Christmas Day Severe Weather
When teaching students how to forecast severe thunderstorms (like the outbreak on Christmas Day...see storm reports below), I've always stressed that they try their best to get a comprehensive handle on the large-scale weather pattern before they start to weigh the mesoscale details. Such a "big-picture" approach means looking at the surface for boundaries, areas of low-level confluence, etc.), 850 mb (low-level jet streams, temperature advection, etc.), 500 mb (short-wave troughs, wind maxima, etc.), and 300 mb (jet streaks, diffluence, etc.).
With regard to 300-mb jet streaks (or 250-mb in the warm season), I pretty much throw out the idealized (and sometimes unrealistic) four-quadrant conceptual model for a straight jet streak. Yes, I realize that this conceptual model is sometimes touted as gospel. Toward the end of my teaching career, however, I presented the four-quadrant conceptual model, but then I told my students to use it at their own risk whenever severe weather was lurking, especially in the case of curved jet streaks.
The storm reports for Christmas Day, 2012. Courtesy of the Storm Prediction Center.
Why would I take such an approach? A seasoned forecaster, Jack Hales, stated (probably more than once) that "People have died in the wrong jet quadrant." By this he meant that low-level uplift and any subsequent initiation of severe thunderstorms are not always constrained to occur in the two most statistically favored quadrants (right-entrance and left-exit regions).
Let's assume that there's an upper-level jet streak slated to pass over a region where ingredients in the lower troposphere appear to be coming together for an outbreak of severe thunderstorms. Focusing my attention on the corresponding quadrant of the 300-mb jet streak (the quadrant above where the "low-level ingredients" are coming together), I look for reasons why this specific quadrant might not be favorable for the development of storms. In other words, I automatically assume from the get-go that this quadrant will support deep, moist convection. Then I look for reasons why it might not be favorable. If I can't find any good reasons why it can't, red flags immediately go up in my mind.
Such was the case for the outbreak of severe weather over the Deep South on Christmas Day. A cyclonically curved jet streak (00Z NAM analysis of 300-mb isotachs, valid at 7 P.M. CST on December 25) was rounding the base of a vigorous short-wave trough. To see this relationship, check out the 00Z NAM model analysis of 300-mb isotachs (color filled this time) and 300-mb heights (lime green contours). Note that the streak's maximum wind speeds were greater than 120 knots at this time.
The 00Z NAM model analysis of 300-mb isotachs (green contours) on December 26, 2012 (7 P.M. CST December 25) and the corresponding 00Z mosaic of composite reflectivity. Larger image. Courtesy of Penn State.
The short wave supported a surface low-pressure system and its associated cold front (see 00Z surface analysis). To convince yourself that you can throw out any preconceived notion about the infallibility of the four-quadrant model of a straight jet streak, check out (above) the 00Z NAM model analysis of 300-mb isotachs and the corresponding mosaic of composite reflectivity (larger image). Yes, the severe thunderstorms occurred in the right-exit region, where the fallible four-quadrant model "maintains" that upper-level convergence occurs (and, concomitantly, downward motion beneath the right-exit region).
When I added the 300-mb streamlines to the isotachs (00Z NAM analysis), a diffluent pattern emerges (note how the streamlines in the vicinity of the jet streak spread out). Above the earth's surface, diffluent streamlines don't automatically translate to divergence. That's because the wind vectors can produce speed convergence (see idealized schematic), so we have to be careful here. Looking at the 00Z NAM model analysis of the 300-mb vertical motion field (blue contours) in concert with the 300-mb isotachs (green contours), we can see that there was upward motion in the right-exit region (so the diffluent pattern was divergent). To get your bearings, I point out that the dashed, blue contours represent negative values (in microbars per second) of upward motion, which is consistent with pressure decreasing with time. The elongated area of upward motion at 300 mb corresponds to the area where severe thunderstorms were occurring.
Here's a summary based on the NAM model analysis of the weather pattern at 00Z and the corresponding radar data. Note the thunderstorms occurred in the diffluent right-exit region of the 300-mb jet streak.
To be fair, note the bull's eye of strong upward motion in the left-exit region of the jet streak over Arkansas. Still, the lesson learned here is that the four-quadrant model is highly idealized and sometimes fails in outbreaks of severe thunderstorms like the one on Christmas day.
Here endeth the lesson.