Strong upper flow support been noticably absent this season?

[Mike Johnston]

I've noticed with many of the severe setups this season, we have not seemed to observe very strong flow at 200 - 300mb - at least over much of the traditional plains chase territory. In particular, I cannot recall any setups where divergence in the jet was a significant contributor to lift. Here and there, we see forecast diffluence (where deceleration of velocity offsets directional divergence), but mainly the jet just seems to crap out upon reaching the plains.

Since this is my first season of intense study, I have a question for those of you with more seasons under your belt: is this absence of strong upper air dynamics unusual for the chasing season, or is great divergence more a special circumstance that we just haven't been blessed with thus far?

 

[Jeff Snyder]

Mike,

I would say that the bigger issue this year has been lmiited moisture. It seems that the moisture scarcity has been the ruin of many potential chase days this year. The repeated Gulf scouring cold fronts has meant that we haven't had true Gulf moisture into the Plains until just the past week or two. Usually, we see at least a few days in April with mid-upper 60 Tds, but not this year. Several of the 'cold core' days earlier in MArch and April could have been very good had we had the moisture and resultant instability. I can't imagine what April 25th would have been like with upper 60 Tds and 3000-4000 CAPE. And what happens when moisture is lacking? We either end up with weak storms or no storms at all (aka - cap busts, much like 2002).

But you are correct... On the days that we have had at least sufficient moisture (such as April 21st), we haven't had the upper support (like 20kts at 250mb on the Topeka and Springfield soundings that evening). This is often the biggest quandry with tornadoes and supercells -- the juxtaposition of good instability and strong shear just doesn't happen very often. As we've seen, some years it happens much less frequently than others. If I recall correctly, there have only been two days this year with signficant warm-sector tornadic supercells -- April 21st and May 12th, both of which had relatively weak mid-upper level support (weak flow in the 250-500mb layer). Oddly enough, those are two days which had sufficient moisture.

EDIT: Forgot about May 11th in NE/IA. But if memory serves me correctly, those supercells didn't really produce nice tornadoes, despite the awesome structure and radar representation.

 

[Mike Johnston]

Yes, I see the juxtaposition of factors really hasn't come together in classic fashion yet, and recognize this can be a rarity some seasons. I guess the crux of my question has to do with upper flow divergence. As I understand, lift can be created by either convergence at the surface or divergence in the upper levels. I remember in my younger years when I lived in Wichita, the TV meteorologist Mike Smith (now of WeatherData, I believe) used to explain this jet stream divergence, and how it could lead to severe weather, to his viewers very well. From memory, this seemed to be a fairly common occurence, but I just haven't noticed this phenomenon this season. I see 100kt+ jets swirling all over the place out over the Great Basin, etc., but never seem to work their way to the plains - am I simply expecting something which doesn't occur very often, or has this factor been unusually absent?

 

[Jeff Snyder]

Mike,

You are correct, the past few "systems" have cut-off, sat over the western U.S., and sheared off into the northern US, making minimal impacts on the central and southern plains weather. Most of the time, we look for a nice jet streak to round the base of an upper-level trough and nose itself out into the plains. For what it's worth, remember that transverse circulation about a jet streak results in divergence aloft over the left-front (left-exit) and right-rear (right-entrance) region of a jet streak. There was a publication by one of the SPC mets which found that the vast majority of signficant tornadoes occur in the exit region of jet streaks, and, in fact, there were more tornadoes in this study that occurred in the right-exit region (subsident region by transverse circulation) than the right-entrance region. Throw in the fact that many jet streaks are curved if it's rounding the base of the trough, and we tend to have enhanced upward motion in the left-exit region (due to transverse circulation and DPVA), enhanced subsidence in the left-entrance region (again, due to transverse circulation and DNVA), while the remaining two sectors (the right entrance and right exit regions) may experience negligible vertical motion due to the offsetting contributions from the transverse circulation and differential vorticity advection.

I'm glad you made the distinction, because many people tend to confuse divergence and diffluence. Divergence, as you correctly noted, combines both a speed influence and a directional influence. Winds may "fan out" over the plains (in a fashion known as diffluence), but there is often speed convergence over the same area. For example, we tend to expect diffluence downstream of a jet streak, yet if we are downstream of a jet streak, we had speed convergence. Sometimes these two contributions cancel out, other times directional divergence (diffluence or confluence) overweighs the speed convergence to result in net divergence aloft.

Because many of the big "systems" this year have not been progressive troughs, but rather giant upper-level cut-off lows, the flow in the plains has been unclimatological. With the cut-off lows like we have seen this year, divergence aloft tends to be negligible, since the flow about the low tends to be rather homogeneous in a circular sense. Typically, this isn't the case, and we do see at least a few events with the classic "jet streak coming out of the southwestern US and into the Plains". Alas, this hasn't happened much this year, and in fact there is a direct relationship between the lack of this upper-level setup and the lack of moisture in the plains. Istead of a deep western US long-wave trough, which usually induces lee troughing and broad return flow, we've had a persistent eastern US trough and Hudson Bay low (reminscient of January), which has allowed for these Canadian highs to invade the continental US east of the Rockies. The upper-air pattern has been such that these large continental airmasses have been able to push all the way through the Gulf and, in some cases, well into the Caribbean and Central America. Thus, we have an unfavorable upper-air pattern which has been a signficant cause of the lack of Gulf moisture in the plains. And, we see, the two big issues this year, moisture and the lack of progressive upper-level troughs and southwest flow aloft, are tied together.

EDIT: I was afraid to write "invade" since that's very similar to "INVADERS" in Corso-ese. :lol:

 

[Karen Politte]

Yes, my vote for the season spoiler goes to two issues:

Lack of moisture (and the reasons causing it), and blocky/non-progressive type pattern.

I'm glad I have a job to work at when it's down-time.....most of the season's been down.

KR

 

[Joel Wright]

Yes this season has sucked so far.

Maybe there'll be an unusually large summer outbreak this year.