Warm fronts and the snow pack

[Jeff Duda]

For my own knowledge, I've been told that a front always lies in a pressure trough. Margules' Frontal Slope Equation and relative minimums and all that. Is this not correct?

I'm pretty sure I've analyzed fronts before that were not coincident with a pressure trough/kink. Anyway, the definition I was taught in school matches what the NWS glossary says:

Front
A boundary or transition zone between two air masses of different density, and thus (usually) of different temperature. A moving front is named according to the advancing air mass, e.g., cold front if colder air is advancing.

The above is also consistent with the AMS Glossary's definition. Their definition does mention that there are several features, including a pressure trough, that commonly accompanies fronts. But I think just the temperature gradient alone suffices to define a front.

I see I'm already arguing about semantics here

 

[Rob H]

I see I'm already arguing about semantics here

Having guys like you and Stan Rose post here with the technical details is one of the benefits I like about ST, which is why I actually chuckled when you both said don't get hung up on the definition. Don't worry about it, no one's arguing

I think it was something Patrick Marsh said on a discussion about drylines here a few years ago and it always stuck with me. Irrelevant to the primary topic in this thread, but I was just curious for my own knowledge. I hate holding onto bad or incomplete information in my head.

edit: ah, here it is
http://www.stormtrack.org/forum/archive/index.php/t-24486.html

 

[Skip Talbot]

Skip, do you remember if there was a snowpack in the morning up there along I-72? I figured it had retreated far to the north by morning, given the overnight/morning thunderstorms, and that only the deeper drifts remained in the I-72 corridor at 2PM. Due to that data hole between I-72 and Macomb, I was guessing on the location of the front at the time, assuming it was farther south due to the fog/low clouds visible to the north of I-72. That fog may very well have been due to the snowpack. Very interesting that these could have been "snowpack boundary" tornadoes.

It had retreated overnight, although I wasn't really out and about until I left the house to chase. I chased to almost Champaign county and there was substantially more snow to the east, however. I had to turn around once because there was a big drift across my grid road. At that point I realized that these storms had to be sucking stable parcels so I might as well give it up.

Going off the wind barbs, the warm front in your surface chart looks like it's quite a bit further north to the east. To the west, it seems the warm front is still pendant to the cyclone and perhaps it's coincidence the snow line is also right there. Is the snow pack re-enforcing the warm front like a storm complex can or does the front typically just overrun the snow pack?

Semantics aside, I'm more interested in how this situation impacted my chase. Did the snow line enhance the directional shear available to supercells that approached it at all, or did it simply create a sharp cut off where the SBCAPE went to zip. I'm under the impression based on some of these surface charts and model plots that it's the latter and that it was largely coincidence that the backing winds of the warm front were near the snow line at the time of the west central IL tornadoes. Does the snow line have the negatives of a warm front (sharp cut off of SBCAPE) and none of the positives (increased helicity)?

 

[James Gustina]

edit: ah, here it is
http://www.stormtrack.org/forum/archive/index.php/t-24486.html

Always fun to relive my n00b days. It is interesting to think back to that, considering a pressure gradient/trough does not a boundary make necessarily though the two are usually mutually inclusive.

On the front debate though, the snowpack might have sharpened the boundary like an MCS, but the SE sfc winds looked largely coincidental considering the sfc cyclone was not in the immediate area so as to greatly affected the wind fields over a specific area of the boundary. It didn't seem like the snowpack line enhanced the kinematics, but it was just absolute dumb luck that everything came together so perfectly at the right time. The best upper-level dynamics kicked in while the winds were beginning to back just south of the front/boundary thingy.

 

[Jeff Duda]

Semantics aside, I'm more interested in how this situation impacted my chase. Did the snow line enhance the directional shear available to supercells that approached it at all, or did it simply create a sharp cut off where the SBCAPE went to zip. I'm under the impression based on some of these surface charts and model plots that it's the latter and the it was largely coincidence that the backing winds of the warm front were near the snow line at the time of the west central IL tornadoes. Does the snow line have the negatives of a warm front (sharp cut off of SBCAPE) and none of the positives (increased helicity)?

Unless the wind shift was coincident with the snow pack boundary, i don't see how it would've amplified directional shear. I was watching the mesoanalyses and noticed a bullseye of 1500 SBCAPE being analyzed just north of STL as the first storms were going tornadic. As I posted on FB, I think this was in response to some clearing just ahead of the front. The clearing was limited to C IL. So it was a matter of a CAPE gradient that probably made the difference on the day.

A snow line is not a synoptic warm front. There's a 1992 paper on non-classical mesoscale circulations (NCMC) that discusses the impacts of snow pack boundaries (among other things). If I recall correctly, that paper basically said each of the NCMCs they investigated caused very little change in mesoscale flow.

 

[Paul Hadfield]

ILX summary buried on the SWOP page.

http://www.crh.noaa.gov/ilx/?n=swop-022014

What a night/day