Short-Range Ensemble Forecast (SREF) Derived Winter Weather Impact Graphics

This page displays automated SREF-derived probability graphics for winter weather event attributes considered significant to the public. They are designed to address the aspects of winter weather events beyond accumulation - specifically duration, timing, and intensity. Additional probability graphics provided are directed at assisting a NWS forecaster during the winter weather forecast process.

Please note the following:

The image content is derived strictly from SREF output (21 members - 3 hourly output out to forecast hour 87). There is no forecaster intervention to the probabilities displayed.

The SREF is run four times per day, initializing at 03Z, 09Z, 15Z, and 21Z. Images on this page will always represent the latest available guidance. Data is typically available approximately 4-5 hours after initialization (i.e. the 15Z SREF images should post by 20Z).

All forecast times are in UTC (Z). If you'd like to know how UTC (Z) relates to your local time, click on this conversion table.

Viewing the forecast graphics

Please use the menu on the right-hand side of this page to begin viewing the graphics. Clicking on any button will automatically remove the text in this window and replace it with a single graphic or forecast loop.

The following are set as defaults initially:

Area - CONUS (contiguous United States).
Forecast Hour - Loop. If chosen, a loop of forecasts from 3-87 hours (in 3-hour time steps) will load.
The probability that the winter precipitation rate will reach or exceed 1 inch per hour.

Note that if you wish to view the default options, simply click on one of the already selected buttons.

Once a single graphic or loop is loaded:

Images can be zoomed by clicking the ZOOM button, and then left mouse-clicking on the image. While in ZOOM mode you can roam or pan around the image by dragging the left mouse button. Revert back to the original image by clicking RESET.
While looping, you can use the Start/Stop and frame advance buttons to control the loop.

Product Description

NOTE - In each SREF member, the precipitation type (snow, sleet, freezing rain, or rain) is determined by taking the "dominant" type from five precipitation type algorithms: NCEP Baldwin/Schichtel, Revised NCEP, Ramer, Bourgouin, and Explicit Microphysics. The precipitation type used in the creation of these graphics is is determined by counting the dominant precipitation types from all 21 members, and the precipitation type with the highest count is taken as the SREF mean precipitation type for that time period.

INTENSITY: Intensity is provided via two parameters - visibility (probability that the surface visibility is 1/2, 1/4, or 1/8 of a mile or less in winter precipitation) and precipitation rate (probability the winter precipitation rate is at least 1, 2 or 3" per hour). Because the SREF output has a 3-hour temporal resolution, the precipitiation rate is calculated by the 3-hour accumulated precipitation divided by 3 hours. Both visibility and precipitation rate are calculated only where winter precipitation (Snow (S), Freezing Rain (ZR), or Ice Pellets or Sleet (IP)) is detected.

DURATION: The duration of a winter precipitation event (probability an event will last at least 12, 24, or 48 hours). If a break in winter precipitation exceeds 12 hours, the "duration" clock is reset to zero at that forecast hour.

TIMING: All products are provided as loops to help discern timing of event attributes.

OTHER IMPACTS:

Accumulation on Roads - depicts the calibrated probability of road sensors detecting S, IP, or ZR assuming winter precipitation occurs (conditional probability) . These are shaded relative to normal (normals available by month) such that values less than 1 (warm colors) imply road temperature may be a significant limiting factor for winter precipitation accumulating on a road surface. The method does not consider traffic volume, whether roads have been chemically treated, availability of snow removal equipment, etc. Winter precipitation rate (intensity) is not factored into this display. Therefore, regions depicted in warm colors may still accumulate winter precipitation when the winter precip rates are high. For reference, the SREF mean 32F forecast ground temperature is plotted (dashed line).. and the solid line represents where all SREF members predict ground temperature 32F or colder. More details can be found at road-snow.pdf

Freezing rain accumulation of at least .01" - depicts the probability of a surface accumulating .01" or more of ZR.

Blizzard Criteria - depicts the probability of blizzard criteria being met (where both the 10m wind speed is >=30kts and surface visibility is < 1/4 mile - in falling snow or ice pellets for at least three hours).

Exceedance of NWS Winter Storm Warning Criteria - depicts the probability accumulations of combined snow/sleet and freezing rain will meet or exceed 12 or 24h NWS Winter Storm Warning criteria. IMPORTANT !!! Freezing rain 12- and 24-hour criteria are the same. Additionally winter storm warning criteria has only been obtained partially over the CONUS. Eventually the grid of warning criteria will expand for the whole CONUS.

Event Mean Accumulations - depicts the ensemble mean event total accumulation. However, unlike precipitation accumulated over a defined time period, the event mean total is reset to zero whenever a 12 hour or greater break in winter precipitation is detected. Therefore areas of event mean accumulation will appear more dynamic as compared to strict accumulation of winter precipitation that does not account for breaks. The mean is calculated across all members - zero values are included in the mean. These graphics are for reference only and are not "THE" accumulations used in the derivation of the impact graphics.