June 6, 2004 North Dakota supercell

June 6, 2004 North Dakota Supercell

Meteorology:

The mid-latitude westerlies had relaxed across much of the U.S. by the first week of June. After several days of chasing in marginal deep-layer shear environments (associated with the weak westerly winds aloft), we were once again hungry for big shear and to get it we had to go north. The northern plains were looking very tempting this day thanks to a strong but compact upper-tropospheric shortwave trough moving eastward into Montana during the day, overspreading North Dakota with strong southwesterly flow aloft (Fig. 1). The question this day was whether sufficient low-level moistening and associated destabilization would occur to result in deep convective initiation over the region. Strong shortwave ridging and associated mid-tropospheric subsidence warming exacerbated our concerns about convective initiation, biasing us even farther north. Early afternoon surface observations (Fig. 2) revealed a well-defined low-level moisture surge over eastern South Dakota and increasing surface winds were advecting this moisture northwestward. It was reasonable to expect that at least lower 60 F dewpoints would materialize over western North Dakota by late afternoon beneath a strong southwesterly mid-level flow, resulting in at least moderate amounts of CAPE and strong deep-layer shear, an environment conducive to supercell storms given thunderstorm development. We became increasingly confident as the dryline noted in the 17Z observations moved east into western North Dakota by late afternoon, increasing low-level convergence and ascent in that region where low-level moisture continued to increase. The key strategic decisions that had to be made were (1) how far northwest do we go and, eventually, (2) given that the tail-end supercell would likely develop near or over the sprawling Ft. Berthold Reservoir northwest of Bismarck, do we drive westward north or south of this lake? Given our original concerns about convective initiation and the fact that mid-level flow (and therefore deep-layer shear) increased to the north, we went north on Hwy 83 out of Bismarck. As we drove north, an isolated severe thunderstorm developed rapidly near Killdeer in Dunn County and began moving east-northeast (see this nascent storm in the visible satellite image in Fig. 3). After carefully determining that the updraft would likely remain north of the lake, we headed west out on Hwy 37.

Route and Maps:


Fig. 1: 22 Z 500 mb "analysis" from the SPC mesoanalysis page, revealing a strong mid-tropospheric wind maximum approaching North Dakota.	Fig. 2: A 17Z surface chart reveals a dryline over eastern Montana and strong low-level moist advection underway over the Dakotas.	Fig. 3: Visible satellite image showing CB developing over northern Dunn County (circled). This would morph into the gigantic supercell that we chased for several hours into central North Dakota.	Fig. 4: Our route this day, highlighted in blue.

Chase Summary: given in caption format beneath each image.

Video Captures


Our first view of the developing supercell, looking west on Hwy 1804 about 10 miles SSE of Roseglen, ND. Note the strong separation between the precipitation core (heavy shaft in the lower right) and the updraft base. This is a function of strong shear - the stronger southwesterly winds aloft displace the precipitation core downshear away from the updraft. Note also the high base and strongly striated appearance of the updraft.	A closer image of the updraft base; note the first of many RFD occlusions in the rear of the base. Such a feature is a hallmark of supercells.	Striations within the updraft become even more pronounced as the updraft itself takes on a cylindrical shape - all manifestations of the storm's strong mid-level rotation.	As the storm moves northeast, we do too. Here we have relocated to a point along Hwy 4 south of Douglas in McLean county looking west at a large wall cloud that has formed within the rear flank updraft base. Note the huge donut hole carved into the storm by the RFD; such an updraft base is called a horeshoe or donut base in storm chasing parlance.

One of the many transient funnel clouds that formed beneath the wall cloud.	A larger perspective of the updraft reveals an impressive amphitheater-shaped structure.	The optical vault (the interface between the storm's forward flank core and main updraft region) is also incredible.	This raggedy funnel cloud had a noticeable and persistent dirt whirl beneath, confirmed by chasers on the other side of the ridge that blocked our view beneath. Therefore, this is actually a confirmed tornado. We were not really in tornado-viewing mode at this time - rather we were desirous of staying far enough ahead of the storm to be able to take in its large-scale structure the entire time.

A closeup of the funnel cloud.	This is a shot of the storm's pregnant, menacing forward-flank updraft base.	As the occlusion progresses, a more laminar funnel protrudes from the wall cloud.	Soon the rotation increases within the funnel cloud as the occlusion strengthens and the funnel attains a classic truncated elephant trunk shape as it protrudes at an angle out of the wall cloud.

This shot and the next are of the same funnel cloud.		We maneuvered through the town of Max, North Dakota where the tornado sirens were in full wail. We jumped east because the low-level mesocyclones were now "jumping" - continually and rapidly reforming eastward as the rear-flank downdraft strengthened and undercut all new attempts. This usually signals the end of any long-lived tornado potential since as soon as the rotation refocuses, it is undercut by outflow. This image shows one of several occluded wall clouds that formed during this stage.	The next several images show the storm's impressive optical vault, along with horizontal lightning that flickered within the vault region. Whenever you see such a supercell and nightfall is close, plan to get downstream of this feature and set up for lightning photography because the lightning here is usually unparalleled in frequency and intensity. Too bad this was during daylight!

Note: we followed this storm east of Harvey, ND until after dark. In fact, at Harvey we stopped for a quick bathroom break with one of this storm's many mesoscylones hot on our heels. While parked at the gas station, our inflow winds increased to over 30 kts out of the east-southeast. Knowing that the storm's RFD was pretty powerful, it was clear that there was some tornado potential so we bailed out of town, heading east. Then we stopped and looked back and over the north part of town we witnessed a lot of RFD dirt blowing. However, we did NOT witness any damaging ground-based circulations (e.g., a tornado). A tornado did make it into the LSR but we did not witness anything that looked tornadic in Harvey, merely RFD winds.