Late Spring HSLC Tornadoes across the Carolinas and Virginia: 4-5 May 2017

In association with a high amplitude closed upper low over the Mississsippi Valley, and a retreating wedge front at the surface, a mainly late night outbreak of wind damage and tornadoes occurred in a classic high shear, low CAPE (HSLC) environment, beginning late evening on May 4 and lasting beyond 8am on May 5.  There were a few tornado reports earlier in the day farther south that may not have occurred technically in low enough shear to meet the original CSTAR-defined thresholds for SBCAPE and MLCAPE, but most if not all in northern SC, and all of NC and VA did. This review will just focus on a couple of these tornadoes that were rated EF1 and not on the EF0s or the numerous reports determined to be straight line winds.

Synoptic U/A maps:

 

Fig1

Fig 1. 500 hPa Hgt/Temp  0000 UTC 05 May 2017.

Fig2

Fig 2. 850 hPa Hgt/Temp/DewPt  0000 05 May 2017

 

Surface frontal and large scale radar evolution:

Fig3

Fig 3. WPC surface analysis and radar mosaic, 0000 UTC 05 May 2017.

 

Fig4

Fig 4. Same as Fig 3 but for 0900 UTC.

SPC summary of national severe reports:

Fig5a Fig5b

Fig. 5.  SPC reports from 1200 UTC 04 May 2017 – 1200 05 May 2017 (left); and same for 05 May – 06 May (right).

 

Tornadoes that occurred between 8pm (May 4) and 8am (May 5) almost all appeared to occur in HSLC environments based on the Sherbun and Parker (2014) definition (SBCAPE of 500 J/kg or less, MUCAPE of 1000 J/kg or less, and 0-6km bulk wind difference of 18 ms-1 or more), when eyeballing the SPC mesoanalysis regional images.  Maps of the tornadoes (with all other reports removed) between 8pm – 8am are shown below.

Fig6

Fig. 6. All tornado reports between 0000 UTC (8pm) 04 May 2017 and 1200 UTC (8am) 05 May 2017.

 

Fig7

Fig. 7. Same as Fig 6 but zoomed in on northern NC and southeastern VA, and with the EF1 tornadoes highlighted along with time of touchdown and approx. path length labelled.

 

Following are some SPC mesoanalysis fields and radar images associated with the Rockingham Co NC EF1 at around 3am (the far SW tornado in Fig. 7 above):

Fig8

Fig. 8. MLCAPE at 0700 UTC 05 May 2017. Approximate location of Rockingham Co NC EF1 indicated by purple star.

 

Fig9

Fig. 9. Same as above but with 0-6km Bulk Shear Vector and magnitude.

 

Fig10

Fig. 10. Same as above but for 0-1km Storm Relative Helicity (SRH).

 

Fig11

Fig. 11. Same as above but for Sig Tor Parameter (STP).

 

Fig12

Fig. 12. Same as above but for SHERBE parameter.

 

Fig13

Fig. 13. Same as above but for Modified SHERBE (MOSHE).

 

Summarizing the above for the Rockingham Co NC EF1, the STP and SHERBE, while both showing underwhelming values in the location where the tornado occurred, at least indicated this was near the nose of a ridge for these parameters, but the MOSHE more clearly showed a maximum with values of at least 2.5.

Radar images for Rockingham Co NC EF1:

 

Fig14

Fig. 14. KFCX Z/SRM images at 0703 UTC (about 10 min before tornado touchdown) near Eden. Top images are the 0.5 deg slice, bottom ones are the 1.3 deg slice. Storm is about 45 nm from radar, and radar beam at 0.5 is about 6,000ft AGL. Radar is to the northwest.

 

Fig15

Fig. 15. Same as above but for 0711 UTC (about the time of touchdown near Eden).

 

Following are similar mesoanalysis fields as shown above but for 6am (was not able to capture 7am for most of these) and with the locations of the two Dinwiddie Co VA EF1s (a little south of Richmond) and the Orange Co VA EF1 (northeast of Charlottesville).

 

Fig16

Fig 16. MLCAPE at 1000 UTC (6am) May 05 2017. Approximate location of the two Dinwiddie Co EF1s (southern-most purple star) and of the Orange Co VA EF1 (northern-most purple star).

 

Fig17

Fig. 17. Same as above but for 0-1km SRH.

 

Fig18

Fig. 18. Same as above but for STP.

 

Fig19

Fig. 19. Same as above but for SHERBE.

 

Fig20a

Fig 20a. Same as above but for MOSHE.

Fig20b

Fig 20b. Same as above but for 1100 UTC (7am). [For some reason this was available at 7am but none the other fields were.]

 

Radar from Dinwiddie Co VA storm that produced two EF1 touchdowns, these are associated with initial touchdown. Radar configuration suggests more of a bow echo with circulation near comma head of bow, which was a common storm mode/configuration for several of the other tornadoes early this morning.

 

Fig21

Fig. 21. KAKQ radar at 1045 UTC, about 6 min before EF1 showing circulation coincident with comma-head region of bow echo. This location is just under 40nm from radar. Upper left panel is 0.5 deg, upper right 0.5 SRM, lower right is 0.5 NROT, and lower left is 1.3 Z.  

 

Fig22

Fig. 22. Same as above but at 1051 UTC (right at time of tornado touchdown), and lower left image is now CC showing subtle tornado debris signature (just to the left of the “M” in McKenney).

Note that a second tornado (also EF1) touched down in northern Dinwiddie Co. less than 20 min later with the same storm, and this was just after the bow echo signature went through a “Broken-S” evolution, the circulation briefly tightened again, and another TDS was observed as well (not shown).

 

Conclusions:

This is just a sampling of a few of the tornadoes associated with a classic High Shear Low CAPE (HSLC) environment, that were part of a nighttime outbreak of tornadoes and widespread severe weather on 4-5 May, 2017, and in association with a deep amplitude nearly vertically stacked upper-level trough and  retreating wedge front at the surface. Most of the tornadoes (if not all) that occurred between 8pm May 4 and 8am May 5 appeared to form in environments that easily fit the HSLC criteria from Sherbun and Parker (2014). A closer near-storm environmental analysis using surface observations and modified RAP soundings may be needed for each of these tornadoes in order to confirm this.

A closer look at the EF1 tornadoes in NC and VA showed that while they occurred on the northern edge of ridges in the analyses of traditional composite parameters (such as STP, and even the SHERBE), the Modified SHERBE (or MOSHE) seemed to show a better signal at the location of these tornadoes with values well above 1.0 in most cases. The caveat here is that the time shown for most of the analyses (1000 UTC) is about an hour before the tornadoes that are overlaid with them, but yet the 1100 UTC MOSHE analysis (which was available) fits pretty well with the tornadoes that occurred around that time.

Radar analysis of the storms associated with these tornadoes showed very shallow reflectivity signatures (and in fact, lightning activity was generally non-existent with them). In fact, in most cases there were only subtle signatures suggestive of a tornadic threat , such as small-scale bow echoes and in one or two cases evolving through “Broken-S” signature. Velocity fields did show weak to moderate circulations with many storms before the tornadoes touched down, but including some storms that did not produce known tornadoes.  Many of the tornadic storms were within 30-40nm of the nearest radar, but even so these signatures were often subtle, especially in terms of reflectivity structures.

 

 

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Modified SHERB forecast plots now available

Forecast graphics for the modified SHERB (MOSH) and SHERBE (MOSHE) parameters are now available at the following links for the RAP, NAM, and GFS:

http://storms.meas.ncsu.edu/users/mdparker/rap/hslc.html
http://storms.meas.ncsu.edu/users/mdparker/nam/hslc.html
http://storms.meas.ncsu.edu/users/mdparker/gfs/hslc.html

shrbtiles.01.gif

Example 1-hour forecast RAP 4-panel of (top left) SHERBS3, (top right) SHERBE, (bottom left) MOSH, and (bottom right) MOSHE, valid 13Z 25 August 2017.

We have noticed that the calculated values show relative consistency between models, and the spatial footprints of enhanced values tend to be similar to those of the SPC Mesoanalysis. However, the values on the SPC Mesoanalysis are generally higher than those that we have calculated, again suggesting that there may be some discrepancies in the way SPC is calculating the parameters. I plan to touch base with SPC soon to ask about their progress on calculating individual terms of the MOSH/E parameters in order to determine where the differences arise.

Please feel free to utilize these plots as we transition into the HSLC season and share any insights you have!

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Collaborative Effort to Account for the Impact of the August 21st Solar Eclipse on Operational Forecasts in the Mid Atlantic and Southeast

Map of the percentage of sun obscuration during the eclipse shown in AWIPS.

Map of the percentage of sun obscuration during the eclipse shown in AWIPS.

Meteorologists recognize that solar eclipses in the past have had a notable impact on the sensible weather in the regions in which they occur. These impacts can include a decrease in surface temperature, reduction and changes to surface winds, lowering of surface pressure, changes in cloud cover and more. National Weather Service (NWS) meteorologist in the Southeast and mid-Atlantic have been working collaboratively to account for some of these impacts on official NWS forecasts during the eclipse on Monday, August 21, 2017.

eclipse temp training

Image highlighting the some details of the Eclipse Temperature smartTool and Procedure for GFE.

Surprisingly, most operational numerical weather prediction (NWP) systems do not account for the changes in incoming solar radiation from the sun during an eclipse and the resultant changes in the weather. This is an issue as NWS forecasters provide forecasts of temperature, winds, and other fields at hourly time steps and the eclipse impacts need to be captured by forecaster intervention over model guidance. The effort to provide details on the eclipse impact on weather in our region began with the development of hourly temperature reductions based on past eclipse events and factoring local climatology proposed by Frank Alsheimer at WFO CAE. Additional WFOs in the CIMMSE area collaborated and provided input on reductions while working within the temporal confines of the National Digital Forecast Database. Joshua Weiss at WFO ILM, examined data from the 1970 and 1984 eclipses in the Southeast and created a GFE smartTool and Procedure to provide a more scientifically sound and consistent process to edit the hourly temperature forecasts.

meteograms

Official hourly forecasts of temperature and sky cover from three locations in adjacent WFOs CAE, ILM, and RAH. The temperatures were adjusted using the Eclipse Temperature GFE procedure. Columbia, SC is in the eclipse totality while Lumberton, NC and Wadesboro, NC reach 97% obscuration. Differing temperature reductions during and after the eclipse are influenced by various factors including differing amounts of cloud cover.

The GFE Procedure incorporates the forecast temperature and diurnal range without the eclipse impact, whether a location is in the total or partial eclipse, and the amount of cloud cover. Nearly a half dozen WFOs in the Southeast will be using this tool which should lead to a more consistent, more scientifically sound, and accurate forecast. In addition, shapefiles of eclipse information were developed for use in some applications, GIS maps detailing eclipse obscuration percentages were installed in AWIPS, and finally methodologies and strategies were shared via a Google document. This effort builds on the history and relationships built across the CIMMSE domain. The event is a great example of many NWS meteorologists and WFOs working together to provide enhanced forecasts and service.

hrrx.example

Screen shot of the High-Resolution Rapid Refresh (HRRRx) experimental real-time weather forecast web page the eclipse.

It is also worth noting that some experimental and non-operational NWP systems will account for the eclipse. NOAA/ESRL/GSD made changes to the 3km experimental HRRR (HRRRx) to include code to account for the sun-obscuration from the eclipse (details… https://esrl.noaa.gov/gsd/learn/hotitems/2017/eclipse2017-hrrr.html). For real-time HRRRx experimental forecasts, including the effects of the eclipse starting Saturday night looking ahead 48 hours with the 00 UTC model run, visit https://eclipse2017.noaa.gov. Some of the selected weather fields available include downward solar radiation, cloud fields and 2-meter temperature, for the HRRRx (with eclipse effect), the operational HRRR-NCEP (without eclipse effects and some other differences) and HRRRx – HRRR-NCEP difference fields.

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A pair of HSLC-related presentations: NOAA VLab Forum and AMS Mesoscale Conference

Over the last few months, I have provided two presentations on recent HSLC-related research. In June, I was the presenter at NOAA’s VLab Forum, where I provided an overview of the ongoing HSLC CSTAR project based at NC State University. A recording of this presentation can be found here.

The next month, I traveled to the AMS Conference on Mesoscale Processes in San Diego, where I presented an update on my ongoing idealized modeling work. Of particular interest, I have identified the chain of processes appearing to result in the development of strong, low-level vortices within simulated HSLC QLCSs and determined how low-level shear vector magnitude and low-level lapse rates could affect these processes.


Simulation-based research continues, and I intend to complete and defend my dissertation later this fall. Please let me know if you have any questions or comments!

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Operational NWP Resolution and Sensitivities Study Using HSLC Event Hindcasts Summary

At last month’s CSTAR workshop, I shared the results and analysis of my thesis project as well as some thoughts on related future work. I would like to provide a summary of my and Dr. Lackmann’s work with this post for those who could not be there and as a refresher for those who were.

The problem I focused on is that severe convection that forms in  High Shear, Low CAPE (HSLC) environments is difficult to predict and is dangerous. There are two approaches to improve HSLC severe convection predictability: the first is the use of environmental predictors such as the SHERB (Sherburn and Parker 2014) and the MOSH (Sherburn et al. 2016) and the second is using Numerical Weather Prediction (NWP) models to predict convection which is what myself and Jessica King did (King et al. 2017). This research project sought to answer two research questions:

1. At what horizontal grid spacing, if any, does an NWP model provide operationally useful information about explicit low instability severe convection?
2. Can a NWP model properly differentiate between a HSLC event case and a HSLC null event case given the proper initial conditions?

Dr. Lackmann and I  looked at two cases, a event case February 24-25, 2011, a null case, December 25-26, 2009. I ran these cases using the WRF model at convection-permitting grid spacings (Figure 1) and then evaluated how well each domain represented the severe weather using severe proxy metrics: half-hourly maximum updraft helicity (1-4km), half-hourly maximum 10-meter wind speed, half-hourly maximum updraft speed, and composite radar reflectivity (Figure 2). In order to compare the different domains on an “equal playing field” so to speak, we conformed the 1.2-km and 400-m domains to a grid with a 3.6-km grid spacing over the area encompassed by the 400-m domain. The severe proxy metrics were calculated on the higher resolution domain first before this interpolation. This process is effectively sub-sampling. The distributions of the severe proxy values of these interpolated domains were compared to one another both in the same case and between the two cases (Figure 3).

The results of this analysis are as follows:
– Based on this study, the recommended horizontal grid spacing to run a NWP model in a way that provides operationally useful information about low instability convection is 3.6-km.
– A larger gain in detail is observed between the 3.6-km and 1.2-km domains than between the 1.2-km and 400-m domains.
– Overall, the event case and null event case are statistically significantly different. It is important to note that there is a difference between differentiating and distinguishing; the model had difficulty distinguishing between the two cases.
– Thresholds of severe proxy parameters, especially updraft helicity, should be adjusted to handle finer model resolution.

I look forward to receiving your questions and comments as well as participating in any discussion that follows. Please feel free to ask me questions in the comments section or to send an email to lrblank@ncsu.edu. Thank you!

modelconfig_02242011
Figure 1: Model domain set up for the event case February 24-25, 2011

02252011_03utc_refc_comp
Figure 2: NEXRAD and conformed model domain composite radar reflectivity for the event case. Time shown is 03:00 UTC on 02/25/2011.

eventvnull_uh
Figure 3: The severe proxy conformed half-hourly maximum updraft helicity distributions for the event case and null event case at each convective permitting domain.

References:

King, J.R., M.D. Parker, K.D. Sherburn, and G.M. Lackmann 2017: Rapid Evolution of Cool Season, Low CAPE Severe Thunderstorm Environments. Wea. Forecasting. doi: 10.1175/WAF-D-16-0141.1

Sherburn, K.D., and M.D. Parker, 2014: Climatology and Ingredients of Significant Severe Convection in High-Shear, Low-CAPE Environments. Wea. Forecasting, 29, 854-877, doi: 10.1175/WAF-D-13-00041.1

———-, ———–, G.M. Lackmann, and J.R. King, 2016: Composite environments of severe and non-severe high-shear, low-CAPE convective events. Wea. Forecasting. Doi: 1175/WAF-D-16-0086.1

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Updated List of NC State-NWS CSTAR Publications

King_Parker_CSTAR2017

Figure depicting the change in surface based CAPE in the hours prior to severe convection in HSLC environments as described in King et al., 2017.

Shown below is a list of publications and abstracts developed as a part of the most recent NC State-NWS CSTAR project entitled “Understanding and Prediction of High Impact Weather Associated with Low-Topped Severe Convection in the Southeastern U.S.” Note that additional presentations are scheduled for upcoming conferences including the AMS 24th Conference on Probability and Statistics in July and the 17th Conference on Mesoscale Processes in July. In addition, manuscripts are being composed for multiple projects including the predictability study using ensembles and dynamical-statistical downscaling project.

Publications (most recent first):

1) King, J. R., M. D. Parker, K. D. Sherburn, and G. M. Lackmann, 2017: Rapid evolution of cool season, low CAPE severe thunderstorm environments. Wea. Forecasting, 32, 763-779. | PDF |

2) Sherburn, K. D., M. D. Parker, J. R. King, and G. M. Lackmann, 2016: Composite environments of severe and non-severe high-shear, low-CAPE convective events. Wea. Forecasting, 31, 1899-1927. | PDF |

Conference abstracts (most recent first):

1) Sherburn, K. D., and M. D. Parker, 2016: The origins of rotation within high-shear, low-CAPE mesovortices and mesocyclones. 28th Conference on Severe Local Storms, AMS, 7-11 November 2016, Portland, OR. | Recorded presentation | Manuscript |

2) Sherburn, K. D., and M. D. Parker, 2016: Insights from composite environments of high-shear low-CAPE severe convection. 28th Conf. on Severe Local Storms, AMS, 7-11 November 2016, Portland, OR. | Poster handout | Manuscript |

3) Blank, L., and G. Lackmann, 2016: Operational predictability of explicit high shear, low CAPE convection. 6th Conference on Transition of Research to Operations, AMS, 11-14 January 2016, New Orleans, LA. | Recorded presentation |

4) King, J. R., and M. D. Parker, 2015: Conditioning and evolution of high shear, low CAPE severe environments. 16th Conference on Mesoscale Processes, AMS, 2-6 August 2015, Boston, MA. | Recorded presentation | Manuscript |

5) Sherburn, K. D., and M. D. Parker, 2015: Examining the sensitivities of high-shear low-CAPE convection to low-level hodograph shape. 16th Conference on Mesoscale Processes, AMS, 2-6 August 2015, Boston, MA. | Recorded presentation | Manuscript |

6) King, J. R., and M. D. Parker, 2014: Synoptic influence on high shear, low CAPE convective events. 27th Conference on Severe Local Storms, AMS, 2-7 November 2014, Madison, WI. | Poster handout | Manuscript |

7) Sherburn, K. D., and M. D. Parker, 2014: High-shear, low-CAPE environments: What we know and where to go next. 27th Conference on Severe Local Storms, AMS, 2-7 November 2014, Madison, WI. | Recorded presentation | Manuscript |

8) Sherburn, K. D., and M. D. Parker, 2014: On the usage of composite parameters in high-shear, low-CAPE environments. 27th Conference on Severe Local Storms, AMS, 2-7 November 2014, Madison, WI. | Poster handout | Manuscript |

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The Utility of Non-NWS Upper Air Observations – NC State Soundings Support Severe Weather Operations at NWS Raleigh on May 4th and 5th, 2017

NC State sounding from 0103 UTC 05/05/2017 during a severe weather event.

There are several non-NWS organizations that take upper-air observations and share them with NWS forecasters. These organizations largely consist of universities, military installations or research laboratories. The motivation for taking these observations include research efforts as well as educational and training activities.

One example of such an organization is the “Sounding Club” at NC State University which is lead by students and overseen by professors within the Atmospheric Science department.  Some of the goals of the organization according to their website are to help students obtain hands-on experience in addition to collecting data for projects. In addition to professional development and project support, the soundings have been used by NWS forecasters to support operational needs.

A pair of soundings from the NC State Sounding Club were very helpful during the evening and overnight hours on May 4th and 5th when there was concern for severe convection (SPC reports).  NWS Forecaster Michael Strickler noted the utility of the soundings at approximately 01Z and 03Z on 05 May…

The soundings revealed that the low 70s/low 60s surface temperatures and dew points were already supportive of surface-based convection even prior to the arrival of the mT air mass from the southeast, but with only weak instability and low equilibrium levels. This information provided radar operators confidence of severe wind and tornado potential. The sounding also revealed the character and strength of the low level shear, with a greater degree of confidence relative to that of WSR-derived VWP winds/hodographs that tend to fluctuate/become noisy and are consequently often times less reliable. Pre-QLCS storm motion vectors, as indicated by the soundings, also proved beneficial from a pre-warning situational awareness standpoint.

NWS Chat message from 0525 UTC (0125 AM EDT) 05/05/2017 describing the near term environment.

UNC Asheville sounding from 0300 UTC 01/07/2017 during a winter storm that affected much of the Virginias and Carolinas.

NWS Raleigh forecasters obtain the soundings from the NC State Sounding Club in near real-time either through email, posts to the CSTAR mailing list, or via social media.  In addition to NC State, there are several other organizations that take upper air observations.  UNC Asheville has taken upper air observations during many winter storms in recent years as a part of their Sounding-based Experiment on Mixed Precipitation Events (SEMPE) project. These soundings have been used by the NWS offices in Greenville-Spartanburg, Blacksburg and Raleigh among others.

In the Southeast, we are aware of several organizations taking supplemental observations; some of them are shown in the list below:

  • NC State
  • UNC Asheville
  • UNC Charlotte
  • University of South Carolina
  • University of Alabama Huntsville
  • Mississippi State
  • University of Louisiana-Monroe
  • Simmons AAF Fort Bragg
  • Redstone Arsenal
  • NOAA Lab in Oak Ridge, TN

These observations have a great deal of potential to support NWS warning and forecast operations. Unfortunately, they are often underutilized for a variety of reasons including a lack of awareness and coordination of the sounding operations, limitations in the dissemination and display of the soundings, and an inability to ingest the data into AWIPS.

There is a desire to collect and organize these grass roots observations into a process to improve the awareness, communication, access and display of these observations among NWS forecasters. Collaborators with the NC State CSTAR group will be reaching out to those taking the observations to gauge interest in improve the flow of the data to the NWS and perhaps elsewhere.

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