Webinar on the Environmental Conditioning of Cool Season, Low Instability Thunderstorm Environments in the Tennessee and Ohio Valleys and Southeastern U.S.

In late April, Jessica King, presented a summary of much of her CSTAR research simulating severe and non-severe high shear, low CAPE convective events. The presentation focused on the examination of the rapid destabilization that occurs in the few hours leading up to severe convection in the simulated events and the mechanisms responsible for the rapid changes in CAPE. Links to the PowerPoint slides and the recorded webinar are available at the bottom of the post. Some notes from the presentation are shared below.

Introduction
fig1Low instability severe thunderstorms are heavily concentrated in the Southeast especially in the Ohio, Tennessee, and Mississippi Valleys. These events tend to occur in the cool season, often in the overnight and early morning hours, and frequently with a convective mode of mini-supercells or QLCSs. The occurrence of severe convection in low instability environments can often be explained by the presence of synoptic scale forcing and mid-latitude cyclones. The research goal was to determine mechanisms by which environmental conditioning occurs in severe and non-severe high shear, low CAPE thunderstorm events.

Simulated Environments
fig1.5Jessica conducted real-data simulations of high shear, low CAPE events when there was at least a “slight” risk for severe storms and the SfcOA (SPC mesoanalysis) CAPE ≤ 1000 J kg-1 and 0-3 km shear ≥ 18 m s-1. She developed a 3-hour time series analysis for separate points in a 7×7 grid for the three hours prior to severe convection. The 0-1km wind shear showed some discrimination between severe and non-severe events with values higher in severe events, especially at night. The wind shear tended to remain relatively steady over time while the CAPE increased over time.

Calculating Contributions to CAPE
fig3Advection of high theta-e air often leads to an increase in CAPE. A plot of 3-hour change in SBCAPE up to the event time shows a much larger increase in SBCAPE prior to the event for severe events with a smaller increase for non-events. An increase in CAPE can be realized by 1) increasing surface temperature 2) increasing surface moisture or 3) decreasing temperature aloft. The mechanisms for destabilization varied significantly among all environments with significant destabilization occurring in the 3 hours prior to severe events. The change in surface moisture was a positive contribution to increase in CAPE for all cases. Warming near the surface was important in destabilizing all of the severe events as well, and cooling near the surface may be detrimental in the non-severe events. The increase in surface temperature was noted for both daytime and nocturnal cases.

Synoptic Forcing for Ascent
fig4Forcing for ascent can be driven by processes such as warm advection, lifting by a front or boundary, and cyclonic vorticity advection aloft. These processes can lead to the release of potential instability. The release of potential instability through layer lifting occurred prominently in 4 of the simulated severe events. In these 4 cases, the 3km vertical velocity increases significantly and the 0-3km lapse rate became less negative in the hour before the severe event as the synoptic forcing approaches.

SHERB
The SHERBS1 and to a lesser extent the SHERBS3 were discriminators between the severe and non-severe events. The skill of the SHERBS1 was likely a result of the 0-1km shear serving as a good differentiator. Work is underway to modify the SHERB parameters to perhaps include a term to account for the release of potential instability.

Future Work and Looking Ahead
• Investigate larger scale observations and climatologies to identify recurring synoptic-to-mesoscale patterns in cool season, low instability events in the Southeast.
• Examine the vertical distribution of CAPE.
• The recognition of patterns in high-resolution model data and observations that indicate a low CAPE environment has the potential to evolve into a severe-weather producing environment.
• The availability of datasets and tools to examine data in the highest temporal resolution possible is critical. In many events, some of the higher resolution data doesn’t show a supportive environment 3 hours in advance of the event but a rapid change in low-level moisture or temperature can occur just ahead of the line that provides the needed buoyancy. Sub-hourly time scales or other perspective may provide an opportunity to anticipate these events.
• There may be an R2O pathway with the near-storm situational awareness environment tool that is under development which provides a means for detailed monitoring of the environment.
• There was also an interest in requesting the SPC produce some 1 or 2 hour change fields for surface temperature and moisture.

Resources
PowerPoint slide deck
Video recording of webinar

 

Posted in Convection, CSTAR, High Shear Low Cape Severe Wx, Uncategorized | Leave a comment

Interesting Findings from Two Fall HSLC Cases (23 Nov. 2014 & 18 Nov. 2015)

CSTAR folks,

We wanted to post a review of several HSLC cases that had some significant operational utility here at FFC to garner further insight, comments, or discussion you may have (included some discussion points at the end).  Both happen to be November events, with the first (23 Nov. 2014) providing a unique outlier to some previous local research, and the second more recent case (18 Nov. 2015) lending to a situation where the SHERBS3 may have been the heaviest hitter of the available predictive severe parameters.

CASE 1

23 Nov. 2014 QLCS Tornadic Event: Persistent Regeneration of Weak Tornadoes with Pronounced Tornado Debris Signatures

  • Poster presentation at 2016 AMS Annual Meeting
  • Coordinated topics/previous research with Jason Deese (FFC), John Banghoff (Ohio State Univ.), Steve Nelson (FFC), and Dr. Gary Lackmann (NC State Univ.)

The tornadic development across central Georgia on the afternoon of 23 Nov. 2014 was not the typical case one would expect in the Southeast U.S. for two reasons:

  1. Persistent northern bowing segment or “broken-S” QLCS convective mode resulting in six separate tornadoes
  2. Pronounced tornadic debris signatures (TDS) seen with five of the six tornadoes, some of which lofted debris to a significant height above the ground more than previously documented with weak tornadoes (Banghoff and Nelson, 2014)
Fig1

Figure 1. Surveyed event tornadoes and damage points.

The synoptic setup consisted of a negatively titled upper shortwave trough tracking northeastward across the southeast CONUS (Fig. 2) with an attendant low pressure system drawing strong low level Gulf moisture advection (Fig. 3).

Fig2_500mb

Figure 2: 500 mb analysis (2000z)

Fig3_850mb

Figure 3. 850 mb analysis (2000z)

A parent surface high pressure system situated off the mid-Atlantic coast had previously resulted in hybrid cold-air damming (CAD) along the eastern slopes of the Appalachians, and allowed for the periphery of the wedge of cold air or “wedge front” to be present across central Georgia (Fig. 4).

Fig4_sfc

Figure 4. Surface analysis (2000z)

The combination of these influencing features provided a HSLC environment for convective development (Fig. 5).  A special 18z upper air release also came out of TAE and indicated HSLC parameters even away from the wedge influence (Fig. 6).

Fig5_cape_shr

Figure 5: Instability and shear parameters (2000z)

Fig6_TLH_params

Figure 6: Special 18z TLH raob instability and shear parameters

Tornadogenesis was observed to consistently occur along the wedge front as it rapidly retreated northeastward across central Georgia ahead of the aforementioned system.

Fig7_tors_a

Fig7_tors_b

Figure 7: Radar snapshots of 6 tors at time of genesis

Dual-polarization radar data were analyzed for each of the tornadic cells to assess strength of rotation (Vr and NROT) and max TDS heights (used GR2Analyst).

Fig8_radar

Figure 8: Method of using dual-pol radar data to analyze TDS height

Fig9_TDS_ht

Figure 9: Resultant TDS heights during 6 tornado lifespans

Fig10_Vr

Figure 10: Resultant Vr and NROT values during 6 tornado lifespans

Much of this data were also analyzed during the event in real-time to assist in enhanced wording of the tornado warnings (prior to implementation of Impact-Based Warning (IBW) wording).

Fig11_ex

Figure 11: Examples of enhanced wording used in real-time warnings

The analyzed TDS heights primarily stayed in a 6-11 kft range, which is more common to the significant EF2 category observed with previous research (Fig. 12).  While the surveyed tornadoes in this event mainly fit in the weak EF0-EF1 categories, it is proposed that such anomalously high TDS heights were due to the presence of abundant fall foliage and lofted leaf debris combined with subsequent tornadic updraft regeneration.

Fig12_adjusted_TDS

Figure 12: Annotated from Entremont and Lamb, 2013

It is also proposed that the wedge front provided a nearly steady source of low level streamwise vorticity available for tilting into the vertical within the convective updraft as subsequent downdrafts instigated persistent tornadogenesis by bringing vorticity to the surface.  Presence of the front thus compensated for the lack of surface based instability in the HSLC environment and helped focus tornadic development. This serves as an extension to previous research on wedge front influence in conversely low shear high CAPE environments (Fig. 13).

Fig13_wfc

Figure 13: Annotated from Baker and Lackmann, 2009

Trends in observed radar data and associated near-storm environment from this particular case provide unique utility in operations.  The findings not only extend the proposed effect of wedge front influence on convection in HSLC environments, but also present an upper bound of TDS height correlation to tornado strength during the fall season.  This provides aid to awareness and enhanced wording in warning decisions. Warning operators could justify a seasonal adjustment to the threshold for tornado damage threat tags with the newly implemented IBW structure (Fig. 14).

Fig14_ibw

Figure 14: Decision Aid utilizing previous TDS research from Banghoff and Nelson

Since the 2016 AMS Annual Meeting, Keith Sherburn provided some archived SHERBS3 and SHERBE plots near the time of initial tornadogenesis for the case (Fig. 15).

Fig15_sherb

Figure 15: SHERB parameters during tornado lifespans (2000-2200z)

Interestingly, the SHERBS3 parameter captured the threat better than the SHERBE as critical values nosed farther north and east near the storm locations.  The SHERBE actually had an opposite trend in central Georgia as it weakened and diminished in spatial coverage. While SHERBE calculations set the effective shear magnitude to zero where CAPE is absent, it is noted this could miss events where CAPE is under-forecast/diagnosed.  In a rapidly retreating wedge situation, this is more likely to occur, especially if guidance struggles to resolve the strength, timing, and extent of the wedge to begin with.  It’s hypothesized the SHERBS3 has potential to more likely illustrate the favorable environment interacting with the wedge periphery in an HSLC environment.

***********************************************************************

CASE 2

18 Nov. 2015 QLCS Tornadic Event

A strong upper low pressure system and associated surface front brought a squall line of storms across north and central Georgia during the late afternoon and evening hours of 18 Nov. 2015.  Three tornadoes formed in the northern part of a pronounced bowing segment along the squall line as it tracked across portions of Coweta, Fulton, and DeKalb counties (Fig. 2). The first two tornadoes resulted in EF-1 rated damage, with one near Palmetto and another near Fairburn. The final tornado was a very brief EF-0 in DeKalb County near Tucker (specifics listed below). Some observed TDSs in radar imagery allowed for enhanced wording of the warnings.

C2_fig1_sfc

Figure 1: Surface analysis

C2_fig2_radar

Figure 2: Radar imagery of northern QLCS bowing segment

There were also indications of a lingering wedge front/retreating warm front near the northern notch of the bowing segment, where there were still some ageostrophic roots to a 1030+mb parent high off the NE coast, and perhaps some in-situ reinforcement occurring from upstream solar sheltering seen in the 2000z obs plot below (Fig. 3).  This boundary could have been interacting with the near-storm environment, providing a more steady source of low level streamwise vorticity necessary for tornadogenesis and subsequent regenerations.

C2_fig3_sfcobs

Figure 3: Surface analysis and obs

 

First tornado (Coweta Co. 448 PM EST):

Rating: EF-1
Peak Wind: 105 MPH
Path Length: 0.2 MILES
Path Width: 200 YARDS
C2_fig4_tor1

Figure 4: Radar imagery of first tor

C2_fig5_tor1_path

Figure 5: Surveyed path of first tor

Second tornado (Fulton Co. 459 PM EST):

Rating: EF-1
Peak Wind: 95 MPH
Path Length: 2.6 MILES
Path Width: 200 YARDS
C2_fig6_tor2

Figure 6: Radar imagery of second tor

C2_fig7_tor2_path

Figure 7: Surveyed path of second tor

Third tornado (DeKalb Co. 547 PM EST):

Rating: EF-0
Peak Wind: 75 MPH
Path Length: 0.12 MILES
Path Width: 100 YARDS
C2_fig8_tor3

Figure 8: Radar imagery of third tor

C2_fig9_tor3_path

Figure 9: Surveyed path of third tor

The environmental parameters also fit the bill for HSLC.  Below are reanalysis plots for 0-6 km bulk shear, 0-1km SRH, SBCAPE, SHERBE, and SHERBS3 compared to the SPC storm reports at times near the initial tornado occurrence (2000z and 2100z in Fig. 10 and 11 respectively).

C2_fig10_params

Figure 10: Instability and shear parameters (2000z)

C2_fig11_params

Figure 11: Instability and shear parameters (2100z)

Again, both SHERBE and SHERBS3 have critical threshold values north of the instability axis near the areas of tornadogenesis, and the SHERBS3 had better coverage than the SHERBE.  This still is in support of the SHERBS3 parameter potentially being a greater predictive parameter in retreating wedge front situations with limited CAPE.

 

**********************************************************************

 

Initial discussion thoughts from both case analyses…

  • Did any other office have some notable observations or analysis from either of these past two events (more likely from 23 Nov. 2014 since 18 Nov. 2015 was quite localized)?  Perhaps another event with a similar setup or outcome?
  • Any further insight into the following proposed operational applications:
    • Raising the Fall seasonal threshold for TDS height and enhanced wording correlation for Impact-Based Warnings
    • Influence of wedge front given HSLC environments
    • Potential for SHERBS3 to capture severe threat better than SHERBE in rapidly retreating wedge cases

 

Posted in CIMMSE, Convection, CSTAR, General Information, High Shear Low Cape Severe Wx | 3 Comments

Potential HSLC severe event Thursday, March 24th, including VORTEX-SE deployment

A potent upper-level trough (Fig. 1) and associated surface cyclone (Fig. 2) moving through the eastern CONUS will bring a chance for severe high-shear, low-CAPE (HSLC) convection to portions of the Ohio and Tennessee Valleys on Thursday. This event is especially noteworthy because in addition to potentially affecting several collaborating CWAs, the VORTEX-SE project (including CSTAR students Jessica King and Keith Sherburn from NC State) will be operating in and near Huntsville, AL.

gfsUS_500_avort_036 (1)

Figure 1. 500 hPa absolute vorticity (1/s) and geopotential heights (m) for 0000 UTC 23 March 2016 GFS, valid 1200 UTC 24 March 2016. Credit College of DuPage.

gfsSE_sfc_thetae_042

Figure 2. 2-m theta-E (K) and 10-m winds (kt) for 0000 UTC 23 March 2016 GFS, valid 1800 UTC 24 March 2016. Credit College of DuPage.

 

Several features of this setup are consistent with recent HSLC research of severe events. The narrow plume of low-level high theta-E air stretching from near the Gulf of Mexico northward through Tennessee (Fig. 2) is similar to that found in Jessica King’s high-resolution simulations and contributes to increased CAPE in this area, particularly in the NAM (Fig. 3). Additionally, forecast soundings suggest continuous advection of high theta-E in the low-levels throughout the event (not shown). Coupled with a rapid decrease in theta-E with height and favorable “ball cap”-shaped hodograph (both shown in Fig. 4), the thermodynamic and kinematic setup appear favorable for severe HSLC convection. Though best upper-level forcing for ascent is projected to lift northward away from the area of interest as the event goes on (note location of vorticity maximum in Fig. 5 compared to Fig. 1), other signs point to this being a severe event.

 

namSE_con_sbcape_039.gif

Figure 3. SBCAPE (contoured, J/kg), SBCIN (hatched, J/kg), and 10-m wind (kt) for 0600 UTC 23 March 2016 NAM, valid 2100 UTC 24 March 2016. Credit College of DuPage.

06_NAM_039_34.54,-86.81_severe_ml.png

Figure 4. 0600 UTC 23 March 2016 NAM forecast sounding for northern Alabama valid 2100 UTC 24 March 2016.

gfsUS_500_avort_042.gif

Figure 5. As in Fig. 1, but valid 1800 UTC 24 March 2016.

 

Convection-allowing NCAR ensembles suggest the possibility for rotating updrafts (Fig. 6) within a broken line of convection (Fig. 7). As noted, VORTEX-SE will be operating on this project with its multiple mobile sounding and radar teams, sticknet array, lightning mapping array, and disdrometers, in addition to other fixed instrumentation. More information on the project can be found here.

I plan to write a follow-up blog discussing VORTEX-SE operations during the event following its conclusion. However, this is an important and exciting step in our aims toward improving the understanding and forecasting of HSLC convection, as this (if it verifies) will be the most-sampled HSLC severe event in history.

Screen Shot 2016-03-23 at 8.43.42 AM.png

Figure 6. 0000 UTC 23 March 2016 NCAR ensemble paintball plot of hourly maximum 2-5 km updraft helicity > 50 m2/s2 valid 2000 UTC 24 March 2016.

Screen Shot 2016-03-23 at 8.42.34 AM.png

Figure 7. 0000 UTC 23 March 2016 NCAR ensemble paintball plot of 1-km AGL reflectivity > 40 dBZ valid 1900 UTC 24 March 2016.

Posted in Uncategorized | 1 Comment

NWP Systems Reference Updated for ECMWF Changes

nwp.basic

Snapshot of the basic NWP configuration and setup reference. 

On March 8, the European Centre for Medium-Range Weather Forecasts (ECMWF) upgraded the Integrated Forecast System (IFS) with a significant reduction of the horizontal resolution of the high-resolution (HRES) and ensemble (ENS) forecasts. The upgraded horizontal resolution is now around 9 km for the HRES from the previous 16 km grid spacing. Details on these and other changes to the modeling system related to the implementation of IFS cycle 41r2 is available at the following URL: https://software.ecmwf.int/wiki/display/FCST/Implementation+of+IFS+cycle+41r2

With this significant change, we wanted to share our updated NWP reference that Shawna Cokley from WFO Raleigh maintains. The reference provides details on the run time, number of forecast hours, horizontal grid spacing, vertical levels, and output frequency along with details on how the modeling system handles convection, microphysics, radiation, initial/boundary conditions, date of recent upgrade and more. Links to the full reference as well as a basic overview reference are shown below. If you have any comments or notice any items that need attention, please let us know.

Basic NWP Configuration and Setup Reference

Full NWP Configuration and Setup Reference

 

Posted in Uncategorized | 1 Comment

CSTAR Project Presentation from the AMS Annual Meeting

In January, Lindsay Blank traveled to the 96th Annual Meeting of the American Meteorological Society in New Orleans to present a talk on her CSTAR HSLC project. This presentation won 2nd place in the oral presentation category in the 6th Conference on Transition of Research to Operations Student Competition. A recording of this presentation is available at the following link:

Blank and Lackmann,”Operational Predictability of Explicit High Shear, Low CAPE Convection”
Presentation

If you have any questions or comments on this presentation, please feel free to address them here or contact Lindsay through email (lrblank@ncsu.edu).

Posted in CSTAR, High Shear Low Cape Severe Wx, NWP | 1 Comment

Observational Resources for the Potential Severe Weather on Wednesday 24 February 2016

 

spc.outlook

SPC Day 2 categorical and probabilistic outlooks for Wednesday.  

The latest Severe Weather Outlook from the SPC indicates an enhanced risk of severe weather on Wednesday, 24 February, across central and eastern NC with a slight risk across much of the mid and south-Atlantic with a region of significant severe weather possible in the NC Coastal Plain and the Eastern Piedmont. I thought it might be helpful to share some of the new or irregularly available observations data that will likely be available on Wednesday including GOES-14 Super Rapid Scan 1-minute data, the NCEP “Fire Weather Nest” 1.33km high-res nest run model, MESO-SAILS nearly 1-minute radar data, and special upper air soundings.

 

GOES-14 Super Rapid Scan Operations for GOES-R will be centered over the Southeast on 24 February, 2016. Super Rapid Scan Operations will provide 1-minute satellite imagery to support multiple research and GOES-R/S user readiness activities. Additional background information including training and links to online imagery is available at: http://cimss.ssec.wisc.edu/goes/srsor2016/GOES-14_SRSOR.html. Imagery including visible, infrared, and water vapor is available on the web at the links below…

20160224.nest

NCEP Fire Weather Nest domain for the 00 UTC, 06 UTC, and 12 UTC runs on 24 February.

The NCEP “Fire Weather Nest” 1.33km high-res nest model run will be centered over a portion of the severe weather threat area for the 00 UTC, 06 UTC, and 12 UTC runs on 24 February. The Fire Weather Nest will be centered at 35.5N, -79.0W as shown in the image to the right. You can access forecast fields from the 1.33km high-res FWN run at the following URL: http://www.emc.ncep.noaa.gov/mmb/mmbpll/firewx/

 

The Multiple Elevation Scan Option for Supplemental Adaptive Intra-Volume Low-Level Scan (MESO-SAILS) capability for the Doppler radar is being testing at 13 CONUS locations including Raleigh, NC (KRAX) and Morehead City, NC (KMHX) which are both in the severe weather threat area. MESO-SAILS will allow radar operators to add two, three or four low-level elevation scan updates per volume scan which depending on other factors could result in a base radar scan almost every minute which would allow excellent sampling of the low-levels and a unique perspective on the convection.

In addition, several special upper air soundings are anticipated tomorrow. These have been schedule for 15Z at Greensboro (KGSO) and likely a few other NWS locations. Some universities in the area may release soundings with NC State likely to do so and perhaps others from UNC Asheville and possibly UNC Charlotte. Finally, the Simmons Weather group at Fort Bragg and Simmons AAF may release some soundings on Wednesday.

Posted in Convection, High Shear Low Cape Severe Wx, NWP, Satellite | Leave a comment

NWFS Collaboration Group authors “Northwest Flow Snow Aspects of Hurricane Sandy” in Feb. 2016 Weather and Forecasting

Just a short note to let readers know that several of us who have been collaborating on NW Flow Snow issues in the southern Appalachians now for a number of years have had our manuscript on the northwest flow snow associated with the remnants of Hurricane Sandy (Oct 2012) published in Weather and Forecasting. This will appear in the February issue, and for now is available as an early release version to WaF subscribers here:

http://journals.ametsoc.org/doi/pdf/10.1175/WAF-D-15-0069.1

My sincere appreciation goes to all the co-authors for the hard work in putting this together and final modifications leading to publication: Doug Miller, David Hotz, Pat Moore, Baker Perry, Larry Lee (yes, even in retirement!), and Daniel Martin.

We will likely move on to other NWFS topics for awhile, including HiRes model validation for typical NWFS events, but the opportunity for simulating many of the forcing mechanisms and moisture sources associated with the historic Sandy snowfall is something that some of us may still pursue.

Steve K.

 

Posted in Uncategorized | 1 Comment