Last Throes of Marginal/Null 10 December 2012 HSLC Event

Here are a few last screen captures from the marginal/null 10 December 2012 event.  One of the most interesting reflectivity structures near our (GSP) forecast area occurred toward 23 UTC as the leading, shallow convective lines were dissipating into a larger light to moderate stratiform rain shield over the lower SC piedmont.  In the first figure below, a very shallow and short line segment bowed out as it moved into Edgefield County, SC. (CAE forecast area.)  There was only a small area of 50+ dBz returns with this feature, which remained entirely below 10 kft.

KGSP 0.5 degree Reflectivity from 2248 UTC 10 December 2012.

KGSP 0.5 degree Reflectivity from 2248 UTC 10 December 2012.

The KCAE radar had a good radial look at a descending rear inflow jet.  You’ll notice a weaker reflectivity notch just behind the bow in the KGSP reflectivity image above, and a 45 kt base velocity max around 3500 feet AGL in the KCAE product image below.

KCAE Base Velocity from 2251 UTC 10 December 2012.

KCAE Base Velocity from 2251 UTC 10 December 2012.

SHERB values were noticed to be diminishing throughout the late afternoon hours.  I manually ran the SHERB tool based on the latest 18 UTC NAM and captured the grid below, valid at 23 UTC.  It is interesting that the corridor of 1.0+ SHERB values along the border between the GSP and CAE forecast areas nicely depicts the alley that the bowing feature moved through.

SHERB valid at 23 UTC 10 December 2012 based on 18 UTC NAM.

SHERB valid at 23 UTC 10 December 2012 based on 18 UTC NAM.

Please note that I am working toward making the SHERB smart tool for GFE more available to interested offices.  I have just recently uploaded awips1 (SHERB_Tool) and awips2 (SHERB) versions of the smart tool to the NWS Software Collaborate Portal (SCP) at:

https://collaborate.nws.noaa.gov/trac/nwsscp/browser/scp/Gfe/Smarttools/Sherb/trunk

I hope to have full documentation for the tool available on the SCP by January 1, 2013, along with the required SCP Wiki page.  In the meantime, please note that the smart tool output may differ slightly from the N.C. State web display of the parameter for the following reasons:

– The smart tool can produce output from the GFS, NAM, RAP, or a blend of any of these three models.  The NCSU version is currently NAM-only.  (Matt and Keith, correct me if I’m wrong.)

– The smart tool uses surface wind and temperature grids from the GFE Fcst database.  The NCSU version uses model (NAM) surface fields.

– The smart tool does no smoothing of resultant grid.  Every 2.5 km grid box (or local GFE resolution) has its own computation of SHERB.  (Values are rounded to the nearest tenth of a SHERB.)  I understand that Matt and Keith are smoothing the NCSU grid somewhat.

– The smart tool is finding 3 km wind and temperature data by stepping up coarse GFE profiles containing data every 25 mb.  The 3 km data are thus only an approximation of the actual 3 km values based on the coarseness of the gridded model profiles.  Matt and Keith may have another method of determining layer values depending on the vertical resolution of their data.

It probably would not be a bad idea for us to pick a lat/lon point and capture the fields from a particular event from the same model at the same time for both the GFE smart tool and NCSU method so that we can double-check the math in the smart tool and get a feeling for the magnitude difference in the various terms between the two computation methods.

Please let me know if you have any questions about the tool or if you need help installing it.

Harry G.

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This entry was posted in CIMMSE, Convection, CSTAR, High Shear Low Cape Severe Wx. Bookmark the permalink.

One Response to Last Throes of Marginal/Null 10 December 2012 HSLC Event

  1. Jonathan Blaes @ WFO RAH says:

    Harry,

    Thanks for sharing and for providing the documentation and links. Gail got the SHERB tool and grid up and running in our GFE. If a grid-point evaluation/comparison is needed, we would be happy to participate either through reading values out of the grids or through some formatter.

    Thanks, JB

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