Winter Storm Watch vs. Warning: When does the National Weather Service upgrade its alerts?

Metro Detroit's snow system is shaping up to be one to remember. Tuesday afternoon, a little after 12, Southeast Michigan was put under a winter storm warning by the National Weather Service. 

Up to 14 inches of snow could fall, well above the threshold for upgrading the alert. Prior to that, rain will fall tonight which could make conditions slippery even before the snow falls.

MORE: Up to 14 inches of snow expected to dump on Metro Detroit

The difference between a watch and a warning has to do with risk and the potential for serious weather. 

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Winter Storm Watch

The NWS issues Winter Storm Watches when significant and hazardous weather could potentially arrive within 48 hours. Whether the severe conditions actually do occur is besides the point, only that it's possible.

Under the weather service's definitions, hazardous winter weather includes a combination of:

  • At least 5 inches of snow/sleet falling within a 12-hour period, or 7 inches of snow/sleet falling within a 24-hour period.
  • Enough ice accumulation to cause damage to trees or power lines
  • A life-threatening or damaging combination of snow and ice, along with wind

Winter Storm Warning

NWS alerts upgrading severe weather in the winter from a watch to a warning includes all the factors listed above, but with much more certainty they will occur. 

If serious snowfall or ice coverage is imminent or falling in real time, the weather service will likely upgrade its alert. 

The threshold is higher in the state's Upper Peninsula, where its five westernmost counties need to see 6 inches of snow predicted in a 12-hour period or 8 inches of snow within a 24-hour period to trigger an alert.