HSLC Case Study Methodology

1. Consult the “Case Study Responsibility” list to see your case studies. Start with the case that you think will be most beneficial to the success of the project.

2. Your home WFO or other WFOs that were involved in your case may already have something written up, providing you with a nice launching pad. Consult the comprehensive High Priority Case List, which lists all of the WFOs within the study area that reported severe weather on your day in question, as well as the total number of reports and the time of the first report and last report.  Contact your fellow collaborators and see if they’ve done any prior work on your case study.

3. Synoptic Overview- Our focus is on mesoanalysis and radar analysis, so this section does not need to be too detailed. Focus on key synoptic features at the surface and the mandatory levels. SPC has nice upper air analysis maps in their archive located here. The SRRS site at NCDC has a good archive of regional and national surface analyses. Also, I like to subjectively select a “representative” RAOB and analyze it.

I highly recommend RAOB for displaying sounding data. It is extremely versatile and can display just about any sounding file format that has been invented (including Bufkit).

4. Collect all of the reports for your case. It’s easiest to use the NCDC Storm Events database. However, I have found several errors in this database. Some may find it more useful to just download the Storm Data publication from NCDC.

5. You need to perform a QC to make sure that all of the severe weather events associated with your case are LC. All of your reports should be associated with HS, but with many of the cases, you’ll probably find at least some of the reports actually occurred in HC environments.

Download the Google Earth SPC mesoanalysis data for your case from the NC State server. If all the data (i.e., the time range) you need is not there, e-mail Matt Parker. Instructions for viewing the data can be found on the same server, as well as a “parameters key.”

You may want to start by simply looking WFO-by-WFO. If it is relatively obvious that MOST of the reports from a certain WFO were HC (i.e., sbCAPE > 1000 J/jg), you can simply eliminate that WFO’s reports from the case. (Contact Justin if you need a shapefile of the participating CWA boundaries for importing into Google Earth).

Another possibility (if you are GIS-handy) is to create shapefiles of the severe reports and import them into Google Earth, overlaying them on the mesoanalysis data. You’ll eventually need to do this anyway, so that you can include an attractive map of the severe events in your case study. The easiest way to do this is to download shapefiles of wind, hail, and tornadoes from the SPC GIS server. These files contain all reports since 1955, so they are quite large. Anyone who is familiar with ArcView/GIS should be able to extract the required data from these files relatively quickly. Doug Berry of CHS has offered to assist anyone who requires GIS help

6. We will be performing careful radar analysis of all tornado reports. Also look for any “high impact” downburst events. Look through event narratives in Storm Data for phrases such as “structural damage,” “widespread” downed trees and power lines, etc. If the number of events that you select for close examination seems overwhelming, it’s okay to .

7. QC the locations of your “high priority” reports by reading through the event narratives (assuming the narratives have adequate detail). You will also want to eventually check the times against radar data.

8. Perform mesoanalysis (at least every three hours) using SPC mesoanalysis data. Discuss key “map” features, such as mesoscale boundaries, mesolows, instability axes, etc. Also discuss how the shear and instability picture (and related fields) changes with time. It is usually best to incorporate the mesonanalysis discussion into your radar analysis discussion. Although we will eventually need to be on the same page, we will leave this section “open-ended” for now.

9. Collect RUC soundings for your “high priority” reports. What you’ll need to do is download archived RUC data in GEMPAK format from Iowa State… http://mtarchive.geol.iastate.edu/. Navigating the archive should be self-explanatory. Once you have collected the GEMPAK files, send Justin the file(s) along with a spreadsheet consisting of the Lat/Lon and time of each report. Again, we’re only doing this for the reports that you will be analyzing closely with radar data (i.e. tornadoes and “high impact” downbursts.

10. Modify the RUC soundings using the surface ob nearest the location of your report. You can download surface ob data from a number of locations. I like to use the ASOS 5-minute data archive from NCDC. This includes ASOS data ONLY.

We will compare our modified soundings to SPC mesonalysis data via the SPC relational database.  Document any differences in shear and especially instability parameters between the modified soundings and the relational database in a spread sheet.

11. IF you have time, document shear/instability parameters of your “non-high priority” reports in a spreadsheet via the SPC relational database. You can request this data from Justin.

12. Radar analysis: Perform a full base data analysis of each “high priority” report. Your case study narrative should include an analysis of every volume scan within 15 minutes prior to the report (to roughly coincide with NWS GPRA lead time goals) and for at least one volume scan after the event ended. Each volume scan should be described subjectively (i.e., notches, hooks, bows, echo overhang, etc) as well as objectively (explicit values of velocity, storm relative velocity, and rotational values, etc.) Include images of four-panel displays of the key elevation slices in your narrative. Include cross sections as deemed necessary. I used custom color curves for all of the base data products. (Consult the images in the Radar Analysis template case study to see these color curves. If you don’t have these, e-mail Justin. If you have custom color curves that you like better, share them!)

If any significant areas of shear are identified (I define significant shear as anything >= 10 x 10-3 s-1), calculate rotational shear values and create time series plots. Also include time series plots of rotational velocity. Begin the time series plots about an hour prior to the event or when organized rotation first developed in order to show the longer term trends. These calculations can be performed by using the “Marker” or “Home” tool in GR2 (right click on background) to calculate the distance between two range gates. Remember, you will have to manually set the storm motion vector to get accurate storm relative velocity data.

Calculate these values by locating the updraft, then choosing the peak inbound and peak outbound values associated with the updraft, even if the rotational couplet is not purely rotational (i.e., there is a convergent or divergent component). However, if the inbound/outbound pair is more convergent/divergent (determined subjectively) than rotational DO NOT calculate the shear parameters. If you have an especially “tricky” calculation to make, it may benefit the project to share it with the entire group for discussion.

Also include an evaluation and time series plots of the GR2 “normalized rotation (NROT)” product.

13. Now you need to collect NWS false alarm Tornado Warnings for your case study. Go to the NWS Verification Website. If you do not have an account, create one. After you log in, on the left hand menu, click on “Verification”à”Severe Weather.” Under “Storm-based Tornado and Severe Thunderstorm Warning Verification” click “Stats on Demand Interface” for cases after October 2007. Click “Stats on Demand Interface” under the next headline for cases prior to this date.

Within the ensuing menu, you will enter a start and end date for your case. Under the “WFO” section, list all WFOs within the study area, as it is not out of the question that some WFOs issued TORs, but did not receive any severe reports. (AKQ, CAE, CHS, FFC, GSP, HUN, ILM, LWX, MHX, RAH, RNK). Under “Match Type” select “Tornado Only.” Under “Report Type” select “Detailed.” Click “Get stats” and you will see a summary of verification statistics at the top of the next page. Near the bottom, you will see a table of all TORs issued by the selected CWAs, followed by a table of all tornado reports. False alarm warnings will have a “—“ in the “Verifying Event” column.  Make a spreadsheet of all the unverified warnings. You may need to perform another mesoanalysis QC to eliminate those TORs issued in HC environments.  IF you click on “TOR“ in the first column, a window with the warning text will pop up, giving you an idea of the basis for the warning and the “triggering” event.

14. Examine radar data for all of the False Alarms. Choose at least two and no more than five False Alarms for detailed examination. Choose the warnings associated with the most impressive radar signatures. Perform a full radar base data analysis of convection that prompted these false alarm TORs, using the same methodology as in XII. Begin your analysis with the volume scan that prompted the warning and continue it for at least 12 minutes after the warning was issued.

15. Modify the nearest RUC sounding for the cell that prompted the False Alarms that you chose. Follow the same methodology as in IX and X. (The lat/lon you provide should be the location of the cell at the time the warning was issued).

This entry was posted in High Shear Low Cape Severe Wx. Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s