MCS Crossing the Appalachian Mountains – Updated & Expanded

On June 18th, a small cluster of thunderstorms developed near St Louis at around 02 UTC. During the next 24 hours, these thunderstorms would grow into an MCS that would move across the OH Valley and the southern Appalachians before a portion of the MCS reached the Atlantic coast near Wilmington NC at 01 UTC on June 19. The MCS reached the higher elevations of the southern Appalachians during the midday hours at around 18 UTC before moving into the lee and reorganizing during the early afternoon hours.

This MCS was responsible for numerous reports of wind damage reports across the central and western Carolinas, southwestern VA, and northeastern GA. A large number of the damage reports in central NC were associated with strong winds out ahead of the convection. In fact, there was one fatality across Anson County when the strong winds ahead of the thunderstorm snapped a large tree branch that killed a woman. There were very few reports of wind damage west of the Appalachians.

The near term forecast of this system and its impact across central NC was difficult. Many of the high resolution convection allowing models showed the main area of convection with the greatest reflectivities shifting south across the southern extent of the Appalachians across GA and southern SC with very little organized convection in western or central NC. An analyzed MCV was progged and observed to move east across PA/MD and generate convection in those areas.  Central NC appeared to be in the gap between the better forcing associated with the MCV and the less obstructed cold pool moving around the perimeter of the southern Appalachians.

CSTAR funded research at NC State from Letkewicz and Parker on this topic – Forecasting the Maintenance of Mesoscale Convective Systems Crossing the Appalachian Mountains, noted that forecasters are encouraged to take into account the thermodynamic in the lee of the mountains first and foremost.

Their results expand on this and include some other items to consider:

  • The environment that the MCS is moving into is more important for its maintenance then one in which it initially developed.
  • Neither the large scale pattern or the radar presentation of the approaching MCS, provide operationally useful information in determining whether an MCS would survive the crossing of the southern Appalachians
  • Crossing cases tended to have a thermodynamic environment in the lee that was more favorable for convection (characterized by higher CAPE and lower CIN) where MUCAPE and MUCIN are recommended as the most important operational ingredients
  • Crossing cases tended to be characterized by a weaker mean wind and shear in the downstream environment. This is a bit counter intuitive and may be explained by
    • a weaker shear would provide a better balance with a weakened cold pool in the lee and/or
    • a slower mean wind would result in weaker down slope flow, and thus less convective suppression in the lee.
  • Idealized model simulations showed that when there is a suitably favorable thermodynamic sounding, convective systems show little sensitivity to changes in the wind profile in terms of their maintenance.

The evolution of this MCS event (shown in a regional reflectivity animation) is largely consistent with findings of Letkewicz and Parker. The MCS reorganized in the lee in a region of enhanced MUCAPE and then weakened further east as the instability decreased significantly.

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One Response to MCS Crossing the Appalachian Mountains – Updated & Expanded

  1. skeighton says:

    Thanks for posting this Jonathan! I was oblivious while on vacation in the Southwest US (no MCS’s to worry about there in June). This also fits the local climatology we have done here at Blacksburg, with a favored crossing time during the afternoon or evening hours. Interestingly, it appears that the main MCS that influenced NC and parts of SW VA and SC had weakened to below severe limits just before it crossed and really intensified right after it crested the Blue Ridge. It seems that the swath of reports from southern TN into Nrn GA and then parts of SC was the second MCS later in the afternoon and into the evening (and really only clipped the southern edge of the Appalachians). I’m not sure that the first one would fit our original definition of a severe MCS upstream due to lack of reports immediately upstream of the Appalachians, but the second one might barely have been contained within our study domain. For more on our local study (with help from SPC), see the following link:

    http://www.erh.noaa.gov/rnk/Research/MCS_Appalachians_Manuscript_final.pdf

    Steve K

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