The simulated IR imagery showed the cold front’s areas of convection on the leading edge of the storm in our target area of Virginia and Maryland on Thursday afternoon.

It matched up spatially with what we were seeing in reality on the rapid scan GOES IR imagery at the same time stamp, 20Z.

The area in blue on the simulated IR indicates cloud tops colder than -60C. This shading doesn’t show up on the real IR image at all, but the cloud tops do have temperatures below -50C in the same convective regions. It looks like the simulated IR is going to overdo the convection, especially for the southern cell over central Virginia, but I thought I’d keep an eye on it to see if a convective cell spawned a severe warning in that area.

The simulated image valid at 21Z shows the strongest convection has shifted further east and is concentrated into one cell.

That cold cloud top maximum in northern Virginia is much smaller and not nearly as cold in the real GOES IR image from 21Z.

During this time period, a line of strong to severe thunderstorms was pushing through the DC Metro area.

Comparing the radar data to the simulated IR at the same time stamp, it appears that the small clusters of convective cells were not well resolved by this product. In fact, the clusters of storms to the south and west of DC were either completely missed by the simulated imagery, or the placement was off by about 50 miles (clouds too far to the southwest to be a match for the convective cells).

This was a day where we had limited tools for severe weather forecasting in the DC Metro area. The threat for hail and tornadoes was very low. ProbSevere, convective initiation, overshooting tops, and PGLM products were rendered useless because of a lack of convection and lightning parameters.

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

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.

Michael Scotten

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.

I’ve been following the performance of the OUN WRF through the day today, and thought I’d go ahead and give a brief description of how it’s doing.  Given the overall lack of any strongly convergent boundaries, these storms have just been developing under the cold core of the upper-level trough.  Therefore, you’d expect a bit of placement issues.  We see some placement issues with this convection, but overall I think the model is handling it pretty well.  It has the storm mode correct with discrete structures and bowing segments being observed both in reality and within the model.

An interesting feature that the model did pick up on is the gravity waves seen across portions of OK and Texas.  Looking at satellite, sure enough there was a large area of gravity waves, with some mid-level echoes showing up with them.  Pretty cool feature that the model was able to pick up on.

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

In the image above is the vLAPS model maximum base reflectivity is evaluated verses observed radar over the Mid-Atlantic region. The top left pane is 0.5 deg reflectivity from the LWX radar at 2017z. The top right pane shows the 2 hour forecast of the 18z run of the vLAPS model surface layer maximum base reflectivity with 1km resolution. Overall the model continues to forecast convective mode very but location is still a bit off.

The model shows a wavy convective band extending north-south to the west of Washington DC along with discrete individual cells between the line of storms and the Chesapeake Bay. Location of the band of storms in the vLAPS model is about 20-40 miles too far west. The location of the highest model reflectivity correlates with the stratiform rain behind the leading edge of the bowing line not handling the eastward progression of the bowing segments very well.

Shawn Smith

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:

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.

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.

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.

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

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.