NAM AIR QUALITY DIAGNOSTIC DISCUSSION NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD 1013 AM EDT MON JUL 02 2012 *** The following information pertains to initialization errors noted on the 12Z NAM that may negatively impact the performance of the 12Z CMAQ output as used for air quality forecasting. Specifically, errors that apply to temperature, winds, relative humidity and precipitation are included. This discussion only applies to geographical regions of the CONUS where air quality is currently forecast to be degraded (i.e. CODES: YELLOW/ORANGE/RED/PURPLE/MAROON). *** ...INTERIOR CA VALLEYS/DESERT SOUTHWEST... THE 12Z NAM INITIALIZATION DEPICTED TEMPERATURES APPROXIMATELY 10 DEGREES TOO COOL ACROSS SECTIONS OF THE SOUTHERN CA VALLEYS AND SOUTHERN AZ. DIRECTIONAL ERRORS WERE ALSO EVIDENT OVER SOUTHWEST CA WITH OBSERVED WINDS AS MUCH AS 50 DEGREES OFF FROM FORECAST VALUES WITH VARIABILITY FROM LOCATION TO LOCATION. ...CENTRAL/SOUTHERN ROCKIES/GREAT BASIN... THE 12Z NAM 2M TEMPERATURES WERE AS MUCH AS 10 DEGREES TOO LOW IN VALLEY AREAS OF WESTERN CO/EASTERN UT/NORTHERN AZ...AND 5-9 DEGREES TOO COOL IN VALLEY AREAS OF NM. ...SOUTHERN PLAINS TO THE SOUTHEASTERN U.S... THE 06Z AND 12Z NAM DID NOT FORECAST THE LARGE COVERAGE OF SHOWERS ACROSS SOUTHWEST AR/SOUTHEAST OK. THE COVERAGE/AMOUNTS OF RAINFALL OVER NORTHWEST AL IS MORE THE 06Z/12Z NAM FORECAST. OBSERVED PRECIPITABLE WATER AMOUNTS OVER NORTHERN AL AND NORTHERN GA AREA ABOUT 0.1 TO 0.2 INCHES HIGHER THAN INITIALIZED BY THE 12Z NAM. ...MIDDLE MISSISSIPPI VALLEY INTO THE OHIO AND TN VALLEYS/LOWER GREAT LAKES/MID ATLANTIC... THE 12Z NAM DID NOT PICK UP ON THE SHOWERS DEVELOPING NEAR THE SOUTH CENTRAL KY/NORTHERN TN BORDER. MODEL ESTIMATED PRECIPITABLE WATER VALUES ARE 0.1 TO 0.2 INCHES LESS THAN OBSERVED. PETERSEN