EWP Blog

This was a less than ideal 0-3 km shear orientation for this QLCS overall, but a section of the line to the northeast of KDGX (Jackson, MS) did have a slightly more favorable orientation.

Above we see an initial NMDA detection at 2039 UTC associated with the tightening couplet. There is NTDA detection to the northwest associated with a broader cyclonic circulation that may at first looked like a developing bookend vortex, but this didn’t last given semi-discrete storm mode.

As the couplet starts to pivot anticylonically and tighten a bit, we see agreement in placement with the NMDA and the NTDA. The >20% shown in the NTDA legend refers to a slider setting to filter out lower probabilities. Having the filtering feature is great and made even better by having the legend provide that extra information.

The next available time shows accurate progression of the NTDA detection with respect to the now even better defined couplet (note: the NMDA had not updated since the previous image).

-marfafront

Starting to see more discrete circulations that are most likely very shallow and probably too small to be well resolved at much greater range than this. The example suggests utility in having the NMDA and NTDA icons displayed together because in this situation they’re basically identifying the same features while bringing different information to the table.

For example, the NMDA is looking at the 0.5 through 1.3 cuts while the NTDA uses multiple moments (including dual pol) at the 0.5 cut and then draws upon a set of a analogs to calculate probabilities. Using the NMDA/NTDA in this fashion is sort of like using a “two model ensemble” to build confidence in detections.

-marfafront

2037Z

A QLCS is moving across far northeastern Mississippi. The KGWX radar is shooting parallel to the gust front on a lengthy segment of this QLCS, which has a fairly well-defined (if not overly sharp) leading edge / shear axis. This is a case in which the NMDA is suggesting that there are multiple weak mesocyclones lined up along this shear axis, when in reality this is (mostly) just a line of forward propagating convection. At the time of this screen capture, the one NMDA detection with a higher AzShear value (in orange) does appear to be legitimately associated with a mesocyclone.

On the other hand, a tight circulation did develop along another QLCS segment further south, where it was intersecting the outflow from the northern segment. NTDA picked up values >50% here, and did so in advance of the circulation tightening up to the extent that it can be likely called tornadic. This was a good detection for sure.

2052Z

A weak/shallow circulation developed in Oktibbeha County MS, on a kink in the line. This feature is just to the right of the word “Oktibbeha” in the image below. However, despite there being a number of false alarms on the NTDA algorithm (including one just to the north), this feature did not generate a detection on either the NTDA or NMDA. Forecasters in our group thought that this feature, while probably unlikely to be actively producing a tornado, was probably the kind of thing you would want the algorithms (or at least the NTDA) to put some kind of small probabilities on. For the purposes of NMDA, it should be noted that this weak circulation did extend through the lowest 3 or 4 slices on KGWX.

–Insolation

0119Z

The supercell cluster coming into Wayne County, Indiana, is exhibiting solid rotation on the lowest slice. NTDA from KIND/KILN are both showing values in the 80% range. Both radars are also indicating two separate mesocyclone detections, and a qualitative analysis of the storm does suggest two separate updrafts and storm cores. As one would expect, the southern core (the newer development) is driving the NTDA detection which matches with the location of the southern NMDA detection. This makes sense conceptually, as the tornado potential will be much greater with the unimpeded inflow associated with the southern core.

0157Z

A broken cluster of supercells is now entering the state of Ohio. Radar data is largely quite mottled and the majority of the NMDA detections are weak, but there are a lot of them — including in places where conceptually you would not really expect a circulation to exist. There are data artifact issues, related somewhat to the distance from the KILN radar, and also perhaps related to the complex interaction between different parts of different supercells and storm clusters. This would likely be a case where extremely weak / transient detections might be desired to be toggled off under a certain threshold. NMDA detections on the strongest circulations have not necessarily been stronger detections, but NTDA detection probabilities have been tracking fairly well with where tornadoes appear most likely to occur.

0237Z

Both the NTDA and NMDA are making high-end detections in Miami County, Ohio, near West Milton. The NTDA has had values >95%, and at times >99%, with this tornado. Perhaps more interesting is that this is the first case I have yet seen in which the NMDA has shown such a consistent, high-end, well-tracked detection across a span of 10-15 minutes — with extreme (>0.03) values of AzShear. Overall, the NMDA algorithm doesn’t seem to trip into the higher values very frequently, so it really calls your attention to it when it does.

0302Z

A messy storm evolution has occurred in Auglaize County, with some kind of hybrid QLCS / supercell structure based on Z/V data. Although qualitatively, the velocity data is not particularly impressive, the NTDA has done a very nice job at consistently detecting a tornadic signal from around 0241Z to 0251Z. A tornado did occur at this location, including a TDS that was apparent along the same track of the NTDA detection just a few minutes later. This is a very good detection by NTDA, correctly identifying a tornado in an area where eyeballing the velocity data would not yield as much certainty.

–Insolation