Many of you are noting that our Storm Total Snowfall graphic shows that we expect places will receive 6+ inches of snow and are asking the question: Why has there been no Winter Storm Warning issued? Answer: The criteria for issuing Winter Storm Warnings for snow is not a simple 6 number/pixel on the picture. The forecasters should have an 80% or better confidence that a county-wide average of 6 or more inches of snow will occur in 12 hours or less. The criteria for a warning for any 24 hour period is slightly more at 8 inches or more as a county-wide average. The Storm Total graphic routinely covers a period of more than 12 hours, and sometimes even more than 24 hours. Note that, at this point, there are no locations which meet Warning criteria (confidence, timing, amounts). In order to issue a Winter Storm Watch, the forecasters must have at least a 50% confidence that Warning criteria will be met. A Watch is meant to be a pre-cursor to a Warning. There is currently a Winter Storm Watch for a few counties in the Southeastern part of our forecast area of responsibility. Of course, our forecasters have the discretion to issue Warnings and Advisories at times when the strict criteria may not be met -- due to a high confidence that weather conditions will lead to an unusually high impact to the area. Situations like these may include: a storm which may occur unusually early or late in the season, or when the conditions will be dangerous during a particularly high travel day or time, or when there will be a combination of many weather hazards that, when taken as a whole, will create unusually hazardous/dangerous conditions. We do collaborate with our neighboring offices when making forecast and warning decisions. Note that there are different criteria in place for warnings in locations outside of Central PA. Also, timing of things like the storm onset, heavy snow, etc., factor into our warning decisions - and when these decisions are made. The main factor for issuing Watches, Warnings and Advisories is forecaster confidence that conditions will be met. At times, due to higher uncertainty in a particular situation, warning decisions will be made at shorter ranges. The Watch, Warning and Advisory criteria for Central PA (and surrounding locations) are described in more detail on our web page: weather.gov/ctp/wwaCriteria Our goal is to make forecasts which will provide scientifically sound forecasts so that you may make prudent decisions, stay safe, and minimize travel impacts and economic losses. The official snowfall forecasts in 6-hourly blocks may be seen on weather.gov at: graphical.weather.gov/sectors/ctp.php
Posted on: Sun, 25 Jan 2015 21:34:20 +0000
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