Potential HSLC Event Early Next Week

The medium range models have been consistent in sweeping a strong cold front across the region early next week. An extended period of deep SW flow is expected to result in an anomalously warm and moist air mass ahead of the front. As usual, shear will be ample, but weak lapse rates and extensive cloud cover will limit instability.

6 Dec 2012/0600 UTC GFS forecast valid at 1800 UTC on 10 Dec 2012. Images are: (upper left) pmsl (blue), b.l. dewpoint (green), surface wind barbs;  (upper right) 1000-850 hPa divergence (color-filled, colder colors are convergence) and 850 hPa wind barbs; (lower left) sbCAPE (color-filled, values in image are generally <200 J/kg), 0-3 km lapse rate (yellow), and 0-3 km bulk shear; and (lower right) 500-300 hPa omega (warm colors are UVV) and 300 hPa isotachs)

6 Dec 2012/0600 UTC GFS forecast valid at 1800 UTC on 10 Dec 2012. Images are: (upper left) pmsl (blue), b.l. dewpoint (green), surface wind barbs; (upper right) 1000-850 hPa divergence (color-filled, colder colors are convergence) and 850 hPa wind barbs; (lower left) sbCAPE (color-filled, values in image are generally < 200 J/kg), 0-3 km lapse rate (yellow), and 0-3 km bulk shear; and (lower right) 500-300 hPa omega (warm colors are UVV) and 300 hPa isotachs.

This is a good opportunity to unveil some of the output of the SHERB smart tool that GSP is running in our GFE.

Output from the SHERB smart tool, with input from GFS forecast data valid at 1800 UTC on 10 Dec 2012.

Output from the SHERB smart tool, with input from GFS forecast data valid at 1800 UTC on 10 Dec 2012.

Comparison of the model output with output from the SHERB smart tool indicates that the strong deep layer forcing, which is immediately along and slightly upstream of the surface wind shift is displaced from the area of maximum SHERB. It is interesting to note that the primary reason for this area of enhanced SHERB is due to the fairly steep low level lapse rates (seen in the lower left panel above) present across the southern Appalachians and foothills, whereas model lapse rates are fairly weak across the remainder of the region. Unfortunately, I forgot to overlay model QPF in the image above, but if I had, it would show that the area of steep low level lapse rates was in a QPF-void region, whereas the convective scheme was activating in most other areas within the weakly unstable/weakly forced warm sector.  So, is the GFS being too aggressive in developing precip within the warm sector? Even if it is, considering that there should be extensive low cloud cover, is it being too generous with the low level lapse rates in the “un-overturned” air mass? It will be interesting to see the NAM’s take on this. Stay tuned…

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3 Responses to Potential HSLC Event Early Next Week

  1. Matt Parker says:

    Hi Justin,

    Thanks for this! It is interesting that GFS lights up the SHERB over the higher terrain. I have noticed this tendency in the NAM and even the mesoanalyses. We’re not sure if there is some physical reason for the SHERB ingredients to be truly enhanced locally, or if this is an artifact of the fixed layers that are used to compute the SHERB (which may cause problems when the surface elevation is higher). Can you archive these plots run by run? It will be interesting to compare to ours once it makes it into the NAM’s forecast horizon.

    Cheers,
    -Matt

  2. MC says:

    The GFS has not been consistent at all run to run over the last several days however, so this is deep speculation considering its fickle tendency presently.

  3. justingsp says:

    Matt,

    We’re not equipped to archive these GFE images. However, I will pass this along to Pat who’s working night shift this week. We’ll try to grab as many screen captures as we can. I noticed the correlation of the higher low level lapse rates with the higher terrain as well. It also appears that the higher 0-3 km shear values correlate well (although not quite 1:1) with the high terrain.

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