Archive for March 10th, 2012

Evening Update on Sunday’s Severe Weather Potential

Published on Saturday, 10 March 2012 21:30

David Reimer

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Good evening!

I hope all of you are staying warm and dry this evening. If you don’t live in Texas or the surrounding areas, we’ve been cool and wet all day with nearly constant precipitation. We definitely need it though to help improve our drought conditions. You’re not here to read about some rainy day, so lets dive into tomorrow!

[caption id="attachment_6620" align="aligncenter" width="600" caption="Chance of severe weather occurring within 25 miles of any given point on Sunday"]width=”600″ height=”354″ />[/caption]

This graphic uses data from the Storm Prediction Center’s Severe Weather Outlooks. This shows the probability of experiencing severe weather within 25 miles of any given point on Sunday. The orange line shows where a standard risk of severe weather is in effect. Locations inside that orange line have a 15% chance of experiencing severe weather within 25 miles of any given point (such as a house). The red line shows where the Storm Prediction Center has placed an elevated risk of severe weather, meaning there is a 30% chance of experiencing severe weather within 25 miles of any given point (such as a house). When you think about it that way, folks inside that elevated risk have a nearly 1 in 3 chance of having severe weather occur within 25 miles of their location on Sunday. In this case, severe weather is defined as tornadoes, large hail in excess of 1 inch in diameter (quarter size hail), and damaging straight-line winds over 58 MPH.

[caption id="attachment_6621" align="aligncenter" width="600" caption="0Z NAM: Dewpoints at 7 PM Sunday"]width=”600″ height=”450″ />[/caption]

Keep in mind since we switch over to DST (Daylight Savings Time) tonight, that all times move forward an hour. That includes zulu time! That means that instead of 0Z being 6 PM, it will now be 7 PM. At 7 PM CDT tomorrow, the NAM (North American Weather Model), has dewpoints at or above 60°F roughly along and south of Interstate 40 in Arkansas, with dewpoint values approaching 65 degrees on the Louisiana/Arkansas state line. For the early part of March, those moisture values are high enough to support organized, surface-based thunderstorms.

[caption id="attachment_6622" align="aligncenter" width="600" caption="0Z NAM: 3 KM Storm Relative Helicity"]width=”600″ height=”450″ />[/caption]

Instead of showing you two or three different graphics showing winds at different altitudes in the atmosphere, I’m going to switch over to a composite graphic. This image shows 3 kilometer Storm Relative Helicity. That means this algorithm takes into account the storm direction and speed, and puts an image together showing how much spin is available in the atmosphere with those factors considered. Generally, values at or above 250 meter squared/second squared (just a fancy scientific term, just remember 250) will be able to support organized thunderstorms, including supercells and other types of thunderstorms. We’ll have those values in place across all of Arkansas tomorrow, with the higher values located along/north of Interstate 40. However, values will be high enough across all of the state and into northern Louisiana.

[caption id="attachment_6617" align="aligncenter" width="600" caption="0Z NAM: SBCAPE (Surface Instability Values) at 7 PM Sunday"]width=”600″ height=”450″ />[/caption]

Wind shear is necessary to support an organized severe weather event, but instability is just as important. I’m confident there will be enough wind shear to support severe weather on Sunday, but instability values are more of a question due to widespread shower activity most of the day on Sunday. As of now, weather models are converging on a solution similar to the graphic shown above. This graphic shows the Surface Based Convective Available Potential Energy (Just think instability values) for 7 PM on Sunday. Notice that the instability is generally located along and south of Interstate 30 in Arkansas. If we had values above 1,000 J/Kg further north in Arkansas on Sunday, I have no doubt that it would be a very active, potentially dangerous severe weather day. I’ll get into the details on why that doesn’t look likely in my next little segment, but as of now it looks like instability values will be highest in Southern Arkansas and Louisiana. The highest wind shear values will be north of this instability, but acceptable wind shear values will be co-located with the instability in northern Louisiana and Southern Arkansas. This is where I’m most confidence in the chance of severe weather on Sunday.

[caption id="attachment_6618" align="aligncenter" width="600" caption="0Z NAM: Precipitation totals from 1 PM to 7 PM Sunday"]width=”600″ height=”450″ />[/caption]

This graphic shows the projected rainfall amounts from 1 PM to 7 PM on Sunday. Notice that much of the state of Arkansas and East Texas have widespread precipitation falling during the afternoon hours. This generally has a stabilizing effect on the atmosphere and tends to reduce severe weather potential. If this widespread rainfall and cloud cover wasn’t in place tomorrow, Sunday would be a major tornado outbreak across Arkansas. However, this widespread precipitation should keep this event from going nuts. That said, precipitation totals are less towards Southern Arkansas and Louisiana, allowing for instability values to increase. If you correlate this rainfall graphic to the instability graphic above, you’ll see they have a big effect on each other.

[caption id="attachment_6619" align="aligncenter" width="600" caption="0Z NAM: Energy Helicity Index at 7 PM Sunday"]width=”600″ height=”450″ />[/caption]

Finally, this graphic combines all the images I posted above. The Energy Helicity Index takes into account the instability amounts and the wind shear present. Generally speaking, any value over a 1 is significant. Values for our major tornado outbreak are closer to 5 and 6. The 0Z NAM is peaking a value of 1.60 on the Arkansas/Louisiana border tomorrow at 7 PM. This shows there is definitely a threat of severe weather, but at this point this does not look like a major outbreak.

Based on the amount of wind shear present in the atmosphere (lots of spin) along with the projected instability amounts (fairly marginal, but possibly still adequate) I’m expecting the threat of a few tornadoes, along with damaging winds and large hail in Southern Arkansas and Northern Louisiana on Sunday. The most likely timeframe for this potential severe weather will be during the afternoon and early evening hours. Further to the north across Central and Northern Arkansas, some of the storms may become severe with large hail and damaging downburst winds, but unless we get surface based instability further north, the tornado threat will be confined to areas with the surface based instability values.

Based on current data, I’m going to take a risk and head out to Southern Arkansas to chase on Sunday. Jenny Brown and Chelsea Burnett will be joining me on this chase! The Live Video Stream will be available tomorrow, so be sure to follow us as we chase!

- David