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J. Paul Dallavalle and Lance F. Bosart

Abstract

Results are presented for a synoptic-dynamic investigation of certain polar anticyclones originating in northwestern Canada.. A composite, consisting of three similar high pressure systems, is constructed. This cold anticyclone weakens while it moves southeastward towards the Gulf of Mexico. Along the Gulf Coast it curves northeastward and reintensifies as it becomes a warm, dynamic anticyclone. During a second composite case the high moves rapidly southeastward before dissipating quickly over the warm waters of the Gulf of Mexico. No further intensification or recurvature occurs. In both instances, cold air penetrates deeply into the southern latitudes.

An upper tropospheric ridge over northwestern Canada and Alaska appears important in strengthening the polar anticyclone. The center of the high moves southward if uninterrupted northwesterly flow is established aloft. The cold air preceding the high pushes towards the Gulf of Mexico as the trough downstream of the Canadian ridge deepens.

The eventual history of the anticyclone seems to depend on the interaction between the low-levellee side trough which forms over Idaho and Montana after the anticyclone's passage and the deep trough in the middle and upper troposphere over the East Coast. A weakening East Coast trough and an eastward propagating lee side trough are associated with rising heights in the middle troposphere over the central and eastern United States. Consequently, an anticyclone is supported dynamically as it builds northeastward from the Gulf of Mexico. If, however, the East Coast trough is pronounced, strong northwesterly flow dominates the central United States, the effects of the lee side trough are minimal, and the cold anticyclone moves over the Gulf of Mexico where rapid weakening occurs.

In the middle and upper troposphere, vorticity advection is most important in causing anticyclonic tendencies over and downwind of the surface anticyclone. In contrast, divergence usually dominates in producing similar tendencies in the lower troposphere over the surface high. The tipping term of the vorticity equation may also be critical in lower levels near frontal regions or orographic barriers.

In the lower troposphere subsidence is generally maximum on the east side of the surface anticyclone in a region or pronounced cold advection. In that area descent of 3 to 6 μbċs−1 may occur. Subsidence over the high center is weak, seldom exceeding 1 or 2 μbċs−1.

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Gordon A. Hammons, J. Paul Dallavalle, and William H. Klein

Abstract

For the last few years the National Weather Service has been producing centralized guidance forecasts of calendar day maximum and minimum temperatures by applying multiple regression equations derived statistically from numerical model output. At fist the equations were developed from a six-month stratification of the numerical forecasts, but later we were able to stratify the dependent data into three-month seasons. At the same time we added a number of new potential predictors. These two changes increased the skill of the automated guidance. Here we discuss the dependent data statistics for the three-month season equations and compare their forecasts with those made by the older six-month equations. Finally, we present verification statistics on the objective guidance for the fall and winter seasons from August 1973 to February 1976.

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Mary C. Erickson, J. Paul Dallavalle, and John S. Jensenius Jr.

Abstract

No abstract available

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Gary M. Carter, J. Paul Dallavalle, and Harry R. Glahn

Abstract

The production of interpretive weather element forecasts from dynamical model output variables is now an integral part of the centralized guidance systems of weather services throughout the world. The statistical forecasting system in the United States probably generates the most extensive suite of operational products, although other nations including Australia, Canada, France, Italy, The Netherlands, and the United Kingdom also routinely provide guidance for many weather elements and locations.

The United States' statistical guidance system has evolved throughout the past 20 yr. The two principal formulation methods that have been employed are the model output statistics (MOS) and “perfect prog” approaches. These techniques have advantages and disadvantages that influence both aggregate and specific day-to-day performance characteristics of the associated weather element forecasts. Verification results indicate that forecasts from both statistical approaches provide useful guidance for most weather elements and projections for locations throughout the contiguous United States and Alaska. The MOS forecasts have generally been superior to the perfect prog guidance; the drawback to MOS is the necessity to rely on a relatively stable numerical prediction model. As dynamical models change and increase in skill, the perfect prog approach may be preferred for some applications.

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Gary M. Carter, J. Paul Dallavalle, Albert L. Forst, and William H. Klein

Abstract

Currently, two sets of automated numerical-statistical forecasts of maximum/minimum (max/min) temperatures for calendar day periods are produced in the day-to-day operations of the National Weather Service. The “early” guidance forecasts are based on output from the Limited-area Fine Mesh (LFM) model, while the “final” guidance relies primarily on predictions from the hemispheric Primitive Equation (PE) model. This paper describes recent improvements to the early guidance surface temperature prediction system.

The Techniques Development Laboratory recently developed new early guidance equations to forecast calendar day max/min temperatures for projections out to approximately 60 h and hourly temperatures at 3 h intervals out to 51 h for approximately 230 stations in the conterminous United States. A combination of LFM model output, surface weather observations and climatic factors were used in this development. We derived three sets of temperature prediction equations for both the 0000 and 1200 GMT forecast cycle as follows: 1) max/min equations for the first (24 h) period and 3 h equations for projections of 6 to 27 h;2) max/min equations for the second (36 h) period and 3 h equations for projections to 27 to 39 h; and 3) max/min equations for the third (48 h) period and 3 h equations for projections of 39 to 51 h. To enhance consistency among the various max (or min) and 3 h forecasts, all the equations within each set are comprised of the same 10 predictors. We also derived a separate set of 60 h max/min equations.

Comparative verification indicates that, in sharp contrast to past results, max/min forecasts from the new early guidance system are now better than those from the final guidance system. In addition, the automated 3 h temperature predictions are superior to persistence forecasts based on 3 to 6 h old temperature observations, as well as to persistence forecasts based on reports taken 24 h earlier.

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Eli Jacks, J. Brent Bower, Valery J. Dagostaro, J. Paul Dallavalle, Mary C. Erickson, and James C. Su

Abstract

In this paper, we describe the development and use of new nested grid model (NGM)-based model output statistics (MOS) guidance that has been available since 26 July 1989 for 204 stations in the contiguous United States. The new guidance, which replaced the NGM-based perfect prog package that had been operational since May 1987, consists of forecasts of max/min temperature, probability of precipitation, cloud amount, and surface wind. Guidance for all four elements is available for projections of 1 and 2 days from 0000 and 1200 UTC. The limited-area fine-mesh model (LFM)-based MOS guidance package is still available and was not affected by this change. Verification on independent data shows that NGM-based MOS and LFM-based MOS temperature forecasts are about equally accurate and that both sets of MOS guidance are clearly superior to the NGM-based perfect prog guidance. For the probability of precipitation, the NGM-based MOS guidance is consistently more skillful than the perfect prog guidance, and usually more skillful than the LFM-based MOS guidance. For cloud amount, the NGM-based MOS forecasts are more skillful than either the LFM-based MOS or the NGM-based perfect prog. Finally, the NGM-based MOS and perfect prog wind forecasts are about equally skillful, and both sets are superior to the LFM-based MOS guidance.

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Mary C. Erickson, J. Brent Bower, Valery J. Dagostaro, J. Paul Dallavalle, Eli Jacks, John S. Jensenius Jr., and James C. Su

Abstract

In the spring of 1990, the National Meteorological Center (NMC) tested several modifications to the Regional Analysis and Forecast System (RAFS). In order to compare the proposed version of the RAFS to the current operational RAFS, NMC ran both systems in parallel for a 4-week period. Simultaneously, the Techniques Development Laboratory (TDL) applied the operational RAFS-based Model Output Statistics (MOS) equations to output from both the operational and proposed (parallel) versions of the Nested Grid Model (NGM) to generate two sets of MOS forecasts. Our goal was to determine the impact of RAFS modifications on the NGM MOS forecasts. At the end of the 4-week test period, we verified both the operational and parallel NGM MOS forecasts. Virtually no differences in accuracy or skill existed between the operational and parallel MOS forecasts of max/min temperature, probability of precipitation, and surface wind. The skill of the cloud amount forecasts, however, deteriorated significantly. The NGM 300-mb relative humidity field changed markedly as a result of the RAFS modifications, and this change affected the cloud forecasts. Since the cloud cover forecasts were the only NGM MOS products adversely impacted by the new parallel RAFS, we rederived the cloud equations without the 300-mb relative humidity. These equations were implemented operationally in September 1990. When the new RAFS is implemented, we expect that the impact on the current NGM MOS guidance will be minimal.

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Edward A. Zurndorfer, Joseph R. Bocchieri, Gary M. Carter, J. Paul Dallavalle, David B. Gilhousen, Karl F. Hebenstreit, and David J. Vercelli

Abstract

In this paper, we present trends in the verification scores of the Techniques Development Laboratory's (TDL's) operational guidance forecasts and the National Weather Service local forecasts made at Weather Service Forecast Offices (WSFO's). Verification statistics for objective guidance and subjective local forecasts of probability of precipitation (PoP), precipitation type, surface wind, opaque sky cover, ceiling height and maximum/minimum (max/min) temperature are shown for the cool and warm seasons as these guidance products became operational during the period 1970–77.

TDL's forecasts are based on the Model Output Statistics (MOS) technique. The primary input to the MOS prediction equations comes from forecast fields from the Limited-area Fine-Mesh, Trajectory, and/or Primitive Equation models. For some equations, surface observations and other variables such as station elevation are also used as predictors.

Verification shows that through the years TDL's aviation/public weather guidance forecasts have compared very favorably with local forecasts produced at WSFO'S. The guidance forecasts are usually better than the local forecasts for precipitation type, surface wind and opaque sky cover; the opposite is true for PoP, max/min temperature and ceiling height. The verifications also show that local and guidance forecasts have generally improved for all elements over the years.

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