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Norman W. Junker and James E. Hoke

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 model's 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.

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Norman W. Junker, Russell S. Schneider, and Stephanie L. Fauver

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. The scale and intensity of the rainfall appeared to be related to 1) the magnitude of the warm advection, 2) the 1000–500-mb mean relative humidity, 3) the breadth of the axis of stronger values of moisture transport feeding northward into a surface boundary, 4) the strength of low-level moisture flux convergence, and 5) the length of the low-level moisture flux convergence that was aligned along the mean flow upstream from the location of the rainfall maximum. The latter finding suggests that propagation plays an important role in modulating the scale and intensity of rainfall events.

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David A. Olson, Norman W. Junker, and Brian Korty

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 understanding of precipitation processes. The verification results have been used to track individual and group progress.

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Norman W. Junker, James E. Hoke, and Richard H. Grumm

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.

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Norman W. Junker, Michael J. Brennan, Frank Pereira, Michael J. Bodner, and Richard H. Grumm
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William E. Gartner, James E. Hoke, Norman W. Junker, and Louis E. Wolf

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Norman W. Junker, James E. Hoke, Bruce E. Sullivan, Keith F. Brill, and Francis J. Hughes

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 during the cool season.

The smooth orography in both models has a strong impact on the models’ precipitation forecasts. Each model overpredicted the frequency of heavier precipitation over the southern Appalachians, over portions of the Gulf-facing upslope areas east of the Rocky Mountains, and to the lee of the Cascade and Sierra ranges of the West. The NGM underpredicted the frequency of heavier amounts on the Pacific-facing windward side of the Cascade Range of Oregon and Washington.

Model performance also seems to be related to the synoptic situation. Threat scores were higher when the midlevel westerlies were more active, with the highest threat scores found north of the most frequent track of cyclones during the cool season.

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Norman W. Junker, Richard H. Grumm, Robert Hart, Lance F. Bosart, Katherine M. Bell, and Frank J. Pereira

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 also typically associated with an atmospheric river with anomalously high precipitable water (PW) and 850-hPa moisture flux (MF) within it. The standardized PW and MF anomalies associated with the heavy grouping were higher and were slower moving than in either of the lighter bins.

Three multiday heavy rainfall events were closely examined in order to ascertain whether anomaly patterns could provide forecast utility. Each of the multiday extreme rainfall events investigated was associated with atmospheric rivers that contained highly anomalous 850-hPa MF and PW within it. Each case was also associated with an unusually intense negative geopotential height anomaly that was similarly located off of the west coast of the United States. The similarities in the anomaly pattern among the three multiday extreme events suggest that standardized anomalies might be useful in predicting extreme multiday rainfall events in the northern Sierra range.

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Eric Rogers, Thomas L. Black, Dennis G. Deaven, Geoffrey J. DiMego, Qingyun Zhao, Michael Baldwin, Norman W. Junker, and Ying Lin

Abstract

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.

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