How can warm fronts enhance supercells?

[Tyler Burton]

Hello, I was wondering if someone could explain how warm fronts can enhance supercells, and how to forecast the potential for a storm latching onto one.

 

[Jeff House]

Good question. My two favorite targets are warm front / dry line intersection and outflow boundary / dry line intersection.

The warm front offers several advantages over just anywhere on the dry line. First low level winds are normally more backed, enhancing the low level wind shear and turning with height. Vorticity is locally higher on the front and at boundary intersections. Moisture convergence can be enhanced on the WF. Storms can slow down and turn right along a WF, the slow part helpful for chasers. Turn should be anticipated. Risks of WF include crossing into the cooler air, and/or numerous cells on top of each other.

Forecasting the latch-on, vs the cross and die, requires looking upstairs. I prefer gradually turning wind with height from south to at least SW; more west the better. High-res models will give storm motion, but I like to look at the winds myself. Unidirectional from the south is not favored. Cold core is an exception, but I defer to others there. I have rarely even tried cold core set-ups.

I actually prefer the OFB over the WF. Usually the OFB has less risk of such cool air on the north side. OFB has a better chance of destabilizing on both sides, while still maintaining other advantages. I normally chase in May, when subtle features like an OFB can light up. Other chasers prefer the WF. Either one increases odds over just anywhere on the DL.

Often high-res models will have better cells near where they forecast the WF/DL and OFB/DL intersections. I do not take the placement literally. I do look for the concept; then, follow the boundary intersection through the day. Use surface obs and visible satellite.

 

[Tyler Burton]

Good question. My two favorite targets are warm front / dry line intersection and outflow boundary / dry line intersection.

The warm front offers several advantages over just anywhere on the dry line. First low level winds are normally more backed, enhancing the low level wind shear and turning with height. Vorticity is locally higher on the front and at boundary intersections. Moisture convergence can be enhanced on the WF. Storms can slow down and turn right along a WF, the slow part helpful for chasers. Turn should be anticipated. Risks of WF include crossing into the cooler air, and/or numerous cells on top of each other.

Forecasting the latch-on, vs the cross and die, requires looking upstairs. I prefer gradually turning wind with height from south to at least SW; more west the better. High-res models will give storm motion, but I like to look at the winds myself. Unidirectional from the south is not favored. Cold core is an exception, but I defer to others there. I have rarely even tried cold core set-ups.

I actually prefer the OFB over the WF. Usually the OFB has less risk of such cool air on the north side. OFB has a better chance of destabilizing on both sides, while still maintaining other advantages. I normally chase in May, when subtle features like an OFB can light up. Other chasers prefer the WF. Either one increases odds over just anywhere on the DL.

Often high-res models will have better cells near where they forecast the WF/DL and OFB/DL intersections. I do not take the placement literally. I do look for the concept; then, follow the boundary intersection through the day. Use surface obs and visible satellite.

Thanks for the reply.

 

[Ethan Lang]

I too prefer the warm front/ dryline or outflow intersection. I chase a lot on crappy road networks so anything that will favor a slower storm motion is preferred. Also it may be just me but when I mostly chase in NE it seems as though any cold front always ends in busts or after dark MCSs. Every tornado I've ever seen I believe has involved a warm front.

 

[James Wilson]

OFB/DL intersection ... did someone say DDC!!!

Those cold fronts have been a long pain in my butt they just love CRASHING a party.

 

[Marshall Stoner]

I think the type of warm front is important. Usually a weak warm front is better than a strong one because there can still be some surface based instability north of the boundary. A leftover east-west oriented outflow boundary is usually just as good if not better than an actual warm front. Often the most unstable CAPE will be elevated just north of an outflow boundary, so storms start off elevated then "drill down" into surface based CAPE as they develop more rotation. An E-W oriented boundary helps back the winds on it's north, but you don't want too strong of a temperature gradient or surface based air will be too dense to lift and any storms that form will simply stay elevated for their entire lifespan.

Leigh Orf did some super-high-res simulations that showed pretty clearly that the ALL the air entering the tornado originates on the cooler outflow side of the storm, but it still manages to be lifted rapidly due to the strong dynamic lift of the mesocyclone combined with some SBCAPE that remains even after heavy precipitation. Clearly a tornado can still form even when the hottest and most unstable air is not being pulled in at the surface. The air entering the updraft from the surface can be cooler, just not so cold that it can't be lifted at all.

 

[Marshall Stoner]

I've also read something that land-spout tornadoes on the high plains often originate from a hot unmodified boundary layer under a rain-free base (and often never develop a condensation funnel all the way to the ground as a result), but big supercell tornadoes almost always involve the pulling in of slightly cooler air.