Unfortunately, it's not over yet. The SPC convective outlooks currently have moderate risks of severe weather posted for the next three days. It's round after round of severe weather.
I want to quickly look at the model forecasts to show why we aren't getting out of the woods yet. Here is this morning's European model 500mb analysis. I've drawn dashed lines through two troughs--one in the central US which is causing tonight's severe weather in the lower Mississippi Valley and the other just now impacting the Pacific northwest with a strong jet streak right behind it.
|Fig 1 -- ECMWF 500mb analysis of winds (colors) and heights (contours) at 12Z, April 25, 2011. From the HOOT website.|
|Fig 2 -- ECMWF 24 hour forecast of 500mb winds (colors) and heights (contours) at 12Z, April 26, 2011. From the HOOT website.|
|Fig 3 -- ECMWF 48 hour forecast of 500mb winds (colors) and heights (contours) at 12Z, April 27, 2011. From the HOOT website.|
But today I wanted to focus more on observing severe weather on radar, mostly in describing squall lines and bow echoes. Tornadoes are one major element of severe weather. However, the strong winds in squall lines cause tremendous amounts of damage too--and over a much larger area. What causes the very strong winds at the surface in these lines of storms?
Let's re-visit the basic model of a thunderstorm. As a storm (or several storms) move from west to east, they are fed by warm moist air rising in the updraft region. As that air rises, it cools and the water vapor condenses into rain. The now colder, rain laden air sinks rapidly back down to the surface in the downdraft region. Often the lift needed to get that warm, moist air initially rising is provided by convergence along a boundary like a cold front which helps move the storms along. Think of the cold front like a big snow plow, moving along and forcing warm moist air up and into the storm as it plows forward. Notice that the cold air sinking in the downdraft region sinks behind the cold front--this influx of cold air at the surface actually helps reinforce the cold front boundary as the cold downdraft air spreads out. In this way, the storms can actually become self-sustaining as their own downdrafts spread out and lift up surrounding warmer, moist air, further feeding the updraft.
|Fig 4 -- Schematic of a cross-section through an idealized thunderstorm. The storm is moving to the right.|
Remember in my previous discussions about tornado formation how the low-level winds just above the surface tend to really increase in strength right around sunset. This phenomenon is called the "nocturnal low-level wind maximum" or the "low-level jet" (these actually are different things, but the nocturnal low-level wind maximum is a kind of low-level jet...). So let's say that we have a strong jet of winds aloft just off the surface, like what we often see around sunset. What would that do to the storms?
Let's imagine a low-level jet of strong winds approaching the storm from the rear. What happens as the storm interacts with the jet? The rapidly sinking air in the downdraft region of the storm forces that jet of winds downward until it runs into the surface. This is what tends to give squall lines their particularly powerful straight-line winds--a low-level jet of air that is pushed down to the surface. In this case, the jet is often called the "rear inflow jet". Not only does this interaction bring those very strong winds down to the surface, but all that air rushing in behind the storm can force the cold front or outflow boundary (that snow plow that is helping to lift up the warm-moist air in the updraft) to surge forward rather quickly. Thus, the storms in the area where the low-level jet is intersecting the boundary will tend to rush forward faster than storms further from the center of the jet. This is what creates the "bow echo" structure so often seen on radar--the storms "bow out" or rush forward where the strong winds of the center of the low-level jet are being forced to the surface, causing those particular storms to surge ahead. No wonder that the strongest winds are often found on the leading edge of a bow-echo.
Smull and Houze, 1987 , Houze et. al 1989 or Biggerstaff and Houze 1991.So how does this look on radar?
Here's a reflectivity image of a squall line/bow echo from last week as it approached Champaign/Urbana, IL:
|Fig 6 -- KILX 0.5 degree base reflectivity image from 0002Z, 4/20/2011.|
|Fig 7 -- KILX 0.5 degree base velocity image from 0002Z, 4/20/2011.|
I wanted to zoom into the velocity image for a moment to talk more about properly interpreting radar velocity images. Remember--the radar can only measure air moving toward or away from it--so if we try to determine wind direction, those are our only two options. I talked in a previous post about how strong inbound air (green colors) right next to strong outbound air (red colors) can be evidence of rotation. But here we have strong reds and greens right next to each other along the leading edge of the line. This is NOT rotation--this is convergence. Here's a closer view:
|Fig 8 -- Zoomed in version of figure 7. The circled areas indicate potential rotation.|
Here's another example. This is from this afternoon's squall line moving through Arkansas. Here's the reflectivity image.
|Fig 9 -- 0.5 degree base reflectivity from KLZK, 0101Z April 26, 2011|
Here's the base velocity image corresponding to what we see above.
|Fig 10 -- 0.5 degree base velocity from KLZK, 0101Z April 26, 2011|
|Fig 11 -- Zoomed in figure 10, Slight rotation is implied in the circled area.|
You can see that not all bright reds next to bright greens on radar velocity images mean rotation--often they mean convergence or divergence. You must keep in mind where the radar is located and what that means for the implied wind directions to accurately assess the radar velocity image. The simple rule is this--draw a line directly out from the radar location to your area of interest. If the line separating the reds and greens is perpendicular to the line drawn straight out from the radar, then it is implied convergence or divergence. If the line separating the reds and greens is parallel to the line drawn straight out from the radar, it could imply rotation (particularly if it's a very small, localized region). Here's the above image with that guideline applied to it:
|Fig 12 -- Figure 11 annotated to describe areas of convergence versus rotation.|
Here's another, more advanced but still cool thing that we can see on radar. With certain radar viewing software, we can examine vertical cross-sections of the reflectivity and velocity fields shown by the radar and compare them to the idealized schematics I drew above.
Here's a reflectivity cross-section through the heart of the squall line in figures 9-12 as it approached Memphis this evening:
|Fig 13 -- Base reflectivity cross-section from KNQA at 0313Z, April 26, 2011.|
Things get more interesting when we look at the velocity cross-section:
|Fig 14 -- Storm-relative velocity cross-section from KNQA at 0313Z, April 26, 2011.|
Hopefully you can now look at squall lines and bow echoes on radar and understand a little bit more of what's going on. The powerful, straight-line winds associated with bow echoes have very distinct radar signatures that can provide advance warning of their approach. Furthermore, sometimes rotation can be embedded in that leading line, but it takes a keen eye to spot the velocity signature corresponding to that.