Moderate Risk: 3/26/17

The Storm Prediction Center (SPC) has issued a Moderate Risk today for central and southern Oklahoma.

Associated with the Moderate Risk is a 10% tornado threat ring just south and east of Oklahoma City.

The SPC is basing this threat on the compact shortwave trough that is passing through Oklahoma today.  Combining this with the developing mid-latitude cyclone moving out of the Texas Panhandle and moisture moving in from the southeast, it is shaping up like a typical spring Great Plains severe weather event.  However, a similar storm system passed through the area just two days ago, so the biggest concern is whether the moisture return will be enough to spawn severe weather.  A layer of clouds will accompany the warm front, which in normal severe weather terms would hamper diurnal heating.  However, given the shallow moisture, the clouds may actually prevent mixing, keeping the boundary layer moist and the severe threat higher than would otherwise be expected with cloud cover.

Synoptically speaking, the 300 mb NAM chart (from Unisys) shows a shortwave trough pushing through northern Oklahoma.  This allows for a weak jetstreak to move through the threat area from southwest to northeast by 00 Z.

The 500 mb NAM chart shows some strong Positive Vorticity Advection (PVA) into the threat area, as vorticity moves out of the Texas Panhandle into western Oklahoma by 18 Z.

The 700 mb NAM chart shows some moderate Upward Vertical Velocities (UVV) by 18 Z.  The UVV are not as strong as I was expecting, given the PVA and the Warm Air Advection (WAA) shown on the 850 mb chart.

The 850 mb NAM chart shows the northern advancement of the warm front into southern and central Oklahoma by 18 Z. Warm Air Advection (WAA) is present throughout the threat area as winds blow across the thermal gradient from warm to cold.

The 12Z upper air sounding from Norman, OK, shows no Convective Available Potential Energy (CAPE) or Convective Inhibition (CINH) at this time.  It also shows a HUGE thermal inversion, and the 0-3 km average lapse rate is only 1.5 C/km.  However, the air mass over the target area is only part of the picture.

The hodograph from Norman, OK, shows that shear is adequate. The deep-layer shear was 44 kts, and the low-level shear was 15 kts. In terms of shear, this is enough to support rotating storms and tornadoes.

The 12Z upper air sounding from Fort Worth, TX, tells almost the same tale, however.  There is a little more moisture at 850 mb, but otherwise, there is no CAPE, no CINH, a large thermal inversion, and relatively dry air.  The 0-3 km average lapse rate is only 2.2 C/km at this location.

The deep-layer shear was 45 kts, and the low-level shear was 29 kts. Once again, this shear is strong enough to support rotating storms and tornadoes.

Similar soundings are found in Shreveport, LA, and Little Rock, AR. I don’t see where there is any moisture return, thus far in my analysis.

TwisterData.com shows the moisture will push up through Texas as strong winds move moisture from the Gulf of Mexico.  There is a weak dryline across western Oklahoma, and the moisture gradient doesn’t get much tighter today than what is shown here for 19 Z.

TwisterData.com also shows the CAPE barely reaching 2000 J/kg in northern Texas by 21 Z.

Currently, the surface observations (from the SPC Mesoscale Analysis Map) show strong winds from the south and southeast.  Also, the dewpoints have risen to the low 60’s in eastern Texas, and into the upper 50’s throughout southeastern Oklahoma.  This looks a little more promising than I have seen in today’s forecast so far.

The Supercell Parameter is expected to increase to 16 over the next six hours, between Dallas, TX, and very southern Oklahoma, along the I-35 corridor.

The visible satellite imagery shows some convective-looking clouds forming to the west of the I-35 corridor in south-central Oklahoma.

The enhanced infrared satellite imagery shows that much of the cloud cover over the state is of moderate thickness.

The water vapor satellite imagery shows the mid-latitude cyclone rotating in the eastern Texas Panhandle at this time.  Moisture at the 700 mb to 400 mb level is not very deep in eastern Texas, but this system will be based entirely on surface moisture.

Overall, I expect there will be storms today.  I think this is an interesting scenario; so far, I have not been impressed with the moisture return, but with the cloud layer, perhaps the moisture will be funneled along the boundary layer, rather than mixing through the entire column.  There is some evidence of this, based on the surface dewpoints across Texas and southern Oklahoma, which continue to climb.

I don’t expect today to be a very chase-able scenario, however.  The potential for large hail is high, and I expect storms to quickly morph into bowing segments as the cold front approaches.  They may initially start out as semi-discrete cells, so the chasers will have to hit early and not get caught in the linear or bowing segments.

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 College of Dupage – SATRAD

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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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