How do I use the PWPF GIF viewer and plumes products?
||Model (Cycle - For Day Shift)
|10 ||NCEP GEFS ensemble members (06Z) ||Randomly Selected |
|25 ||ECMWF ensemble members (00Z) ||Randomly Selected |
|10 ||Canadian ensemble members (00Z) ||Perturbations 1-10 |
|1 ||NCEP Short-Range Ensemble Forecast (SREF) NMMB member (09Z) || |
|2 ||NCEP High-Resolution Rapid Refresh (HRRR) runs (06Z and 12Z) ||Through Day 1.5, SREF NMMB and GEFS Members used for Days 1.5-3 |
|2 ||Canadian Regional Model runs (00Z and 06Z) ||Through Day 2.5, CMCE members used for rest of Day 3 |
|2 ||NCEP North American Mesoscale 3km CONUS Nest runs (06Z and 12Z) ||Through Day 2, Parent NAM and SREF NMMB member used for Day 3 |
|4 ||NCEP Hi-res WRF ARW runs (12Z) ||Through Day 1.5, Time-lagged GFS, GEFS, and SREF NMMB members used for Days 1.5-3 |
|1 ||NCEP Hi-res FV3LAM (12Z) ||Through Day 2, Time-lagged GFS used for Days 3 |
|1 ||NCEP Global Forecast System (GFS) (12Z) || |
|1 ||ECMWF Deterministic (00Z) || |
|1 ||NCEP Global Ensemble Forecast System (GEFS) Mean (06Z) || |
|1 ||WPC Forecast || |
Upon loading the page, the QPF of the latest available cycle (see Date/Run cycle FAQ below) will display. Users can view previous runs by picking a Date and Run of their choosing. Available precipitation types include QPF (default), snow, and ice (see snowfall total FAQ below). Select a member to view by clicking or hovering your mouse over the 2-digit number of a model 'core' (i.e., ECMWF/EPS, GFS/GEFS). The '00' member for ECMWF and GFS is the deterministic run. The first 3 rows will display the Day 1,2, and 3 24-hr precip totals. The 4th row displays the 72 hour accumulated totals. The columns correspond to the model cores. On the far right, the ensemble mean (MEAN), WPC, NDFD, and NBM forecasts are available for comparison. The 'Verification' toggle allows one to see how each member performed. The verification has a ~4 day delay, because the verification grids (Stage IV, NOHRSC) often change as the grids are updated/QC'ed.
This page functions like the SPC SREF plumes. On page load, the 6hrly QPF for DCA is displayed. Users can select different point locations by using the map below the plumes. Other fields include 6hrly snow and ice, along with run-total accumulations for each. Users can pick and choose which ensemble members they wish to display. There are 'model core' buttons that will display just the model core (EPS, GEFS, SREF, NWS, etc). The NWS option includes forecasts from WPC, NDFD, NBM, HREF (mean [HRFM] and probability matched mean [HRFP]), and the Bias Corrected Multi-Model Ensemble 5 KM (MMEBC5K; Keith Brill). The dProg/dt option will display the WSE means for the currently selected run and the 3 previous runs (WPC, NDFD, NBM, MMEBC5K, and HRF not included in the calculation of the mean).
What exactly do the Date and Run Cycle represent?
Why don't the 24hr snowfall totals displayed on the viewer match those I see on Pivotal Weather, WeatherBELL, or Tropical Tidbits?
The PWPF process is run 6 times per day, as described in the table on the right. The initial time is based on the WPC Winter Weather Desk timing.
The components of the PWPF displayed on these websites represent the data that is used in the 'Prelim' and 'Final' runs of the PWPF process. The ensemble components are updated (if data exists) and the process is run automatically during the 'Update', though these updates to the contributing members are NOT displayed on these webpages.
For example, images and plumes corresponding to the 11/19/2018 12:00 UTC cycle are used in the 12z Preliminary and Final PWPF process (run around 06z and 0830z) and will be available on these pages before the 12z PWPF process (around 04-05z).
Note that the initialization time in the figures and plumes corresponds to the WPC WWD timing, not the contributing member itself. In the future (mid- to late-December, 2018), more information about a contributing member is planned to be included in the viewer image labels (initialization time of any given model, which specific ensemble member, etc).
Are the ensemble members the same each run? For each P-type? What are the corresponding EPS & GEFS ensemble members?
The following steps are used to prepare raw ensemble data for use by the PWPF process:
- PRISM-based downscaling is applied to the quantitative precipitation forecasts (QPF) from each model.
- At each grid point, the precipitation type determination for the NCEP models is based on the dominant type algorithm (Manikin, 2005). Precipitation type for non-NCEP models is determined by applying a simple decision tree algorithm using surface temperature, and temperatures on the 925-hPa, 850-hPa, and 700-hPa mandatory isobaric levels.
- A unique snow level is also determined for each member based on the height of the 0.5 C Wet Bulb surface. At each grid point, a top down search is conducted to find where the wet bulb temperature first exceeds 0.5 C. It is then found where the snow level intersects the 5-km topography from the Realtime Mesoscale Analysis (RTMA). Grid points above the snow level are flagged as snow in the final precipiation type mask for each member.
- Each member's snowfall is based on the QPF, the precipiation type, and a snow-to-liquid ratio (SLR). The 6-h SLR at each grid point is based on a modified version of the SLR from the National Blend of Models (NBM).
- For each member, the freezing rain accumulation is based on the QPF, the precipiation type, and an ice-to-liquid ratio (ILR). The ILR is based on a simplified version of the Freezing Rain Accumulation Model (FRAM; Sanders and Barjenbruch, 2016) which uses precipiation rate as the sole predictor.
- A binormal (Toth and Szentimrey, 1990) probability distribution or density function (PDF), which allows skewness, is utilized for the PWPF. The fitting of the binormal distribution is a method of moments approach. The WPC forecast is the mode of the distribution. The placement of the WPC forecast in the ensemble order statistics determines the skewness of the distribution. The variance of the distribution is matched to the variance of the ensemble. The WPC deterministic forecast is included as an additional member of the ensemble for the computation of the variance. This fit is done at each grid point; so, the probability density function (PDF) varies from grid point to grid point.
See the PWPF Product Information
page for more information on the PWPF and Snow-to-Liquid Ratio Information
for detailed explanations of the SLR processes used by WPC for snow and ice forecasts.
This process can result in a blocky apperance of the snow and ice accumulation grids. This can be traced back to coarse resolution of the models that this process is applied to, along with summing up 6 hourly grids to create the 24 and 72 hour accumulation graphics.
'HIRES' (high-resolution) members and the 'CMC' (Canadian) models are used for the first 1.5 days, and then suplemented by a variety of other models at farther projections. This can lead to odd looking discontinuities when viewing accumulations (particularly on the plumes page).
The ensemble members for the ECMWF EPS and GEFS are randomly chosen for each cycle of PWPF. These randomly selected members are used for all forecast hours and precip types for that run. The randomly selected EPS & GEFS members information will be available for the 2019-2020 season.
Why are some members unavailable?
The PWPF process runs in chunks depending on model availability. Some components are available earlier than others. Use the viewer's 'Help Overlay' to get an idea of when you can generally expect GIFs from a model chunk component to be available.
Check the P[Q/W]PF Status Page
for the current condition of the viewer system.