2013-08-29 Pulse Storm, Part I: Synoptic Setup

Fig. 1: The base of the pulse cell ~4 nm to the NNW, looking N from 37.234N, 80.434W (the Math Emporium) at 1713Z. Taken with a Canon EOS 550D with Magic Lateran custom firmware and a 70-300mm f/4.0-f/5.6 lens at 70mm, f/5.6, 1/125s. and ISO 200. Image shot as RAW (CR2) and postprocessed with white balance 5750K and color correction to match the original scene.

On the afternoon of 29 August, 2013, a small summer pulse storm struck Blacksburg, VA. It was relatively unremarkable, being a common occurrence in the dynamically stagnant environment of the Mid-Atlantic warm season and providing only a brief respite from the sweltering summer heat index. However, such convection is worth documenting as a regular feature of the summer months in Blacksburg, and, as despite the overly-long focal length of the lens for the subject, and the user's relative inexperience with still photography (versus video production), the final image still managed some level of interest (Fig. 1).

Fig 2: WPC surface chart for 12Z, with radar overlaid. Note slowly stagnating dynamics as the low and front recede SE.

  Our analysis of this case begins, as it must, with the consideration of the synoptic scale environment and broad scale features as depicted through the relevant surface charts and upper-air analyses. At 12Z on the 29th, a stationary boundary was draped from west northwest to east just south of Blacksburg, extending toward an area of low pressure off the coast of Virginia to our east (Fig. 2). Weak high pressure well to the northwest was slowly pushing in behind the boundary, and a warm front associated with another low pressure system lay far to the north.

 Fig. 3: 500mb upper air plot at 12Z. Note the deep LW trough receding off the coast and no apparent shortwaves present.

In the upper levels, a deep 500mb longwave trough slowly plowed off the eastern seaboard, with no sign of any substantial shortwaves in the vicinity (Fig. 3). Thus, any convection that might occur would be expected to organize around mesoscale boundaries or orographic features in the presence of relatively high CAPE and low-level moisture, due to the lack of apparent significant forcin.

Fig. 4: 250 mb upper air plot at 12Z. Note moderate jet max and relative orientation to Blacksburg.

In a pattern of generally meridional flow, a relatively moderate 250 mb jet max is apparent to the north-northwest of Blacksburg, with the latter situated peripheral to the right exit region of the jet max, implying some amount of upper-level convergence and thus minimal upper-level support for ascent (Fig. 4). Even at 250 mb, winds aloft are relatively weak (<50 kt), implying that shear (and vertical momentum transfer) will probably not play much of a role in any convection later in the day.

Fig 5: WPC surface analysis for 21Z, depicting southeastward progression of front and high/low pressure areas.

The WPC surface analysis at 21Z reveals the progression of the features previously discussed (Fig. 5). The low has shifted well off the coast, and its accompanying boundary, now pushing south as a cold front, has continued well south of Blacksburg. As the area of high pressure to the northwest begins the influence the region, another, secondary low pressure center developed to the southwest along the cold front. These relatively unfavorable synoptic conditions, advecting cooler, drier air from the north and limiting surface lift, further limited the potential for any organized convection at this time other than diurnally enhanced showers and storms fueled by local features in the surface wind field, such as low-level convergence, moisture pooling, as well as post-frontal upper-level forcing, all of which did in fact occur.

Fig. 6: SPC 500mb UA analysis. Note progression of features from previous chart, as well as weak shortwave over RNK.

The 500mb analysis at 00Z reveals the expected changes from the previous, as well as one more interesting feature (Fig. 6). The deep longwave trough has continued to progress off the coast, leaving a broad ridge across most of the central CONUS. Further, the pattern over Blacksburg continues to exhibit relatively weaker meridional flow. However, a weak but apparent shortwave trough is visible, with its axis over or just to the east of Blacksburg. This features could have provided some amount of upper-level lift just at the time it was required for the initiation and enhancement of diurnal convection, helping storms temporarily overcome the relatively unwelcoming environment at the surface.

Fig. 7: SPC UA analysis at 250 mb. Note enhanced velocities over Blacksburg and position relative to jet maxima.

A general continuation of the previous pattern is also observed at 250mb, again albeit with one additional feature that may have aided storms on this day (Fig. 7). Flow has become more sharply meridional with the progression and slight deepening of the longwave trough, with again enhanced upper-level winds to the north of Blackburg. However, despite a reduction in the jet maxima, Blacksburg appears to be located closer to the center axis of the jet, and is experiencing stronger upper-level winds, resulting in less inhibiting factors for convection around this time, shortly before which it did in fact occur.

Fig. 8: SPC analysis of 12Z RAOB from WFO Blacksburg. Note already elevated thermodynamic parameters and low shear.

The 12Z morning sounding from Blacksburg already provides a number of clear indicators toward the likelihood and mode of localized, pulse-type convection later in the day (Fig. 8). The Skew-T chart shows a nearly saturated 1000-700 mb layer (although likely somewhat contaminated by fog or low/altostratus, due to the saturated profile around 700 mb) capped by a weak temperature inversion and topped by significantly drier midlevels. Around 1413 J/kg of SBCAPE already exist with virtually no cap, while the generally more relevant MLCAPE value already nears 800 J/kg--fairly substantial for Blacksburg this early in the morning--with around 25 J/kg of CIN, which could easily be eroded as day heating progresses. DCAPE is also fairly elevated for this hour, at just over 800 J/kg, but any strong downdraft potential on this day would not eventually be realized due to poor storm organization and relativity weak low-level lapse rates, barely wet-adiabatic. Kinematics are also fairly weak, with less than 100 m2/s2 of SRH and less than 25 kt of 0-6km shear, likely leading to marginal storm organization and maintenance.

Fig. 9: SPC 00Z evening sounding analysis of Blacksburg office RAOB data. Note greater CAPE and reduced CIN vs prev.

The 00Z evening sounding, launched a few hours after storms had already formed, is largely unchanged in main substance besides the addition of diurnal heating. This would act to suppress the mid-level inversion, steepen lapse rates near the surface, and thus boost CAPE and erode CIN. This indeed being the case, we find a SBCAPE value nearing 3000 J/kg and a MLCAPE figure near 2000 J/kg, both relatively high for this far east, as well as virtually no CIN and again minimal shear. This implies pulse storms would indeed form and could potentially produce stronger winds along with heavy rains due to abundant low-level moisture apparent in the Skew-T. However, two important points must be noted. First, the sounding is apparently contaminated by convection or some other such activity, due to the virtual saturation of a significantly deep layer; also, shear is again week preventing organization of a strong cold pool producing damaging winds, or promoting "training" structures promoting flash flooding (especially when coupled with the relatively unremarkable P-WATs).

And here, I will end things for the moment, not because I wish to leave my (few) readers handing, but rather because I am both tired enough to make continuing very difficult, but yet still conscious enough to not trust myself to write properly should I continue at this hour.

Stay tuned over the next few days for Part II: Mesoanalysis and Part III: Storm-scale Radar Analysis, coming to a weather blog near you. Until next time, this has been C. A. M. Gerlach, and I'll see you on the WIDEnet.