Summary of August 7, 2023 Giant Hail and Flash Flooding

On Monday, August 7^th, 2023, an anomalous trough (for early August) moved from the Upper Ohio Valley to the Mid-Atlantic. Embedded within the trough was a pair of 500-mb speed maxima of around 50 knots. One over the Mid-South and the other from eastern Kentucky and Tennessee into western Virginia and North Carolina. Notably, a 300mb jet became divergent over Virginia/North Carolina with a maxima of 70 knots at 00z (8 pm) over far southwest Virginia.  At the surface, winds were generally southerly over the local area and southeasterly from northern Virginia north due to a warm front lifting into Pennsylvania and New York. In the wake of morning convection, clouds cleared with stratus (due to warm air advection) lingering into the early afternoon to the lee of the mountains in the Piedmont. This led to MLCIN (aided by a weak EML around 900 mb). MLCIN eroded everywhere by the mid-afternoon with the most persistent area in north-central North Carolina. This CIN in the southern Piedmont into north-central North Carolina likely aided in the split of storms to the north (where the greater forcing was located) and to the south (where the greater CAPE was located). This area tends to see splits like this in situations where forcing lifts north while convection in the Carolinas cuts off the inflow for central Virginia. Once the sun came out, the radiative heating allowed for a deepening of the lee trough, aided by the large scale trough approaching the area. By mid-afternoon storms initiated over the mountains of western Virginia and gradually moved east-northeast.

A supercell developed over Bath County just after 3 PM, quickly becoming severe warned. This supercell moved northeast before turning right in Augusta County. From this point on, this supercell moved East-northeast, passing north of the city of Charlottesville in Albemarle County. This initial supercell produced 3” (baseball sized) hail in Augusta County and 2.5” (tennis ball sized) hail in Albemarle County. Meanwhile, a second storm developed over southwestern Nelson County at 4 PM and moved northeast into northwestern Louisa County. This storm stayed rather weak, splitting in Albemarle County. The left split merged with the initial supercell north of Charlottesville while the right split struggled until it reached Louisa County. The Louisa storm quickly strengthened and became a supercell as it approached Route 33 around 5:50 PM. The supercell exhibited a BWER (bounded weak echo region) just after it crossed Route 33 with a VROT of 39 knots. At this point, a tornado warning was promptly issued for Louisa. The supercell continued to move east-northeast while the low level mesocyclone moved east-southeast. Part of the reason for this is because the storm finally began merging with the now two storms to the north (including the initial supercell). The rotation peaked at 5:59 PM with a VROT of 42.7 knots gate-to-gate but did not produce a tornado. The cell merger completed over Spotsylvania County as it began to exhibit the classic kidney bean supercell structure on reflectivity as cloud tops reached 55,000 ft tall. Broad rotation continued to be evident aloft with another tornado warning issued for Caroline County. The VROT aloft (1.3 degree tilt) peaked at 47 knots. A wall cloud and funnel was spotted from a storm chaser, however, the hook never tightened enough for a tornado. As the powerful supercell approached Bowling Green, the hook continued to weaken and it became apparent that the near-term tornado threat was over. However, the supercell remained a very impressive 55,000 ft tall and reflectivity took on a flying eagle (V-notch) structure. At its peak near Bowling Green, 48 dbz reached 50,000 ft, 62 dbz reached 40,000 ft, 66 dbz reached 35,000 ft, and 70 dbz reached 27,000 ft. For reference, the freezing line (0 C) was 15,000 ft with the -20 C line at 26,000 ft. This is very impressive for a Mid-Atlantic storm. MESH reached around 3” with MRMS hail estimates of 2.09”. As such, the severe thunderstorm warning was upgraded to a considerable tag with 2” hail in the warning. However, not even the MESH (which commonly overestimates hail size) was close to how large the hail actually got. Several 4-4.5” giant hailstones were found just south of Fort A.P. Hill with one hailstone measuring in at a whopping 4.75”. This is the largest hailstone ever found in our CWA and the 3^rd largest ever in the state of Virginia. The largest hailstone ever recorded in Virginia was 5” recorded both on April 23^rd, 1968 in Lee County and July 2^nd, 1968 in Loudoun County. That makes this the largest hailstone measured in Virginia in 55 years and the first time since May 9^th, 2003 (in Sussex County) that our region recorded a hailstone greater than 4” in diameter. A truly remarkable event.

Additionally, very heavy rain fell across the Maryland Eastern Shore due to training storms which resulted in significant flash flooding in Cambridge. MRMS estimates of 2.5-3.0” of rain fell within three hours in Wicomico County (including Salisbury) and 3.5-4.0” of rain in Dorchester County around Cambridge. A Flash Flood Emergency was issued for Cambridge due to knee to waist deep water along portions of Washington Street and numerous road closures requiring multiple swift water rescues around Cambridge. Flooding also occurred in Wicomico with two cars stranded. Additionally, a mesonet station in Cambridge measured 4.11” of rain between 8 and 11 PM with 3.55” of that total occurring in just one hour between 8 and 9 PM.