Wednesday, March 30, 2016

Severe Weather Discussion for March 30, Posted March 30

A severe weather event is forecasted to unfold across the Plains and Mississippi Valley today, March 30th.

           Large upper level low currently stationed across the Western US will provoke a potential severe weather event in the Plains and lower Mississippi Valley today. Situation is complicated as convection later in the day will be heavily influenced by morning convection, residual cloud cover, as well as dryline placement later in the day.

               Stout upper level low centered across Nevada and Utah will begin to flatten as energy currently located in the Dakotas is ushered northeastward in pace with the jet stream, which will be arching north of mid-level ridging currently stationed in the lower Mississippi Valley. This energy in the Dakotas will be drawn towards another system in south-central Canada, resulting in a band of elevated vorticity extending from Canada back to Nebraska at 12z today. Further, the vorticity max currently in southern California, extending in a semicircle offshore the West Coast into Oregon, will keep the upper level low positively tilted and aid in this flattening-out.
    Moisture flow will continue to amplify across the Southern Plains, with water vapor imagery showing the subtropical jet feeding a moisture plume into the region. As such, cloud development occurred across the area earlier Tuesday and continues to persist at the time of posting. To this extent, continued moistening of the planetary boundary layer through isentropic ascent, notable 850-millibar moisture plume advection, and substantial low-level warm air advection will result in areas of drizzle to showers overnight, particularly across Texas and Oklahoma. 3/30 00z KOUN sounding showed strong temperature inversion between 800-millibar and 700-millibar levels, and I suspect this inversion will be strong enough to suppress anything more than broad, generally-weak precipitation. Surface plots at 0700z show areas of mist and light rain across central and eastern Texas, in line with favored model guidance. Model guidance troubles will be discussed more in depth in the following sections. Uncertainty with this particular subject of morning precipitation will feed into the morning and afternoon discussions below.

Early Morning to Noontime…
         Convection-allowing model guidance has been promoting the formation of showers and thunderstorms along the Red River, most notably in southeast Oklahoma in the mid-morning hours today, following expected overnight precipitation and continued moistening of the lower levels. Not completely convinced we see hail threats as advertised by some, including the threat of up to tennis ball-sized hail as per NWS OUN in their hazardous weather outlook posted yesterday. However, as the atmosphere continues to become more conducive to severe weather through continued warm air and moisture advection overnight, will not rule out severe weather tomorrow morning, especially given expected values of elevated instability that may favor stronger storms. I do believe hail is a primary threat with these storms, especially in their elevated nature, but again, I am not completely sold on the idea of severe and/or significant hail.
         Primary point of this timeframe discussion is to what extent precipitation is ongoing by and after 12z. Model guidance has been hinting at potential convection across portions of Oklahoma, especially south and east in the morning hours, but has also left the door open for weaker, more broad precipitation in central and northern Oklahoma. Regardless, expect overcast cloud cover given continued lower-level warm air and moisture advection, as well as taking into account 3/30 00z soundings across Texas and the western Gulf Coast. I am not taking the HRRR model into account for this discussion, as although it is currently handling surface precipitation (or lack thereof) rather well, it brings surface temperatures across Oklahoma and Texas into the 80s and 90s, well above current and more-realistic temperature forecasts introduced by the NWS. Additionally, am not using the NCAR ensemble, as all members have weak, broad, and scattered precipitation at initialization (3/30 00z) across Texas, which was proven to not be the case when compared with 3/30 00z radar. In terms of morning precipitation, am currently favoring 00z 4km WRF-NMM model, as well as SPC SREF system, as both models handle initialization conditions at the surface with regards to precipitation coverage well, and perform well with expected high temperatures later in the day. Given high model uncertainty with this event, however, making these two forecasts ‘favored’ is not exactly as affirmative a word as it is made out to be.
          In all, expectation is for precipitation, possibly convective in nature, to be ongoing in portions of Texas and Oklahoma by daybreak today and in the morning hours in general. Most favored region for this activity, as promoted by model guidance and SPC probabilistic thunderstorm outlooks, is southeast Oklahoma into northeast Texas, east into Arkansas. Weaker and more broad precipitation also possible for central Oklahoma, where uncertainty is greatest.

Afternoon and evening…
        Remarkably complex situation will unfold for the afternoon and evening hours. It has been noted that the upper level low in question has been progressing east more slowly than anticipated yesterday, and this has been feeding into a further-west trend with the dry line positioning later on in the afternoon today. Case in point, 4.0km WRF-NMM guidance shows convective initiation in the afternoon hours along the dry line, which is projected to be planted almost immediately east of the Oklahoma panhandle in due north-south orientation. This is the most westward solution I’ve seen, and while it has handled overnight precipitation well so far as of this posting, and the upper level low has been progressing slower than thought, the far westward positioning of this dry line is something to monitor. To speak more frankly, this solution is suspect, although this model still remains among favored guidance as of posting time. 3/29 21z SPC SREF takes a nice middle ground between this westward solution and further-east guidance, placing the dryline approximately along a north-south line of Lawton, Oklahoma to Burlington, Oklahoma by the afternoon hours. As such, thunderstorm initiation along the dryline would threaten the Oklahoma City metro area, and eventually may threaten the Tulsa region.
        Concerns have been brought up frequently in discussions around the lack of convective initiation on the dry line in some model guidance, despite the presence of an uncapped, supercell-favorable storm environment. This would be the result of a lack of trigger mechanism, it appears, as some guidance shows weak convergence along the dry line. I am personally opposed to this solution, and while it may very well pan out, I am more pressed to believe favored guidance, which does initiate convection along the dry line in the afternoon hours.
In the event convection does initiate along the dry line, it is expected that thunderstorms would quickly become supercellular, with all modes of severe weather possible. The new SPC Day 1 outlook has placed central and southern Oklahoma in the Enhanced Risk of severe weather, primarily for potentially significantly large hail, in addition to damaging winds and a non-zero, yet relatively low tornado threat. Although it still retains unrealistically-high surface temperatures tomorrow afternoon, new runs of the HRRR model continue to gradually increase the coverage and intensity of convective initiation along the dry line in central Oklahoma. Initiation here is along the Lawton-Burlington line previously mentioned, but I am not willing to put stock into the HRRR currently given its unrealistic surface temperatures.
       Outside of Oklahoma, best tornado dynamics outlined by SPC Day 1 projection and SPC SREF guidance will be placed in Louisiana/Arkansas/Texas area, where a 10% hatched delineation has been posted in the Day 1 outlook. Despite this, as mentioned previously, non-zero tornado threats will exist in Texas and Oklahoma in addition to these more favorable dynamics.

        To summarize, potential severe weather event is on the table today for the Southern Plains into the lower Mississippi Valley, with high uncertainty still included in the forecast. Broad, weak precipitation expected to develop and move into Texas and Oklahoma overnight into the morning hours will likely temper a full-on severe weather outbreak for the Plains, but afternoon clearing and favorable dry line positioning may allow the formation for strong to severe storms in the afternoon across central and eastern Oklahoma. Widespread convection is expected for the eastern Texas / northern Louisiana / Arkansas region, where tornado dynamics will be best. Large hail will be a day-long threat for the Oklahoma area, particularly with a round of thunderstorms in the morning that may form in the midst of the weak, broad precipitation to produce hail, and then again in the afternoon when potentially-supercellular development occurs along the dry line.