Using Mesoanalysis to Message a Decreasing Severe Weather Threat in Nebraska: 6/21/2020 - Operations Proving Ground

We often focus on how mesoanalysis allows for the enhancement of messaging in scenarios in which the severe weather threat is locally increasing. But mesoanalysis can certainly prove useful in providing clues that a particular severe threat may underperform original expectations as well. Such was the case on the afternoon of June 21, 2020 across central and eastern Nebraska (NE).

Early in the day it seemed as though the potential for severe convection over the eastern half of NE was on the increase. The 13Z Day 1 Convective Outlook from the SPC indicated an Enhanced Risk across eastern NE premised on a 30% probability of severe hail (Figure 1). This was a categorical upgrade from the previously issued 06Z Day 1 Convective Outlook which outlined a Slight Risk. The 1630Z convective outlook contained further adjustments that reflected an increasing threat for severe wind in portions of NE. Specifically, the 30% wind probability was extended into northeast NE (Figure 1, bottom right) after collaboration with the Omaha WFO. This extension was based on the 12Z HREF updraft helicity forecast (Figure 2) and RAP point forecast soundings (Figure 3). Although concern was given to ongoing elevated thunderstorms extending from eastern NE to north-central Kansas, consensus of CAM data as of mid-morning suggested that the activity would dissipate by midday and the environment along the front over northeast NE would become quite conducive for organized storm modes by mid-afternoon.

By early afternoon, surface mesoanalysis revealed a growing corridor of drying in portions of eastern NE; dew point values dropped into the upper 50s and low 60s (Figure 4). However, despite the drying boundary-layer, an 18Z RAP forecast sounding valid at 21Z for Norfolk, NE still indicated a moderately unstable and weakly capped air mass coincident with a wind profile supportive of organized storm modes (Figure 5). Shortly after 18Z, a couple of storms formed ahead of the surface front in northeast NE in a favorable thermodynamic environment. Thus, the following was messaged in NWSChat by NWS Omaha (18:30Z):

“Recent storm initiation over northeast NE may have occurred along an outflow surge associated with the early-day storms. Objective fields suggest the air mass has become nearly uncapped in that area with MLCAPE estimates of 2000-2500 J/KG. The wind profile is sufficient for supercells capable of very large hail. The tornado threat is non-zero.”

Between 20 and 21Z, data from a special 19Z sounding launched at NWS Omaha (Figure 6, left) revealed a mean mixing ratio (lowest 100 mb) of only 10.6 g/kg despite a surface dew point of 69. That, coupled with a warm nose associated with a capping inversion at the base of an EML, and poor lapse rates (i.e., < 7 C/km) in the 3-6-km layer, resulted in MLCAPE of ~500 J/kg with MLCIN of ~-200 J/kg. In contrast, the 1-hr forecast from the 18Z RAP at KOMA (Figure 6, right) indicated a mean mixing ratio of 12.7 g/kg, a much cooler EML base (centered around 800 mb), and a similar lapse rate profile which resulted in MLCAPE of ~2100 J/kg with little to no MLCIN.

While the large discrepancy between the observations and model forecasts was noteworthy, focus soon turned to isolated storms that developed along the front east of O’Neill, NE around 2130Z. Amid pressure falls, surface winds were backing and strengthening ahead of the front from south-central NE, northeast toward Norfolk, NE (Figure 7), which gave continued reason for the concern of severe hazards in this area.

However, as of 22Z thunderstorm development continued to struggle, and there was a notable lack of any cumulus growth in the open warm sector (Figure 8); both indications that something was awry. Meanwhile further south, radar and satellite trends showed a considerable increase in convective activity in northwest Kansas and along Interstate 70 in central Kansas (Figure 8). As a result, an expansive anvil-level cloud shield began to spread north into south-central Nebraska (Figure 8), which was yet another sign that potential instability and convective development would be limited. Messages in NWSchat from both WFO Omaha and WFO Hastings began to convey to partners the uncertainty.

“As of 5 PM, convection continues to struggle to initiate and/or intensify along the surface boundary from Knox County toward Ord, NE. Visible satellite indicates agitated cumulus along that zone, and it still seems quite likely an intense storm or two will evolve within that regime. Interestingly, the absence of any boundary-layer-based cumulus within the open warm sector suggests that weak subsidence/capping may still be present. Or, that the source region for some of the warm-sector air parcel trajectories is convectively overturned air from KS storms.” - NWS Omaha, 22:17Z

“Cirrus cloud shield seeping north into southern Nebraska is acting to stabilize the atmosphere and limit northward development. Also, clouds/convection earlier today worked over a bit of the area. Attention is up along the convergence near the front north of Kearney. Not discounting anything just yet but short term trends for rapid, widespread development in very near term is limited. Storms working across north central Kansas still strong at times and the Watch will remain for the time being.” - NWS Hastings, 22:30Z (grammatically corrected)

Figure 8: GOES-16 Day-Cloud Phase RGB valid 6/21/20 from 14:01Z to 23:56Z.

Scattered thunderstorms did eventually initiate along the surface trough moving into south-central Nebraska. This was expected as the air mass near the boundary was more unstable due to greater solar insolation. However, by this time widespread expansion appeared unlikely as the surface trough was moving into the more stable air mass in the wake of convection just a few hours before (Figure 9). Furthermore, short-term CAM guidance suggested expansion of storm development along the trough was unlikely. Given this understanding, this sentiment was expressed through a tweet from NWS Hastings around 23Z (Figure 10). A couple of these storms did manage a severe report or two, however, as alluded to in the tweet, the storms weakened before making much progress to the east. Shortly before 01Z the Severe Thunderstorm Watch cancelations began for portions of southeast NE. Other portions of the area near the front would follow.

Storms are beginning to develop along the front in Nebraska, right now it looks kinda shaky as to whether they will hold together or not...the large cirrus shield from the storms in KS is extending far north into NE and limiting the potential of these storms. #NEwx #KSwx pic.twitter.com/P3PJb7Pc8F

— NWS Hastings (@NWSHastings) June 21, 2020

Figure 10: Tweet sent by NWS Hastings at 23:02 6/21/20.

In the end, lingering morning thunderstorms likely played a key role in starting a chain reaction which resulted in very little severe convection in eastern Nebraska. The remnant air mass over the area was more stable than previously anticipated due to the post-convective regime and cloud cover. This was evident in a comparison of a RAP forecast thermodynamic profile to the 19Z observed Omaha sounding. A lack of a warm-sector cumulus field then further signaled that the environment was less than ideal for robust convection. And by late afternoon, cirrus from thunderstorm anvils to the south overspread south-central and eastern Nebraska which significantly limited any chance of destabilization. Mesoanalysis was essential in revealing these clues which allowed meteorologists at both the Hastings and Omaha WFOs to convey the increasing uncertainty and the potential for a less significant event to partners and the public.

Authored by Corey Mead (WFO Omaha), Mike Moritz (WFO Hastings), and Ryan Difani (OU CIMMS/OPG). Special thanks to Brian Barjenbruch (WFO Omaha), Kim Runk (OSTI), and the OPG team for their contributions, reviews, and support.