A Preliminary Assessment of the Utility of TAMDAR Aircraft Sounding Data in Monitoring the Cap Evolution Prior to the June 7, 2005 Severe Weather Outbreak
Gene Brusky, NWS Green Bay
I. Introduction
The purpose of this overview is to illustrate the utility of the TAMDAR aircraft sounding data during a convective episode in the NWS Green Bay (GRB) forecast area. The TAMDAR aircraft sounding data was invaluable in helping to monitor the evolution of a low-level stable layer (cap) during the course of the day. The goal of this brief discussion is to encourage forecasters to utilize TAMDAR sounding data in their assessment of convective potential.
II. Discussion
During the early morning of June 7, 2005, a weakening linear MCS was approaching the GRB forecast area from eastern Minnesota. As the system pushed across southern Minnesota into far western Wisconsin, it produced some widely scattered wind damage. By 15Z, the MCS extended from near Eau Claire (EAU) to just west of LaCrosse (LSE) with the convection primarily elevated in nature (Figure 1). Downstream of the approaching MCS, the atmosphere appeared to be destabilizing as temperatures climbed into the upper 70s to lower 80s over southwest into central Wisconsin with surface dew points generally in the mid to upper 60s (Figures 1 and 2). Based on RUC forecast soundings, SPC issued Severe Thunderstorm Watch # 425 for parts of central and east-central Wisconsin from 1530Z to 2100Z as there was concern that the cap would erode quickly with continued heating and the convection would subsequently become surface-based as it moved into central Wisconsin. However, examination of a few morning TAMDAR aircraft soundings at CWA (point A) and ATW (point B) revealed a rather impressive cap over the watch area (Figures 3 and 4) that appeared quite a bit stronger than what the RUC was forecasting, suggesting that the convection would likely remain elevated and weaken. In fact, the cap did remain quite strong through early afternoon with the convection gradually weakening as it approached the Green Bay (GRB) forecast area (Figures 2 and 5). In the wake of the weakening convection, skies cleared and surface dew points fell slightly over parts of central Wisconsin (Figures 5 and 6) with the 1923Z CWA TAMDAR sounding indicating the cap actually strengthening slightly around 900 mb (Figure 7). A comparison of the 1932Z CWA TAMDAR and 19Z RUC2 sounding indicated the RUC2 had indeed significantly underestimating the strength of the cap (Figure 9).
Later on in the afternoon where skies had cleared over west-central Wisconsin, the atmosphere quickly recovered and destabilized ahead of a rather strong shortwave disturbance and associated surface trough approaching from Minnesota (Figures 6 and 10). Examination of subsequent CWA TAMDAR sounding indicated that the strong cap had rapidly eroded by 2336Z (Figures 11 and 12) with severe convection exploding over parts of central and northwest Wisconsin (Figures 13 and 14). Severe convection had also broken out over northern Wisconsin possibly in the vicinity of a weak frontal boundary near the Wisconsin-Upper Michigan border. These storms quickly became supercellular over Forest County and generated impressive mesocylonces prompting the issuance of several Tornado Warnings. On a side note...examination of the surface data, radar data and visible satellite imagery hinted that a possible lake-induced boundary had pushed as far west as eastern Langlade and Forest counties helping to initiate the storms in that area. These storms were probably the most impressive of the event and generated baseball hail and substantial wind damage. It has still yet to be determined as to whether the damage was associated with intense downburst winds or a tornado, as the storms had moved across a rather remote area. In light of recent studies by Jon Davies, the early evening CWA TAMDAR soundings (Figures 11 and 12) suggested that the LFC heights were rather high (5-6 kft) which would reduce the potential for significant tornadoes (F2 or greater) over the forecast area.
III. Summary
The routine monitoring of TAMDAR aircraft sounding data over central and east-central Wisconsin provided forecasters valuable insight into the evolution and strength of a cap during the day of June 7, 2005. Morning TAMDAR soundings upstream from an approaching linear MCS, revealed a rather dry airmass with a substantial cap in place over the forecast area such that any convection moving into the forecast area later in the day would likely remain elevated and weaken. Later in the day, the TAMDAR soundings also revealed the subsequent rapid erosion of the cap with severe convection exploding over the forecast area ahead of a shortwave disturbance and associated surface trough. Comparison of the TAMDAR soundings to RUC2 soundings indicated that the RUC2 had underestimated the strength of the cap early in the day, and was too slow in forecasting the subsequent erosion of the cap and rapid destabilization that lead to the development of severe convection by late afternoon over the northern and western portion of the forecast area.
Gene Brusky, SOO KGRB
Updated 6/13/05
Discussion Figures 1-15 Below
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Gene Brusky, SOO KGRB
