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NOUS41 KWBC 161933
PNSNMC
NATIONAL TECHNICAL INFORMATION MESSAGE 96-6
NATIONAL WEATHER SERVICE HEADQUARTERS WASHINGTON DC
300 PM EST FRI FEB 16 1996
TO ALL USERS OF NWS PRODUCTS
FROM JOSEPH R. BOCCHIERI
CHIEF... SCIENCE AND TRAINING CORE
SUBJECT CURRENT WINTER SEASON BIAS AND RECENT CHANGE
TO THE AIR FORCE DAILY SNOWDEPTH PRODUCT
(THE FOLLOWING INFORMATION WAS FURNISHED BY KEN MITCHELL
OF THE NATIONAL CENTERS FOR ENVIRONMENTAL PREDICTION)
SINCE EARLY 1993... BOTH THE NGM AND THE ETA MODELS
HAVE USED THE AIR FORCE DAILY N. HEMISPHERE SNOWDEPTH
ANALYSIS (47-KM GRID) IN AN EFFORT TO IMPROVE THE
QUALITY OF THEIR COOL SEASON LOW-LEVEL TEMPERATURE
FORECASTS. THIS SNOWDEPTH ANALYSIS IS SUPPORTED AND
PRODUCED BY AIR FORCE GLOBAL WEATHER CENTRAL (AFGWC) AT
OFFUTT AFB IN OMAHA... NE.
FOLLOWING THE 94-95 WINTER SEASON... AFGWC
IMPLEMENTED SEVERAL CHANGES TO ITS SNOWDEPTH ANALYSIS
ALGORITHMS. EARLY THIS WINTER SEASON... BASED ON
FEEDBACK FIRST RECEIVED FROM THE ALASKAN REGION... NCEP
NOTIFIED AFGWC THAT THESE RECENT CHANGES WERE CAUSING A
NOTICEABLE POSITIVE BIAS IN THE ANALYZED EXTENT OF
SNOWCOVER. WHILE AFGWC EMBARKED ON TESTING ALGORITHM
IMPROVEMENTS... NCEP INSPECTED THE AIR FORCE PRODUCT AT
LEAST WEEKLY AND PERIODICALLY FORWARDED EXAMPLES OF THE
BIAS TO AFGWC. IN NCEP'S JUDGEMENT HOWEVER THE BIAS
WAS NOT LARGE ENOUGH NOR WAS THE AREA AFFECTED LARGE
ENOUGH TO WARRANT GOING BACK IN THE NGM OR ETA MODELS
TO THE FORMERLY USED LESS FREQUENT (WEEKLY) AND COARSER
GRID (190-KM) SNOWCOVER PRODUCT FROM NESDIS.
THE POSITIVE SNOWCOVER BIAS HAS BEEN MOST
NOTICEABLE IN CASES OF RELATIVELY MILD SOUTHERLY FLOW
1-4 DAYS AFTER A SIGNIFICANT SYNOPTIC SNOWFALL. IN
THESE CASES... THE AIR FORCE PRODUCT HAS BEEN TOO SLOW
IN CAPTURING THE RECEDING SNOW BOUNDARY. THUS... IN
THESE CASES... THE NGM AND ETA MID-DAY LOW-LEVEL
TEMPERATURE FORECASTS HAVE BEEN TOO COLD (E.G. NGM FOUS
T1 TEMPERATURE)... BY AS MUCH AS 15 C IN WORST CASES.
THE ASSOCIATED NGM-BASED MOS TEMPERATURES WOULD HAVE
HAD CORRESPONDING ERRORS. UNLIKE THE NEWER ETA MODEL
... WHICH CAN MELT SNOWCOVER DURING ITS
FORECAST... THE NGM MODEL KEEPS ITS INITIAL VALUE OF
SNOWCOVER CONSTANT. THEREFORE... THE NGM IS MORE
SENSITIVE TO THE CITED SNOWCOVER BIAS.
AS OF FRIDAY FEBRUARY 16... 1996... AFGWC HAS
IMPLEMENTED CHANGES TO ITS OPERATIONAL SNOWCOVER
ALGORITHMS WHICH SHOULD ELIMINATE THE CITED BIAS. NCEP
IS MONITORING THE AIR FORCE SNOWDEPTH ANALYSIS FOR THE
EFFECTS OF THESE CHANGES WHICH SHOULD GREATLY REDUCE
THE PARTICULAR NGM FOUS AND MOS ERRORS DESCRIBED ABOVE.
NWS FORECASTERS WITH THE PCGRIDDS CAPABILITY CAN
MONITOR THE AIR FORCE PRODUCT INDIRECTLY BY INSPECTING
THE NGM 00-HOUR PCGRIDDS PRODUCT "0000SNWD". THIS
PCGRIDDS RENDITION IS THE RESULT OF A) AN INTERPOLATION
FROM THE 47-KM AIR FORCE GRID TO THE 85-KM NGM GRID...
B) CONVERSION OF SNOWDEPTH TO A BINARY CONDITION OF
SNOW (100 PERCENT) OR NO SNOW (0 PERCENT)... AND C)
CROPPING OF THE NGM GRID FOR PCGRIDDS.
THE COMPLETE AIR FORCE PRODUCT IS AVAILABLE IN ITS
ORIGINAL FORM OR ON THE ETA GRID VIA INTERNET FTP FROM
THE NCEP INFORMATION CENTER (NIC). FOR FTP INFORMATION
CONTACT Y. LIN OF NCEP AT 301-763-8161 OR E-MAIL
WD22YL@xxxxxxxxxxxxxxxxx.
THE AFGWC TECHNICAL POINT OF CONTACT FOR THE
PROCEDURES AND METHODS OF THE AIR FORCE SNOW ANALYSIS
IS DR TOM KOPP AT 402-294-3395 OR E-MAIL
RKIESS@xxxxxxxxxxxxxxxxxxxxxx.
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