Norman W. Junker

Changes to the Operational ''Early'' Eta Analysis / Forecast System at the National Centers for Environmental Prediction

This note describes changes that have been made to the National Centers for Environmental Prediction ( NCEP ) operational ‘‘early’’ eta model. The changes are 1 ) an decrease in horizontal grid spacing from 80 to 48 km, 2 ) incorporation of a cloud prediction scheme, 3 ) replacement of the original static analysis system with a 12-h intermittent data assimilation system using the eta model, and 4 ) the use of satellite-sensed total column water data in the eta optimum interpolation analysis. When tested separately, each of the four changes improved model performance. A quantitative and subjective evaluation of the full upgrade package during March and April 1995 indicated that the 48-km eta model was more skillful than the operational 80-km model in predicting the intensity and movement of large-scale weather systems. In addition, the 48-km eta model was more skillful in predicting severe mesoscale precipitation events than either the 80-km eta model, the nested grid model, or the NCEP global spectral model during the March ‐ April 1995 period. The implementation of this new version of the operational early eta system was performed in October 1995.

Evaluation of 33 Years of Quantitative Precipitation Forecasting at the NMC

Abstract The National Meteorological Center (NMC) initiated Quantitative Precipitation Forecasts (QPF) and an intensive QPF verification program in 1960. These forecast products have evolved from a manual effort, relying on extensive forecast experience to one that placed much greater reliance on the interpretation and modification of numerical models. Verification graphs show steady improvements in forecast accuracy, especially for the longer-range forecasts, which in this context am those in the 24–60-h range. During the 1960s the Threat Score (TS) for day-2 forecasts for 1 in or more of precipitation averaged approximately 0.07. During recent years, that score has nearly doubled, and the 36–60-h period forecast in 1993 had a TS comparable to that for the 12–36-h period during the 1960s. Improvement in accuracy is probably related to a number of diverse factors including improved numerical models, increased forecaster knowledge of the strengths and weaknesses of the operational models, and an increased ...

A Study of Heavy Rainfall Events during the Great Midwest Flood of 1993

Abstract A synoptic–dynamic climatology was constructed using all 24-h 2-in. (50.8 mm) or greater rainfall events in nine states affected by heavy rains and flooding from June through September 1993 using 6- or 12-h gridded analyses from the Regional Data Assimilation System and geostationary satellite imagery. Each of the 85 events was assigned a category (0–4) based on the areal coverage of the 3-in. (76.2 mm) or greater observed precipitation isohyet. A variety of meteorological fields and rules of thumb used by forecasters at the Hydrometeorological Prediction Center are investigated that may help identify the most likely location and scale of a convective precipitation event. The heaviest rain usually fell to the north (downwind) of the axis of highest 850-mb winds and moisture flux in an area of 850-mb warm temperature and equivalent potential temperature advection. The rainfall maximum also usually occurred to the north or northeast of the axis of highest 850-mb equivalent potential temperature. Th...

Use of Normalized Anomaly Fields to Anticipate Extreme Rainfall in the Mountains of Northern California

Abstract Extreme rainfall events contribute a large portion of wintertime precipitation to northern California. The motivations of this paper were to study the observed differences in the patterns between extreme and more commonly occurring lighter rainfall events, and to study whether anomaly fields might be used to discriminate between them. Daily (1200–1200 UTC) precipitation amounts were binned into three progressively heavier categories (12.5–50.0 mm, light; 50–100 mm, moderate; and >100 mm, heavy) in order to help identify the physical processes responsible for extreme precipitation in the Sierra Nevada range between 37.5° and 41.0°N. The composite fields revealed marked differences between the synoptic patterns associated with the three different groups. The heavy composites showed a much stronger, larger-scale, and slower-moving negative geopotential height anomaly off the Pacific coast of Oregon and Washington than was revealed in either of the other two composites. The heavy rainfall events were...

Seasonal and Geographic Variations in Quantitative Precipitation Prediction by NMC's Nested-Grid Model and Medium-Range Forecast Model

Abstract This paper assesses the performance of the National Meteorological Center (NMC) Nested-Grid Model (NGM) during a period from March 1988 through March 1990, and the NMC medium-range forecast model (MRF) in two 136-day tests, one during summer made up of two 68-day periods (19 July–25 September 1989 and 20 June–28 August 1990) and one during winter and early spring (12 December 1989–26 April 1990). Seasonal and geographical variations of precipitation bias and threat score are discussed for each model. Differences in model performance in predicting various amounts of precipitation are described. The performance of the NGM and MRF varied by season, geographic area, and precipitation amount. The bias of the models varied significantly during the year. The NGM and MRF overpredicted the frequency of measurable precipitation (≥0.01 in.) across much of the eastern half of the United States during the warm season. Both models, however, underpredicted the frequency of ≥0.50-in. amounts across the South dur...

Performance of NMC's regional models

Abstract This paper details the performance characteristics of the two regional dynamical models used at the National Meteorological Center to forecast for North America. Strengths and weaknesses of these models—the limited-area fine-mesh (LFM) model and the nested grid model (NGM) of the Regional Analysis and Forecast System (RAFS)—are presented in terms of their ability to predict such fields and features as 500-mb heights, surface lows and highs, precipitation events, and the diurnal cycle. The systematic characteristics of the models are emphasized. Overall, the NGM was found to be more accurate than the LFM. Nevertheless, the LFM is a valuable forecast model because of its accuracy and longevity in providing operational guidance.

An Examination of Nested Grid Model Precipitation Forecasts in the Presence of Moderate-To-Strong Low-Level Southerly Inflow

Abstract The performance of the nested grid model (NGM) in predicting heavy rain is assessed for those cases in the cool season when moderato-to-strong low-level southerly inflow from the Gulf of Mexico is present. This study indicates that the NGM underpredicts precipitation maximum for heavier rainfall events, with the underprediction more common at 32°N than at 40°N. The NGM is also shown to have a slight slow bias in moving heavy precipitation bands to the east. Two case studies illustrate the models difficulties in predicting heavy precipitation but also show that the NGM offers useful information in predicting major rainfall events. Several possible reasons for the NGM underprediction of heavy rainfall over the southern United States are presented.

Comments on “A Climatology of Significant Winter-Type Weather Events in the Contiguous United States, 1982–94”

Branick (1997) presents results from a sophisticated national winter weather climatology study and suggests possible applications to operational forecasting. We at the National Weather Service’s Hydrometeorological Prediction Center (NWS’s HPC) appreciate the author’s efforts and agree with and support many of the author’s findings and recommendations. We also look forward to the possibility of accessing this database and applying it to the HPC’s winter weather forecast program. We concur with the author’s recommendations regarding the need for a standardized method for reporting winter weather, something meteorologists at the HPC have desired for some time. A standardized reporting system [e.g., the suggested use of local storm reports (LSRs) by all NWS offices] would be invaluable for supporting real-time and poststorm forecast verification, responding to frequent media queries, and preparing and issuing HPC’s operational storm summaries, as well as poststorm research and case studies. We also strongly agree on the importance of and the need for effective long lead time forecasts (greater than 12 h) of winter weather. In section 5, the author states, ‘‘if NCEP is to provide effective [winter weather] guidance to WFOs, it will be necessary to establish guidance products that cover lead times of 12–24 h.’’ We agree that long lead time is extremely important. This statement, however, may leave readers with the impression that no winter weather guidance currently exists. In fact, forecasts of heavy snow, which the author points out accounted for 80% of the reports in the sample and deems the ‘‘dominant hazard,’’ have been issued for a number of years by HPC for lead times beyond 12 h. The HPC, in part formerly the Weather Forecast Branch of the National Meteorological Center, issues two 12-h forecasts that indicate the potential for heavy