The Storm Prediction Center has outlined a severe weather threat on Saturday for a large swath of the Central and Southern Plains, including cities such as Dallas/Fort Worth and Oklahoma City, among other metropolitan centers.
The SPC outlook on the left denotes the risk of severe weather in one of two categories. The first category is a 15% chance of severe weather within 25 miles of any given point on the map. This risk is equivalent to a Slight risk in shorter-term outlooks. The second category, which would be shown in red, denotes a 30% chance for severe weather, and is usually issued in the long-term outlooks when forecasters feel there is a good threat for a severe weather event. As you might expect, the 30% risk area is not issued all that often.
The mid-level prognosis on Saturday includes a negatively-tilted shortwave pushing into the Central Plains, amidst a large trough encompassing the Rockies. This set-up is great for those who have been waiting for summer-like weather in the Central and East US, as we see how far north the contour lines are forced with the ridge in the eastern half of the country. With this set-up, however, comes the severe weather threat for the Plains. The Storm Prediction Center has indicated that all modes of severe weather are likely for the area outlined in the top image, including damaging winds, large hail, and even the threat of tornadoes. Looking over model guidance right now, it looks as if the tornado threat is rather low. Despite this, and perhaps it's just me being more optimistic with severe weather forecasts lately, I wouldn't be surprised if we saw more than a handful of tornado reports on the Storm Prediction Center database when this is all set and done. An outbreak isn't expected, but the threat is there; it's just the severity that's in question.
The upper air pattern is rather indicative of where the severe weather may be most probable. In the jet stream forecast on the right, we see the subtropical jet stream in Baja California and Mexico, as well as a jet streak rounding the base of the trough from eastern California to New Mexico. These two ribbons of wind combine somewhere along the US/Mexico border near west Texas, and make a move into the Southern Plains. This sequence is interrupted by an area of divergence poking through in Kansas, where the two streams diverge again. This diverging of jet streams generally puts the area between the streams under the gun for severe weather, since rising air is disrupting the jet stream and forcing it to split.
The instability forecast, namely the Lifted Index outlook in the contour lines, shows a very ripe environment for thunderstorm development across Kansas, Oklahoma, and Texas, where Lifted Index values could dip below the -8 mark.
When the forecasted Lifted Index drops below zero, the atmosphere is considered to be unstable. When it drops below -4, the atmosphere is considered to be very unstable. Numbers below -8 can indicate significant instability, while any positive numbers indicate a stable environment, and a low thunderstorm threat.
Summary
- Severe weather is possible on both Friday and Saturday, May 8th and 9th.
- The Central and Southern Plains are at risk of all modes of severe weather on both days.
Andrew
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