The week starting on April 17th was a rather event week of severe weather. There was a total of 274 hail reports, 68 wind reports, and 20 tornado reports in the two-day span between April 19th and April 20th. This is however not the end of the severe weather streak. A high amplitude long-wave trough is currently positioned over the central US is expected to propagate eastward over the Appalachian Mountain range and produce more extreme weather along the east coast states. The 4/21 12Z HRRR model run (Figure 2) shows the trough becoming negatively tilted at 4/23 0Z as the jet streak at the base of the trough intensifies. Those are synoptic features that are often associated with severe weather. The Storm Prediction Center (SPC) has a slight risk outlook for 4/22 Saturday (Figure 1) spanning from Maryland to the to South Carolina. The primary severe weather risk is expected to be damaging winds and hail. Although the Tornado risk was marginal, it is not out of the question.
As the shortwave energy of the trough intensifies, a surface low is projected to form near Kentucky around 12Z of 4/22 and intensify downstream of the trough. This is the result of a combination of trough/ridge and jet streak dynamics. At the base of the trough, vorticity is maximized due to the curvature of the thermal wind. Therefore, the region downstream of the trough experiences positive vorticity advection and UVM forcing according to Term A of the QG Omega equation. Upward vertical motion promotes divergence aloft and surface convergence due to the mass continuity principle. Moreover, the presence of a strong jet streak at the base of the trough dynamics promotes ageostrophic divergence and upward vertical motion in the left exit region of the jet.
In the 4/21 12Z HRRR model run, the surface low is expected to intensify to 997 mbr by 0Z of 4/23. The center will move NE parallel to the 500 mbr height contours. As the surface low intensifies, a cold front form and advects cooler air from the NW. By using the 1D frontogenesis equation, the vertical forcing along the cold front can be diagnosed primarily with term B, A, and C. Although sign analysis of HRRR surface temperature at 18Z of 4/22 (Figure 3) reveals the shearing term (A) to be negative (-*+), the confluence term (B) is positive (-*-) and likely at a much larger magnitude. The diabatic term is inconclusive due to extensive cloud cover on both sides of the front during a time of day time heating. And overall positive sum of the 1D frontogenesis equation implies that frontogenesis is occurring at that point in time. This is associated with upward vertical motion ahead of the cold front which promotes convective activity ahead of the cold front.
Figure 3: 12Z 4/21 HRRR run, valid for 18Z 4/22. 2m Temperature (f, colors)and surface winds (kt). Source: Pivotal Weather.
This particular synoptic set up is conducive of extreme weather, because the presence of a strong jet streak at 500 mbr is usually associated with strong 0-6km wind shear. The wind shear values in Virginia in particular are up to 70 knots, which is sufficient for supercell development if given sufficient instability. The strong southerly flow ahead of the cold front also increases 0-1 km wind shear. Despite the strong southerly flow, the moisture and instability from the Atlantic Ocean is inadequate compared to the Gulf of Mexico. CAPE values are forecasted to be around 500 J/kg, which is barely enough to fuel convection. However, tornadic supercells should not discounted due to the potential for a much different outcome if diurnal heating generates more instability than originally anticipated. The convective storms forecasted on Saturday is expected to form a squall line with embedded supercells according to the 4/21 12Z HRRR run. Damaging wind gusts and hail are likely with some chance of weak QLCS tornadoes.
