How we Forecast Winter Storms ~ WEATHER 101 with AL

VPpromo-KaprielianBy: Al Kaprielian – November 2015

With summer over and fall here, we now look ahead to the winter. In this month’s article, I am going to talk about winter storms and how Meteorologists forecast them. Winter storms can bring snow, sleet, and freezing rain.

There is a difference between sleet (ice pellets) and freezing rain. Sleet falls as a solid. This occurs when there is a warm layer in the upper atmosphere. When the snowflake hits this warm layer, it melts and then refreezes in the cold layer below 32 degrees Fahrenheit below the warm layer. The snowflake refreezes into an ice pellet, or sleet. This is not hail. Hail is used in the summer with thunderstorms due to cold air aloft.

Freezing rain falls as a liquid, and the drops freeze after they hit the ground. The ground temperatures are at or below 32 degrees, and all the upper levels of the atmosphere are above 32 degrees. Freezing rain is worse than snow because you have less traction on the roads. A lot of freezing rain over .25” of ice accretion can cause weight to build on power lines and trees causing them to fall. The result can be widespread power outages. Remember the big ice storm of December 2008. Snow falls when all levels above the ground are below 32 degrees, the ground temperature can be above 32 degrees.

How do Meteorologists know when a winter storm approaches? We look at weather maps to see how the jet stream (winds 20 to 40 thousand feet above the ground) are flowing. Usually, most of our winter storms come from the Gulf of Mexico.

The major criteria for us getting a winter storm is if the northern branch of the jet stream (the polar jet) phases with the southern jet (the subtropical jet). If they phase then the storm will most likely come close enough to Cape Cod & the islands to give us a winter storm. If the two jet streams do not phase, or phase too late to our east/southeast over the ocean, then the low pressure system will track too far south of our region and we miss the storm.
The Mid-Atlantic region, including the Washington DC/ Baltimore areas, would be close enough to get a snowstorm. Also, the track of the low pressure system is critical because our region is so close to the Atlantic Ocean. Remember, the ocean warms slower than the land in the spring/early summer, and cools slower in the fall/early winter. One reason is that evaporation is always occurring over the ocean.

This is a cooling process in that it removes heat from the air. Another reason is that the ocean is always moving and this causes more heat from the sun to warm the ocean one degree versus the land areas. The jet stream causes the low pressure to track in a given direction. If the low pressure tracks to our west over the Great Lakes or New York State, then we are on the warmer side of the low pressure. The reason is the winds will be blowing in off the warmer ocean waters. This means more rain than snow. If the low pressure tracks over Cape Cod, Nantucket, or just south of Nantucket, this puts the Merrimack Valley on the west/northwest side of the low-pressure. This is the colder side of the storm and would mean more snow than rain. The exception would be Cape Cod & the Islands where there would be more rain than snow because they would be closer to the storm and the warmer ocean waters.

How about this winter, will it be as snowy and cold as last winter?? Possibly not?? The reason is this year we are in an El Nino, which occurs when the waters of the South Pacific Ocean are warmer than normal. El Nino can affect the path of the jet stream. With El Nino, there is more of a trough in the jet stream over the western US. This means above average rainfall for California and also the SW US. With El Nino, the jet stream can be farther north over the eastern US, along with more ridging (high pressure) and less troughing (low pressure). This can cause New England to have less snow and not as cold a winter compared to last year. This is not definite, but a good possibility. Last winter we had a large upper level ridge (high pressure) over Alaska.

This caused warmer weather in Alaska and the western US. To the East of the upper level ridge over Alaska, the Northwest winds brought very cold air from the Artic region. Also there was more troughing over the eastern US which cause more storms to form, hence above average snowfall. Time will tell what this winter will exactly bring, but no two winters are always the same. From this article I hope you have a better understanding of how Meteorologists predict winter storms.