June 15 TWISTEX Update

Status: Preparing for Deployment Tommorrow
Location: Terril, IA

11:30 am Update: Official word is out that TWISTEX will be deploying tomorrow and Sunday over areas of the Dakotas. Marginal day expected on Saturday, however with the threat being there on Sunday in the Dakotas we will string it into a 2-day trip. 12z NAM continues to show promise in central SD tomorrow along a stationary warm front, sufficient instability and shear should allow thunderstorms that do develop to become severe and possibly tornadic. Target will likely be somewhere near the dead-center of South Dakota.

On Sunday the target is still up in the air, as we could potentially have two surface low pressure systems along the cold front that is likely taking over the Dakotas. Capping may be a concern south of the SD/NE line and even some parts of southern SD may be under to stout of a cap. Currently the SPC has picked up on areas of ND/MN as the best potential. We'll have to just leave the target for this day open as too many options exist at the moment.

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Forecasts are becoming increasingly favorable for TWISTEX deployment this weekend, with latest thoughts including Saturday as a chase day for the Dakotas. Tonights' NAM run shows good shear values, with westerly winds aloft from 200mb through 500mb with slight speed shear. Winds then turn to the southwest at 700mb, and then southeasterly at 850 with even some straight easterly winds in the western Dakotas. This combined with southeasterly/southerly winds at the surface finish off good low level shear values that seem conducive for tornadic storms. Moisture and instability should easily be sufficient for thunderstorms as well, the current negative seen in forecasts could be a good focus point for development. Models seem to throw to MCS's into the picture, one over central SD/ND and another over southwestern Minnesota & adjacent areas. Nonetheless, feel that with this type of wind fields and combination of other severe parameters it could very easily warrant a chase opportunity.

The threat on Saturday comes before the trough works into the plains even, with Sunday bringing in increased wind speeds at the upper levels. A good 20-30 kts increase at 250mb and 500mb levels, with only slight changes in direction to a southwesterly component. 700mb level does show some concern, first with wind speeds only favoring a speed shear component and not much directional; secondly the temperatures are shown to be 12-15 C throughout South Dakota. This would be a good cap over the area, however the last event in western SD with temperatures well above normal capping strength didn't seem to have a problem. This system is different, but still hard to look down on the event just because of that at the moment. Instability is much greater on Sunday, with moisture and lifted indicies higher as well. Shear is in question though, with winds at 850mb and surface coming from the southwest/south. Despite some flaws, the day on Sunday still has potential for tornadic storms in the Dakotas and possibly Minnesota. The system is likely to be shown faster in the models, thus expect the eastern part of the Dakotas if not central Dakotas to be the best target on Sunday. Sunday does seem like a chase day if the team is already out from a Saturday chase in the region as well.

Monday the cold front begins its' quick move across the plains, thus with the lack of shear and the simple movement of just a cold front across the plains it is unlikely that a chase will ensue on this day as well. However, this day is still quite a ways out and forecasts due change. This day will remain questionable for deployment in my mind...