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The dramatic stage collapse that occurred at the Ottawa Bluesfest July 17, 2011 was the result of a well defined squall line that moved through the area. Forming a storm line classically described as a “bow echo” on radar displays, and known meteorologically as a “mesoscale convective system (MCS)”, the squall line is a tight line of convective thunderstorm cells that push out in a bow shape due to the strong outflowing winds that develop, particularly in the center of the line.

The storm line characteristically has a strong elevated rear inflow jet along its rear that create a low pressure differential which further enhances the jet flow. Once this flow gets to the leading edge of the convective updraft, it descends and flows outward from the squall in strong straight line winds. Bow echos are not rare – they are a very common product of summer heating across the north American continent, and can produce some very dramatic dark and fearsome skies. While tornadic activity can develop between the cells, its not a common feature on these storm lines. When the storm lines become large, long and very powerful (winds greater than 58 mph) they are referred to as Derechos.

EWR logged most of the composite reflectivity scan of the bow echo/squall line that passed over Ottawa and caused the stage collapse at the festival. The following radar reflectivity animation shows its dramatic passage across Ottawa. A few frames are missing in the animation; we were monitoring other areas when we noticed the bow echo and had to retrace for scans.

Bow Echo of the July 18, 2011 "Bluesfest Storm"

(Click on the image if it doesn’t self-animate)

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Probably. Environment Canada will make the official determination, but the radar evidence from last night suggests a small strong F0 and perhaps a weak F1 one was quite likely. A compact but intense cell crossed into Southwestern Ontario east of Sarnia about 7PM in the evening July 23, 2011.

We watched the cell develop strong verticality over the next couple of hours with high hail densities evident high in the updraft. We noted an MDA event (“mesocyclone detection algorithm” – an automated detection of updraft rotation in a thunderstorm, converting it from an ordinary thunderstorm to a supercell) over this cell about an hour after it crossed the lake shore. The reflectivity radar trace indicated a large upper hail core.

The overall presentation of the cell didn’t suggest a classic tornadic mesocyclonic cell. However, when we looked at the storm relative velocity profile on the lowest tilt we found the couplet (yellow arrow) shown in fig. 1. The couplet didn’t show on further upward tilts, indicating some degree of rotation was occurring near or at the base of the cell. Since the distance between KDTX and the location of the couplet is about 80 miles, the SRV couplet indicated a velocity disturbance around 5000 feet. Since the beam is narrow, and above the ground, it cannot determine how low the disturbance went.

Figure 1. Storm Relative Velocity couplet, July 23, 2011, Lambton Co.

A velocity couplet in a storm relative velocity profile indicates a rapid change in the direction component of the velocity vector (velocity, by definition, is the combination of speed and its direction context). Storm relative velocity profiles measure changes within a wind profile, and therefore will show when there is a significant change in direction of wind, like occurs in a tight rotating air column).

Though the scale is not shown in fig. 1, the wind shift in the couplet can be measured. The light green portion of the couplet measures to -44 knots, to +15 knots for the immediately adjacent red bin. This translates to a wind shift (rotation) occuring at approximately 120 km/h. This measured amount puts the event at the high end of the F0 scale (64-116 km/h and the low end of the F1(117-180 km/h) range. The couplet doesn’t indicate whether or not any rotation actually reached the ground. Damage evaluation will confirm further if the duration was enough to consider a full fledged low power tornado. The SRV couplet survived on the radar scan for more than one cycle, indicating the event would possibly have been in play for several minutes.

Update 2011-07-22: Due to strong vertical shear Dora has reduced to category 2, continues on its predicted path.

Hurricane Dora in the Eastern Pacific gains strength to Saffir-Simpson Category 4 and is expected to begin to swipe coastal Baja California today. It remains far enough offshore to mostly be a threat at surf levels. Dora is expected to begin to decline on Friday as it enters over cooler water in its northwesterly journey.

See Atlantic and Eastern Pacific Hurricane Activity for more details.

[ What is the Saffir-Simpson Scale of hurricane categories?]

Goodbye Bell and DSL,
No more life in internet hell,
We’ll miss you not a wit,
Losing sleep and patience,
bit by bit,
EWR is again all able,
thanks to the benefit of going cable.

Well, a poet, I’m not… But we finally buried the last of the feeble service that the mighty Bell Canada once was. Sir Alexander would be disappointed. I’m under no illusions that the cable industry is the complete answer – it does cost a bit more, but it offers me 3-6x faster web access with the ability to go even faster if necessary, and the lines are new(ish) in this area, and as a bonus, I can power my HDTV in my office here too…
EWR should be fully operational going forward. Since hydro costs are still climbing, EWR may still be offline when no weather is threatening, but hopefully gone are the midday drop-outs and stalled updates, and hopefully, the missed alerts. This spring was not pretty.
It’ll take a bit to get caught up on some of the peripheral stuff like hurricane season now upon us and some of the other projects that were on the burner.
Now would be a good time for me to mention that if any of EWR is useful to you, a visit to the donate button would be appreciated…

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