Finally, after completing some site maintenance and resolving an issue with our router, I can finally post the video of the tornado at Grand Forks, ND on Thursday, June 17. The video shows an entire occlusion cycle of the tornado, which was high-contrast with a classic cone shape. Enjoy!

Thursday afternoon, optimal conditions were in place for severe storms over the northern and central Plains states. Boundary level moisture conditions were forecast to be excellent, with dew points in the high sixties/low seventies over parts of North Dakota to mid-eighties dew points over Minnesota. A high amplitude shortwave trough was forecast to translate northeastward through the region, with strong cyclogenesis expected to commence at the surface in association with the upper level system.

After analysis of the 1800Z GFS and RUC model forecasts, I decided on northwestern North Dakota as my initial target area. Instability for the region was expected to reach CAPE values of 2500-3000 j/kg by afternoon peak heating, and surface winds were forecast to be backed out of the southeast at 35-40 kts, yielding good wind shear and helicity values for supercells and tornadoes. I departed Winnipeg for Grand Forks at 10:00 AM CDT.

Stopped for lunch just after arriving in Grand Forks at around 2:00 PM. Light convection from earlier in the morning had mostly dissipated, but cloud cover was still intact over the area. I was initially worried that this could be problematic for the afternoon heating and initiation of storms later in the afternoon, but visible satellite imagery showed the clouds were thinning out and moving on to the northeast. Temperatures were in the low seventies already, and surface winds were from the east southeast at thirty knots. The wind shear was good, all that was needed was a little more surface heating to get things started.

By around 4:00, convection was beginning to develop rapidly further south, with numerous cells starting to appear on the GRLevel3 data feed. There was a storm to the west of Grand Forks near Devils Lake that was showing pronounced rotation for approximately fifteen minutes, but it lost intensity and dissipated a short time later. I downloaded a quick update of surface conditions, and decided to stay put Grand Forks, as instability was good and winds were almost easterly at 40 knots.

At 4:00 PM, the cold front moving in to southwest North Dakota could clearly be seen on GRLevel3, as it was producing a well-defined squall line with embedded supercells. I held out hope that some discreet storms would develop in the warm sector ahead of the cold front, before the whole show turned into a mess of linear storms, and no hope for tornadoes.

By 4:30, a robust supercell appeared to be developing just to the south of Grand Forks, between Northwood and Reynolds, moving to the northeast at about 40 knots. GRLevel3 showed robust rotation at the center of the storm, with good gate-to-gate shear apparent on the lowest tilts of the radial velocity scans from the Grand Forks NWS. I decided to attempt an intercept just south of Grand Forks, and proceeded south on I-29.

I was totally surprised and frustrated by an unforeseen construction delay on I-29. The west half of the once twinned highway was closed, and traffic slowed to a crawl as the roadway was pinched down to just two lanes in a drenching rainfall. As I crawled along with the traffic, I noticed what appeared to be a very dark wall cloud to the west, so low that it appeared to be touching the ground. I knew I had to get the car turned around so I could parallel the storm as it traveled north, but there was no place to even stop the car, let alone turn around. Finally though, I saw a chance to pull off the road and grab a quick U-turn. It was a turnoff where a new bypass was being built, but the road wasn’t paved, and was the consistency of a swamp. I should have gotten stuck, but the car somehow managed to swim through the mud and get back on to solid pavement.

When I finally got the car turned around, I saw a vigorously rotating wall cloud just to the west of the city, and clearly visible form the interstate. Small vortices were appearing just beneath it, then evaporating like smoke before they could touch the ground. Several other chasers were stopped near me on the shoulder of the highway, admiring the spectacle and taking video and photographs. Just before 5:00, a well defined funnel cloud appeared, and snaked its way toward the ground. It was a high-contrast funnel cloud, and the sun could be seen shining through the clear slot to the southwest. A debris cloud appeared just below the tip of the slender funnel. Touchdown!!

The tornado went through several occlusion cycles, touching the ground and retracting back into the wall cloud. Although it wasn’t a large tornado, it put on a spectacular show before it finally roped out after about eight minutes.

All told, a fantastic chase day, and well worth the short distance traveled. I was saddened today, however, to learn that the tornado I witnessed destroyed a home, and fatalities occurred in neighboring Minnesota along with heavy property damage.

My heartfelt condolences go out to the affected families.

A series of powerful tornadoes carved a deadly path through the southeastern U.S. Saturday, killing at least 10 in Mississippi and leaving dozens injured.

The storms were part of a large scale disturbance and cold front that swept through the southern U.S., spawning a series of tornadoes as it moved through Texas on Thursday.

The worst of the damage occurred at Yazoo City, Mississippi, about 370 kilometers north of New Orleans. The 1.2 kilometer-wide funnel claimed ten lives, including three children, as it tore through an estimated 100 homes.

At last report, thousands were still without electricity as rescue workers continued to comb through the debris for survivors.

Tornadoes and injuries were also reported in Louisiana, Arkansas and Alabama.

Satellite image courtesy NOAA

Departures from the normal range of sea surface temperatures in the central and eastern equatorial Pacific Ocean, coupled with observed patterns of convective weather in the tropics indicate a strengthening El Nino pattern that should persist at least through the end of the Northern Hemisphere winter.

Sea surface temperatures in the eastern Pacific have ranged as much as 1.5 C higher than average, while below the surface, anomalies ranging from 1.5 to 5.0 C have been observed in the upper 300 m layer, patterns that are consistent with an established El Nino event.

Temperatures at the ocean’s surface have a large influence on the weather above, as thermal energy is transferred from the water to the air. Persistent areas of higher pressure consistently form in the atmosphere above warmer ocean waters, a scenario that NOAA forecasters expect to unfold over the coming months.

Most of the forecast models used to predict the strength and duration of El Nino suggest that it could last into the March-May 2010 period, with its intensity likely peaking within the moderate range and lasting through the Northern Hemisphere winter.

That could be very good news for people who enjoy mild temperatures and less snow in the north-central United States and southern portions of the Canadian Prairie Provinces, where the potential impact of El Nino is expected to include above normal temperatures and below normal precipitation during the winter months.