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Figure 1. Storm reports from the storm prediction center (SPC) website. Types are color coded as in the key in the lower left of the images.

15 July 2012 Severe Weather Event 1. Overview

Severe weather affected the Mid-Atlantic area on 15 July 2012 (Fig. 1). With the

exception of a tornado in Bradford County, Pennsylvania and in St Mary’s County,

Maryland, the event was dominated by low-end wind damage and reports of downed trees.

The severe weather was triggered by a short-wave moving over through the larger-

scale ridge over eastern North America (Fig. 2). This wave lowered the 500 hPa heights to

near normal by 0000 UTC 16 July 2012 (Fig. 2e). The short-wave brought a surge of

moisture and above normal precipitable water (PW) in Pennsylvania (Fig. 3). The air behind

the wave was relatively dry and the PW values fell to near normal by 0000 UTC 16 July

2012. In the warm season, high

PW values are often associated

with high values of convective

available potential energy

(CAPE). Not surprisingly there

was a surge of CAPE of between

1200 and 1500 JKg-1 over the

region between 1800 UTC 15 July

and 0000 UTC 16 July 2012 (Fig.

4).

In addition to the

relatively favorable CAPE, there

was a weak surge of strong low-

level flow over the anticyclone to

the southeast. This produced a

surge of modest 1 above normal

winds at 850 hPa (Fig. 5). This produced modest shear in the 10m to 850 hPa layer (Fig. 6)

which went across Pennsylvania between 1800 UTC 15 through 0600 UTC 16 July 2012.

The shear was stronger in the 10m to 500 hPa layer peaking between 25 and 30kts. The

combination of strong shear, over 20kts in the lower 1km and CAPE over 1500 JKg-1 is a

good indicator of strong severe events with the potential to produce tornadoes in

Pennsylvania when accompanied by lifting condensation levels below about 1000m.

The deep moisture and strong winds produced a surge of high moisture flux over the

region (Fig. 7). Moisture flux is often used to predict heavy rainfall and flooding but is also a

useful tool in severe weather events. Modest moisture flux was present over Pennsylvania on

15 July 2012.

This paper will document the severe event of 15 July 2012. The radar images are

shown to represent the key convective aspects of the event.

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2. Data and Methods

All radar data used were obtained from the National Climatic Data Center (NCDC) and

displayed using the Gibson-Ridge software package (GR2-Analysist). Storm reports from the

Storm Prediction Center (SPC) and the NCDC storm databases were used here. The NCDC

storm data were put into a Placefile which were then used as overlays in GR2Analyst to match

storm types to tornado reports. The NCDC storm report latitude and longitude had to be

converted to degrees from degree and minute data and local times were converted to UTC.

The large scale pattern was reconstructed using the North American Regional Re-analysis

(NARR:Rutledge et al 2006) data. These data were displayed using GrADS (Doty and Kinter

1995).

3. Radar

Two bow echoes affected central Pennsylvania. The first bow echo developed around

2100 UTC (Fig. 8) and tracked across Pennsylvania, New Jersey, and to NYC and Long Island.

This track can be viewed on radar (not shown) and implied for the storm reports in Figure 1.

Once this feature generated a cold pool and low-level jet it was relatively long-lived.

Like many west-to-east bows, this one began to quickly take shape as the convection

moved into a valley near Weikert, Pennsylvania (lower left Fig. 8). Over nearly 20 years of

WSR-88D use in central Pennsylvania this region of the State is a region of bow echo

development and may be favored due to the terrain and funneled flow of moisture up the more

north-south Susquehanna Valley. The initial winds as the cold air descended into the valley near

Weikert were about 30 kts (Fig. 8 upper right) and the storm relative motion (SRM) showed

convergence.

The rapid evolution of the bow is shown in Figure 9. By 2130 UTC winds along the line

of storms were over 40 kts. Experiences shows that bows with 40 to 45kts will occasionally

produce reports of downed trees and wires. The winds with this bow would peak near 50 kts in

Schuylkill County near Tamaqua. No wind damage was observed in Schuylkill County but wind

damage was observed near Sunbury where the leading edge of high winds, around 40kts, was

approaching at 2130 UTC.

4. Summary

A severe weather event developed in the Mid-Atlantic region and brought damaging

winds to central Pennsylvania. The event occurred as a short-wave moved through a strong ridge

over the eastern United States. A surge of high PW air with modestly enhanced flow over the

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anticyclone over the western Atlantic lead to the development of several bow echoes and a few

isolated large thunderstorms.

One bow echo developed over central Pennsylvania in a general west-to-east valley and

accelerated eastward. This bow produced several reports of wind damage from near Sunbury

Pennsylvania to the New City Metropolitan area.

A second bow developed in southern Pennsylvania and produced some very isolated

wind damage in the Harrisburg region. Earlier in the event there were shorter lived storms which

pushed 60dBZ cores to near 30fkt. The -20C level, often used to pulse storms was near 26kft

during the event.

Two storms produced short-lived isolated tornadoes. Neither was in the State College

forecast area and are not examined here.

5. References

Craven, J. P., and H. E. Brooks, 2004: Baseline climatology of sounding-derived parameters associated

with deep moist convection. Natl. Wea. Dig., 28, 13–24.

Doty, B.E. and J.L. Kinter III, 1995: Geophysical Data Analysis and Visualization using GrADS.

Visualization Techniques in Space and Atmospheric Sciences, eds. E.P. Szuszczewicz and J.H.

Bredekamp, NASA, Washington, D.C., 209-219.

Davies, J. M., 2006a: Tornadoes in Environments with Small Helicity and/or High LCL Heights. Wea.

Forecasting, 21, 579–594. doi: http://dx.doi.org/10.1175/WAF928.1

Davies, J.M.. (2006b) Tornadoes with Cold Core 500-mb Lows. Weather and Forecasting 21:6, 1051-

1062

Online publication date: 1-Dec-2006. Abstract . Full Text . PDF (1512 KB)

Grams,J.S, R. L. Thompson, D. V. Snively, J. A. Prentice, G. M. Hodges, L. J. Reames. (2012) A

Climatology and Comparison of Parameters for Significant Tornado Events in the United States.

Weather and Forecasting 27:1, 106-123 Online publication date: 1-Feb-2012.

http://journals.ametsoc.org/doi/pdf/10.1175/WAF-D-11-00008.1

Grams, J. S.,W.A.Gallus Jr., S. E.Koch, L. S.Wharton,A. Loughe, and E. E. Ebert, 2006: The use of a

modified Ebert–McBride technique to evaluate mesoscale model QPF as a function of convective

system morphology during IHOP 2002. Wea Forecasting, 21, 288–306.

Markowski, P. M., J. M. Straka, and E. N. Rasmussen, 2002: Direct surface thermodynamic observations

within rear-flank downdrafts of nontornadic and tornadic supercells. Mon.Wea. Rev., 130, 1692–

1721.

Rutledge, G.K., J. Alpert, and W. Ebuisaki, 2006: NOMADS: A Climate and Weather Model Archive at

the National Oceanic and Atmospheric Administration. Bull. Amer. Meteor. Soc., 87, 327-341.

Markowski, P, Y. Richardson, E. Rasmussen, J. R. Davies-Jones, R. J. Trapp, 2008: Vortex Lines within

Low-Level Mesocyclones Obtained from Pseudo-Dual-Doppler Radar Observations. Mon. Wea.

Rev., 136, 3513–3535. doi: http://dx.doi.org/10.1175/2008MWR2315.1

Schoen, J.M W. S. Ashley. 2011: A Climatology of Fatal Convective Wind Events by Storm Type.

Weather and Forecasting 26:1, 109-121. Online publication date: 1-Feb-2011.

Abstract . Full Text . PDF (1569 KB)

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Trapp, R. J., S. A. Tessendorf, E. S. Godfrey, H. E. Brooks, 2005: Tornadoes from Squall Lines and Bow

Echoes. Part I: Climatological Distribution. Wea. Forecasting, 20, 23–34. doi:

http://dx.doi.org/10.1175/WAF-835.1

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Figure 2. GFS 00-hour forecasts of 500 hPa heights and 500 hPa height anomalies in 6-hour increments from a) 0000 UTC 15 July through f) 0600 UTC 16 July 2012. Return to text.

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Figure 3. As in Figure 2 except for precipitable water (mm) and precipitable water anomalies. Return to text.

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Figure 4. As in Figure 2 except for CAPE(JKg-1) shading showing values over 800,1200,1500, and 2400 JKg-1. Return to text.

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Figure 5. As in Figure 2 except for 850 hPa winds (kts) and 850 hPa wind anomalies. Return to text.

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Figure 6. As in Figure 2 except for 850 hPa to 10m bulk shear (kts) estimating shear in the lowest 1.5km layer. Return to text.

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Figure 7. As in Figure 2 except of 850 hPa moisture flux. Return to text. -

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Figure 8. KCCX radar at 2103 UTC 15 July 2012. Upper left shows 0.5 degree reflectivity, upper right shows 0.50 degree velocity, lower right shows storm relative velocity and lower left shows the terrain. Arrow shows origin of bow and its approximate path. Return to text.

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Figure 9. As in Figure 8 except showing 0.5 degree reflectivity and base velocity data upper image is 2112 UTC and lower image is 2130 UTC. Return to text.

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Figure 10. As in Figure 9 except for 2144 and 2212 UTC. The green bow is drawn in to approximate the feature of note with higher winds in the 40 to 48kt range behind the bow. Return to text.

NWS State College Case Examples

NWS State College Case Examples