Disclaimer:

The analyses found here are not official NWS products.

Production of these files is not operational and missing data may occur. Termination of this data may occur without warning or notification.

WPC Snowfall and Freezing Rain Analysis

1. Introduction

The WPC requires an analysis of both snowfall and freezing rain to verify the products of the Winter Weather Desk.

2. Verifying Data

There are four sources of data for the verification: the Co-operative Observer (COOP) reports of 24-h snowfall totals, the Community Collaborative Rain, Hail & 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) available from the National Precipitation Verification Unit (NPVU) prior to 01 March 2012 or 6-h QPE from the Environmental Modeling Center (EMC) Climate Calibrated Precipitation Analysis (CCPA) on and after 01 March 2012. 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.

Another potential source of verification data, WFO storm report bulletins, is not available to the automated data assimilation procedure. This source cannot be used because the data is not transmitted in any standard format, but instead must be gleaned manually from text messages.

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.

Fig 1 shows an example of this analysis for the 24-h event total ending at 1200 UTC 23 December 2004. Since the analysis exists only at grid points completely surrounded by data within a relatively small radius of influence, there are numerous points with missing values. This is why there are holes in the analysis. The tan colored areas mark where there are observations of sub-threshold values (~0). The analysis for the 24-h accumulation of freezing rain for the same time period is shown in Fig 2. Again, many points have missing values because no data are available.