Harry R. Glahn

An Operational Model for Forecasting Probability of Precipitation—PEATMOS PoP

Abstract A dynamical-statistical model for use in probability of precipitation forecasting out to 60 h is described. This model has been fully operational twice daily in the National Weather Service (NWS) since January 1972. The acronym for the model is PEATMOS PoP, for Primitive Equation And Trajectory Model Output Statistics, Probability of Precipitation. All inputs to the PoP model are provided at the National Meteorological Center by the normal outputs of the Primitive Equation and Trajectory models. We have continued to modify and adjust the model in the various ways described in order to improve its performance. An evaluation of performance shows that the accuracy of the forecasts produced by this model has continued to increase from year to year. PoP forecasts going to the public from NWS local offices have also improved with time. This improvement is likely the result of better guidance (PEATMOS PoP), although there was a slight adjustment period that lasted only a few months when the objective gu...

A Local AFOS MOS Program (LAMP) and its Application to Wind Prediction

Abstract The Techniques Development Laboratory has a project called the local AFOS MOS Program (LAMP). Its purpose is the development of a system which can produce at any hour of the day in a Weather Service Forecast Office (WSFO) environment Model Output Statistics (MOS) forecasts for essentially all locations for which the WSFO makes routine forecasts. These forecasts will be for most weather elements and for projections of 1 to about 20 hours. Inputs will include centrally produced MOS forecasts, hourly observations, radar data, and a few forecast fields from the National Meteorological Centers primary guidance model. LAMP includes three rather simple advective models: one to forecast sea level pressure, one to forecast 1000–500 mb moisture and precipitation, and one to forecast sensible weather such as ceiling height and precipitation type. This paper describes LAMP, its three advective models, and the results of experiments in surface wind prediction. It is concluded that LAMP wind forecasts are con...

Recent Developments in Automated Prediction of Ceiling and Visibility

Abstract The history of the National Weather Services development efforts in terminal weather prediction for aviation is discussed and results from recent experiments involving three approaches are presented. In one approach, single-station equations for predicting the probability of specific ceiling and visibility categories are developed. The equations are based upon weather observations at the local terminal only and are derived by using the Regression Estimation of Event Probabilities screening technique. In another approach, Model Output Statistics (MOS) is used to develop probability forecast equations for ceiling and visibility. MOS consists of determining a statistical relationship between a predictand and the forecast output of numerical prediction models. The statistical relationship is determined by screening regression in this paper. The two numerical models used are the National Meteorological Centers (NMC) primitive equation (PE) model and the Techniques Development Laboratorys Subsynopti...

Objective Prediction of Cloud Amount Based on Model Output Statistics

Abstract We have applied the Model Output Statistics (MOS) approach to the prediction of cloudiness. Final guidance warm and cool season forecasting equations were developed by screening forecast fields from the primitive equation and trajectory models. We derived separate equations for each of 233 stations to estimate the probability of clear, scattered, broken and overcast conditions 12 to 48 h in advance. The same predictors were used in all four equations for any given station and projection. In like manner, we also derived a set of early guidance equations for the warm season by screening forecasts from the limited-area fine mesh model. Here, separate equations were developed for 230 stations and projections of 6 to 24 h. Weather parameters from surface reports were also included as potential predictors for the first two forecast projections to provide the latest observed conditions for the early and final guidance systems. We verified both experimental and operational cloud forecasts made from the f...

Use of Model Output Statistics for Predicting Ceiling Height

Abstract The model output statistics (MOS) technique consists of determining a statistical relationship between the forecast output of numerical prediction models and a predictand. This paper presents some results obtained in applying the MOS technique to the prediction of ceiling height by means of screening regression. Data from 3 winter seasons and 95 eastern U.S. stations are combined in a generalized operator approach to develop multiple regression equations. The potential predictors subjected to screening include surface variables observed at 0700 GMT and forecast output from both the National Meteorological Centers primitive-equation model and the Techniques Development Laboratorys subsynoptic advection model. Prediction equations are developed for 5-, 11-, and 17-hr forecast projections representing ceiling height forecasts valid at 1200, 1800, and 2400 GMT, respectively. Ceiling height is treated both as a categorized and as a continuous predictand. Where ceiling height is categorized, the regr...

Objective Estimation of the Conditional Probability of Frozen Precipitation

Abstract A system is developed which produces objective forecasts of conditional probability of frozen precipitation for the conterminous United States. Development of the system consists of two basic steps, in each of which the MOS (Model Output Statistics) concept is used. First, for each of 186 stations, we find a “50%” value for each of three variables predicted by the National Meteorological Centers Primitive Equation (PE) model: 1000–500 mb thickness, boundary-layer potential temperature, and 850-mb temperature. For instance, we find the value of the 1000–500 mb thickness which indicates a 50–50 chance of frozen precipitation at a particular station, provided precipitation occurs. These 50% values are determined by using the logit model to fit data from three winter seasons, September 1969 through March 1972. Secondly, the deviations from the 50% values are determined for each station for each variable; the relative frequency (for those cases when precipitation occurred) of frozen precipitation is ...

Testing the Limited Area Fine Mesh Model for Probability of Precipitation Forecasting

Abstract Experimental objective forecasts of probability of precipitation (PoP) were made and verified for a large number of United States cities in order to make preliminary tests of the usefulness of the limited area fine mesh (LFM) model. The Model Output Statistics (MOS) technique was used to derive the forecast equations for the winter and summer seasons. Predictors were selected from forecast output of the Primitive Equation (PE), Trajectory (TJ) and LFM models, and from the sine and cosine of the day of the year. Forecast equations were developed with data from only one winter and one summer season (“small sample equations”) from PE predictors only, TJ predictors only, LFM predictors only, and various groupings of these. The small-sample equations were compared on independent data with each other and with operational equations developed on a much larger data sample. It was found that the small-sample combinations that included the LFM gave better results than the operational equations in winter. Ho...

Verification and Further Development of an Operational Model for Forecasting the Probability of Frozen Precipitation

Abstract An automated system for forecasting the conditional probability of frozen precipitation was put into operation by the National Weather Service in November 1972. The Model Output Statistics (MOS) concept was used to develop the system, and both teletypewriter and facsimile products have been distributed to field offices twice daily. In this paper, guidance forecasts from this system are compared to subjective (local) forecasts prepared at Weather Service Forecast Offices. The local forecasts have been archived since September 1973 as part of a combined aviation/public weather forecast verification program within the National Weather Service. The comparative verification between the guidance and locals for two different data samples shows the guidance has produced better forecasts for the 18, 30, and 42 h projections. In attempting to improve the operational system, we experimented with the Regression Estimation of Event Probabilities (REEP) screening technique. The operational system had been deve...

The Equivalency of the Tangent and Secant Lambert Conformal Map Projections

Abstract There is a popular misconception that the secant form of the Lambert conformal map projection is “better” than the tangent form. It is shown here that the two forms are equivalent; they are different only in the sense that the scale of the map quoted is usually true at the two secant latitudes for the secant projections and at the single tangent latitude for the tangent projection.

Progress in the Automation of Public Weather Forecasts

Abstract Complete automation of public weather forecasts will exist when forecasts are prepared entirely by computer and are disseminated to the final user through associated communications facilities. Preparation of such a forecast must start with observations of various kinds and proceed through the collection and analysis of those observations, the running of numerical and statistical models to arrive at digital forecasts of the pertinent weather elements, and the preparation of the forecast in its final format. Observing the atmospheric variables used as input to forecasting models is far from automated, although much has been done in this area in recent years. The assembly and analysis of the observations have been largely automated at national centers, although work must continue on incorporating new types of observations and making more efficient use of those already available. Numerical models are being run which do a good job of predicting the future state of certain atmospheric variables. Statis...