Chase Log 2006
The Year of No Tornadoes!
4579 Miles Total
Further east, relatively moist surface air could be found in southeasterly flow over Kansas heading towards southwest Nebraska. There, a trough axis separated that from drier air over Colorado, and the pattern was complicated by a couple of outflow boundaries in Kansas. We figured if the moisture made it to sw Nebraska at the same time that the incoming mid-tropospheric trough changed the shear profile, we'd have a chance to see a couple of supercells.
The RUC forecast exactly that, with favorable buoyancy showing up in the forecast CAPE field east of the trough axis, surmounted by 45 knots of 500 mb flow. SPC had outlooked the area for a slight risk, with a 5% tornado risk in the area.
What eventually occurred is that the rich dew point air over Kansas "mixed out" as it marched towards Nebraska. In addition, very strong surface souteasterlies (20-30 mph) underran 700 mb southerlies that were actually weaker, and then the progged 500 mb southwesterlies did verify. Unfortunately, that wind profile produced a backing wind shear vector with height, favoring left moving storms.
We were debating a northeast Nebraska target and a target from Hebron to Belleville. These two target areas werenortheast of the incoming midtropospheric trough: (a) the further north one went, the surface wind field was more backed (with better low level shear), but progged mid and upper tropospheric winds were nearly parallel to the surface winds....we felt that storms there would be developing in unidirectional shear; (b) further south, the low level shear was not nearly as great, but the 0-6 km shear vector was southwesterly and about 6 X 10-3 s-1 progged for the afternoon. We were also very uneasy about the CAPE "hole", which ended up verifying. That was evident in a number of the morning soundings in the area, which showed a very shallow moist layer.
SPC had a slight risk for the area, with a 5-10% tornado within 25 miles of a point probabilistic forecast for the area.
The storms that developed in the Grand Island area were clustered together with competing outflows. However, the southern most of those tended to be discrete, and there was a persistent development area near Kearney, a couple of times having a "hookish" look to it. Storms roared northeastward (35 to 45 mph), and were high-based. A check of the dew points showed why the latter was the case...the originally mid 60 dew points had mixed into the high 50s, just as the RUC had forseen.
We knew May 25 would be a down day, as a ridge temporarily rode over the northern Plains. The next trough was set to come out on May 26. So we spent the night of May 24 in Lincoln, Nebraska, and then drove west to North Platte Nebraska.
We were hoping that the upper 50 to low 60 dew points that had overspread the area overnight would not mix out (this was not to be). Thus, the warm front might serve as a source of lift for the east southeasterlies, and the outflow boundary could either serve as a source of parcel lift or a source of vorticity for storms that might form west of the area on the Palmer Divide. As it happened, the intiation occurred near the intersection of the outflow boundary and the warm front, but the moisture was very shallow, and the storms were high based. Ultimately, we observed beautiful supercells, but with high LCLs, no rotating wall clouds. To emphasize this, I measured a temperature of 99F and a dew point of 57F in the inflow area of these storms. Hardly conducive for tornadogenesis.
Later the storm progagated and had new developments on its right flank. This is a picture of the strongest mesocyclone cloud signature. Note the beautiful striations, and the rain curtains wrapping around the updraft area. The corresponding radar showed a shear signature of 103 mph across the hook echo (on the west part of the radar image). Here's a shot of Thom taking pictures of this impressive feature.
North of us was another mesocyclone. Here's a shot of Thom, Chuck and Vickie Doswell, with the cloud features in the background. And, a spectacular shot of the multiple midlevel cloud bands (cigars) that were being pulled into the various rotation centers.
Towards sunset, we had a wonderful view of the backlit cloud features with the major mesocyclone. And, as a final treat, that area had a downburst crash through the circulation...with a vibrantly colored cloud band to its east.
All in all, this was a spectacular day for storm chasing. No tornadoes, but a very photogenic and classically structured supercell.
On the morning surface map, steep lapse rates in an elevated mixed layer had overspread all of the Central Plains. But the deep moisture (generally in the green shaded area bounding the 60F+ dew points) was well east of the best (or any) deep layer shear (generally within 150 km of the arrow. SPC had outlooked a bit of the area for tornadic storms.
Scott Landolt had joined me, and we headed toward the northern Nebraska Panhandle, an area in which the RUC suggested a mesolow would circulate easterly flow not evident on the morning analysis back west into an upslope area of eastern Wyoming and western Nebraska, an area in which the prognosis had 500 mb flow more than adequate for rotating storms by the late afternoon...though the buoyancy, due to lack of deep low level moisture, would be meager.
Storms did develop, first in isolated fashion, and then more numerous. The early storms had signs of rotation, but by the afternoon, the area of western Wyoming had erupted with many storms, some with transient mesocyclones, that eventually coalesced into a derecho/MCS. In this radar image you can see the bowed segments, and the shear signatures of either mesocyclones or the surging gust front. We ducked into Chadron for the night as the bow echo came slamming through with frequent cloud to ground lightning, 3/4" hail, heavy rain, and winds gusting to 65 mph. There may have been some forward flank mesocyclones and weak tornadoes with the line as well.
In Chadron, the bow echo caught up to us. Looking to the west, we could see the tiered forward flank, with the suggestion of some rotation, just ahead of heavy precipitation, and dust being kicked up by the 60 mph outflow winds. Out to the west, we could see cloud features on the forward flank that had vortices. That shelf was roiled as it passed directly over us and showed curling cloud features consistent with the Threatnet Wagonwheels seen on the radar.
The boundary northwest or southeast along the KS-se CO border south to the western OK Panhandle, it could act as a focus for subsequent convection. Both the RUC and the NAM 12 UTC runs have 30 knot or greater southwest flow at 500 mb from the extreme western OK Panhandle and north to Palmer Divide and east to about the GCK longitude. I overlaid that on top of the surface southeasterlies and the CAPE field (slightly different for the NAM and the RUC, though) and got a box that covers se CO to the western OK Panhandle. In that box the progged surface flow for each model shows either easterlies in the OK Panhandle (or southeasterlies from the NAM) and southeasterlies in eastern CO. That gives an effective shear of 40 knots or greater, particularly in the OK Panhandle.
Soundings in the area showed the usual...a steep lapse rate in the elevated mixed layer (for Denver and OKC) surmounting a very shallow mixed layer (for OKC). As usual, we're concerned about the quality of the moisture that will make it back underneath the strengthening flow at 500 mb by the afternoon. SPC had outlooked our area for a slight risk, with a 5% probabilistic forecast for tornadoes.
Storms fired on the Raton Mesa and eventually one became a supercell...subsequently to become outflow dominated and a part of yet another MCS. We targeted the storm near the border, and, eventually, got a shot of a wall cloud (see below).
Shortly after that, a new storm developed on the south flank of the first (you can see it in the first radar above), and our storm became outflow dominated, with a shelf that connected south to the southern storm. Here's another view of the shelf from a vantage point to the south looking back to the rain core which had overwhelmed our storm. The southern storm had a rotating base and looked good briefly visually and on radar.
We dropped south to get a better view, but the storm kept on getting undercut by outflow, although each outflow push had a lowering on its southern side, that looked briefly interesting in each instance. Further east, othet storms were developing and splitting, with rain cores punching through updraft areas. Here's a spectacular example.
Thunderstorms erupted west of Denver, and one storm moved out onto the Palmer Divide, seemingly on a boundary which might make it a classic supercell, with a tornado. Although it briefly looked good, the storm quickly became outflow dominated and merged with the other cells in the area to form yet another MCS.
This is a shot looking north on the now outflow dominated main storm, with its shelf ominously extending southward. The purples, greens and blues were striking. The storm eventually propagated southward and had a mesocyclone out to our east, although it was outflow dominated. The shelf had downbursts of precipitation very evident.
In fact, the survey of the couple of tornado reports that occurred during that time (off the Cheyenne Mesa on the Wyoming/Colorado border) indicated that what had been reported as tornadoes were, in fact, gustnadoes, as we saw on May 23. (These are shear-induced whirlwinds that have nothing to do with cloud layer rotaitonal processes...hence, are not tornadoes, and have more in kin to dust devils). Such as it is.
We made one more attempt on June 2 in a somewhat favorable pattern in eastern Colorado, but even that "threat" evaporated quickly during the day. Chuck and Vickie went home, and Scott and I drifted north into the southern portions of the Nebraska Panhandle, on the chance that pattern forecast by the models for June 3 would firm up. But with each successive run of the models, the threat of supercells, let alone tornadic supercells, seemed more and more remote.
That was it. Scott and I decided to "...call it a chase..." I got an early flight out of Denver on Saturday, June 3.
Good luck to all my colleagues who are still in the Plains hoping to wrest some science knowledge of tornadic supercells out of this unfavorable weather pattern. Thom and I, along with Scott (and Chuck and Vickie) will be back next year. Till then, all the best.
June 3, 2006