3.5.1 Squall Line Motion

For all types of squall lines, line motion is a result of both the advection of individual cells within the line and propagation due to the triggering of new cells. The overall propagation speed of the entire squall line tends to be controlled by the speed of the system cold pool with new cells being constantly triggered along its leading edge gust front. At midlatitudes where some mid-level potentially negatively buoyant “dry” air is nearly always available, an average cold pool speed is on the order of ~ 20 m/s (40 kt).  Nearly everyone has experienced the cool, often refreshing and gusty winds that accompany thunderstorm passage. However, with many organized systems these winds become much stronger and potentially damaging as will be discussed later.

3.5.2 Long Line Motion

Click the image above to view animation.

Within squall lines composed of primarily ordinary cells, each cell will generally move with the 0-6 km mean wind with new cells triggering in the downshear direction of the low-level vertical wind shear vector, along the cold pool gust front. For very long squall lines (greater than 200 km or 110 n mi in length), individual cells may move at an angle to the line, but the net motion of the line usually stays perpendicular to its initial orientation, independent of the direction of the mean wind or mean wind shear vector. This was true during the scenario case where the cells moved to the northwest, but the long line overall moved eastward.

It may be difficult to assess cell motion within solid squall lines, as shown in the radar reflectivity loop above.  Derived radar products, such as the NEXRAD storm track indicator, 1-hour precipitation, or storm total precipitation may help. Notice the southwest-to-northeast oriented tracks in the STP product shown below for the same squall line that was animated above.

3.5.3 Short Line Motion

Click the image above to view animation.

However, for shorter lines (where the heavy precipitation is less than about 100 km or 55 n mi long), the systems tend to reorient themselves over time to be perpendicular to the mean low-level vertical wind shear vector. The lines then propagate in the direction of the shear vector as new cells are more easily triggered along the downshear gust front.

3.5.4 System Motion and Back Building

System motion can also be affected by variations in the environmental conditions along the squall line. For instance, more cells may be triggered in regions where the CAPE is greater or LFC is lower, thereby promoting squall line propagation toward the region with more favorable environmental conditions. This image shows a common scenario where cells are first initiated to the north where the “cap” or convective inhibition (CIN) is smaller, but over time cells are triggered further southwestward, as the gust front intersects the preexisting boundary. This setup can lead to “back building.”

Back Building Definition:

Back building refers to the situation where new convection persistently develops on the rear side of an existing storm or MCS. This may occur as a result of inflow being lifted by an outflow boundary that has become nearly stationary while the old cells move downstream or by gradual elimination of the cap on the inflow side of the system. It can cause a storm or the most vigorous parts of an MCS to propagate far to the right of the mean flow or typical cell motion and often results in little movement of the region of heaviest rain rates, producing flooding.

3.5.5 Review Question: Thunderstorm Formation

Assume it is mid-afternoon on 6 May 2003. Your job is to forecast thunderstorm development for north-central Texas during the remainder of the afternoon and early evening. At 2010 UTC low-level reflectivity data from the Fort Worth radar (KFWS) indicate a northeast to southwest boundary from central Oklahoma to the west of your area of concern.

To complete this exercise, study the radar loop and the accompanying data products and circle the area(s) where you expect storms to form in the next three to four hours. You will have the chance to compare your prediction to the experts'.

Click the image above to view animation.

Review Question Feedback

Expert Answer:

As shown in this radar loop, a line of thunderstorms did in fact back build along the boundary to the southwest into north-central Texas during the late afternoon and early evening.

Click the image above to view animation.

Expert Discussion:
This real life case closely matches the classic back-building scenario depicted in the conceptual model below. Not only does the CAPE increase to the south, but also there was evidence that the CIN (or cap) was weaker to the southeast than the northwest. This was apparent in the radar loop by noting that the thunderstorms initially developed over Oklahoma, not in Texas. The morning wind profile shown in the soundings indicated that the direction of mean vertical wind shear was toward the northeast.

End of Section 3.5: Squall Lines - Squall Line Motion