A Brief Look Back at the SHERB Parameter During the Christmas Eve 2014 High-Shear Low CAPE Event

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Fig 1. SPC storm reports for 24 December, 2015.

First, I need to note that I was not on the forecast desk on December 24th, but I did help with the forecast during a few of the days preceding the event.  The blog post below is not intended to be an in-depth summary of the event but an opportunity to share a few images and comments about the event.

18_500mb

Fig 2. SPC analysis of 500 hPa heights, winds and temperatures at 1800 UTC on 24 December, 2014.

This event was characterized by a highly amplified upper trough across the Mississippi Valley during the afternoon of  12/24 with strong winds through a deep layer of the atmosphere (see image to the right). At the surface, deepening low pressure was moving across the Ohio Valley during the afternoon of  12/24 with a cold front extending south across the Tennessee Valley into the northern Gulf of Mexico while an inland penetrating warm front was moving across the Carolinas.

Fig 3. The 30 to 33 hour forecast of the SHERBS3 parameter from 12 UTC on 12/23 and 09 UTC on 12/23 valid at 18 UTC on 12/24 from the GFS, NAM, SREF, and Canadian models as depicted in AWIPS-2.

The 30 to 33 hour forecast of the SHERBS3 parameter valid at 18 UTC on 12/24 from the GFS, NAM, SREF, and Canadian models are shown to the right as depicted in AWIPS-2. The SHERB is a composite parameter composed of the surface to 3-km shear magnitude, the 0-3-km lapse rate, and the 700–500-hPa lapse rate intended to separating significant High-Shear Low CAPE Event (HSLC)  severe reports from non-severe HSLC storms using a threshold of 1(with significant severe weather more likely for SHERB above 1).

Fig 3. The 36 to 39 hour forecast of the SHERBS3 parameter from 12 UTC on 12/23 and 09 UTC on 12/23 valid at 00 UTC on 12/25 from the GFS, NAM, SREF, and Canadian models as depicted in AWIPS-2.

Fig 4. The 36 to 39 hour forecast of the SHERBS3 parameter from 12 UTC on 12/23 and 09 UTC on 12/23 valid at 00 UTC on 12/25 from the GFS, NAM, SREF, and Canadian models as depicted in AWIPS-2.

The 36 to 39 hour forecast of the SHERBS3 parameter valid at 00 UTC on 12/25 from the GFS, NAM, SREF, and Canadian models are shown to the right as depicted in AWIPS-2.

Two areas of elevated SHERB values can be seen in the forecast images, the most prominent is located across the upper Tennessee and Ohio Valleys and another located across the eastern Carolinas. From the various sets of model guidance, the SHERB was most consistently forecast to be greater than 1 across eastern Kentucky, Ohio and West Virginia. The SHERB was forecast to be near but generally below 1 across the eastern Carolinas.  The SPC Day 2 Outlook also included these areas in the marginal severe weather threat.

Regional radar imagery from 1630 UTC and 2230 UTC on 24 December, 2014.

Fig 5. Regional radar imagery from 1630 UTC (top) and 2230 UTC (bottom) on 24 December, 2014.

Regional radar imagery from 1630 UTC (top) and 2230 UTC (bottom) on 24 December, 2014 is shown to the right. The convection across the Carolinas occurred earlier in the day and was multi-celluar. The Ohio Valley convection occurred later in the day, primarily as a narrow convective line. HSLC severe weather was observed in the Ohio Valley along with a single severe weather report in southeastern NC.  There were two tornadoes reported on 12/24 including an EF1 tornado in Castle Hayne NC at 1530Z and an EF0 tornado in Lancaster, OH that occurred at 2216Z. In addition, there were more than 20 thunderstorm wind damage reports in the Ohio Valley.  A full summary of storm reports are available from the SPC web site.

SPC analyzed SBCAPE from 1600 UTC (top_ and 2200 UTC (bottom) on 24 December, 2014.

Fig 6. SPC analyzed SBCAPE from 1600 UTC (top_ and 2200 UTC (bottom) on 24 December, 2014.

The 0-6km shear values were very high during the times in which severe weather occurred with shear values ranging between 50 and 60 kts across southeastern NC and generally between 80 and 100 kts across the Ohio Valley. The SPC mesoanalysis of 0-6km shear values valid at 19 UTC on 24 December (approximately halfway between the two tornado events) indicate the strong deep layer shear across the eastern United States. Surface-based instability was very limited. The SPC mesoanalysis SBCAPE values were very low across both areas, analyzed at less than 250 J/Kg.

Plots of the SPC analyzed SHERBS3 parameter at 1600Z near the time of the Castle Hayne, North Carolina EF1 tornado at 1530Z (left side of first image), and the SHERBS3 parameter at 2200Z near the time of the Lancaster, Ohio EF0 tornado which occurred at 2216Z (left side of second image), along with all of the severe weather reports for that day (right side of both images).

SPC analyzed SHERBS3 parameter at 1600Z near the time of the Castle Hayne,  North Carolina EF1 tornado at 1530Z (left) along with all of the severe weather reports for that day (right).

Fig 7. SPC analyzed SHERBS3 parameter at 1600Z near the time of the Castle Hayne, North Carolina EF1 tornado at 1530Z (left) along with all of the severe weather reports for that day (right).

 

 

 

 

 

 

 

 

SPC analyzed SHERBS3 parameter at 2200Z near the time of the Lancaster, Ohio EF0 tornado which occurred at 2216Z (left). The other severe weather reports in Ohio generally occurred between 21Z and 00Z. All of the severe weather reports for that day are shown to the right.

Fig 8. SPC analyzed SHERBS3 parameter at 2200Z near the time of the Lancaster, Ohio EF0 tornado which occurred at 2216Z (left). The other severe weather reports in Ohio generally occurred between 21Z and 00Z. All of the severe weather reports for that day are shown to the right.

 

 

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One Response to A Brief Look Back at the SHERB Parameter During the Christmas Eve 2014 High-Shear Low CAPE Event

  1. Seth Binau says:

    Great writeup on the Christmas Eve QLCS, Jonathan! This strongly forced but very shallow line of convection in Ohio was a classic cool season HSLC QLCS – very little lightning (most of the event had none) but quite a bit of wind damage. TDWR radars (TCVG/TCMH) showed numerous mesovortices, many of which were just a little too weak (rotational velocity, spectrum width) and without spatiotemporal continuity of rotation in both the vertical and horizontal to have a tornado warning placed on them. There was one brief (30 second) EF0 tornado near Lancaster that rolled over a small truck and lifted roofing from a storage building and did quite a bit of tree damage. A very tricky event in the warning phase with tor/no-tor decisions with the presence of many marginal mesovortices.

    SHERB was really good this event. I looked at SHERB a lot the previous day and had been noting the NAM really expanding/increasing the values over central Ohio in the afternoon which ended up verifying well – was a good awareness signal. That’s a couple of events now where we’ve noted that when SHERB values rapidly expand in magnitude AND spatial coverage – those events are good severe wind/qlcs tornado events for us. Typically occurs coincident with deepening synoptic cyclones that we see this response in SHERB fields.

    I wish I would have used SHERB more in the short term mesoa assessment in the late morning/early afternoon than I did – but we were looking at it on many shifts prior and our overnight AFDs from a few days prior were mentioning SHERB and the possibility for fine line of wind-producing convection. As Jonathan points out, there was good agreement across the NWP spectrum of increasing SHERB signal through the day across Ohio – which ended up verifying extremely well.

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