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Harris-Mann Climatology Article Archive

Title: Warnings and Advisories For the Winter Season

Author: Meteorologist
Published: 11/15/2017

Right now, the National Oceanic and Atmospheric Administration, NOAA, says that we’re currently experiencing weak “La Nina” conditions. La Nina is the abnormal cooling of sea-surface temperatures in the south-central Pacific Ocean. During La Nina years, our part of country often experiences snowier than normal winters. Cliff and I still expect about 85 to 90 inches of snow for the 2017-18 season.

If the cooler than normal sea-surface temperature event holds on to life into next year as many forecasters are predicting, then the chances of a White Christmas are pretty good. I’ll have more details on that forecast in the coming weeks.

During the upcoming winter season, it’s a good idea to know what’s in store weatherwise, especially if one is planning to hit the roadways to go to the mountains or visit friends and relatives. The mountain passes, especially over the Cascades, can change very quickly and the roads can become extremely slick in a short period of time.

For much of the Inland Northwest, the mountains generally refer to any elevation above 3000 feet. A Winter Weather Advisory, Snow Advisory, Winter Storm Warning, and a Heavy Snow Warning are usually the most common statements issued by the National Weather Service, sometimes days in advance.

A winter weather advisory is issued when a precipitation mix of snow, sleet, freezing rain and strong wind events are expected. The advisory is upgraded to a Winter Storm Warning if snowfall in the valleys is expected to exceed 4 inches in a 12 hour period in addition to the sleet, freezing rain or wind.

In the mountains, the expected snowfall must exceed 8 inches in that time frame to prompt a warning. If the precipitation is expected to be all snow, a Snow Advisory is issued when 2 4 inches is likely in a 12 hour period. When more than 4 inches of snow is forecast for the valleys (8 inches in the mountains) in a 12 hour period, we’ll see a Heavy Snow Warning. For early or late season storms in the mountains, like in April or October, lesser snow amounts can also prompt warnings.

Another type of advisory one might see, especially during an El Nino year, when sea-surface temperatures are warmer than normal, is the Freezing Rain or Sleet Advisory. These are issued any time the surface becomes hazardous due to those types of precipitation. When more than a half inch of sleet is expected, a Heavy Sleet Warning is issued.

An Ice Storm Warning is issued when the area is threatened by more than a quarter inch of ice. A Blizzard Warning is rare in the Inland Empire, but is issued when visibility due to blowing snow is reduced to a quarter mile or less and winds are 35 mph or stronger. Also, we can have blizzard-type conditions even after it has stopped snowing, especially if there are strong winds that reduce visibilities.