With a big trough in the west ejecting waves of energy into the Plains early this week, at first glance, both Monday 3-21-11 and Tuesday 3-22-11 all looked to be possible severe weather days over the KS/NE/MO/IA area. But the "cap" (warm temperatures advecting/moving northeastward from the desert southwestern U.S. in southwesterly flow aloft, called the "elevated mixed layer" or EML) made all the difference between the 2 days. Monday was a bust, with the EML completely inhibiting surface-based thunderstorm developing. But a strong wave in upper flow on Tuesday quickly eroded the EML and allowed storms to fire, some becoming tornadic over Iowa (see pics above from my storm chase with Shawna on Tuesday in the Bridgewater-Greenfield, Iowa area)
When looking at model forecasts for upcoming days, 700mb temperature forecasts can be useful in gaging when the "cap" may be too strong for storms to form. I like to use the chart above as a very rough guideline east of the high plains. In March, 700mb temperatures around 5 deg C (see the chart) are usually warm enough to completely inhibit storm development, unless one is in the high plains with a moist easterly upslope wind to provide mechanical lift (for example, last Saturday evening 3-19 around Lubbock TX - not shown -, where a couple supercells formed), or daytime temperatures become unseasonably warm (for example, 90's F).
The first 700mb forecast graphic above shows NAM and RUC forecasts for Monday evening (00 UTC 3-22-11) from the UCAR online site. Notice how temperatures 5 deg C and warmer on both models were forecast to extend all the way to the NE/SD border, blanketing the central plains in warm air aloft. As a result, Shawna and I did not chase on Monday, with MLCAPE all "tucked" in under this warm air aloft. No daytime storms formed across the KS/NE area due to this strong cap, even though appreciable MLCAPE was present south of a warm front (see the first panel of Earl Barker's RUC CAPE forecast graphic above at the same time). The 2nd 700 mb forecast graphic above shows NAM and RUC forecasts for Tuesday evening (00 UTC 3-23-11) that were dramatically different from the day before, with a strong upper wave moving through the plains, removing the "cap" over KS/NE/IA as much cooler temperatures aloft poured in (notice how the 5 deg C line was forecast to move well southeastward from the day before). This energy and cooling aloft easily triggered thunderstorms in northern KS and eastern NE during the afternoon that moved eastward into Iowa where combinations of MLCAPE (see the 2nd panel of Earl's RUC CAPE forecast above) and low-level shear (not shown) were favorable to support a few tornadoes.
Use the 700 mb temperature chart above as only a rough guide, and not a hard and fast rule. The most helpful thing is to watch _trends_ (for example, how the temperatures around 5 deg C and above moved out into the plains and increased aloft on Sunday 3-20 and Monday 3-21 on model forecasts, then retreated again with the approach of the strong wave on Tuesday 3-22). And don't think that temperatures cooler than the guidelines in the chart mean that storms will defintely occur! On Sunday 3-20 (not shown), 700 mb temperatures cooler than 4-5 deg C were over northern MO and IA, but storms did not develop along a weak frontal boundary. This was probably due to subsidence (sinking motion) behind a short wave that had moved across the Midwest earlier in the day, and boundary layer moisture that was not that deep or rich.
Hopefully the above helps some chasers be more aware of the trending of temperatures aloft and the "cap"/EML over the plains... that can really impact storm potential.
- Jon Davies 3/23/11
