Twin States Weather Blog

Davies Time!

* I have a confession to make...I missed the previous talk to get a quick bite of breakfast. McDonald's is only about a half mile up the road, and I was quite hungry. Now, back to the Symposium...

* It's Jon Davies' turn to take the stage. This guy is on another level when it comes to tornado research; he is one of the brightest minds in the entire science.

* Some of this is very technical and uses parameters with which you may not be familiar. Here's a severe weather parameter glossary, you can reference it for any help you might need. You're also welcome to e-mail me with questions about any of these values.

Severe Weather Parameter Glossary

Here's a running summary of his presentation...

* Most important ingredients for tornadic supercells is CAPE (instability) and SRH (storm rel. helicity).

* Low-level mesocyclones develop when horizontal vorticity (rotation) in the atmosphere gets tilted vertically. These mesocyclones are often precursors of tornadoes.

* EHI (Energy Helicity Index) is an indicator of tornado potential - it combines SRH and CAPE. EHI values over 2.0 indicate a potential for low level mesocyclones.

* Problems with EHI are centered around when there is large SRH and small CAPE, especially during cool seasons. Also, cold core tornado situations (500 mb closed lows) give the index problems. Another "wild card" is the development of tornadoes with nonsupercell/nonmesocyclone tornadoes.

* According to work by Dr. Erik Rasmussen, EHI computed over the lowest 1 KM of the atmosphere is quite important.

* Deep layer shear helps organize storms and strengthen updrafts.

* Stronger supercells are supported by 0-6 KM Deep Layer Shear values in excess of 30 knots. Values above 40 knots are particularly supportive.

* Lower cloud bases (we call this the LCL) have importance in tornado development.

* Significant tornadoes usually occur with LCL heights at or below 1500 m. This is not a hard, fast rule (many of these are not). In fact, tornadoes often occur in the northern Plains with higher bases. Where we live, LCL heights are almost always low during severe weather days.

* Low LFC heights and sizable CAPE in low levels (roughly below 3 km) have some relevance in tornado development.

* In some of his recent research, Davies found that a majority of F1-F4 tornadoes occur with MLLFC (Mixed Layer Level of Free Convection) heights between 500m and 2000m.

* STP (Significant Tornado Parameter) is very important, as is EHI...these are used to cross-reference various atmospheric conditions.

* Basic environment ingredients for supercell tornadoes are instability, enhanced horizontal vort. near ground, deep layer shear (0-6 km), relatively low cloud bases, (low LCL), and sizable CAPE in the lowest part of the atmosphere.

* Here are some rough numbers to use...Remember, nature doesn't know parameters, these are only rough values!

0-1 ML EHI 2.0 - 3.0
ML CAPE 500-1000 J/kg
0-6 km shear 30-35 kts
MLLCL heights below 1200-1500m
0-3 KM MLCAPE 40-60 J/kg
MLLFC less than 2000 - 2500 m.

It should definitely be noted that these are NOT exact forecasting tools. Getting too specific into parameters and numbers is not the correct way to approach forecasting tornadoes. The atmosphere does not recognize thresholds, there are many exceptions...

* Look for areas of focus and convergence where storm development might be expected and then assess how the parameter fields may affect that area.


* Fit the parameter fields with the surface pattern.

posted by WTOK Weather Staff @ 9:29 AM