Saturday, September 20, 2008

High Plains Severe Threat

The SPC has placed a slight risk of severe weather over the High Plains. The risk tomorrow doesn't look as robust as on Monday, but there is still a risk of thunderstorms able to produce strong updrafts with hail and isolated winds. A strengthening low-level jet in the event will allow the storms to transition into a greater wind threat, but the upper-level energy will have pulled to the north by this time, likely reducing the chance of damaging winds.

SPC Severe Weather Outlook for Sunday
The NAM 500 mb chart tomorrow at 18Z shows one large upper-level trough in the west with two very apparent short waves. The first will be moving through the High Plains by 18Z (or near noon local time). This will be the initiator of the storms during the mid-day time frame, or early afternoon. Decent difluence will be present, along with a good vearing wind profile which will support rotating storms. However, with this shortwave moving through this area by 18Z, the energy will move off to the north before the greatest heating, thus keeping the severe threat low. The greatest threat should be in western South Dakota, southwestern North Dakota, and western Nebraska.

NAM 500 mb 18Z Sunday

Monday's shortwave looks much more potent and moves through the Dakotas during the 0Z-6Z time frame (6pm to midnight). This is much more ideal for severe storm development and I believe the severe threat will be greater on Monday slightly further east than on Sunday from north central Nebraska into the central Dakotas and then east into the overnight hours. The greatest threat will be hail and strong winds, but with a forecast shear profiles suggesting strong rotation, supercells will be likely along with isolated tornadoes. However, I wouldn't expect a major tornado outbreak as dewpoints will not be very high enough (low to mid 50s).

SPC Severe Weather Outlook for Monday

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