Sunday, February 28, 2016

March 1, 2016 Severe Weather Discussion for Oklahoma

This is a severe weather discussion valid for the morning of March 1, 2016 for the state of Oklahoma.

Models used: 00z NAM, 00z NAM-4km, 00z GFS, 21z SREF

Energy that will trigger this event is currently pushing into the Pacific Northwest / British Columbia region, moving directly into a small ridge positioned immediately due east of the system. Expectation is for the main body of energy to ride down on the lee side of a ridge that will build in across the Pacific Northwest as the energy continues moving eastward. Immediately south of this main piece of energy, at approximately 18z 2/29, emergence of higher mid-level vorticity values will commence across the Southwest, immediately ejecting southeastward into the Southern Plains. It will be this new piece of energy that will help to trigger the potential severe weather event.

Concentrated lower-level warm air advection will commence in the morning hours on Monday, with 850-millibar winds out of the west by 12z. Winds shift to southwesterly in the late morning, marking the warm air advection as high pressure shifts to the east, away from the Plains. Warm air advection aloft intensifies by 21z, and continues to build in magnitude through the late evening on Monday, up until the early morning hours on Tuesday when the potential severe weather event begins to unfold. Overall wind field for this event is messy. 500-millibar jet streak will round the base of the main trough as the event unfolds, reaching speeds AOA 80 knots in Colorado, as per latest model guidance.

Despite muddying of synoptic environment from previous discussion to this one, overall severe weather set-up has not deteriorated, and has actually strengthened in a handful of regards. Thunderstorms in this event will need to be elevated, due to strong temperature inversion consistent on forecasted soundings around the 900-millibar level. Surface CINH depicted at approximately -265 j/kg on latest NAM guidance, certainly conducive to a stable environment. However, 850-millibar to 500-millibar lapse rates of 7.8 degrees Celsius/km show a much more favorable environment above the PBL. As such, MLCAPE of just below 1400 j/kg with 87 j/kg MLCINH displayed at 06z Tuesday over KOUN. Wind profile shows substantial veering, perhaps with slight backing near 500-millibars, but overall much improved from previous model runs a few days ago. Surface to 3km storm-relative helicity values over KOUN progged at 435 m2/s2 from latest guidance, in addition to 0-6km shear around 50kts exemplify this improved wind field from previous model runs. LCLs outlooked at around 700 meters, certainly supportive of tornado formation, though perhaps slightly high in my opinion for any real tornado potential. SARS product over KOUN at 06z from 00z NAM depicts 65% chance of tornado, utilizing favorable MLCAPE and improved wind fields. Still skeptical of PBL temperature inversion, as well as depicted isolated storm coverage on 00z NAM-4km.

Most concerning point about this event, in addition to PBL temperature inversion, is projected coverage and evolution of convection. 00z NAM-4km projects evolution of thundershowers in extreme northern Texas, intensifying into either a group of strong cells or a single severe thunderstorm by 04z Tuesday, crossing into Oklahoma by 05z. Complex appears to strengthen and begin to bow out by 07z Tuesday, and begins to impact OKC/OUN at 08z, albeit in a slightly degraded severity. Complex continues to bow out and degrade through 12z, turning to the right and bowing out in the process as it moves into southeast Oklahoma. Scattered strong thunderstorm cells begin to fire at approximately 10z in eastern Oklahoma out ahead of the weakened segment of thunderstorms, and these could also pose a risk for marginal severe weather, in line with the latest Storm Prediction Center outlook (Slight Risk across most of central Oklahoma). In terms of probabilities, latest SREF suite places a 50% to 70% likelihood of thunderstorms occurring in an area with a supercell composite at or above 1. SREF members notably less enthusiastic with MLCAPE values over Oklahoma for this potential event, on the order of AOA 500 j/kg from about KOKC south. This uncertainty calls into question the bullish appearance of the 00z NAM, but will wait for the 03z SREF suite to see if uncertainty holds or is reduced using new 00z data. Reduction in uncertainty is certainly a possibility, as SREF suite will likely be influenced by more aggressive 00z NAM solution.

Overall, situation is a difficult one to assess. Not entirely convinced mixed layer will be conducive to severe weather event, especially given unfavorable temperature inversion around 900-millibars. In addition, very scant coverage by 00z NAM-4km only adds to concerns about any convection actually forming to begin with. Do believe that if convection is able to form, strong thunderstorms will become a good probability given favorable wind fields and potentially-favorable mixed layer environment. Severe thunderstorm formation also possible in the event convection can form and be sustained. Tornado risk somewhat low but not entirely zero in my opinion, given slightly high LCLs despite favorable wind profile, including veering winds, rather high 0-3km SRH values, and good surface to 6km shear values. Best threats will include hail and strong winds, with perhaps a low threat for a tornado if mixed layer environment can be fully realized.