Dangerous Weather Event Fennel: Ice Everywhere

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The forecast for today shows freezing rain possible from Arkansas to the Atlantic coast. Forecast courtesy of NOAA.

Decoded Science has named Dangerous Weather Event Fennel, which will last until the current cold weather over the eastern two-thirds of the US subsides — probably more than a week from now.

Several factors make this an especially serious situation:

  • The cold is widespread; many areas are at risk.
  • Ice offers less traction than snow to tires and shoes, and is contributory to automobile accidents as well as personal accidents.
  • A wide swath of advancing moist air from the Gulf of Mexico assures a protracted period of freezing rain.
  • The extreme cold makes it more difficult to effectively treat roadways.
  • Where daytime warmth is enough to melt snow, re-freezing occurs at night.
  • The flow of cold air under bridges can make them freeze when roadways are free of ice.

Density Decides Who’s On Top

Cold air is denser than warm air. This fact is critical to the movement of air masses, the energy balance of the atmosphere, precipitation patterns, and whether the temperature at the surface is above or below freezing.

  • When cold air advances, it wedges under warmer air and displaces it, forcing the warm air to rise.
  • When warm air advances, it cannot dislodge the denser cold air, so it rises above it.
  • In either of the above cases, the rising warm air, often moisture-laden, will produce precipitation. The amount of precipitation depends on the amount of water vapor in the warm air.
The weather map for last night at about a mile aboe the ground shows a flow of air northward from the Gulf of Mexico. Analysis courtesy of NOAA.

The weather map for last night at about a mile above the ground shows a flow of air northward from the Gulf of Mexico. Analysis courtesy of NOAA.

In all cases of moving air masses of different temperatures, cold air stays near the ground, while warm air is lifted above the cold.

Since the higher latitudes receive less solar heat than the lower latitudes, there is always a contrast of air masses from north to south. This is a high potential energy state; the energy is released (as kinetic energy of wind) as the air masses are redistributed, with the cold air ending up below the warm.

Eastern And Central US Perfectly Situated For Overrunning Ice Storms

Cold air from Canada often spills southward across the eastern 2/3 of the US, funneled by the Rockies to the Gulf and Atlantic coasts. Warm and humid air is always lurking in the Gulf of Mexico. This is the ideal setup for an overrunning ice storm situation.

Overrunning is the term meteorologists use when warm air flows over cold, typically near a warm front. However, since the cold air is very hard to dislodge, the overrunning may occur well into the cold air mass.  If the vertical structure of the temperature profile is of a certain type, the precipitation will fall as freezing rain.

Precipitation Types During An Overrunning Event

Overrunning can cause several different kinds of precipitation:

  • When the overrunning warm air is below freezing, precipitation will fall as snow all the way to the ground.
  • When the cold air layer is very deep, but the overrunning warm air is above freezing, precipitation will either form as rain in the warm air or as snow above the warm layer and melt as it falls into the warm air. In either case the raindrops will re-freeze before they hit the ground. The resulting precipitation is normally called ‘sleet,’ though the term ‘ice pellets’ is also used.
  • If the cold air at the surface is shallow enough, the raindrops will supercool (cool to below freezing but remain liquid) and freeze when they hit a solid surface.

Whom Will Fennel Affect?

The current situation will create overrunning conditions that could produce freezing rain from Texas north to the central plains, and from the mid-south to the midwest and northeast:

  • Today freezing rain will affect most of Arkansas; southern portions of Missouri, Illinois, Indiana, and Ohio; the central Appalachian Mountains; and extreme eastern Oklahoma and Texas.
  • Monday, freezing rain will affect many of the large cities of the northeast, including Washington and New York.

Even in places where the daytime heating is enough to raise temperatures above freezing, after dark the temperature will quickly fall and the remaining water will re-freeze on roadways and sidewalks.

How Long Will Fennel Last?

This forecast for next Saturday shows high pressure over the eastern US. Clear skies and light winds will lead to temperatures above freezing in the daytime and well below freezing at night. Forecast courtesy of NOAA.

This forecast for next Saturday shows high pressure over the eastern US. Clear skies and light winds will lead to temperatures above freezing in the daytime and well below freezing at night. Forecast courtesy of NOAA.

The major threat of freezing rain will end Monday night as the area of frozen precipitations exits New England, although there is some chance of freezing rain in the Carolinas on Tuesday with the advance of a new cold front.

However, as clear weather and a new push of arctic air cover much of the eastern portion of the country, the threat of melting and re-freezing increases.

Clear skies will allow the temperature to rise above freezing during the day, even where readings are well below freezing at night. Snow banks will partially melt and the liquid on sidewalks and streets can re-freeze very rapidly after sundown, as the earth radiates away its heat.

The next chance for significant freezing rain will occur after next weekend, as the warm, humid air from the Gulf of Mexico tries to push northward once again.

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