May 17, 2013

Friday May 17 Target---NW Nebraska

The "Road Less Traveled".   Panorama shot on Highway 61 in the Sand Hills, somewhere north of Hyannis

Initial Target--Through the Sand Hills

The morning weather map shows that yesterday's warm front has drifted north to the South Dakota border.

Southerly winds have brought mid 50 dew points into most of Nebraska east of a dry line that extends from the center of a cyclone in ne CO southward.

As the main trough in the middle and upper troposphere (see 500 mb chart posted) continues to evolve, pressures should fall in that low, drawing southeasterly winds across most of Nebraska, and bringing even 60+ dew points into the state.

 Given the winds aloft, southeasterly surface winds are very favorable for the kind of deep layer shear (see hodograph) that encourages supercellular convection.

Right now, the best combination of this deep layer shear with the buoyancy most favorable for deep convection (see sounding) is projected to be in the red box drawn on the surface chart.  There are also several aspects of the deep layer shear in combination with the projected low level shear suggestive of relatively long lived  rotating storms, although they may tend towards the HP mode.

The High Resolution Rapid Refresh Model (HRRR) depicts storm initiation in the eastern Nebraska Panhandle around 20 UTC (3 PM CDT).  I've included that graphic and also what the HRRR depicts that echo doing by 0000 UTC (7PM CDT).  The model creates a very intense mostly isolated storm that appears to be a supercell, having moved northeastward to just south of the South Dakota border.

So our target is not far today.  Also, it does look like the highest risk for tornadic supercells tomorrow will be along the KS-Nebraska border, so whatever happens today, we will be in good position for tomorrow's storms.

The Chase

Panorama of Alliance, Nebraska elevated supercell, view towards the northwest..   Detail of the updraft base is given below.  Inflow bands at midlevel and a large low level "tail" inflow can be seen on the right side of the image.

The cell we targeted was east of Alliance.  It had the familiar
hook and flying eagle presentation of supercells.
We drifted west to Ogallala as the first round of initiation began to occur in eastern Wyoming.  We drifted north on state highway 61 through the Sand Hills and into Cherry County.

Cherry County has only a few roads, so it is a target area dreaded by chasers.  The early initiation was pretty much as the HRRR depicted (as shown in the last section.  The early storms congealed into a small line, and we attempted to get on "tail end" charlie.

That we did.

That portion of the line was broken up into somewhat discrete cells.  These were rotating and presented as small supercells on radar.  We targeted the south cell that had somewhat of a couplet on storm relative velocity plots.

The storm also had a moderate couplet on it, as shown on the
North Platte Doppler radar storm relative velocity profile
(The light red (outbound) next to the dark green (inbound).

By the way, it's worth it to point out that our decision not to activate Mobile ThreatNet was a good one.   ATT has excellent coverage through most of the Plains, and good enough coverage even in the traditional "holes" (like the Sand Hills) for RadarScope to get files (even though at exactly the same location GRLevel was not able to receive files.

In addition Thom had an iPhone using Verizon, and we were often able to alternate phones when GRLevel was not able to get files, and one of the other, Verizon or ATT was not in range.

As we dropped southeast of that storm, we could see it was elevated.  It was clearly a supercell, but the the surface dew points were too low and the condensation was developing four or five thousand feet above the ground.  There we could see the familiar horsehoe base, with a Rear Flank Downdraft cut.

The panorama of this storm (shown above)  shows that it had midlevel cloud bands extending into it and a large inflow band at lower midlevels.  It was indeed a supercell, and somewhat picturesque.  However, it did not have a chance of becoming tornadic.

Final view of "tail end Charlie" showing detail of horseshoe base, indicative of a mesocyclone in the lower midlevels of the storm.   The rear flank downdraft cut is also visible at the center of the horseshoe and lined up well with the radar presentation (examples above).