Tonight’s Winter Weather Threat

Not much change in the thinking of the winter weather threat tonight. The National Weather Service has issued a Winter Weather Advisory for Marion, Winston, Blount, Etowah and Cherokee Counties in the Birmingham CWA for tonight. This advisory goes into effect at midnight and will last until at least 6 AM.

Cold air continues to settle in from the northwest with high pressure centered over northern Mississippi that continues to bring an early spring cold snap to the Southeast. Low pressure diving towards the Gulf Coast today will be bringing an increase of moisture and uplift across the Southeast especially later tonight. Already seeing moisture levels increase in the upper-levels as clouds are on the increase. This moisture will interact with very cold temperatures in the upper-level of the atmosphere, providing all the right ingredients for late season winter weather threat.

When forecasting a winter weather event, it is extremely important to look at every weather model and all the forecast tools that are available. I like to use Bufkit for model interpretation as it provides a descent look at the atmospheric column and shows how the models see the weather event through the forecast period.

First, thing I look for is if there is moisture in the snow growth region. The snow growth region is the area of the atmosphere as the name implies, is the most ideal temperature range for snow to develop and grow. The most ideal region is -12 to -18 degrees Celsius, but just outside that -10 to -20 degrees Celsius snow growth can still occur. If there is moisture in the snow growth region, it is then important to see if there are any layers of the atmosphere above freezing, and if so, how much above freezing  and how deep are the layers.  This will determine if the snow will melt before reaching the surface. Also check to see if there are any layers below these melting layers that are below freezing.  If there are, that means the precipitation that has melted could freeze back into sleet and graupel. Lastly, look at the surface temperatures. Taking into account these aspects of the atmospheric column, we can then formulate a descent forecast for winter weather events.

For this event, there will be moisture in the snow growth region. The mid and lower levels of the atm0sphere will be very dry today and this evening. Moisture will be increasing in the upper- levels and as it condenses and falls it will begin to moisten up the layers below and with the evaporative cooling, those layers will begin to cool as well. The low will also increase the moisture in the atmosphere.  For our northern counties especially, as the snow begins to fall through the atmosphere early in the event, there are some melting layers and freezing layers, so it appears that the event starts off as a rain/sleet mix. Through out the night, all melting layers in the upper and mid levels of the atmosphere will erode away and we should see a rain/snow mix for the remainder of the event until day break. After that, the column will warm and this will switch over to an all rain event. Several hours of moderate rain/snow could allow for some light accumulations, especially on grassy areas and elevated surface. Could see a few slick spots across the northern third of Alabama.

For areas along Interstate 20, temperatures should stay above freezing and settle into the mid 30s tonight. I think we will see mostly a very cold rain event with some sleet mixing in most of the night. As we head through out the morning, it will switch over to all rain as the atmospheric column warms allowing for complete melting of the snow and sleet. We will see all rain tomorrow and it will be a very raw and wintry day. It will be cloudy, rainy and very cold. Temperatures will be 15-20 degrees below normal for this time of year. You will think that we are back in early January.

Surface temperatures in the Birmingham metro should stay just above the freezing mark tonight. So not expecting any travel issues in our area.

Category: Alabama's Weather, Winter Weather