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Robert E. Hart and Richard H. Grumm

Abstract

A method for ranking synoptic-scale events objectively is presented. NCEP 12-h reanalysis fields from 1948 to 2000 are compared to a 30-yr (1961–90) reanalysis climatology. The rarity of an event is the number of standard deviations 1000–200-hPa height, temperature, wind, and moisture fields depart from this climatology. The top 20 synoptic-scale events from 1948 to 2000 for the eastern United States, southeast Canada, and adjacent coastal waters are presented. These events include the “The Great Atlantic Low” of 1956 (ranked 1st), the “superstorm” of 1993 (ranked 3d), the historic New England/Quebec ice storm of 1998 (ranked 5th), extratropical storm Hazel of 1954 (ranked 9th), a catastrophic Florida freeze and snow in 1977 (ranked 11th), and the great Northeast snowmelt and flood of 1996 (ranked 12th).

During the 53-yr analysis period, only 33 events had a total normalized anomaly (M TOTAL) of 4 standard deviations or more. An M TOTAL of 5 or more standard deviations has not been observed during the 53-yr period. An M TOTAL of 3 or more was observed, on average, once or twice a month. October through January are the months when a rare anomaly (M TOTAL ≥ 4 standard deviations) is most likely, with April through September the least likely period. The 1960s and 1970s observed the fewest number of monthly top 10 events, with the 1950s, 1980s, and 1990s having the greatest number. A comparison of the evolution of M TOTAL to various climate indices reveals that only 5% of the observed variance of M TOTAL can be explained by ENSO, North Atlantic oscillations, or Pacific–North American indices. Therefore, extreme synoptic-scale departures from climatology occur regardless of the magnitude of conventional climate indices, a consequence of a necessary mismatch of temporal and spatial scale representation between the M TOTAL and climate index measurements.

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Neil A. Stuart and Richard H. Grumm

Abstract

Forecasting major winter storms is a critical function for all weather services. Conventional model-derived fields from numerical weather prediction models most frequently utilized by operational forecasters, such as pressure level geopotential height, temperature fields, quantitative precipitation forecasts, and model output statistics, are often insufficient to determine whether a winter storm represents a large departure from normal, or has the potential to produce significant snowfall. This paper presents a method, using normalized departures from climatology, to assist forecasters in identifying long-duration and potentially significant winter storms. The focus of this paper is on anomalous low- and upper-level wind anomalies associated with winter storms along the U.S. east coast.

Observed and forecast low-level (850 hPa) and upper-level (300 and 250 hPa) easterly wind anomalies are compared with a 30-yr (1961–90) reanalysis climatology. Anomalous easterly low-level winds are correlated with enhanced low-level forcing and frontogenesis. Strong low-level winds can also contribute to enhanced precipitation rates. Upper-level winds that are anomalously below normal, represented as easterly wind anomalies, are also correlated with systems that are cut off from the main belt of westerlies, which may result in slower movement of the system, leading to long-duration events. The proposed method of evaluating easterly wind anomalies is shown to assist in identifying potentially slow-moving storms with extended periods of enhanced precipitation.

To illustrate this method, winter storms on 25–26 December 2002 and 2–4 January 2003 will be compared with past historical winter storms. The results suggest that the low- and upper-level wind anomalies in the two recent snowstorms share common characteristics with several record snowstorms over the past 52 yr. Many of these storms were associated with easterly wind anomalies that departed significantly (2 or more standard deviations) from normal. The examination of climatic anomalies from model forecasts may assist forecasters in identifying significant winter storms in the short range (2–3 days) and potentially out to ranges as long as 7 days when ensemble forecast guidance is utilized.

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Richard H. Grumm and Anthony L. Siebers

Abstract

A quantitative assessment has been made of the surface anticyclone forecast errors found in the operational nested grid model (NGM) run at the National Meteorological Center (NMC). Preliminary results covering a period from 1 December 1988 to 31 August 1989 reveal that the NGM predicts the central pressure of surface anticyclones to be too low over much of central and eastern North America during the winter and spring, especially along the track of transient anticyclones. The NGM tends to predict surface anticyclone pressure to be too high over the eastern Pacific and portions of the western Atlantic during winter, spring and summer. Pressure errors grow by forecast length and season. The 48-h forecast errors are larger in magnitude and better defined than the 24-h forecasts. The winter and spring pressure errors are better organized and have larger magnitudes than in summer.

Thickness (1000–500 mb) errors over the anticyclone center indicate an overall warm bias, especially over the North American continent and the adjacent western Atlantic Ocean, where anticyclones tend to be transient. Areas of negative thickness errors (cold bias) are found over the oceans and the elevated terrain of western North America. In general, the model places surface anticyclones too far south and east of the verifying position in the colder months.

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John H. E. Clark, Richard P. James, and Richard H. Grumm

Abstract

The processes responsible for a banded snowfall region during a December 1997 East Coast storm are examined. Conventional data plus a numerical simulation with the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) are used. Calculations of slantwise potential area near the bands suggest that the release of conditional symmetric instability played a role in their formation. The location and timing for the appearance of negative moist potential vorticity (MPV) cannot, however, be reconciled with band formation. A balanced MPV model based on the geostrophic momentum approximation is developed. It provided new insights into the mechanisms of MPV generation. A swath of negative balanced MPV now coincides with the snowbands. MPV sources are proposed that are linked to a vigorous mesoscale updraft near the bands. The updraft occurred on the warm, moist side of a zone of midtroposphere frontogenesis. Negative MPV develops through differential ageostrophic transports of geostrophic momentum and equivalent potential temperature. Of these, differential vertical equivalent potential temperature transport was the most efficient and accounted for the largest fraction of model-produced negative MPV tendencies near the bands. This mechanism was particularly strong in the lower troposphere near the mesoscale updraft.

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Richard H. Grumm, Robert J. Oravec, and Anthony L. Siebers

Abstract

Systematic errors in the nested-grid model (NGM) forecasts of surface cyclones are examined over a two-year period from 1 December 1988 through 30 November 1990. The parameters examined include the location, central pressure, 850-mb temperature, and the 1000-500-mb thickness over the center of the surface cyclone. The mean cyclone position error was typically 150 km at 12 h and 225 km at 24 h, and grew to about 350 km by 48 h. The overall mean cyclone pressure error was −0.57 and −0.68 mb at 24 and 48 h, respectively.

The results show that the skill of the NGM forecasts of surface cyclones displayed both seasonal and annual variability. The seasonal variability is represented by overall smaller errors in the summer and larger errors in the winter.

The NGM tended to overdeepen surface cyclones in all but the summer months. A large part of the pressure error was due to the model's inability to fill cyclones properly and a tendency to forecast systems to deepen when they were observed to fill. About 15% of the time in the winter months, the NGM forecast cyclones to deepen when they were observed to fill.

The NGM had difficulty detecting the initial development of surface cyclones, especially near the elevated terrain of western North America and along the track of transient cyclones. In these same regions, the NGM tended to forecast cyclones that were not observed. There was a preponderance of both nonobserved and nonforecast cyclones over the elevated terrain of North America, indicating that the NGM has difficulty with orographic effects.

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Robert E. Hart, Gregory S. Forbes, and Richard H. Grumm

Abstract

Since late 1995, NCEP has made available to forecasters hourly model guidance at selected sites in the form of vertical profiles of various forecast fields. These profiles provide forecasters with increased temporal resolution and greater vertical resolution than had been previously available. The hourly forecast profiles are provided for all of NCEP’s short-range models: the Nested Grid Model, Eta Model, and Mesoscale Eta Model. The high-resolution forecasts aid in the timing of frontal passages, low-level jets, and convective initiation. In addition, through time–height cross sections of Richardson numbers, forecasters can alert pilots to the potential for clear air turbulence several hours to a day in advance. Further, the profiles are useful in prediction of cloudiness and the dissipation of low-level stratus and fog. Time–height cross sections of wind velocity have proven extraordinarily useful in visualizing and forecasting inversion heights, frontal passage timing, boundary layer depth, and available environmental and storm-relative helicity during convective events.

The hourly model forecasts were found to be exceptionally helpful when combined with hourly surface observations to produce enhanced real-time analyses of convective parameters for use in very short term forecasting. High-resolution analyses of lifted index, CAPE, convective inhibition, moisture flux convergence, and 2-h changes in these fields aid the forecaster in anticipating convective trends. The introduction of model forecast error into these real-time analyses was minimized by using the latest available Eta or Mesoscale Eta Model runs. Therefore, the model data used to enhance the analyses are typically no more than 6–12 h old.

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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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John R. Gyakum, John R. Anderson, Richard H. Grumm, and Elissa L. Gruner

Abstract

An eight year sample of cold-season (1 October through 31 March) extratropical cyclones in the, Pacific Ocean basin is used to study central pressure changes and life cycle characteristics.

We find that over 90% of the cyclones passing through the area of the Kuroshio Current intensify in this region. Corresponding percentages in excess of 60% extend from the Kuroshio, south of 45°N, eastward to 130°W. Mean 24-h central pressure falls of all cyclones exceed 9 mb through the entire basin west of 140°W in the latitude band 30° to 50°N.

A statistical analysis of 24-h central pressure changes is performed on all cyclones within our domain. A frequency distribution of 1996 cases of 24-h maximum deepening reveals statistically significant departures from a Gaussian distribution, with the coefficient of skewness substantially negative. We also find similarly significant departures from normal in a frequency distribution of all 24-h central pressure changes, in spite of the fact that this distribution would be expected to have relatively fewer nonlinear interactions of processes associated with maximum deepening. A stratification of these data into ten degree latitude bands reveals that the ocean-dominated areas south of 60°N all have significant departures from the normal distributions with significantly large negative values of skewness. The land and ice-dominated region between 60° and 70°N has a deepening rate distribution that is approximately Gaussian with coefficients of skewness and kurtosis within the confidence limits of a normal distribution. These results suggest that the underlying ocean surface may be responsible for the significant departures of the pressure change distribution from a normal distribution.

We find that explosively developing cyclones (defined as those systems whose central pressure falls at least 24 mb in 24 h at 45°N) have longer lifetimes than the more conventional lows. Approximately 74% of the explosive cyclones last for at least four days. Only 21% of the nonexplosive cases exist for as long as four days. The vast majority of rapid deepeners commence their maximum intensification within 24 h of their initial formation. Thus, a correct analysis and forecast of a newly formed cyclone appears crucial to a successful explosive cyclone simulation.

Although cyclone formation areas cover vast areas of the Pacific, especially those east of Japan, south of Alaska, and the surroundings of the Kamchatka Peninsula, explosive cyclone formation positions are almost exclusively south of 50°N, concentrated east of the Asiatic continent, and in an area between 150° and 160°W. The “bomb” maximum deepening positions are located in areas slightly to the north and east of their formation positions. Dissipation positions, while concentrated in the Gulf of Alaska, the northeast Pacific, and in an area west of Kamchatka for all systems, are almost exclusively confined to areas north of 50°N for the rapidly deepening cyclones.

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Norman W. Junker, Michael J. Brennan, Frank Pereira, Michael J. Bodner, and Richard H. Grumm
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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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