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June 21, 2007 Oneill, NE
Analysis Data: http://www.directwx.com/index.php?q=node/451
Target: Valentine, NE
Summary: Chase wih the Weather Explorers Post. Caught a long-lved supercell with wall cloud and several funnels along with 1 inch hail (and a turtle)
Setup: Outflow boundary-enhanced weak frontal zone was lying east/west across southern SD. Strong cap would be broken by combination of afternoon heating and some mid-level cooling from a short-wave trough at 500-700 mb coming out of WY. This would induce a weak low-pressure area over northwest NE moving east along the NE/SD border bulging the dryline east south of the low. The goal was to reach the dryline/frontal boundary intersection in order to maximixe convergence and break the cap. An axis of 15C temperatures at 700 mb was forecast to extend from CO through central NE to just south of the Valentine to Yankton area. We would work the northern edge of these warm mid-level temperatures. Dewpoints were in the upper 60's producing forecast CAPE of ~ 4000 j/kg. Wind shear was favorable in this NW flow type pattern with W-WNW winds of 40 kts at 500 mb and S winds of 20 knots at 850 mb. SPC had a high end slight risk out with only a 5% tornado threat at 12Z which was upgraded to Moderate Risk at 16Z (mainly for wind event associated with the ongoing MCS in SD).
When we got up this morning thunderstorms were already raging in SD west of Sioux Falls at 8 am. In fact by 10 am there had already been a tornado warning. This cluster of storms continued to move Southeast toward the NW IA region.
This created a little bit of a dilema as there was some question as to whether these storms might eventually become surface-based and change our target decision.
Christy, me, Jared, Kyle and Rebecca met at Scooters at 11 am then stopped by to pick up Craig. We decided to head north on I-29 since that would take us to our original target. If the ongoing convection was interesting we would check it out on the way. We stopped just north of Little Sioux to observe the severe warned storms to our north but they remained elevated and were moving into a less favorable shear environment so we continued on.
This convection extended back into NE to north of Norfolk. Extensive cloud cover debri was now located over our target area but forecast to eventually clear. Another problem that became apparent was that the outflow boundary was pushing rather far south into NE, away from the better dynamics that were forecast for later in the afternoon.
We turned west on hwy 50 toward Yankton. Clearing was becoming visible to our west which was encouraging. After another hour or so it became clear the outflow boundary had moved too far south to go for a western target and the distance was too far so we decided to turn south on hwy 281 to cross the NE border, then turned west again on hwy 12. giving us more options to go south if we wanted to. Much of the cloud debri had cleared with only some altocumulus and cirrus. Around 5:30 near Spencer a patch of Cu became visibile about 10 miles to our west. While the Cu was flat and showed no development it was an extensive Cu field indicating perhaps some moisture convergence in this area.
We decided to look for a library in Spencer (this would become a running joke later in the day). We sighted the library but as we u-turned to get to it I saw a nice tower going up through the Cu field we had just passed. This of course trumped any desire to go to the library and lead to great excitement in the group as it had started to look like the cap might hold.
Heading back to the west we lost the towering Cu in the Cu field but then were able to see it again about 15 minutes later, now as a nice Cb with an anvil to our west. Other explosive towering Cu could be seen as well indicating the cap was indeed breaking and the CAPE was being realized.
Eventually the base came into view and we could see a shallow wall cloud below the base. This was somewhere near Naper. It was somewhere around this time that we encountered the large turtle crossing the road (I still wish we had gotten a picture of him/her:) Behind this smaller cell there appeared to be another storm and it was at this time the first warning came across, a severe thunderstorm warning, for Keya Paya County for a storm near Springview.
The sky was hazy and it was difficult to see any detail of this storm. Wanting to get south and east of the storm we turned south down hwy 137 and shortly we were able to make out the towering Cu feeding into the storm near Springview. The extensive anvil indicated the updraft was strong. After driving though some of the anvil rain we wer able to seee a beautiful rain-free base with a prominent wall cloud. We turned West on hwy 20 however very shortly we began to hit some small hail, pea size. Since we were driving Jared's fathers van we did not want to risk larger hail and luckliy found a gas station near Newport to take shelter. The cloud base was still somewhat visible through the (close) trees to our west. The hail continued to increase in size reaching dime size. The hail abated some and we attempted to hit the road again to to get into a position to observe the base but once again the hail increased and we drove back the 1/2 mile to the gas station. This time the hail really kicked in with stones the size of quarters. The storm then (of course) became tornado-warned and a funnel was confirmed by observers. Frustrated but not detered we decided to try a different path and headed east on 20 then south. The storm motion was to the Southeast around 15 knots indicating this was a right-turning storm, consistent with the meso-cyclone we were able to observe.
Driving south on 11 we once again were able to view the beautiful structure of the storm that was continuing to exhibit good rotation and a wall cloud.
We stayed on the storm for the next hour and watched a number of funnels descend from the wall cloud but it did not appear that any of these reached the ground. After taking some pictures near dark of the back-lit wall cloud we finally hit the road for home around 9:30 pm, getting back to Omaha around 2 am.
What went right: The target area was valid as this turned out to be one of the best storms of the day. The storm exhibited a nice hook on radar and had a "flying eagle" signature for a short time:
While the earlier MCS that moved into IA did produce an apparent tornado the storm we picked was probably more photogenic and isolated. Having data on the road (Jareds phone and MyCast) was helpful but in the end did not change our target. The ability to get updates from SPC and especially access to the visible satellite imagery helped determine the location of the outflow boundary which would later be important for initiation of the supercell.
What we could have done different: Not much. This was a good chase that went more or less as planned. Model forecasts were too fast to move the surface low east and instead of being in north central NE ended up in northwest NE such that the dryline remained to our west. In the end surface heating and the outflow boundary were enough to overcome the cap. Having a vehicle that could take some hail dings would have allowed us to stay on the storm a little longer. Oh yeah and we should have taken a picture of that snapping turtle!!
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