rather than from the Gulf of Mexico.

Abstract A climatological and composite study of banded precipitation in the northeast United States during the cold season (October–April) is presented. Precipitation systems in the northeast United States in April 1995 and from October 1996 to April 2001 that exhibited greater than 25.4 mm (1 in.) of rainfall, or 12.7 mm (0.5 in.) liquid equivalent, were identified as cases for study. A total of 111 cases were identified during this period, of which 88 had available radar data. Of these cases, 75 exhibited banded structure whereas 13 did not. A band classification scheme was developed from a subset of study cases. Application of the classification scheme to the 88 cases revealed that banded cases can exhibit a variety of banded events during their evolution. Single-banded events were the most common (48), followed by transitory (40), narrow cold frontal (36), multi (29), and undefined (9). Further investigation of the single-banded events highlighted banded structure in the comma-head portion of storms, with 81% of these events exhibiting a majority of their length in the northwest quadrant of the surface cyclone. Composites were calculated for cases exhibiting single-banded events in the northwest quadrant of the surface cyclone and for nonbanded cases to distinguish synoptic and mesoscale flow regimes associated with banded events and nonbanded cases. The banded composite was marked by cyclogenesis and the development of a closed midlevel circulation. This flow configuration was associated with deformation and strong midlevel frontogenesis northwest of the surface cyclone center, which coincided with the mean band position. The nonbanded composite exhibited a much weaker cyclone located in the confluent entrance region of an upper-level jet. The absence of a closed midlevel circulation in the nonbanded composite limited deformation and associated frontogenesis northwest of the surface cyclone. Cross-section analysis through the respective composite frontogenesis maxima showed that the banded composite frontal zone exhibited stronger and deeper frontogenesis and weaker conditional stability than the nonbanded composite frontal zone. Case studies from the northeast United States confirm the composite results, highlighting the importance of deep-layer frontogenesis coincident with weak conditional stability. These results are in qualitative agreement with the Sawyer–Eliassen equation, which predicts that the frontogenetical response will be enhanced (reduced) in the presence of small (large) moist symmetric stability.

The impact of increasing automation of meteorological guidance on manual preparation of operational weather forecasts is discussed. There appears to be some evidence that forecasters are abdicating their meteorological input into the final forecast as automated guidance increases in quantity and quality. Results of a National Weather Service Western Region program to stimulate more forecaster input into the final forecast are given. Suggestions are made regarding “interactive” forecasting using on-station computers in the upcoming Automation of Field Operations and Services (AFOS) era.

In the 23 years since Hurricane Camille devastated Virginia with 27 inches of rain in 24 hours, a major area targeted for hydrometeorological forecast service improvements has been flood and flash flood forecasting. The first attempts to tackle the problems were event driven. Numerous poststorm analyses led to the definition of meteorological criteria often associated with various types of major flash flood–producing rainfall situations. Individual forecast offices attempted to use these techniques with inconsistent success. Additionally, verification was not carried out on a routine or systematic basis. In 1979, the National Weather Service (NWS) Eastern Region began to encourage its offices to use precipitation forecasts routinely to anticipate critical flood conditions, rather than awaiting observations of rainfall. However, the implementation of a broadscale programmatic approach to the routine operational use of quantitative precipitation forecasting faced numerous hurdles. Complexities ran the gamut of operational problems; and broadscale efforts to implement the program floundered. At the same time, public and private sector users continued to request more accurate information with better lead time for response. Academic studies showed that in order to gain enough lead time for effective decision making and response, it is essential to incorporate the uncertainty of the precipitation forecast into flood forecast operations. Within the last five years, the NWS once again introduced the possibility of a disciplined, systematic, scientific application of these ideas in the field of operational forecasting. The NWS modernization has afforded the vehicle to implement these concepts operationally. In parallel, NWS forecasters and university researchers have collaborated on probabilistic approaches to the rainfall forecast problem, integrating theory, method, process, and operations. Based on 20 years of progressive learning and operational experience, the NWS now has the tools, the understanding, and the scientific and operational capabilities to expand the efforts nationally.

Abstract Winter storms are disruptive to society and the economy, and they often cause significant injuries and deaths. Innovations in winter storm forecasting have occurred across the value chain over the past two decades, from physical understanding, to observations, to model forecasts, to postprocessing, to forecaster knowledge and interpretation, to products and services, and ultimately to decision support. These innovations enable more accurate and consistent forecasts, which are increasingly being translated into actionable information for decision-makers. This paper reviews the current state of winter storm forecasting in the context of the U.S. National Weather Service operations and describes a potential future state. Given predictability limitations, a key challenge of winter storm forecasting has been characterizing uncertainty and communicating the forecast in ways that are understandable and useful to decision-makers. To address this challenge, particular focus is placed on establishing a probabilistic framework, with probabilistic hazard information serving as a foundation for winter storm decision support services. The framework is guided by social science research to ensure effective communication of risk to meet users’ needs. Solutions to gaps impeding progress in winter storm forecasting are highlighted, including better understanding of mesoscale phenomenon, the need for better ensemble calibration, a rigorous and consistent database of observed impacts, and linking multiparameter probabilities (e.g., probability of intense snowfall rates at rush hour) with users’ information needs and decisions.

*NOAA, National Weather Service, Eastern Region, Bohemia, New York+American Meteorological Society, Boston, MassachusettsCorresponding author address: Dr. Susan F. Zevin, NOAA, National Weather Service, Eastern Region, Bohemia, NY 11716.