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P2H.9 VERIFICATION OF QUANTITATIVE PRECIPITATION FORECAST GUIDANCE FROM NWP MODELS AND THE HYDROMETEOROLOGICAL PREDICTION CENTER FOR 2005–2007 TROPICAL CYCLONES WITH CONTINENTAL U.S. RAINFALL IMPACTS Michael J. Brennan, Jessica L. Clark, and Mark Klein NOAA/NWS/NCEP Hydrometeorological Prediction Center, Camp Springs, MD 1. INTRODUCTION flooding and river flooding. Considering that more annual fatalities in the United States are The National Weather Service's attributable to flash flooding than any other Hydrometeorological Prediction Center (HPC) weather phenomena during the 30-year period routinely produces quantitative precipitation from 1977-2006 (NWS 2008), and that forecast (QPF) guidance for days 1 through 5 freshwater flooding was responsible for more over the Continental United States (CONUS). than half of the loss of life in the United States This guidance is used by National Weather from TCs in the period from 1970-1999 Service (NWS) River Forecast Centers (RFCs) (Rappaport 2000), HPC is particularly interested and Weather Forecast Offices (WFOs) in the in improving the skill of TC QPFs in the interest preparation of river forecasts and gridded NWS of saving lives and property. The goal of this precipitation products. HPC computes skill study is to explicate the methodology, scores for all its QPFs in addition to verification, and evaluation of TC QPFs at HPC. corresponding numerical weather prediction Section 2 will describe the QPF forecast and (NWP) model QPFs; in particular, threat score verification methodology at HPC. Section 3 will and bias (e.g., Wilkes 2006) will be examined present seasonal verification results for the 2005, here. In 2004, HPC began performing regional 2006, and 2007 hurricane seasons, while verification for its 24-hour QPF products for days section 4 contains individual case study QPF 1 through 3. The introduction of this process examples. Section 5 provides a summary and was the first step in an HPC initiative to assess describes upcoming changes to the QPF QPF performance for specific meso-β scale verification methodology at HPC. systems. By breaking down the skill assessment of QPFs over the CONUS 2. METHODOLOGY regionally, HPC can more accurately asses its skill and the skill of the NWP models at QPF from HPC and three NWP models were forecasting precipitation for specific phenomena, verified for TCs which impacted the CONUS including rainfall associated with tropical from 2005-2007. The models verified included cyclones (TCs) and their remnants. This study the National Weather Service’s GFS and NAM highlights TC QPF verification from both HPC models and the ECMWF model. Table 1 and NWP forecasts from the National Weather provides a list of those TCs included in this Service’s Global Forecast System (GFS), North verification study. American Mesoscale (NAM) model, and ECMWF model guidance for the 2005-2007 Threat score and bias statistics were calculated hurricane seasons. This period was chosen for the 12-36 hour forecast period from the 0000 because in 2005, 24-h QPF from the European UTC model cycle (hereafter denoted as “day 1”). Centre for Medium-Range Weather Forecasts (ECMWF) global model was added to the Threat score is defined as: guidance suite available at HPC. = /( + + COFCTS ) (1), Rainfall from TCs can occur well away from the center of circulation and may persist for days where F is the area forecast, O is the area after the low-level circulation center has observed and C is the area “correct” (i.e., where dissipated. This rainfall often generates flash the area forecast for a given threshold overlaps ______________________________________ with the area where that threshold was Corresponding author address: observed). The units of F, O, and C for a Dr. Michael J. Brennan specified precipitation threshold amount are in 5200 Auth Rd., Camp Springs, MD 20746 km2. Email: [email protected] The day 1 regional verification was performed Bias is defined as: using the Forecast Verification System (FVS; Brill 2008), which is widely used at both HPC = / OFB (2), and NCEP’s Environmental Modeling Center (EMC) to evaluate model performance. where B is the forecast coverage area (F) for a Statistics were calculated for each TC, and particular threshold divided by the observed summary statistics were also computed for each area of that threshold (O), regardless of of the tropical seasons. Additionally, storm-total accuracy in location. precipitation analyses for the TCs presented here were produced at HPC using a At HPC, the day 1 forecast consists of an methodology described by Roth (2008). accumulation of four spatially averaged QPFs at 6-hour intervals. Due to the variable spatial 3. 2005–2007 SEASONAL VERIFICATION resolution among the numerical models, each of the forecasts, including those from HPC, were Skill assessments of HPC QPF and the QPF mapped to a standard grid (approximately 32-km from the models for the 2005-2007 seasons are grid resolution at 60°N) that has been used for shown in Figs. 2-4. For all three seasons, HPC verification at HPC for several years (Charba et outperformed the NWP QPF model guidance at al. 2003). This method ensures a fair most thresholds on day 1. HPC QPF generally comparison among each of the forecast shows the most improvement over the models products. During the three-year study period, for TCs that produce the heaviest, most the numerical models underwent several widespread precipitation. However, HPC tended changes, including upgrades to the GFS (before to have a high bias for amounts over 3 inches the 2007 tropical season; NCEP 2008b) and during the 2005 (Fig. 2) and 2006 (Fig. 3) ECMWF (before the 2006 tropical season), and seasons, over-forecasting at these thresholds. a transition in the NAM from the Eta model to Conversely, the 2007 season (Fig. 4) saw a low the Weather Research and Forecast (WRF) HPC bias for amounts over 3 inches. This may model NMM core prior to the 2006 season have been, at least in part, due to (NCEP 2008a). The verification data consisted overcompensation by HPC forecasters who of a manual, quality-controlled analysis of the were aware of their previous high bias at these Climate Prediction Center’s (CPC) daily rain thresholds. gauge observations, of which there are over 8000 stations reporting across the United States While the NWP models generally do not have and Mexico. seasonal threat scores as high as HPC for amounts less than 3 inches, they show the most In order to adequately differentiate precipitation skill compared to HPC at these lower thresholds. associated with a given tropical system from The NAM is the poorest overall performer, and other synoptic or mesoscale features, the its threat score drops off dramatically as the CONUS was divided into 14 climatologically threshold amount increases. The NAM also has similar regions (Fig. 1). Verification was only an extremely low bias for amounts over 2 inches, performed for those regions in which rarely forecasting amounts that heavy. Overall, precipitation was directly associated with the TC the GFS and ECMWF showed similar skill at or its remnants. The region or regions verified forecasting precipitation at thresholds 1 inch and were chosen on a daily basis to capture the under, with the GFS showing more skill at movement of the system over time. amounts over 1 inch during the 2007 season Determination of the regions was accomplished while the ECMWF was the superior performer at through examining the observed track of the TC, these thresholds during 2006. Interestingly, the radar imagery of the event, and quality- GFS and ECMWF were nearly perfect in terms controlled quantitative precipitation estimate of bias for all thresholds during the 2005 season, (QPE) data from the RFCs over the CONUS. but during the 2006 and 2007 season, the GFS While there was some subjectivity to the displayed a high bias for amounts over 3 inches evaluation of the regions, if it was deemed that a while the ECMWF demonstrated a significant majority of the observed precipitation was not low bias. Perhaps the changes made to the directly associated with the TC, that region was ECMWF prior to the 2006 season contributed to excluded from the statistics for that verifying the decrease in its aerial coverage for date. precipitation at these higher thresholds. 4. CASE EXAMPLES greater (and higher than HPC for the 2 in. threshold). The ECMWF had the highest model Three cases will be presented to highlight the threat score for lighter amounts and generally challenges of forecasting TC rainfall, even in the exhibited the best bias of the model guidance day 1 period. The performance of the model and through the various thresholds. The NAM had HPC QPFs will be discussed along with the the lowest threat score through all thresholds, as particular forecast challenges associated with well as a severe low bias for amounts of 1 in. or that TC. greater. a) Hurricane Rita (2005) On 25-26 September, the remnants of Rita were moving northeastward into the mid-Mississippi Hurricane Rita made landfall on 24 September Valley and interacting with and ultimately 2005 near the Texas/Louisiana border (Knabb et becoming absorbed by a cold front moving al. 2006). After landfall, Rita weakened and eastward across the upper Mississippi Valley moved north-northeast through eastern Texas, (Fig. 8). For the 24-h period ending at 1200 UTC Arkansas, southeastern Missouri and southern 26 September, HPC QPF indicated two Illinois. Rita produced heavy rainfall in South precipitation maxima, one over central and Florida as well as across eastern Texas and northern Mississippi and a second farther north Louisiana (Fig. 5), with the maximum rainfall over southern and central Illinois (Fig. 9a). In amount of 16 in. reported at Bunkie, Louisiana. both of these areas HPC forecast more than 2 in.

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