Identifying false weather returns (anomalous propagation) using dual-polarization radar data

In the wake of rain and storms earlier tonight, a radar phenomena known as "beam ducting" created anomalous propagation (AP, or in plain language, false weather returns).  This is somewhat common in rain-cooled air that is found behind departing precipitation.  One advantage of the brand-new dual polarization radar data is the ability to tell the difference between these false weather returns, or AP, and real precipitation.  The correlation coefficient product returns certain values when the precipitation is real and other values when it is not. See the image below.

On the left is base reflectivity, the product most people are familiar with seeing that shows location of precipitation.  On the right, the corresponding correlation coefficient (CC) product.  The area that is circled in green has a high CC value (reds are near the far right of the color scale on the right panel), indicating that the returns are likely precipitation.  In the larger white circled area, what appears to be precipitation on the left has much lower CC values that are random and speckled.  In this case, the radar returns are not actually precipitation, but AP.  If you could see a loop of the precipitation product, you would see the real precipitation moving smoothly across the screen, whereas the AP jumps around irregularly - another clue that it is not rain.  Hence, we have one benefit of the new correlation coefficient product from dual-polarization radar!

For additional information on how radar beams behave in the atmosphere, visit this page from the NWS Jetstream program.

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For weather information for Memphis and the Mid-South, where and when you need it, visit MemphisWeather.net on the web, m.memphisweather.net on your mobile phone, download our Android app, or visit us on Facebook or Twitter.

"See Text" - what text? Thunderstorm discussions on marginal days

UPDATE 2 (2:30pm): The Slight Risk area has been expanded and now includes the Memphis metro area (see map below).  The main threat is large hail through about 8pm. For more info, click here.

Updated slight risk issued at 2pm by the Storm Prediction Center.

UPDATE (10:30am): A Slight Risk area has been added for Arkansas, roughly south of I-40. The threat for hailstorms is higher in that area. The risk has not increased significantly in the metro. For more info, click here. 

Original Post:

As of 8am, the Memphis metro is under what meteorologists often refer to as a "SEE TEXT" area for this afternoon with regards to thunderstorm development. What the heck does that mean? And what text am I supposed to see, and what does it say?? First of all, check out the first graphic below from the Storm Prediction Center. These are the folks that issue the Slight/Moderate/High risk areas for severe thunderstorm potential. See the words "SEE TEXT" over the Mid-South in the graphic? This means that SPC believes there is a small risk for some type of strong storms, but not enough to post a Slight Risk. The accompanying graphics show where that risk lies and what the threats are. The "text" being referred to is the technical discussion from SPC, which can be found on their website at http://www.spc.noaa.gov/products/outlook/ - click on the outlook that includes the "See Text" area.

The SPC "See Text" area includes the Mid-South for Wednesday afternoon

So what about today's threat specifically? An upper-level disturbance and cold front will move into the area this afternoon. With plenty of low level moisture (dewpoints approaching 60) and a modest amount of instability, especially if we see some sunshine this afternoon that could boost temperatures to 70 degrees, the upper-level impulse will be enough to trigger some scattered convection. According to the graphics below, some storms could pose a threat of large hail and a damaging wind gust. The tornado threat graphic is not shown as it indicates there is almost no threat of tornadoes.

Hail outlook for today - 5% chance of severe hail

Wind outlook for today - 5% chance of severe wind

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For weather information for Memphis and the Mid-South, where and when you need it, visit MemphisWeather.net on the web, m.memphisweather.net on your mobile phone, download our iPhone or Android apps, or visit us on Facebook or Twitter.