FINALLY! A Lightning Jump Detection

Finally on our last day of EWP operations we were able to capture a weak lightning jump with the Lightning Jump Detection Algorithm. This jump was detected from a discrete cell that was lifting north across the western edge of the District of Columbia around 2109z. The jump from 0 sigma to 1 sigma (or 1 Standard Deviation) shows up as the green blotch on the image above. This is overlaid on top of the Flash Extent Density product which measures total lightning in the storm. At this time in the image above the flash density was 10 flashes per km^2 which was overlaid on 0.5 deg KLWX reflectivity of around 52 dBz.

The Tracking Meteogram Tool was used to see the evolution of the Lightning Jump, reflectivity and Flash Extent Density verses time. The take home from this is that a lightning jump or rapid increase in Flash Density within a storm correlates with a rapid intensity of a storm. Note that between 21:06z and 21:08z the Flash Extent Density rapidly increased or “jumped” from 1 flash/km^2 to 10 flashes/km^2 which triggered the Lightning Jump Detection Algorithm to increase from 0 to 1 sigma. During this time the dBz values of reflectivity increased from 20 dBz to greater than 55 dBz in 8-9 minutes. This cell was also somewhat low-topped with echo tops only reaching to around 32kft. Please keep in mind that this is a weak example of just how rapidly a cell can intensify since the jump was only 1 sigma.

Shawn Smith

ProbSevere Underestimates Storm in N AR on May 15

The storm below produced golf ball size hail (around 1.75 in diameter) and had 50 dBZ up to 31157 ft MSL from SRX radar (114 nm to the west southwest). ProbSevere only indicated 13% for severe with 1049 J/kg, 25.4 kt of EBShear, and 0.60 in MESH.  The lack of nearby radar data with the LZK (Little Rock) WSR-88D being inoperable may have significantly impacted the ProbSevere algorithm.

ProbSevere seems to again be underestimating the severe potential and expected hail size.  The environment was characterized with low topped severe storms with mid/upper trough overhead.  The 12Z LZK sounding is below as the last image.

Nearcast Tool – Convective Coverage

Above is the NearCast imagery at 22Z.  Utilizing this imagery, a pretty apparent boundary is evident across portions of MO into northwest Arkansas.  Along this boundary, convection was much more widespread than it was further south.  Further south, despite better instability, there is no indication of any boundary which likely explains the more scattered nature of the convection.  Operationally, seeing this boundary on the nearcast model would give me higher confidence in convective coverage further north versus further south.

22Z National radar Imagery (super hi-res :P)

Forecaster thoughts on how to use ProbSevere

A question was posed to the forecasters today: "How would you use a product like ProbSevere in your warning decision process?"

A common response was that the product is unobtrusive and readily overlaid radar data (all tilts or other radar products). Having a quick look at the CAPE, shear, MESH, and satellite growth rates with a simple scroll-over has been helpful.  ProbSevere can give forecasters a brief overview of which storms to watch and consider more carefully. It seems to have the best utility when considering large hail, and may struggle in environments conducive to low-top storms. The satellite growth rates have helped somewhat this week, highlighting storms before robust radar is present, but may have more utility in rapidly growing storms.

-Cintineo

Pre-frontal Virginia convection

A few storms have fired ahead of the squall line in northern Virginia. While most storms have had very low probabilities of severe (via the ProbSevere model), one storm had a 37% probability (figure 1), owing to 0.87" of MESH, 1220 J/kg of MUCAPE, and 27 kts of effective shear. The IR and visible GOES-East imagery showed that there was a very thick cloud mass over the mid-Atlantic, damping the satellite growth rates. However, 8 minutes after the ProbSevere exceeded 30%, the NWS issued a severe thunderstorm warning (figure 2). Fourteen minutes later, a wind report was recorded in northern Virginia. This demonstrates that ProbSevere was able to identify the "storm to watch" in this region, and the probability of severe could have been aided by earlier temporal information from the satellite growth rates, if they had been observable.

Figure 1: ProbSevere highlights a storm with 37% probability in Virginia, in midst of storms with 1-10% probabilities of severe. This storm later produced a severe wind report, while the lower probability storms have had no reports.

Figure 2: NWS warning (issued at 3:50pm). 

-Cintineo

Simulated Satellite Imagery Perspectives OK/AR on May 15

The simulated satellite imagery performed okay, generally capturing the overall IR and water vapor patterns.  The simulated IR and water vapor depicted linear convection too far south across Arkansas while the actual imagery depicted convection over northern Arkansas.  This product may be more useful for a National Centers such as SPC and WPC which cover the entire nation, but may be a bit too general for a local WFO which relies heavily on small scale features.

Michael Scotten

CI Tool Shortcomings -Thursday 1945Z

I noticed today that with the satellite out of rapid scan mode, the CI tool has not been nearly as effective.  It does not give the type of lead times that we had seen in rapid scan mode.  Here is an example:

1945Z satellite imagery with the CI tool overlayed.

In the above imagery, try to focus across the northern CWA (near the chimney region).  As you can see, very low values of CI are being detected at 1945Z.

Satellite with CI tool overlayed at 20Z.

I’ve circled the area of interest here.  By this time, there’s one small area of 50-60% dBz.  However, at this time, echoes were already beginning to show up on radar.

Satellite imagery with CI tool at 2015Z.

The probabilites finally began to expand and increase in this scan.  However, by now, the radar already had 30+ dBz echoes, which gave very little lead time to the probability of CI.  Given this is a cold core convective case, perhaps the threshold of CI is lower?  I noticed the other day 70% and above tended to result in CI, but perhaps in these type of environments the CI thresholds may be 50-60%.  Either way, it appears that without rapid scan mode, this product’s utility and ability to increase lead time to convection is greatly diminished. Below is the radar slice at 2019Z which corresponds to the latest satellite/CI imagery seen above.

Radar image at 2019Z.

ProbSevere not working out for bowing low-topped liner convection

In the 4-panel image above from 1955z over northern Virginia the ProbSevere model was evaluated on a low-topped bowing line of storms. The developer has noted poor performance in liner convection but wanted to evaluate its utility in storms with damaging winds.

The top left pane shows 0.5 deg reflectivity from LWX radar with the ProbSevere model shapefile overlaid. The top right pane shows 0.5 deg velocity and the lower left pane is enhanced echo tops. The mouseover shows inbound velocities of around  50kts at the nose of the bowing convective line with enhanced echo tops only to 20kft (low-topped). This line of storms is likely producing some damaging winds but the low-topped nature of the convection should preclude any hail. The ProbSevere shapefile of predictors indicated only a 1% probability of a severe storm in an environment with 620 J/kg of MUCAPE and a healthy 36.2 kts of EBShear (High Shear Low CAPE). The MESH product is likely performing well since it only indicated hail to 0.08 inches. This case further supports the lack of utility of the ProbSevere model in liner convection. However, I continue to feel that its best use is for detecting storms which can produce severe hail.

Shawn Smith

On the Usefulness of the GOES-R Convective 4-Panel Procedure

A 4-Panel created by Bill incorporating many of the GOES-R Products

Just a quick post to discuss the utility of this convective 4-panel.  Operationally speaking, I can forsee this procedure being quite useful as it incorporates many analysis tools.  The NearCast tool (top left) can be looked at and compared to ongoing cloud cover/convection to see if storms are forming along/near any boundaries.  In this case, you can see that convection is concentrated in the most unstable airmass, located across Arkansas northwest into eastern Oklahoma.  It is also nice to have the CI tool (top right) with this procedure to diagnose if any of these boundaries have convective potential in the near-term.  I added radar to the bottom-right panel in order to see convective trends and whether or not CI is actually occuring in some of the areas highlighted by the CI tool.  In this case, the overshooting top tool did not have any detections, but would also be of use with more robust convective elements.  Overall, I think this procedure will be of operational use, especially once GOES-R is actually launched and these products increase in overall utility.

- KD

ProbSevere Too Low in Cold Core Low Events with Low CAPE/Marginal Shear

The top image below depicted a few marginal severe storms in western Arkansas,  The storm of interest is the one farthest west between the two other cells that were located slightly to the east.  ProbSevere gave this storm 9% chance of being severe in an environment with 898 J/kg, 27.4 kt of ENShear, MRMS MESH of 0.53 in, 1.27%/min normal vertical growth rate, and 0.03/in glaciation rate.

This storm produced quarter size (1 in diameter).  MESH underestimated the hail size in this case.  I (as well as the WFO Little Rock) did not issue a warning of this storm thinking the hail size was a bit smaller (closer to pennies – 0.75 in or nickels – 0.88 in).  The environment was characterized by the 12 UTC LZK (Little Rock) sounding below (bottom image) with a freezing level of 6579 ft AGL and -20C level at 15572 ft AGL, conducive for the development of hail.

The ProbSevere seemed to be too low in this case, only with a value of 9%. I would personally estimate the ProbSevere closer to 30 or 40% for this storm, especially considering that the reflectivities 16-17 kft AGL were 55-60 dbz (I typically use 60 dbz or greater at the -20C level as a proxy for issuing severe thunderstorm warnings for large hail.)