There is a lot that goes into forecasting winter weather in Texas. One complicated aspect of any forecast is trying to anticipate what precipitation type will be dominant during an event for a given location. There are four precipitation types we deal with regularly during winter weather events in the state. The two most common are rain and snow, but we also have to deal with sleet and freezing rain. Why do some locations receive freezing rain with temperatures in the 20s while other-times someone is getting snow at 32 degrees? Welcome to this week’s Weather Wednesday.
This week’s Weather Wednesday will explain how the temperature profile of the atmosphere dictates a precipitation type. The assumption we’ll use is that the temperature at the surface is below freezing (32 degrees Fahrenheit).
Snow: Besides plain-old rain snow is the second easiest precipitation type to forecast for most winter weather events. In order for snow to occur temperatures in the entire atmospheric column must be at or below freezing – from the cloud all the way to the surface. There are circumstances where a wet snow may occur with surface temperatures just above freezing, but that involves upper-level dynamics that aren’t applicable to this week’s discussion (a hint, perhaps?)
Sleet: Temperatures at the surface are below freezing along with most of the atmospheric column. A thin layer of above-freezing temperatures, usually only one to two thousand feet thick, causes the snowflake to melt as it falls from the cloud. That snowflake becomes a liquid rain-drop. As that raindrop falls below that warmer layer of air and back into a sub-freezing airmass it freezes into an ice pellet. Sleet occurs when the warmer layer of air aloft is thin enough that the raindrop can freeze back into an ice pellet before reaching the surface.
Freezing Rain: Sometimes there is difficulty in determining whether there will be freezing rain or sleet for an event, but other times it’s a clear-cut forecast. Freezing rain occurs when a majority of the atmospheric column is above freezing. Sometimes temperatures a few thousand feet above the surface can actually be in the 40s and 50s! The snow-flake falls from the cloud base into that warmer air and melts into liquid rain. Freezing rain occurs when that rain-drop falls into a thin-layer of sub-freezing air near the surface. The supercooled water droplets will freeze on contact with exposed objects. Those water droplets don’t have time to refreeze back into an ice pellet. This scenario happens in Texas more often because southerly winds above the surface are able to bring warmer air/moisture in from the Gulf of Mexico. Many of our stronger cold fronts may be shallow; that is that cold airmass may only extend from the surface up to one-thousand feet.
Freezing Drizzle/Freezing Fog: While not explicitly a winter weather event on its own, freezing drizzle can easily cause travel chaos. Freezing Drizzle is simply drizzle that occurs when temperatures are below freezing. Those super-cooled water droplets freeze on contact with exposed objects. Ice accumulations from freezing drizzle are minuscule and yet enough to turn bridges and elevated roadways into skating rinks.
Rain: Occurs with surface temperatures above freezing. A snowflake falls from the cloud. Once that snowflake descends into an airmass that is above freezing it melts into a raindrop. That’s how it reaches the surface. If the temperature at the surface is below freezing that rain-drop becomes freezing rain, otherwise it’s just a plop on the head.
Hopefully, that helps explain the different precipitation types we’ve experienced over the last two months. We could do a completely different blog when discussing precipitation types during our warm season, but that hail of a blog will just have to wait for another day.