When you've completed this page, you should be able to discuss where severe thunderstorms usually occur in the context of mid-latitude cyclones. You should also be able to discuss the role of surface boundaries in lifting parcels to the LFC, as well as the concepts of moist advection and moisture convergence.
When I evaluate the lower troposphere for its potential to initiate and support deep, moist convection, I routinely look at several fields, including surface temperatures and dew points (as well as their profiles in the first several thousand feet above the ground). After deciding that the overall thermodynamic environment favors an outbreak of thunderstorms based on relatively high CAPE and low CIN, I then search for surface boundaries. I'm using "surface boundaries" here as a generic "catch-all" term for fronts and mesoscale boundaries such as dry lines, sea-breeze fronts, and outflow boundaries (we'll explore these features in more depth later). My forecasting checklist includes this search because surface boundaries have the potential to lift air parcels to the LFC.
As you already know, low-level lift is often associated with surface convergence (orographic lift can also get air parcels to the LFC). Ultimately, identifying surface boundaries that have the potential to lift air parcels to the LFC boils down to finding areas where there's low-level convergence. Not surprisingly, there are a couple of important tools that help forecasters to detect low-level convergence, and they're the focus of this section.
WPC Analyses, Satellite and Radar Imagery
Where should you start your search for surface boundaries? The surface analyses from the Weather Prediction Center (WPC) are a good place to start. On the most fundamental level, identifying surface fronts should always be a top priority because they can help to lift air parcels to the level of free convection. There's a broader, more philosophical reason, however, for you to sit up and take notice of surface fronts. There's a crusty, old forecasting rule that most severe thunderstorms occur in the warm sector of a mid-latitude low (the region ahead of the low's cold front and on the warm side of the warm front), or on the cool side of a warm or stationary front within 300 kilometers of the front.
This "old school" law of forecasting is probably one of the most important you'll encounter in this course because it can really help you keep your bearings when assessing potential areas for severe weather outbreaks. It won't help you catch every single area at risk for severe thunderstorms, but it will help you find most of them (especially the "big" outbreaks).
Either way, surface boundaries (fronts, troughs, dry lines, outflow boundaries, etc.) are certainly something to keep your eye on because they are typically zones of low-level convergence. However, surface boundaries typically don't initiate storms everywhere. So, at face value, WPC surface analyses don't always show exactly where the surface boundaries are going to be "active." Therefore, we need some additional help in identifying where thunderstorms may erupt along a surface boundary. For assistance, we'll turn to our indispensable old friends, satellite and radar imagery.
Let's use May 10, 2011 as an example. The 21Z surface analysis on May 10, 2011 (illustrated below), showed a dry line and a series of fronts over the Middle West that were associated with an occluding low-pressure system over North Dakota. Were any of these surface boundaries about to initiate thunderstorms? To answer this question, let's turn to satellite and radar imagery.
On the 21Z analysis above, turn your attention to the long dry line that weaved its way from Texas all the way to the southern border of Minnesota on the map. But, a look at satellite imagery shows that the boundary likely didn't stop there. If you look at the 1945Z visible satellite image, you'll see a boundary between clear air and a cumulus field over Minnesota, which likely was the northern portion of the dry line. WPC elected not to analyze it probably because there weren't enough station models to detect the dry line in that area. At 21Z, the stationary front over western Minnesota clearly lay west of the surface boundary represented on satellite imagery (compare the 21Z visible image to a close-up 21Z surface analysis). At any rate, this portion of the dry line started to initiate thunderstorms by 21Z, and a few hours later, some nasty-looking storms were underway (2245Z visible image).
Radar imagery also suggested the presence of this surface boundary over Minnesota. The 21Z composite of base reflectivity showed a line of generally weak reflectivity representing insects, etc. caught in the pattern of low-level convergence associated with this portion of the dry line. Just two hours later, the 23Z composite of base reflectivity shows deep, moist convection initiating along the boundary.
The moral of this story is that while synoptic-scale surface analyses are helpful in identifying large-scale surface boundaries, satellite and radar imagery are critical forecast tools that can point the way to where thunderstorms may erupt along those boundaries (or other subtle boundaries not obvious from a surface analysis map).
There are other ways you can assess low-level convergence and its potential to initiate thunderstorms, and I'll discuss them now in the context of the outbreak of severe weather that occurred over the Upper Middle West on on July 14, 2010.
July 14, 2010
Previously, we explored the role that the 500-mb pattern played in priming the atmosphere for deep, moist convection in the Upper-Mississippi Valley on July 14, 2010 (you may want to revisit the video Case Study on that page). While the 500-mb pattern made the atmosphere generally more favorable for deep, moist convection, it was surface boundaries that determined the specific locations where thunderstorms would erupt. Now we're going to focus on the occluded and cold fronts in Minnesota, which you can see on the 21Z surface analysis. Low-level convergence along these boundaries helped lift parcels to the LFC, causing thunderstorms to erupt (check out the 21Z mosaic of composite reflectivity).
Anticipating the onset of severe thunderstorms, SPC forecasters issued a "Mesoscale Discussion," which they typically do when severe thunderstorms are slated to develop in the next several hours (read more about Mesoscale Discussions, if you're interested). In Mesoscale Discussion #1303, SPC forecasters mentioned the pivotal role of low-level convergence in the initiation of thunderstorms along these two fronts (excerpt below):
STORMS ARE DEVELOPING WITHIN ZONE OF DPVA AND LOW LEVEL CONVERGENCE ATTENDING A PROGRESSIVE VORT MAX AND OCCLUDED/COLD FRONT FROM N-CNTRL THROUGH CNTRL MN.
If you're confused by the mention of "DVPA," don't worry. It's just an acronym for "differential positive vorticity advection," which is a fancy proxy for divergence downwind of the vort max mentioned in the next sentence of the discussion. But, clearly, forecasters at SPC recognized the importance of the occluded and cold fronts in creating low-level convergence that could lift parcels to the LFC.
As you learned previously, forecasters routinely examine charts of surface streamlines to pinpoint lines of low-level convergence. Indeed, the 19Z analysis on July 14, 2010 (above), revealed a line of surface convergence along the occluded and cold fronts in Minnesota; it was one of the key factors that prompted SPC forecasters to issue MD #1303. In a nutshell, the confluence or "coming together" of streamlines in the region corresponds to low-level convergence. Note that we can directly connect confluence to mass convergence at the surface, but not necessarily aloft, for reasons we'll discuss later.
With regard to initiating surface-based thunderstorms, it's important that the air converging along a surface boundary is relatively moist. As it turns out, there are actually two processes typically at work along surface boundaries that initiate deep, moist convection: low-level convergence and moist advection. For the record, moist advection is the horizontal transport of moist air by the wind.
One standard way to measure moisture, which you may remember from previous courses, is mixing ratio. As a reminder, mixing ratio is the ratio of the mass of water vapor to the mass of dry air in a parcel, and is usually expressed in grams per kilogram. Let's use mixing ratio to assess the air that converged along these boundaries to initiate thunderstorms on July 14, 2010. If you check out this analysis of 1000 mb streamlines (a proxy for the surface) superimposed on mixing ratio at 21Z, you can easily see the predominant southerly flow from the Gulf of Mexico that advected moisture far northward over the Upper Mississippi Valley. To get your bearings, the contour interval for mixing ratio is 2 g/kg, and values exceeding 18 g/kg are color-filled in green to indicate moist air. Note the "tongue" of high mixing ratios (very moist air) that extended into Minnesota and south-central Canada.
For convenience, SPC forecasters combine the two processes of low-level convergence and moist advection into a single field called moisture convergence. For the record, the magnitude of convergence in the calculation usually dominates the magnitude of advection, so this field gives forecasters a good proxy for surface convergence. The 21Z surface analysis of this field on July 14, 2010 (below), indicates relatively strong moisture convergence (solid red contours) in Minnesota near the secondary low and along the occluded and cold fronts. Note, just a tad farther east, that there's another pocket of moisture convergence. This pocket coincides with an area where discrete tornadic supercells erupted near eastern Minnesota and the western Wisconsin border. By now you should be getting the sense that moisture convergence near the ground goes hand in hand with initiating surface-based thunderstorms.
These maps from SPC are really useful because they allow you to see surface winds, mixing ratios, and moisture convergence together. For mixing ratios, the contour interval is 2 g/kg and mixing ratios exceeding 16 g/kg are color-filled with dark green in order to indicate the areas that are quite moist. For the record, the units of moisture convergence are grams per kilogram per second. Those units might seem odd to you at first, but keeping in mind that the units of mixing ratio are grams per kilogram, the "per second" makes sense since we're measuring moist air coming together in time.
If you're interested in finding resources for assessing low-level convergence via streamlines and moisture convergence, check out the Explore Further section below, as it has some key data resources for you. Otherwise, up next, we're going to look at some "sneaky" ways that low-level convergence can help initiate thunderstorms. As it turns out, moisture convergence doesn't only occur along fronts. Let's investigate.
Key Data Resources
If you're looking for resources to analyze areas of surface convergence, via current or forecast streamlines, moisture convergence, etc., you may be interested in the following links:
- SPC Mesoanalysis Page: You can get real-time and recent regional Rapid Refresh analyses of surface winds, mixing ratio, and moisture convergence via the "Surface" menu. For archived national images (like the one shown above), you use the National Sector Archive (select your date, then hour, and look for "mcon" in the file name).
- Plymouth State Weather Center: Includes analyses of surface streamlines.
- University of Wyoming: Includes forecasts for streamlines at 1000 mb (a proxy for the surface), as well as the ability to overlay with mixing ratio (among other forecast variables).