EWP Blog

Interesting challenge in today’s Archived Case from WFO ALY. One particular challenge was the presence of Three Body Scatter Spikes (TBSS) present within the inflow region of supercells south of the RDA causing the radar beam to strike hail prior to the rotational signature. Since this phenomenon was more so identified on the lowest tilt, I did not witness any significant anomalously high probabilities from the algorithm, but it did identify a centralized area of rotation (see “stacking” of NMDA and NTDA in images below).

In the meantime, damage from a tornado was being reported in the lead (eastern) cell, which also had lower-end probabilities but within a more trustworthy region of velocity – owing to confidence in the warning. However, using meteorological “common sense” gave me the ability to take the western cell’s algorithm detection less seriously.

Is this a weakness or a strength in the algorithms? Depends on how you look at it. For the lead storm, I was a bit surprised on the lower probabilities, but for the western cell, it gave a prime example on the limitations that exist and where you, the forecaster, come in by having that situational awareness at hand to identify false signals.

– Dusty Davis

Today’s case at EWX presented numerous challenges in a mixed case multicellular/linear convective environment. While closely monitoring for any potential local surges along an eastward surging line segment (already covered with a SVR/TOR POSSIBLE at the time), it was observed several NMDA and NTDA algorithms were lightning up along the front edge of the line.

At this time, I was closely monitoring for any local surges given 0-3km Bulk Shear reached 30-35kts, but the UDCZ (Updraft/Downdraft Convergence Zone) was not line-normal to the shear orientation, meaning it would take a local surge to possibly spin up requiring a proactive tornado warning decision.

This line segment was just west of the EWX RDA at the time, which presented some challenges along the leading edge of the line with ground clutter in an area of low power return causing erroneous velocity (as noted in very high SW).

This was an example of where applying situational awareness to each algorithm is necessary, and false signals can and will appear. This is where the trained eye comes in to further investigate using additional products/scans to gain confidence at what exactly is going on.

– Dusty Davis

A convective complex near the EWX radar developed into a more linear system as it moved east of the RDA. Several low end NTDA detections were made along the line, generally where there were kinks in the velocity field. One detection did end up showing high probabilities of over 80% just north of Gonzales, Texas. This occurred on the northern end of a localized surge in the line. The NTDA peaked at  84% at 21:38Z and again at 82% at 21:42. There was broad rotation in both instances but not a particularly tight couplet. The center of the NTDA detection was over the inflow region of the storm. I’m not sure if this was a tracking issue or maybe I’m misunderstanding the way the polygon is placed . The NMDA showed an azShear Max of 0.0218 /s at 21:38Z but the AzShear Avg never rose above the 0.02 threshold. The NMDA and NTDA were occasionally not co-located.

I’ve overlaid 0-3km bulk shear vectors from the RAP on the screenshots below. The shear direction is nearly parallel to the line before the surge occurred which would not be favorable for tornadogenesis by the three ingredients method. Though there were other ingredients/nudgers that would support tornadic development.

– Gerry Bertier

2138 UTC 18 May 2021 KEWX

KEWX radar was definitely the most active in the mid afternoon on Tuesday.  A QLCS segment formed along or just north of the city with initially multiple circulations along the line with 6 weak to modest NMDA circulations along the line. One circulation looked like a mesovortex on the leading edge along the line (Fig. 1).  The NTDA percentage shot up to 83-84%.  The NMDA was in the medium range with a “high” as AzShear max.  The total tilts from the bottom to top was 0.5-4.0 DEG (7 tilts).   It was at a decent range from the radar at 55 nm.  It definitely looked like a SVR was needed with a tornado tag.  I would have been tempted to issue aTOR.  A consistent time continuity of the MESO would have increased my confidence despite the “High” tornado probability.  The couplet was dominated by outbound velocities based on the SRM data (velocities > 60 kts).  It was nice that the NMDA and NTDA values were co-located. Also notice with the outflow a new cell fires (Fig. 2)  in the second image (SRM velocity data).  The circulation was narrow, compact and gate-gate in terms of the couplet velocities that the WFO KEWX office issued a TOR on that cell.

– Yoda777

KEWX – 22:47

Good example to watch the quality of the velocity data…as it could lead to unrealistically high NMDA and NTDA.

Note the very high SW values in lower left hand corner. NTDA probs were near 87% with max/min velocity diff of 51kt. Granted this signature was nearly 100mi from the radar, with the center of the beam nearly 10K ft AGL…so probably not using this for warning decision…but it was still a good reminder for me to QC the velocity data and ask myself “does this make sense”.

-Beltzer

This was an interesting find during the Archived Case from ILN. Focus was on western portions of Indiana/eastern Ohio at several areas of rotation associated with a multicellular cluster of convection. A tool commonly used to interrogate a potentially tornadic storm’s rotational velocity is the VRshear tool. This tool calculates the Vrot, line distance (end of points), shear (in ms-1) and distance from the RDA. WDTD mentions from their Impact Based Warnings Guidance of around 30kt of rotational velocity being the initial supercell tornado warning threshold. Of course, this has some gray area but is generally expressed as an approximation.

At 0207Z from KILN, a cell was identified to show increasing cyclonic rotation. Vrot was calculated to be at 37kt estimated. NDTA probabilities began relatively low at 21.48% which at this early indication, a warning would have already been required. The next scan at 0208Z showed an increase of Vrot to 38kt, with NTDA probability increasing to 24.85%. One more minute followed at 0209Z showed an obvious increase in Vrot at 44.7kt with NTDA Probability of 32.41%.

This is one of several examples where lowering the current default threshold of 30% was necessary. While tracking of both NMDA and NDTA was very accurate, I was surprised at the lower probabilities. A few calculated parameters were identified to be a potential hindrance at a higher probabilistic value, including high SW (and trending higher) and AzShear Max value actually showing a temporary lowering trend at 0207Z. An interesting situation where having to lower the probabilistic threshold was necessary, and was a bit lower than what was expected but still showed consistency with tracking.

– Dusty Davis