Sandhills Cold Front - SD / NE

30 May 2014

We woke in Rapid City on the west side on South Dakota with the potential of only a ‘see text’ to chase today. But we did have birthday cake as it was Tim’s birthday, so anything on top of this was merely a bonus. The storms, some of which may be severe, were expected to fire along a trailing quasi-stationary diffuse cold front sagging southwest from the low in Canada through to western Nebraska. Although low level southerly flow was weak, it had transported enough moisture up to the high plains. The issue however was the lack of shear. Mid and upper level flow was mostly southerly too with this being parallel to the front, there was also the chance that any storms that fired would be undercut by the front itself.

It wasn’t much to go on but we decided to apply the same strategy as yesterday and sit just behind the cold front nearest the area of best shear. We also tried to track down the area with the clearest skies as this was where convection would be aided by the greatest surface heating. An upgrade to a slight covering the western Nebraska panhandle nearly had us tempted to ditch all strategy and just head to Scottsbluff – a place that almost always gets hit with the weather should there be any in the area, but there was nothing apparent in the latest RAP model update that tempted us with the 3 hour journey straight south.

Instead, we decided to head east to get under some clear skies and ahead of anything that might fire on the front before then heading south into the ‘slight’ risk zone.  Heading east along the I90 gave good views of the area and by the time we decided to head south, numerous cells or cluster of cells had popped up on the Nebraska / South Dakota border and within the rest of slight risk area. One storm became severe warned but it was obvious that none of the storms were going to be that long lived. There was enough moisture and hence modest CAPE in the area but too little shear resulted in them not moving far and just raining themselves out. The small movement they did have was to the north-northwest for most but positioning wasn’t a concern as they weren’t supercellular.

Unfortunately the sandhills areas of northern central Nebraska near the SD border, although very beautiful didn’t allow particularly good views of the storms and the choice of roads were a bit scarce. We managed to find a hilltop from which to photograph a couple of shelf clouds as they moved past, but there was no other structure visible. We ended up becoming surrounded by these small pulse type cells as they popped up all around us.

We waited here for a bit of time until the storms had nearly fizzled out but in the end decided to head back north towards tomorrow’s risk area which looked to be a much more robust setup with a shortwave trough digging in through Montana – a mere 8 hours drive away! We saw some nice horses on route though.

Shelf

Horse

Key points for trailing diffuse cold front in northern plains

-          Little directional shear - pulse type storms and unorganised clusters

-          Modest H7-H5 flow parallel to boundary – very slow north moving cells and some undercutting from front

-          Southern central South Dakota – windy roads but good terrain.

-          Northern central Nebaska - sandhills a pain! 

SPC Day 1 Weather Outlook as at 06:00UTC (01:00 Central Time the night before)

SPC 01:00UTC day 1 forecast for Fri May 2014

SPC Day 1 Weather Reports for the day.

SPC storm reports for Fri May 2014

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