All (updated at 830 pm Thursday 09/20 after some discussion with coworkers),
SPC upgraded much of the coastal Mid-Atlantic to a Moderate Risk on the Day 1 Outlook at 1630Z 09/18/2012, with a 45% probability of damaging winds and a 5-10% probability of tornadoes.
Looking at storm reports from 09.18.2012 it is obvious that this system was more of a non-event over the Carolinas/VA. Although there were numerous damaging wind reports in the northern Mid-Atlantic, the severity and areal extent of wind damage was less than anticipated, and only one tornado was reported (as far as I know), which occurred in Jones County, North Carolina. I summary of the Jones County, NC tornado can be found at:
Why were there so few damaging wind reports and only one confirmed tornado over the Carolinas/VA? What can we learn from this event? I went ahead and created animated gifs of numerous SPC mesoanalysis fields from September 18 in order to take a closer look.
In the upper levels, an amplifying northern stream trough tracked southeast from the upper midwest into the northern MS river valley Tuesday morning, then eastward through the Great Lakes Tuesday afternoon and evening. A deamplifying southern stream upper low, near the TX/LA gulf coast at 00Z Tuesday, tracked ENE into Alabama Tuesday morning, then northeast into the western Carolinas Tuesday afternoon and evening. Additional shortwave energy over the southern Rockies and lower Midwest tracked southeast into the lower MS river valley during the day Tuesday, on the southern periphery of the more amplified northern stream trough progressing east through the Great Lakes.
In the lower levels, a SFC-H85 trough attendant the amplifying northern stream trough and deamplifying southern stream wave tracked slowly east to the spine of the Appalachians by 12Z Tuesday, progressing through the Mid-Atlantic during the afternoon and early evening hours. However, the SFC-H85 trough over the Carolinas was much slower, and did not progress eastward into the Carolina piedmont until late Tuesday evening. The slower progression of the southern portion of the SFC-H85 trough and surface front was likely due to 1) further separation from the northern stream trough over the Great Lakes and 2) upper level forcing upstream of the Carolinas in association with northern stream shortwave energy progressing into the lower MS river valley and Deep South by Tuesday evening.
Looking at the pattern over the eastern US, it appears that deep-layer forcing was most prominent over the northern Mid-Atlantic and New England in closer proximity to the northern stream trough. Further south, deep-layer forcing became increasingly weaker, especially over southern VA and the Carolinas where upper level forcing was confined to small amplitude perturbations in southwest flow aloft for much of the day, until late Tuesday afternoon and evening when the deamplifying southern stream wave tracked into western NC/VA. Low-level forcing was largely absent over the Carolinas and southern VA on Tuesday, with unidrectional (SW) flow downstream of the SFC-H85 trough stalled over the mountains for most of the day.
Convection developed throughout the day over the Carolinas/VA given marginal instability (500-1000 J/kg MLCAPE) and above-normal moisture (PWAT ~2.00″) in the presence of small amplitude perturbations in SW flow aloft. However, given the lack of a pronounced low-level trigger or stronger/more focused DPVA, it appears that updrafts could not survive the strong wind fields and would get sheared apart after several volume scans. This made for a difficult warning decision-making process, especially when low-level rotation would strengthen and it was difficult to ascertain whether or not the circulation would further tighten or the entire cell would weaken and collapse in subsequent volume scans.
I’ve seen this happen before in other events, and it makes me think: In the absence of a good low-level trigger (front, trough, etc) or DPVA (shortwave, MCV, etc) perhaps updrafts developing in high shear and low cape environments are simply too weak for horizontal vorticity to be tilted into the vertical and stretched? I recently talked with Mike Strickler about this, and he had a good point that updrafts may not have been tilted much on Tuesday given that low-level flow (i.e. 925 mb) was similar in magnitude to the mid-level (i.e. 500 mb) flow over central NC, at least until the low-level jet significantly weakened later in the afternoon. Effective bulk shear over central NC between 12-18Z Tuesday (per SPC mesoanalysis) was analyzed at 35-45 knots despite the fact that observed winds were ~50 knots at all levels between 900-500 mb (per the 12Z GSO RAOB), meaning that most of the effective bulk shear was either in the lowest 100 mb or above 500 mb, the latter of which seems unlikely given how the effective shear is calculated. If this is the case, perhaps effective bulk shear was over-analyzed? Radar trends here in central NC were consistent with what you would expect to see in an an environment characterized by strong low-level shear and weak deep-layer shear, with rotation developing almost exclusively in the lower levels and reflectivity values rarely if ever above 55-60 dBZ. If this is the case, perhaps the limiting factor was a lack of deep-layer forcing and a lack of deep-layer shear, the latter of which could have enhanced updraft velocities (and therefore stretching) simply by virtue of the pressure perturbation effects associated with mid-level updraft rotation. This could potentially explain the lack of tornado reports in at least portions of the Mid-Atlantic where the low-level jet was stronger and deep-layer shear may have been similarly negated, preventing the development of strong mid-level mesocyclones (though I haven’t looked at radar data up in the VA/DC/MD area).
Additionally, why was strong low-level flow unable to mix down to the surface in the presence of little (if any) CIN in an environment where at least some mixing was occurring, as evidenced by sustained wind speeds and gusts outside of convection? Several SVRs were issued over central NC Tuesday morning in such an environment and very few wind reports were received. Although convection over at that time was generally shallow in nature (20-30 kft echo tops), 50 knot southerly flow was present as low as 1500-2500 feet AGL, and it was gusting up to 25-30 knots outside of convection. By afternoon, areas of stronger insolation resulted in temperatures reaching 80F in the Raleigh area, in the presence of ~40 knot flow at 1500-2500 ft agl, and convection still had a difficult time transporting stronger flow aloft to the surface. Mike Strickler mentioned that the boundary layer was perhaps too shallow (low-level lapse rates no higher than 5.5-6.5 c/km) and moist for evaporative cooling to have any effect, and that convection may have been too shallow for precipitation loading to be the primary mechanism driving downward momentum transport. Further north where a strongly forced line of convection was present in the Mid-Atlantic during the afternoon, the mean 850-500 mb wind was almost parallel to the squall line, and it is possible that this may also have been a mitigating factor with regard to damaging winds. -Brandon V.