South Dakota

-tvguy

Looking at atmosphere in wake of ongoing MCS which is moving to the east, trying to use tools to determine whether atmosphere will recover in time for a second round of severe weather later today.

SBCAPE from the sectors reporting in south central South Dakota ranges from 500-1000 j/kg along the Nebraska border, to 1000-1500 north of I-90 at 1830Z. A moment ago, a couple of bins in excess of 2000 j/kg emerged along the Missouri River north of Pierre.

On the downside of recovery, heights are slightly rising in south central SD. At 1913Z, the area of 2000 j/kg was reduced under 1500, presumably because a broken cloud shield went through that area. Don't think there was much else going on that would have caused the SBCAPE to diminish.

NUCAPS grids arrived about 1945Z, and we were able to determine what kind of cap existed. We knew there was warm air, but we did not know how warm. We have been seeing a slight increase in towering Cu, but our hopes and dreams were dashed after seeing NUCAPS grids, which reported temps >13C at 700 mb behind the anvil shield of the departing MCS. Not sure what the convective temperature is, but it must be really high.

But when NUCAPS sounding came in it told a scary story, producing SBCAPES >5000 in south central SD. I did a cross section of temperatures in NUCAPS grid. Drew a line A across a thin line of towering Cu. In the middle of the cross section, I could see a thermal boundary right were the line of Cu had developed. (I was unable to use the drawing tool, but the gradient is seen a the bottom of the screen about halfway between the bottom left and bottom right edge of the image.

Convection quickly popped up along that boundary, and so did ProbSevere product, with 80% ProbHail. The object appeared immediately next to the SFD METAR plot, which was 103/65. Immediately to the SW, the measurement at VTN (Valentine, NE) was 109/52.

Thunderstorms grew quickly along that boundary over Winner, SD...

and the strongest core almost certainly produced large hail. Seemingly the only forcing was because of the subtle boundary we had observed earlier. ProbHail was 99%.

Isolated thunderstorm was amazingly resilient, and continued to produce significant lightning as detected on GLM. Since storm now crossing border into Gregory Co (my FSD CWA) I chose to issue a warning for the hail/wind threat. Am concerned about hail size being suppressed by very hot environmental temps, but any melting would also contribute to the wind threat.

ProbSevere developed a second object south of NE border which moved north. It treated the two cells as two different objects. Figured they would be a line, and previous warning expired, grouped them together and formed SVR for the line in Gregory Co.

About 15 minutes later, the dominant part of the storm (highest reflectivities, overshooting top) dropped south across the border into Nebraska. Left the warning expire on schedule.

But as the strongest cell moved down into Nebraska, a trailing cell strengthened, with reflectivities and ProbSevere both increasing. Had to issue a new warning for the cell which followed the Missouri River toward the Nebraska border.

We were watching that part of the storm on satellite, but our viewing appeared to hampered by anvil of the Nebraska storm, which covered up our few of the new updraft. That is unfortunate, because this area is nearly in a NEXRAD void, with beam heights very high from all three of surrounding 88-D's.

Had an interesting event pop up a short time later, as the cluster of thunderstorms was in Nebraska. We saw flashes show up well downstream from the storms, at the edge of the anvil in Yankton, South Dakota. The lightning is not in a place we would have expected, but GLM picked up on it, and then there was also a single NLDN CG (+ charge) just SW of Yankton on the Nebraska side of the border. This, where the FSD NEXRAD was only reporting 20 dbZ. Station plot also reported TR.