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Adam J. Clark, William A. Gallus Jr., and Morris L. Weisman

of forecasts as perceived by human forecasters. The purpose of this study is to demonstrate the usefulness of a neighborhood-based equitable threat score (ETS; Schaefer 1990 ) to compare precipitation forecasts from experimental convection-allowing Weather Research and Forecasting Model (WRF; Skamarock et al. 2005 ) simulations conducted during April–July 2004–08 by the National Center for Atmospheric Research (NCAR) to operational North American Mesoscale (NAM; Janjić 2003 ) model forecasts

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Badrinath Nagarajan, Luca Delle Monache, Joshua P. Hacker, Daran L. Rife, Keith Searight, Jason C. Knievel, and Thomas N. Nipen

generality of the analog-based methods’ performances by extending them to (i) the entire conterminous United States (CONUS); (ii) forecasts from the North American Mesoscale Forecast System (NAM; Janjic et al. 2010 ), Global Forecast System (GFS; information online at http://www.emc.ncep.noaa.gov/GFS/doc.php ), and Rapid Update Cycle (RUC; Benjamin et al. 2004 ); (iii) the near-surface variables 10-m wind speed, 2-m temperature T , and 2-m relative humidity (RH); and (iv) a full year of verification

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John M. Lanicci and Thomas T. Warner

–9 April lid cycle a. High pressure stage The first stage of the lid cycle was characterized by a large surface anticyclone over the Great Plains and western Gulf of Mexico (see Figs. 2a,b ). Cold, dry air poured into the southern plains and gulf during this stage as a strong north–south pressure ridge established itself over this region. Conditions for the anticyclone’s intensification were favorable, as low-level cold air advection was present to the east of the surface ridge axis, while confluent

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Laura A. Stoss and Steven L. Mullen

Abstract

Forecast errors in the 500-mb geopotential height field over North America and adjacent ocean environs are calculated for the National Meteorological Center's Nested Grid Model (NGM). The eight winters 1985/86-1992/93 are examined. Errors are compared for the time-mean flow and for four recurring planetary-scale flow regimes, and the statistical significance of the differences is estimated.

Overall, the NGM produces very accurate 500-mb height forecasts out to 48 h, with every forecast cycle of the study period exhibiting useful deterministic skill at 48 h when averaged over the study domain. During the first two winters of operational NGM implementation, the spatially averaged errors were noticeably greater than in subsequent winters. NGM error was essentially constant during the 1987/88-1992/93 winters.

The bias in the NGM works to erode the asymmetries associated with the wintertime stationary waves. When the errors are categorized by the initial flow configuration, no significant differences are found among the spatially averaged error statistics for the regimes and the winter-mean values. On the other hand, the distribution of the bias varies notably among the regimes, with none of the regimes exhibiting a tendency to weaken the preexisting anomalies of its initial state.

The rms error (RMSE) is dominated by contributions from the random component at all forecast projections. The random RMSE is typically an order of magnitude larger than the bias, except over the Gulf of Mexico and the Caribbean, where they are of comparable size. The random RMSE at 48 h tends to be locally enhanced along axis of the polar jet stream, where its size approaches the observed rms variance due to synoptic-scale transients. Based on that finding, the authors hypothesize that 48 h may be close to the useful limit of predictive skill for mobile short waves at 500 mb.

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Michael C. Coniglio, James Correia Jr., Patrick T. Marsh, and Fanyou Kong

Abstract

This study evaluates forecasts of thermodynamic variables from five convection-allowing configurations of the Weather Research and Forecasting Model (WRF) with the Advanced Research core (WRF-ARW). The forecasts vary only in their planetary boundary layer (PBL) scheme, including three “local” schemes [Mellor–Yamada–Janjić (MYJ), quasi-normal scale elimination (QNSE), and Mellor–Yamada–Nakanishi–Niino (MYNN)] and two schemes that include “nonlocal” mixing [the asymmetric cloud model version 2 (ACM2) and the Yonei University (YSU) scheme]. The forecasts are compared to springtime radiosonde observations upstream from deep convection to gain a better understanding of the thermodynamic characteristics of these PBL schemes in this regime. The morning PBLs are all too cool and dry despite having little bias in PBL depth (except for YSU). In the evening, the local schemes produce shallower PBLs that are often too shallow and too moist compared to nonlocal schemes. However, MYNN is nearly unbiased in PBL depth, moisture, and potential temperature, which is comparable to the background North American Mesoscale model (NAM) forecasts. This result gives confidence in the use of the MYNN scheme in convection-allowing configurations of WRF-ARW to alleviate the typical cool, moist bias of the MYJ scheme in convective boundary layers upstream from convection. The morning cool and dry biases lead to an underprediction of mixed-layer CAPE (MLCAPE) and an overprediction of mixed-layer convective inhibition (MLCIN) at that time in all schemes. MLCAPE and MLCIN forecasts improve in the evening, with MYJ, QNSE, and MYNN having small mean errors, but ACM2 and YSU having a somewhat low bias. Strong observed capping inversions tend to be associated with an underprediction of MLCIN in the evening, as the model profiles are too smooth. MLCAPE tends to be overpredicted (underpredicted) by MYJ and QNSE (MYNN, ACM2, and YSU) when the observed MLCAPE is relatively small (large).

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Richard H. Grumm and Anthony L. Siebers

successively nested interior grids (grid-Band grid-C), each having twice the resolution of thegrid in which it is nested. This study examines the performance of the NGM surface cyclone forecasts ongrid-C and portions of the surrounding grid-B, whichspans all of North America and the adjacent oceans.The data in this study was displayed on the limitedarea fine-mesh (LFM) model grid. When initially introduced operationally, the NGMforecasts contained a cold bias in the lower levels ofthe model, but the

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Kimberly M. Wood, Oscar G. Rodríguez-Herrera, Elizabeth A. Ritchie, Miguel F. Piñeros, Ivan Arias Hernández, and J. Scott Tyo

Pacific exhibit remarkably high levels of axisymmetric organization. Nondeveloping tropical waves, mesoscale convective systems that form during the North American monsoon season, and nondeveloping disturbances that result from the breakdown of the intertropical convergence zone may contribute to these false alarm rates (e.g., Ferreira and Schubert 1997 ; Molinari et al. 2000 ; Serra and Houze 2002 ; Seastrand et al. 2014 ). In addition, Hennon et al. (2013) noted that the eastern North Pacific

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Arthur C. Pike and Charles J. Neumann

must beaccounted for when the question of skill is addressed. This study extends the concept of FDL beyond theNorth Atlantic basin to other tropical cyclone basins.Are forecasts in tropical cyclone basin "A" inherentlymore difficult than those in tropical cyclone basin "B"?Such questions often arise in the comparison of operational forecast errors among basins. For example,unpublished National Weather Service data show thatduring 1979-85, the average 72-h error for North Atlantic official

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Brandon McClung and Clifford F. Mass

could enhance onshore (westerly) diurnal winds from the west, generally opposing downslope flow. 4. Synoptic evolution associated with Diablo–North wind events This section explores the synoptic evolution of 500-hPa geopotential height, sea level pressure (MSLP), 850-hPa temperature, and 925-hPa relative humidity from 72 h prior to 24 h after Diablo–North wind events. Composites were constructed using grids from the North American Regional Reanalysis dataset (NARR) ( Mesinger et al. 2006 ), with a

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Peter M. Caplan and Glenn H. White

NorthernHemisphere winters, for both NMC and the EuropeanCentre for Medium Range Weather Forecasts(ECMWF). During both winters and for both centers,forecasts over Europe appear to have considerably moreskill than those over North America, reflecting eitherthe influence of orography or perhaps the lack of observations upstream from North America comparedto the high density and quality of observations upstream from Europe. While the ECMWF still demonstrates a clear edge over NMC in overall forecastskill, NMC

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