A July example where awareness of 700 mb temperatures east of the High Plains can help resolve conflicting model output regarding severe storms
(discussion by Jon Davies)

Here's a summer 2009 case that shows how using 700 mb temperatures as an estimation of a significant "cap" might be useful input when making a convective forecast in the central U.S. east of the High Plains.   It is also an example of how very warm 700 mb temperatures can be overcome by intense surface heating in the High Plains (near and west of 100 deg W longitude).   The reader can cross-reference this analysis with the discussion of 700 mb temperatures and the "cap" here.

Shown below, the morning NAM-WRF 6 hour forecast of 700 mb height contours and temperatures valid at midday on 7/10/09 (1800 UTC) showed a weak 700 mb trough/short wave moving across western Kansas and central Nebraska toward the Kansas City area.  Ahead of this wave, with plenty of CAPE forecast by the same model over eastern Kansas into western Missouri by 0000 UTC 7/11/09, the NAM-WRF 12 hour forecast of precipitation suggested that a convective complex would form in the Kansas City area by late afternoon, potentially generating severe weather with significant deep layer shear also present (not shown).

<== NAM/WRF 700mb, CAPE, & precip forecasts on 7/10/09

However, looking back at the midday 700 mb forecast above, notice that very warm 700 mb temperatures (15 deg C and greater) were also forecast to spread across much of Kansas, with 12-13 deg C (see dotted red line) as far east as central Missouri.  With the RUC model (not shown) forecasting similarly warm 700 mb temperatures, and morning upper air data consistent with that depiction, the warm air aloft over the Kansas City area should make a forecaster really question the NAM-WRF precipitation forecast, and pause before making a forecast of severe storms over eastern Kansas and western Missouri.  Such warm temperatures on west-southwest flow aloft east of the High Plains suggest that strong daytime convection would be inhibited in the Kansas City area, conflicting with the NAM-WRF depiction of convective storms.

The observed radar mosiac below at 0000 UTC 7/11/09 shows that, with heating and lift, weak convective showers did develop over eastern Kansas and western Missouri during the afternoon.  But these showers were shallow and rooted essentially above a strong capping inversion just below 700 mb on local model analysis profiles (not shown).  True surface-based storms fed by deep boundary-layer moisture from below the strong capping inversion could never get going during daylight hours in the Kansas City area.  Notice the warm observed 700 mb temperatures (> 13 deg C, isotherms plotted over the radar mosaic in yellow) over eastern Kansas and western Missouri.  As a result, no severe storms occurred (see SPC severe reports graphic), even though the NAM-WRF model suggested otherwise.  It is also notable that, several hours later after 0800 UTC during the early morning hours on 7/11/09, with some nocturnal cooling aloft and an intensifying southwesterly low-level jet, severe storms did develop over west-central Missouri just east of the 12.5 deg C 700 mb isotherm (not shown).

<== observed radar mosaic at 0000 UTC 7/11/09  <== SPC storm reports 1800-0400 UTC

Farther southwest over south-central and southwest Kansas and northwest Oklahoma, surface heating and temperatures were extreme (100-110 deg F, see red isotherms plotted over the radar mosaic above).  In this area near and into the High Plains, a few isolated strong storms did form, as the intense surface heating over more elevated terrain was able to overcome capping in areas of convergence, even with temperatures of 15 deg C at 700 mb.  These storms were very high-based with LCL heights above 2500 m (typical for storms forming from such intense surface heating), and resulted in a few severe wind/hail reports.

This case is a good example of how awareness of very warm temperatures at 700 mb east of the High Plains might help a forecaster make a better local severe convective forecast in the face of conflicting model output.  It is also a good example of how 700 mb temperatures often do not work very well as a severe limiting factor over the High Plains and elevated terrain, particularly when intense surface heating is present.

- Jon Davies 8/15/09           back to 700 mb temps & cap page        back to Case Studies/Briefs page