Winter Weather Verification (Day 1-3)
Verification is available starting on 02/01/2015 and is on a 6 day delay
**More about Winter Weather Verification (Click to Show/Hide)**
WPC subjective "visual" verification is available for the Day 1, Day 2, and Day 3 forecasts of the probability of at least 4, 8, or 12 inches of snow. Each forecast is valid for a 24 hour period ending at 00 UTC or 12 UTC and can be compared to either the corresponding total observed snowfall or selected 4, 8, or 12 inch thresholds. For each selected threshold, the observed snowfall exceeding that threshold will be shaded in brown. Verification started on February 1st, 2015 and is available during the winter weather season (September 15th-May 15th). Verification is on a 6 day delay to allow the snowfall observations to come in.
There are four sources of observation data used for verification: the Co-operative Observer (COOP) reports of 24-h snowfall totals, the Community Collaborative Rain, Hail and Snow Network (CoCoRaHS) 24-h snowfall totals, the METAR hourly reports of surface data that include precipitation type, and the multi-stage 6-h quantitative precipitation estimates (QPE) from the Climate Calibrated Precipitation Analysis (CCPA). The QPE values are interpolated to COOP observer locations and used to partition the 24-h totals into 6-h totals. The weather type codes along with temperature and dewpoint temperature data from METAR reports are examined to assign a precipitation type to qualifying station locations. The QPE values are interpolated to METAR station locations to assign a 6-h accumulation to the reported precipitation type. If the type is snow, the climatological ratio of snow to liquid is used to assign the accumulation; for ice pellets the ratio is 2 to 1; and for freezing rain the ratio is 1 to 1. Ice pellets do not constitute a separate precipitation type, but are accumulated with the snow. These methods are the same as those used in generating forecast accumulations. The 6-h amounts are summed to form 24-h accumulations over intervals corresponding to the forecasts.
The 24-h accumulation data described above exist at scattered point locations. The forecast data to be verified are on a grid of regularly spaced points. Therefore, objective analysis is applied to assign the 24-h accumulation values to grid points. The grid used for objective analysis is a subset of the 20-km AWIPS grid (#215). The objective analysis (GEMPAK Barnes program) process affords an opportunity for applying the following quality control procedures: 1. A grid initialized to -.01 is used so that points that are not modified by data can be set to missing. 2. A first guess grid is computed using a Barnes objective analysis with a moderate radius of influence (weights no smaller than [1/e]**5). 3. A final objective analysis is done with a significantly smaller radius of influence. The first guess grid is used as a quality control grid: any station with a back interpolated snow (freezing rain) amount deviating more than 2 (.2) inches from its value is discarded. 4. All negative values are set to the missing value on the final grids.