Slight Risk: 4/16/16

The Storm Prediction Center (SPC) has issued a Slight Risk today for parts of Texas, Oklahoma and extreme southwestern Kansas.

Associated with the Slight Risk is a sliver of 5% Tornado Threat Ring, stretching through western Oklahoma and through north and Central Kansas.

The SPC is basing this threat on several factors. First, an upper-level, closed-low near the Four Corners Region will continue east. As it does, a surface cold front that extends from Nebraska into New Mexico will be forced east as well. The dryline that diminished by late evening yesterday will reform through the Texas panhandle and begin pushing east as well, perhaps just ahead of the surface cold front. Storms will likely develop along the dryline. However, as the cold front catches up to the dryline, the storms will become more linear, with a few embedded rotating storms.

Synoptically speaking, the 300 mb NAM chart (from Unisys) shows the closed-low pressure system passing through New Mexico by this evening. The jet that wraps around the low will exit in the Slight Risk area.

The 500 mb NAM chart shows that while there is plenty of positive vorticity, the advection is a little weak. The winds are projected to blow mostly with the gradient instead of across it. The strongest advection on this chart will be in northeastern Colorado at 0Z Sunday.

The 700 mb NAM chart shows rapidly-rising air, across much of the High Plains this afternoon. In particular, northeastern Colorado and southwestern Nebraska. However, there will be rapidly-rising air throughout the threat area, minus the US-380 corridor.

The 850 mb NAM chart shows only weak Warm Air Advection (WAA) in the Slight Risk area, but strong WAA in northwestern Kansas and northeastern Colorado.

The 12Z upper air sounding from Amarillo, TX, shows some promise for severe weather today. There was 401 J/kg of Convective Available Potential Energy (CAPE) this morning, plenty of moisture at the boundary layer, and a thermal inversion near the surface and again at 600 mb. These inversions may erode throughout the day, or perhaps as this air mass moves east, will affect an area that does not have quite the same profile. The supercell parameter was 4.4, which is a good start to a severe weather day.

The deep-layer shear was 46 kts, and the low-level shear was 29 kts, with mostly directional shear at the low levels and speed shear aloft.

And, just because I am curious, the 12Z upper air sounding from Denver, CO, is not nearly as impressive. First, the surface temperature was only 33 F, and even farther east, and even with strong WAA, I don’t expect it to raise quickly enough to cause many storms to form in this region. Even though the sounding is quite humid, the actual moisture present in the atmosphere is not very high (0.43 inches of precipitable water), as the temperatures are so low. I wish there was a Dodge City or Goodland sounding to check this morning.

The shear is weaker and consists mostly of directional changes at all levels.

Currently, the surface observations (from the SPC Mesoscale Analysis Map) and radar overlay highlight the location of the dryline. First, notice how the dewpoints are in the upper 30’s in New Mexico, but by Amarillo, they have climbed into the mid-50’s. Second, the radar overlay shows a line of developing convection along this boundary as well. This part of the dryline is colinear with the cold front as well; notice the temperature drop and the wind shift.

The Supercell Parameter, according to the RAP, will increase to 4.0 for much of the Texas Panhandle over the next six hours.

The visible satellite imagery shows a very distinct boundary running through the Texas Panhandle this morning. However, much of the threat area is under cloud cover from yesterday’s storms.

The enhanced infrared satellite imagery shows that the clouds over Texas are not very thick, so perhaps they will mix out by this afternoon.

The water vapor satellite imagery shows a center of circulation (an upper-level, closed-low) over the Four Corners region. This circulation is bringing dry air to the mid-levels over the Slight Risk area, which will help to mix out those clouds.

Overall, I would target Childress, TX, this afternoon. The trick would be to watch where there was some separation between the dryline and the cold front. Anywhere they are colinear, the storms will be linear. Where they are separate, there is a chance for discrete supercells. This does not mean that there is no threat where the dryline and cold front are together; there will be embedded supercells along this line, with the favorable shear values. However, the discrete cells will be in places where the dryline is ahead of the cold front.

Thank you for reading my forecast.

The upper air soundings and mesoscale analysis plots are from the Storm Prediction Center website.
The forecasted upper air soundings are from TwisterData.com.
The surface observation and upper level charts are from Unisys Weather.
The satellite data is from NASA – MSFC

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About highplainschasing

This blog is about my tales in storm chasing. My name is Seth Price and I am an instrumentation instructor at New Mexico Tech. My amateur radio call sign is N3MRA.
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