4/28/02 LaPlata, Maryland tornadic supercell environment

The long-track supercell that produced the LaPlata, Maryland tornado originated in the early afternoon over extreme eastern Kentucky and West Virginia (not shown).  The storm entered northern Virginia after 20 UTC.  The 18 UTC 500mb model analysis shows strong westerly flow aloft with an upper trough, and the 20 UTC surface map shows southwesterly surface winds over the VA/MD area, with increasing dew points closer to the coast:
<Eta analysis 500mb height contours & winds 18 UTC  <surface map 20 UTC

Around 2045 UTC, the aforementioned supercell produced a tornado near I-81 in Shenandoah County, Virginia. 
< LWX radar base refl. 2057 UTC

SPC mesoanalysis page estimates of parameter fields at 20 UTC showed a maximum of 0-1 km EHI over northern Virginia (large low-level SRH with the strong wind fields, even with southwesterly surface winds), and deep layer shear of more than 60 kts.   Low-level thermodynamic parameters were also favorable, suggesting good low-level humidity and surface-based storms (low LCL heights, small CIN, and a minimum below 1000 m in the LFC height field).  The significant tornado parameter guideline (Thompson 2002) was also maximized over northern Virginia, suggesting favorable potential for supercell tornadoes:
< 20 UTC 0-1 km EHI   < 20 UTC BL-6 km shear
< 20 UTC LCL height   < 20 UTC CIN and Sig Tor Parameter   < 21 UTC LFC height

The storm did not produce other tornadoes until it crossed into Maryland shortly before 23 UTC.  It then produced a long track violent (F4) tornado about 20 miles south of the Washington D.C. area, stiking the town of La Plata and later moving out into Chesapeake Bay.  Three people were killed.
< surface map 22 UTC  < visible satellite image 2315 UTC
< LWX radar base refl. 2235 UTC & 2306 UTC

Here is the RUC-2 analysis profile for Quantico, Virginia, at 22 UTC, ESE of the supercell before it produced the violent tornado:
< Quantico VA (NYG) RUC-2 analysis sounding 22 UTC

Using relative parameter value strengths:  , the table below summarizes this environment south of Washington D.C. at 22 UTC:   

Shear-CAPE combinations and deep shear were both strong, and low-level thermodynamic parameters were "ok" to "strong", so it is no surprise that the supercell produced a damaging tornado.  With the favorable parameters over northern Virginia on the SPC 20 UTC graphics, it's impossible to say why this storm did not produce tornadoes 2100 to 2230 UTC prior to crossing into Maryland, or why other storms further north and south weren't tornadic.  But the SPC graphics and RUC-2 profile certainly suggested heightened awareness was in order with an environment supportive of significant tornadic supercells across the Virginia/Maryland area.

Here are EHI, LCL, and LFC height from the SPC mesoanalysis page at 23 UTC, still showing strong shear-CAPE combinations (EHI) and favorable low-level thermodynamic parameters over this area: 
< 23 UTC 0-1 km EHI   < 23 UTC LCL heights  < 23 UTC LFC heights

- Jon Davies 7/1/02

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