LIS Soil Temps and the Unfolding Winter Storm…

The current winter storm unfolding across the eastern U.S. may be the storm that launches a thousand blog posts.  Well, maybe at least two or three on this site.  We’ll see.  Anyway, there may be multiple opportunities to assess the utility of a variety of Proving Ground (PG) data sets and imagery during this event.  Although not a PG suite of products, I’m going to start by taking a quick look at the SPoRT LIS soil temperature data that are being delivered to several NWS offices, including my own (Huntsville).

Below is a brief loop of 0.5 degree composite reflectivity imagery from the NWS NEXRAD network.  The great thing about AWIPS II, is that with some manipulation of time options, the radar loop can be layered over the latest LIS soil temperature data.  In this case, the latest data available were from 09Z this morning.


NASA SPoRT LIS 0-10 cm soil temperatures (image, F) valid at 09Z 22 Jan 2016, overlaid with WSR-88D 0.5 degree composite reflectivity, 1324Z-1512Z.  Surface observations in yellow. 

The soil temperature data have been color coded so that white colors represent the transition zone where temperatures are around freezing (28F to 34F), per the model, and impacts may be occurring or beginning to occur.  Of course, there are other factors to consider, such as precipitation type at the surface, and precipitation intensity.  If precipitation is liquid, then soil temperatures in the upper 20s will likely translate into  an icing event.  However, if temperatures are at or just below freezing, latent heat release generally will warm the very shallow layer near the surface, leading to eventual melting of any ice that accumulates.  If soil temperatures are above freezing, say around 34F, snow can still accumulate on the surface, as long as the snowfall rate exceeds the snow melt rate.

Looking at the data and imagery above, it would appear that much of Tennessee is under threat of some type of freezing precipitation at the surface, whether from accumulating snow or freezing rain.  Taking a closer look, the observation at Knoxville starts out at 36F in this loop, and finishes at 35F by the end of the loop with rainfall being reported.  Soil temperatures there are right around 32F to 33F at closer inspection.  So, no real good chance of freezing precipitation at the surface.  But, the data show forecasters that ground temperatures will not have to fall much before there are potential issues, especially as colder air begins to move into that area.  However, per reports from Nasvhille and many surrounding locations in middle and western Tennessee, snowfall is accumulating rapidly where soil temperatures (per closer inspection in AWIPS) are around 31F to 32F.  Meanwhile, reports of accumulating snowfall have been received from portions of northern Mississippi and southwestern Tennessee, where 0-10 cm soil temperatures are generally in the 34F to 38F range.  So, these are locations where the snowfall rate certainly exceeded the melt rate of the snow at the surface.  Nevertheless, the ground will continue to radiate these higher temperatures into the shallow snowpack and some melting from beneath will likely occur through the day.  These are other ways the data can be used…to determine the potential length of lingering impacts from recent snowfall.  Some limitations to the data currently?…they are only at 3 hourly resolution and have about a 2-8 hour latency.  We might want to reconsider higher temporal resolution during times of inclement winter weather, where more rapid updates would be more operationally useful.  Anyway, this is something for the SPoRT modeling team to explore.

These data are becoming a very valuable tool during operations at our office and several others.  Just yesterday, the Raleigh NC NWS office cited the data when assessing the potential for accumulating snow and ice in their forecast area.


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