Prefrontal Troughs and Confluence

This is a sample lesson page from the Certificate of Achievement in Weather Forecasting offered by the Penn State Department of Meteorology. Any questions about this program can be directed to: Steve Seman


When you've completed this page, you should be able to discuss the role of prefrontal troughs and zones of surface confluence in initiating thunderstorms. You should also be able to discuss "deep moisture convergence", and how it can help forecasters determine whether discrete (individual) thunderstorm cells or large, organized "thunderstorm systems" will form.


When you originally learned about the classic model of a mid-latitude cyclone, you learned that the model includes showers and thunderstorms developing along or just ahead of the low's cold front. As you just saw in the previous section, cold (or occluded) fronts serve as classic lines of low-level convergence that can lift parcels to the LFC, initiating thunderstorm formation. But, sometimes, lines of showers and thunderstorms form farther out ahead of cold fronts. Let's investigate.

Prefrontal Troughs

Check out the surface analysis at 06Z on February 11, 2009 (below). At the time, a cold front associated with a low centered over the panhandle of Texas was moving east across eastern Texas.  Ahead of the cold front, a squall line had formed in a prefrontal trough, where low-level convergence played a role in getting air parcels to the LFC. For the record, a prefrontal trough is simply a trough (elongated area of low pressure) preceding a cold front that is usually associated with a wind shift. The storms on February 11, 2009 meant business (06Z composite of radar reflectivity), and SPC eventually issued a Tornado Watch as the squall line roared eastward.

06Z surface analysis on February 11, 2009 showing a pre-frontal trough and squall line.
The 06Z surface analysis on February 11, 2009, shows that a squall line had developed in a trough ahead of a cold front (a prefrontal trough).
Credit: Weather Prediction Center

To affirm the convergence in the surface trough (where the squall line formed), please note the confluence of the wind barbs (southerly just ahead of the trough and west-northwesterly and southwesterly just behind the trough). The corresponding 06Z analysis of surface streamlines leaves no doubt about confluence over eastern Texas along the prefrontal trough. At the surface, this confluence indicates convergence (remember, that's not necessarily true aloft), but can we get a little more quantitative with our assessment of low-level convergence?

Another useful tool for identifying regions where there might be sustained and "dependable" lines or areas of convergence in the boundary layer is SPC's deep moisture convergence (or "deep moist convergence") product. Sometimes, looking at moisture convergence right at the surface doesn't paint the clearest picture of where sustained, meaningful moisture convergence is occurring. So, SPC's deep moisture convergence product averages moisture convergence in the lowest two kilometers of the troposphere. In other words, if there's something happening in the boundary layer, this field usually shows it. Like moisture convergence at the surface (discussed in the previous section), the convergence term tends to dominate the advection term, so you can use this product as a proxy for convergence in the lower troposphere.

As a quick aside, SPC calls the product "Deep Moist Convergence", but I will use "deep moisture convergence." That's because "deep moist convergence" looks a lot like "deep, moist convection, and I don't want you to get these two terms mixed up (it's easy to do).

The 06Z analysis of deep moisture convergence on February 11, 2009 (below), shows that there was ample deep moisture convergence associated with the prefrontal trough over eastern Texas (the red contours represent lines of constant deep moisture convergence).  For the record, the units of deep moisture convergence are grams per kilogram per second (the same as the units of surface moisture convergence).

06Z analysis of deep moisture convergence.
The 06Z analysis of deep moisture convergence on February 11, 2009. Red contours indicate lines of constant deep moisture convergence (dashed contours represent lines of constant moisture divergence). Note the strong, deep moisture convergence associated with the prefrontal trough over eastern Texas. The thin, green contours are lines of constant mixing ratio in the lowest 100 mb of the troposphere.
Credit: Storm Prediction Center

Clearly, the confluence along the prefrontal trough corresponded to strong deep moisture convergence. In case you're wondering, 06Z field of surface moisture convergence was much less impressive, so there's often an advantage in assessing the deep moisture convergence to catch the impacts of moist advection and convergence throughout the boundary layer.

Now that we've seen that prefrontal troughs can be regions of deep moisture convergence, which can lift parcels to the LFC, what exactly causes prefrontal troughs? In many cases, it's the prevailing synoptic-scale pattern. For the case of the prefrontal trough over eastern Texas on February 11, 2009, synoptic-scale forcing was certainly the culprit. At the time, there was a closed 500-mb low trailing the cold front to the west. Farther to the east of the strong vort max associated with the 500-mb closed low, a lobe of relatively high absolute vorticity likely produced enough upper-level divergence to promote surface pressure falls ahead of the cold front. Note that this analysis has isovorts contoured every 2 units instead of every 4 so that the lobe of relatively high vorticity was easier to pick out.

It's often subtle, weaker vort maxima like this one that can lead to surface pressure falls and prefrontal troughs. Of course, another potential cause of prefrontal troughs that you learned about previously is a lee trough, which can form over the Plains (or east of the Appalachians) with relatively fast westerly flow blowing over the mountains. If you're interested in learning more about the possible causes of prefrontal troughs, I recommend the paper "A Review of Cold Fronts with Prefrontal Troughs and Wind Shifts," published in Monthly Weather Review.

Regardless of the specific cause of a prefrontal trough, you should be much more concerned about the potential role a specific prefrontal trough might play in a subsequent outbreak of severe weather. Analyses of mean sea-level pressure, surface streamlines, and analyses of deep moisture convergence (or sometimes surface moisture convergence) will help you identify potential zones of confluence and convergence where parcels may be lifted to the LFC.

With that said, however, sometimes there's confluence that we need to be aware of without an obvious prefrontal trough (or clear surface trough of any type). On February 11, 2009, the prefrontal trough over eastern Texas (where the squall line formed) was pretty obvious on the 06Z synoptic-scale analysis of mean sea-level pressure. But, without such an obvious prefrontal trough, surface streamlines can still help forecasters identify confluent zones.

In such cases, one could argue that surface streamlines are even more useful than analyses of mean sea-level pressure because they cut right to the chase. In the grand scheme of forecasting, determining whether or not a trough happens to be coincident with a confluence of surface streamlines doesn't really matter. The fact that there's confluence and low-level convergence is what matters, and using surface streamlines will allow you to get a quick sense for where there's low-level convergence that has the potential to get air parcels to the LFC.

To highlight the utility of surface streamlines, check out the 21Z analysis of surface streamlines from February 18, 2009 (below):

21Z analysis of surface streamlines from February 18, 2009 showing weak confluence.
The 21Z analysis of surface streamlines on February 18, 2009 shows a zone of weak confluence across Mississippi and Alabama. Note the "squeezing together" of streamlines in the oval.
Credit: Storm Prediction Center

A region of confluence was readily apparent across Mississippi and Alabama as southwesterly streamlines "squeezed together." Meanwhile, if we examine the corresponding 21Z surface analysis, is the region of confluence as obvious? Not really. At the time, much of the Southeast lay in the warm sector of a mid-latitude cyclone centered over Lake Huron, but at first glance, the warm sector looks rather commonplace, with no prefrontal troughs or apparent areas of surface convergence. So, a forecaster that just looked at the analysis of mean sea-level pressure may have easily missed this zone of confluence, which is why I highly recommend incorporating streamlines into your forecasting routine!

As it turned out, this rather weak confluence ahead of the cold front (in the warm sector) produced just enough surface convergence in the warm sector to get air parcels to the LFC and set the stage for discrete supercells to erupt (check out the 2330Z reflectivity). For the record, surface-based CAPE was relatively high in the warm sector, and the vertical wind shear was strong.

Deep Moisture Convergence and Thunderstorm Mode

Were you surprised to see  that "rather weak confluence" helped to initiate discrete supercells, which represent some of the most violent thunderstorms on Earth? I confess that it might seem a bit contradictory at first. Allow me to shed a little light on what seems to be a paradox.

For starters, take a look at the 21Z analysis of deep moisture convergence on February 18, 2009 (shown below). To get your bearings, the red contours represent lines of constant deep moisture convergence, and the thin green contours indicate the average mixing ratio in the lowest 100 mb of the troposphere.  The deep moisture convergence in the warm sector over the Southeast States (where supercells erupted) is generally weak and rather piecemeal (scattered or fragmented).

21Z analysis of deep moisture convergence
The 21Z analysis of deep moisture convergence on February 18, 2009. Please note that deep moisture convergence was rather weak and piecemeal in the warm sector over the Southeast States. Nonetheless, discrete supercells erupted over this region.
Credit: Storm Prediction Center

I realize that "weak" is somewhat of a subjective description, but with a comparison, I think you'll see why I classified it as weak. Compare the deep moisture convergence in the warm sector at 21Z on February 18 (above) with the deep moist convergence over the eastern Gulf and Southeast Coasts 12 hours later at 09Z on February 19. No contest, wouldn't you agree? The strong, organized band of deep moisture convergence along the Gulf and Southeast Coasts was associated with the strong cold front, which had obviously advanced southeastward during the 12-hour period after 21Z.

The transition to stronger, more organized deep moisture convergence translated to big differences in thunderstorm mode (type). At 2330Z, when the deep moisture convergence was weak and piecemeal, discrete supercells were able to erupt (2330Z radar review). But, with much stronger, more organized deep moisture convergence, a large, organized line of thunderstorms developed (0925Z radar, for comparison).

The bottom line here is that strong surface convergence distributed rather uniformly along a surface boundary tends to initiate large "thunderstorm systems" that are more organized (in this case, a long line at 09Z on February 19, 2009). Conversely, weak, piecemeal convergence (confluence) at the surface tends to initiate discrete storms. Not surprisingly, there's more to the story, and I'll fill in all the scientific details later in the course.

Now that you have a better sense for the role that the synoptic-scale surface pattern plays in the development of deep, moist convection, let's move our big-picture overview up to 850 mb. In the meantime, if you're interested in learning how to access analyses of deep moisture convergence, check out the Explore Further section below.

Explore Further...

Key Data Resources

If you want to access analyses of deep moisture convergence, you'll be interested in the following resource:

  • SPC Mesoanalysis Page: You can get real-time and recent regional Rapid Refresh analyses of deep moisture convergence (called "Deep Moist Convergence") via the "Upper Air" menu. For archived national images (like the one shown above), you can use the National Sector Archive (select your date, then hour, and look for "dlcp" in the file name).