question about severe weather setup for 3/10/10

[Todd Friskey]

Okay, I am still learning, have a long ways to go. I have been looking at the models, reading forecast discussions from SPC, NWS Tulsa and Springfield offices. I am also reading what other members are saying in Stormtrack. I too believe the potential for severe weather is there. However, the Tulsa radar shows rain moving from southern OK, towards the northeast this morning. Would not this be a fly in the ointment so to speak for the forecasted severe weather in SE KS, NE OK, NW AR and SO MO? Big question! If this moisture and cloud cover moves up over this area by early afternoon and hangs around, would that not limit the amount of instability? This should play a role in what happens this late afternoon. What if this just becomes a linear event, with embedded severe storms. Correct me if I am wrong, please....
Any help from a veteran forecaster would set this rookie straight.

 

[Skip Talbot]

Hi Todd,

There is often a band of clouds or precip moving through the target area in the morning. More often than not its there, and its always a primary concern to the setup whether or not that is going to clear out in time. Sometimes its obvious, with fast moving bands moving out by morning, but other times, they keep filling back in as the day progresses. You are right, if the clouds and precip don't clear out by later this afternoon, the instability available for severe thunderstorms later will not materialize. There is nice clearing behind the current line though, and it looks the target areas of NE OK and SE KS will see some sun later, although I haven't been following the setup too closely so I don't how fast that's moving through. On Monday the morning squall did indeed move out in time yesterday to destabilize western OK resulting in low topped supercells. Yesterday, however, NW MO was socked in with clouds and rain all day. A few storms did go up, but they were pretty weak. So yes, the clouds and precip you see now is critical to what is going to happen later this afternoon.

 

[Bill Tabor]

Todd, rain and widespread showers can act to stabilze the atmosphere limiting severe. The overcast can also block the daytime heating generating less CAPE (Convective Available Potential Energy). However some rain cover could be good such as part of a lower level jet bringing moisture to an area. Sometimes tornadic areas are foggy / wet in advance. The other issue however, not covered is there are other factors that affect severe weather. The approaching mid level low (vorticity) coming from the west across Oklahoma creates lift and somewhat simulates CAPE. This is one of the primary reasons some of the areas you mentioned did see storms. The biggest limiting factor in my opinion for some of those areas was lack of higher dewpoints. If you had a bit higher dews 55 to 60 and enough convective inhibition to hold off storms until arrival of the vorticity you could have more explosive storms. Actually to mention it as I recall some of these areas did have fairly high convective inhibition (Cinh). My guess this was partly due to lower moisture (dewpoints) in the area and perhaps a temperature inversion further up.

As for linear storms - that has primarily to do with alignment along a surface boundary and the direction the winds at various levels is blowing. Wind shear is one of the primary differences between just a garden variety shower / thunderstorm versus a severe supercell. As I recall the storms in ne TX and AR had better shear in particular 850mb to 500mb crossover winds were better suited to supercells. They also had better moisture, and still some affects of the mid level wave. They also had upper level divergence from the upper level jets causing lift and creating storms.

A good tool for analyzing storms is SPC's Mesoanalysis Tool http://w1.spc.woc.noaa.gov/exper/mesoanalysis/. Another good tool is Tim Vasquez's Digital Atmosphere:
http://www.weathergraphics.com/da/

 

[Brian Stertz]

Rain is not always bad especially this time of year when strong upper systems and associated strong shear compensate for the perceived weaker instability (as compared to mid/late spring). Rain & showers is usually the sign of strong LL warm advection pattern. This can seed the airmass and moisten the lower levels...and then show time. This time of year, it is fairly common to see pre-frontal troughs and confluence lines develop ahead of the actual DL or Pacific front. These often provide the firing line for supercells in the early spring....seems to happen alot in the Lower Mississippi Valley region. Enough babbling...happy learning Todd. I am always in learning mode, because I still see things that make me scratch my head and have been chasing 20 yrs. plus.