Use of the MRMS for Hail Warnings…

Posted in Live Blogs on by Greg Stumpf.

This afternoon, my colleague and I on the “warning desk” for the FWD CWA got to use some of the MRMS data to diagnose the potential for severe hail.  To start, I will say that the products helped with situational awareness, and for efficient determination of what cells needed the most focus, when a long line of cells extended SW to NE across the CWA.  Quickly, our attention became focused on a cell that was about to push into southern portions of the CWA in the Salado-Killeen-Ft Hood area.  In this first image valid at 2030 UTC notice the cell was producing >1.0 inch hail per the MRMS MESH product (lower right).  MRMS Reflectivity at -20C was ~55dBZ, also lending support for the presence of severe hail.

Image 1. MRMS products valid 2030 UTC. Upper left - Merged Reflectivity QC Composite (dBZ), upper right - 50 dBZ Echo Top (kft), lower right - Maximum Estimated Size of Hail (in), lower left - Reflectivity at -20C Isotherm Level.
Image 1. MRMS products valid 2030 UTC. Upper left – Merged Reflectivity QC Composite (dBZ), upper right – 50 dBZ Echo Top (kft), lower right – Maximum Estimated Size of Hail (in), lower left – Reflectivity at -20C Isotherm Level.

This storm was considered to be too far outside the CWA boundary at this point and was moving slowly enough, that a warning was not issued at this time.  However, it caught our attention as the process was made more efficient by the MRMS suite of products.  The next image valid at 2042 UTC indicated that the storm had undergone some temporary weakening, as MESH values fell below one inch (lower right).  Nevertheless, reflectivity values at -20C remained >50 dBZ.

Figure 2. MRMS products valid at 2042 UTC. Upper left – Merged Reflectivity QC Composite (dBZ), upper right – 50 dBZ Echo Top (kft), lower right – Maximum Estimated Size of Hail (in), lower left – Reflectivity at -20C Isotherm Level.

Next, at 2050 UTC, MRMS products indicated that the storm was undergoing some temporary strengthening once again.  Given the cyclic nature of this storm and the reflectivity signature noted in the reflectivity composite, the first warning for the FDW CWA was issued at 2053 UTC.  In this warning, we included the potential for quarter size hail, which was near the highest suggested by the MESH data and other techniques such as Donovan-Jungbluth.

Image 3. MRMS products valid 2030 UTC. Upper left - Merged Reflectivity QC Composite (dBZ), upper right - 50 dBZ Echo Top (kft), lower right - Maximum Estimated Size of Hail (in), lower left - Reflectivity at -20C Isotherm Level.
Image 3. MRMS products valid 2053 UTC. Upper left – Merged Reflectivity QC Composite (dBZ), upper right – 50 dBZ Echo Top (kft), lower right – Maximum Estimated Size of Hail (in), lower left – Reflectivity at -20C Isotherm Level.

We got our first report of severe (quarter size) hail at approx. 2120 UTC.  The next image is the MRMS products valid at 2118 UTC.  Notice at this time that the storm had strengthened and MESH values (lower right) were near 1.5 inches.

Figure 4. MRMS products valid 2118 UTC. Upper left - Merged Reflectivity QC Composite (dBZ), upper right - 50 dBZ Echo Top (kft), lower right - Maximum Estimated Size of Hail (in), lower left - Reflectivity at -20C Isotherm Level.
Figure 4. MRMS products valid 2118 UTC. Upper left – Merged Reflectivity QC Composite (dBZ), upper right – 50 dBZ Echo Top (kft), lower right – Maximum Estimated Size of Hail (in), lower left – Reflectivity at -20C Isotherm Level.

A little later, the warning forecaster had to make a decision about warning continuation.  The MESH showed that the secondary storm that had developed on the western flank of the initial storm had larger hail sizes.  Given other factors, when looking at all-tilts and taking into account the Donovan-Jungbluth technique and corroborating reports, another warning was issued.  This one however included the quickly-developing storm to the west (warning not shown).

Figure 5. MRMS products valid 2136 UTC. Upper left - Merged Reflectivity QC Composite (dBZ), upper right - 50 dBZ Echo Top (kft), lower right - Maximum Estimated Size of Hail (in), lower left - Reflectivity at -20C Isotherm Level.
Figure 5. MRMS products valid 2136 UTC. Upper left – Merged Reflectivity QC Composite (dBZ), upper right – 50 dBZ Echo Top (kft), lower right – Maximum Estimated Size of Hail (in), lower left – Reflectivity at -20C Isotherm Level.

So, why is this imagery potentially useful?  Well, the 2-minute temporal resolution is advantageous, potentially buying lead time.  Also, it helps with the overall SA and the efficiency of the warning process.  See the image below to see the extent of the convection ongoing at the time.

Image 6. KGRK 0.5 degree reflectivity image valid 2149 UTC.
Image 6. KGRK 0.5 degree reflectivity image valid 2149 UTC.

With multiple deep cells in the area, products like the MESH, and -20 C Reflectivity can be very helpful for determining the cells on which to focus.  This has the potential of making the warning process more efficient.

Another thing I would like to point out is the potential for radar failure.  I worked the April 27th severe weather event in northern Alabama, where we lost our primary radar and a secondary radar.  I was using neighboring radars which were equi-distant to the remaining convection late on that day.  A MRMS suite of products would have been very beneficial to the alternative of having to look at data from three other radars.  That’s one more thing I’d like to point out about these products.

Kris and Ashlie

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