ARMOR Radar and Strange Showers

Posted on: 3:50 pm, November 7, 2012, by

We have had an unusual amount of phone calls and questions about what in the world has been falling from the sky today! A very low freezing level (under 4,000 feet), some evaporational cooling, and a little daytime heating at the surface brought spotty showers that were almost summer-like in development. Summer showers develop due to an unstable airmass and a small “kick” to get air parcels rising; that ultimately leads to clouds and precipitation. Today’s brief spike into the lower 50s gave us just a little bit of instability, then as the clouds developed and precipitation fell, temperatures chilled back down into the 40s keeping the lower atmosphere just cold enough for some ice pellets and graupel (precipitation that is like a small, soft hail) to mix in with the rain.

No accumulations of ice are expected; however, as temperatures drop to near freezing tonight, some thick frost and some patchy dense fog are possible. By sunrise, lows will have dipped to the lower 30s, and some of the fog may freeze to elevated surfaces like cars, rooftops and even trees. Highways will not likely be affected, but if freezing fog is occurring where you are, take extra caution when going over bridges!

SPC Mesoanalysis showing relative humidity, freezing level height (in thousands of feet). Most of the Tennessee Valley is in the 3500′ to 4500′ freezing height area.

Freezing heights are important to forecasts because they help us determine what kind of precipitation will fall. Late last week, I wrote on the blog about the possibility of a little wintry mix due to a cold lower atmosphere; the details were not clear, and since there was not much chance of an real winter weather-related problems, we chose not to make a big issue of it.

Another addition to short-term forecasting on days like this is dual-pol radar. The addition of the Hytop and Columbus, Mississippi dual-pol radar upgrades to the data we see from our own Live ARMOR radar is great for determining precipitation type through the use of two “moments” – base reflectivity showing where precipitation is occurring, and the correlation coefficient (“RHO” or “CC”) showing the similarity of the precipitation “targets” (raindrops, hailstones, pellets, snow flakes, etc.)

Here’s a view of ARMOR showing heavy precipitation on the right over top of Morgan City at 3:00 PM. With standard radar data, we assume that since the reflectivity colors are bright orange and red that we at least have some heavy rain and maybe some water-covered snowflakes, sleet or hail. With the “RHO” or “CC” beside it for comparison, we can tell that there is a mixture of precipitation types in the area where the strongest reflectivity is showing up!

The pink and white colors surrounding the Morgan City area are regions of uniform precipitation type at an elevation of about 2300 feet. (Radar does not follow the curvature of the Earth; the beam gets higher with distance away from the radar site). In the heaviest areas of reflectivity, the correlation coefficient drops from around 98% (almost all rain) to around 81 to 94% over much of northwestern Marshall County – that would indicate a good mixture of precipitation “targets” in the area. Since we knew sleet and graupel were being reported with the showers prior to this image, it was easy to tell that the mixed precipitation was still falling even without a report at this exact moment!

Kinda cool if you ask me!

Here are some of the photos we have received today. Anytime you have a weather photo you’d like to share, just send it to photo@whnt.com!

-Jason
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