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

Important -

• The image content is automatically derived strictly from SREF output (21 members - 3 hrly output out to fhr 87).  There is no forecaster intervention to the probabilities displayed.

• Images are generated as soon as SREF mean data is available on the NWS super computer in GEMPAK gridded format (four cycles per day at 03Z, 09Z, 15Z, and 21Z).
  

• 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.

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Product Description

NOTE - In each SREF member, the precipitation type (snow, sleet, freezing rain, or rain) is determined by taking the "dominant" precipitation type from five precip type algorithms: NCEP Baldwin/Schichtel, Revised NCEP, Ramer, Bourgouin, and Explicit Microphysics. The precipitation type used in the creation of these graphics is from the SREF Mean and is determined by counting the dominant precipitation types from all 21 SREF members, and the precip 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 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 3hr temporal resolution,  the precipitiation rate is calculated by the 3hr qpf divided by 3hours.  Both visibility and precipitation rate are calculated only where winter precipitation (S, ZR, or 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 detectng S, IP, or ZR assumming 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 !!! ZR 12 and 24h 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 accumulated QPF, 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 QPF or snowfall 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.