Valleywx.com: Winter Weather Advisory In Effect Through 9am Tuesday

futurecast

Things change by the minute when forecasting snow. It’s a very difficult and complex task. This system is no different. So many variables come into play when dealing with timing and precipitation amounts. Just this morning, we went from a Winter Storm Watch to a Winter Weather Advisory. If we were expecting more snow or ice, then a Winter Storm Warning would be in effect. Technically a Winter Storm Warning is one level higher than a Winter Weather Advisory. Either way, slick roads are in the forecast late tonight into Tuesday morning. Here is a look at our in-house computer model. The advisory is in effect starting at 6pm mainly for the Shoals. If you are in Florence, Red Bay, Tuscumbia, Cherokee, or Phil Campbell, you have the best chance of sleet and snow early this evening. Everyone east of there have a greater chance from midnight on.

One of the biggest questions with the amount and duration in snow/sleet forecasting is MOISTURE.  An optimum temperature (upper levels) for a snowflake to form is around minus 10°C. That’s not an issue. The moisture is. The regional scene shows moisture trying to move eastward from Missouri into Kentucky and Tennessee. The dewpoints are very low. The air there is dry at the surface and upper levels. The snow has a tough time holding together when trying to make it to the surface. Look at the regional dewpoints. The dewpoint in Nashville is 19°. For a snowflake to survive its fall, the temperature and dewpoint need to be the same or nearly the same from the top to the ground. If there is dry air, the snowflake evaporates or doesn’t form at all.

regional temperatures

It will be a battle of the drier air to the north and the moisture coming in from the west. That’s why I believe the snow totals won’t be higher (advisory and not a warning.) The moisture wins in the long term, but drier air holds on in the near term allowing the snow to get a later start. Southern middle Tennessee is closer to the drier air and therefore their totals are lower compared to the rest of the region. If you are south of the Tennessee River, you have the best chance of 1″-2″ snow totals through Tuesday afternoon.

As you can see, the higher percentages for accumulation are in northern Alabama where the moisture will likely be more available. There is a 30%-40% (green) for an inch of snow in Alabama opposed to a 10%-20% (yellow) in southern middle Tennessee.

HPC

Green: 30%-40%;Yellow: 10%-20%

Chance of frozen precipitation on Wednesday:

A stronger secondary storm passes to our southeast Wednesday. It will close enough to produce freezing rain, sleet, and snow for a portion of the Tennessee Valley on Wednesday. Areas along and east of I-65 have the greater chance of accumulation with that system. Here is a look the percentage of snow ending at 6pm Wednesday. Jackson, DeKalb, and Franklin (TN) have the best chance of accumulation.

HPC2

Blue Shades: 50%-60% chance of 1″ of snow.

Bottom Line: Be prepared for snow and sleet to develop this evening into the overnight. Slick roads are in the forecast Tuesday morning. The chances for accumulations are higher south of the Tennessee River opposed to southern middle Tennessee. Be ready for the second storm on Wednesday. The best chance of accumulating snow will be over eastern zones.

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