Sunday Afternoon Update- Soaking Rain Likely Tonight through Tuesday

Radar is continuing to show light precipitation moving into the Southwestern Tennessee Valley, but  most of the rain is struggling to reach the ground due to dry air just above the surface (weather enthusiasts: for more on how we determine how the moisture stacks up at all levels of the atmosphere see the technical postscript at the end of this post).

This dry air will be overcome over the next several hours as more showers are ushered into the region by a strong upper-level low located to our southwest, and widespread rain will begin to affect the Tennessee Valley tonight.

Rainfall totals by morning could exceed one inch in many locations. The image below is a forecast from the HPC of the probability of specific locations getting at least one inch of rain between now and 7AM Monday morning. The highest odds are to the west of us, but Northwest Alabama has about an 80% chance of getting more than inch tonight, while most of the Tennessee Valley has at least at 40% of seeing an inch or more tonight.

The upper-level low responsible for all this rain is a slow mover and will keep rain chances high through at least Tuesday. Here is the HPC’s forecast for rainfall totals from now through Wednesday morning. The deep purple contours indicate the possibility of more that two inches before the system moves out mid-week.

There is a possibility of severe weather tomorrow, with the primary concern being high winds. This a marginal threat and will depend largely on how much sun we will get, but it is a threat you should be aware of. We will keep you updated frequently on all of WHNT’s platforms both on television and online: WHNT.com, Facebook, Twitter, and here atValleyWX.com.

And if you haven’t already done so, make sure you maximize your decision making power with our suite of FREE weather apps:
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-Brandon
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Technical Post Script: Dry Air Indicated by a Model Sounding:

Rapid Update Cycle Model Sounding at Muscle Shoals on a Skew-T Log-P diagram. This plot simulates a weather balloon launch above a point on the ground and is intended to give information on the temperature, dewpoint, and wind speeds of the entire section of the atmosphere where weather occurs (Troposphere).  Credit: NOAA

A pocket of dry air is indicated by this model sounding which is an attempt to simulate weather balloon data that would give atmospheric conditions above a particular point on the ground from the surface through the stratosphere. The two most important variables to consider in this situation are the dewpoint line (blue) and the temperature line (red). When these two lines are close together, the atmosphere is more saturated at a given level and more conducive to raindrops passing through without evaporating; when the two lines are far apart the air is drier (less saturated), and rain drops may begin to evaporate. The thickness of this dry layer plays a key role in how much of the rain can evaporate. Currently, we have a very dry layer that stretches from approximately 3,000 feet to about 10,000 feet above Muscle Shoals. This is a very difficult layer for the light rain drops that the clouds are producing to make it through without evaporating. However, as more drops evaporate in this column of air, the atmosphere at these levels will saturate, and by this evening the entire column of air is expected to be humid enough to allow rain to fall all the way to the surface unimpeded.