Issues with chasing cold frontal boundary setups?

[Andrew Clope]

Hey guys, I was out near Omaha yesterday chasing what I thought could have possibly been a somewhat promising setup. Obviously I was wrong. But I have a few questions.

First, I saw a few folks post up regarding the fact that it was a cold front setup, and that they had no interest in chasing it. Why is this? I know that cold fronts are faster movers than the rest of the boundaries, and that they seem more aggressive in triggering lift/convection. I would imagine you'd want this (to a certain degree). Is the problem that it can be too aggressive? Triggering too much at once and lining out the way it did yesterday? Would it be more desirable to have a slower moving form of lift?

Also, it seems to me that yesterday all of the runs on the HRRR showed the most favorable shear to the East of the river, over in the extreme SW corner of IA. The storms seemed to fire off to the south west, where there was CAPE and lift from the front, but less than favorable shear. I was holding back to the East just south of Omaha and was hoping that A) either the storms firing down in NC KS and SC NE would stay somewhat discrete as they moved into the more sheared environment (which I knew was very unlikely), or B) that the OFB from the storms firing all along that cold front would lift a few out in front closer to where the best shear was. Neither happened, but how else could I have played yesterday? Was I correct in my line of thinking here, or am I missing something?

I ended up calling it at around 530 and headed home, just because I had no desire to get close to that complex just to see (not see) a possible rain wrapped vortex, especially with the numerous reports of golf ball sized hail. And sitting near Omaha, the anvil from the storms back west of Lincoln started casting a cloud cover over the whole area so I figured it'd be hard to trigger anything substantial with no heating anyways.

I appreciate all of the help!

 

[Jeff Duda]

If there's a moderator paying attention to this, this could probably be converted to a MISC thread for 2014-08-31:

First, I saw a few folks post up regarding the fact that it was a cold front setup, and that they had no interest in chasing it. Why is this? I know that cold fronts are faster movers than the rest of the boundaries, and that they seem more aggressive in triggering lift/convection. I would imagine you'd want this (to a certain degree). Is the problem that it can be too aggressive? Triggering too much at once and lining out the way it did yesterday? Would it be more desirable to have a slower moving form of lift?

Yes, yes, and yes. The three-dimensional structure of a cold front is responsible for causing linear storm modes many times. The cold front has a wedge shape to it - thin at the leading edge near the surface, but deepening with rearward tilt - that promotes uniform lifting over a large horizontal distance. Drylines, on the other hand, don't move by advection, but rather by mixing, and typically have less linear lift (although the lift can be just as strong as that from a cold front). That's why you typically see cellular storm modes from a dryline. Every cold front is a little different, and there are cold fronts that are a little less homogeneous, so sometimes you can get discrete cells to maintain themselves ahead of a cold front. Typically, you want to see a significant component of the storm motion vector being away from the boundary, though. As Tim Supinie illistrated in his post in the FCST thread for this day, the storm motion vector had almost no component normal to the movement of the cold front. Rather storm motion more or less kept pace with the front, which does not bode well for keeping storms discrete. Also, if you can get a trigger ahead of the main front (such as a pre-frontal trough), you can get cellular storms ahead of a cold front also.

Also, it seems to me that yesterday all of the runs on the HRRR showed the most favorable shear to the East of the river, over in the extreme SW corner of IA. The storms seemed to fire off to the south west, where there was CAPE and lift from the front, but less than favorable shear. I was holding back to the East just south of Omaha and was hoping that A) either the storms firing down in NC KS and SC NE would stay somewhat discrete as they moved into the more sheared environment (which I knew was very unlikely), or B) that the OFB from the storms firing all along that cold front would lift a few out in front closer to where the best shear was. Neither happened, but how else could I have played yesterday? Was I correct in my line of thinking here, or am I missing something?

It is not uncommon for the best low-level shear and helicity to be present well ahead of the forcing mechanism when a cold front is involved. The NAM and GFS showed days out that a narrow low-level jet stream would be present over E KS and W IA, but displaced from the better mid-level flow (deep layer shear) and from the trigger. Because this region is so far out ahead of the trough axis, large-scale ascent is usually absent where this low-level shear is best, and thus the region tends to remain capped. That is what happened yesterday. I did see the OFB that some were talking about across SE NE. Apparently it just didn't provide strong enough lift to get parcels through the cap there. Seems to me this happens a lot in KS/NE/IA/MO, especially in June. I don't know how many times I chased an OFB left by a morning MCS that had 3000 CAPE and 300 SRH north of it with severely backed surface winds and sufficient mid-level winds for a big time event that cap busted in Iowa. It's such a c--ktease sometimes.

 

[Andrew Clope]

Jeff thanks again for taking the time to explain things. Greatly appreciated.

 

[JamesCaruso]

The three-dimensional structure of a cold front is responsible for causing linear storm modes many times. The cold front has a wedge shape to it - thin at the leading edge near the surface, but deepening with rearward tilt - that promotes uniform lifting over a large horizontal distance. Drylines, on the other hand, don't move by advection, but rather by mixing, and typically have less linear lift (although the lift can be just as strong as that from a cold front). That's why you typically see cellular storm modes from a dryline. Every cold front is a little different, and there are cold fronts that are a little less homogeneous, so sometimes you can get discrete cells to maintain themselves ahead of a cold front. Typically, you want to see a significant component of the storm motion vector being away from the boundary, though. As Tim Supinie illistrated in his post in the FCST thread for this day, the storm motion vector had almost no component normal to the movement of the cold front. Rather storm motion more or less kept pace with the front, which does not bode well for keeping storms discrete. Also, if you can get a trigger ahead of the main front (such as a pre-frontal trough), you can get cellular storms ahead of a cold front also.

.

Hey Jeff,

Thanks as always for that great explanation. IIRC, I was once told by one of the "veteran" chasers/meteorologists (you know, one of the ones that can't be bothered to participate here anymore) that storms can only go up on the cool side of a cold front unless a pre-frontal trough is involved. Do you agree with this, or are there other mechanisms that can allow discrete cells to form ahead of a cold front? It is possible I am not even remembering the conversation correctly but this thread triggered the recollection and I wanted to make sure I understand it. Thanks!

Jim

 

[Royce Sheibal]

I was also out chasing, but on the Nebraska side near Crete. I saw HRRR had predicted some isolated cells out infront of the cold front during prime time (3pm to 6pm) and models predicted adequate shear. The cells that fired blew up on a pre-frontal boundary in a line, but each one was discrete enough to have rotation. The problem was that the shear which was supposed to be nice and loopy, ended up being linear. (so instead of a hook on the hodograph its a straight line from SW to NE). This caused all the cells to form and split quickly, and some were even left dominant.

It also didn't help that storm motions were parallel to the front, as the front quickly caught up to the 2-3 cells in SE neb that finally started organizing. The one cell that did get a tor warning was one of the few right dominants that held together, but it ended up in SW IA and didn't produce. The variability of shear is incredibily critical in these cases, and although the models weren't terribly wrong, they were just wrong enough.

 

[Jeff Duda]

Thanks as always for that great explanation. IIRC, I was once told by one of the "veteran" chasers/meteorologists (you know, one of the ones that can't be bothered to participate here anymore) that storms can only go up on the cool side of a cold front unless a pre-frontal trough is involved.

By the strict letter of the law (assuming a perfectly wedge shaped cold front), that is technically true because the lift due to the cold air mass moving in doesn't start until the front edge of the front passes. However, not all cold fronts are textbook, and, while I don't know 100% the exact answer, I suspect that the mesoscale circulation associated with many cold fronts does promote lifting ahead of the front as well. Since cold fronts occur along a local minimum in the pressure field (i.e., the "kink" in the isobars), there must be some factors pulling air mass out of the column right on top of the front at the surface. Those factors should also include upward motion to conserve air mass.

Of course, there are fronts that act more like battering rams than wedges (the cold air mass is very deep right at the leading edge instead of gradually deepening rearward). I suspect weaker fronts like those that occur during the summer are shaped more like this. Typically it's the really cold/Arctic air masses that can be very thin. It would be nice if there was proper instrumentation out there to observe the vertical structure of cold fronts on a regular basis so these hypotheses could be tested more thoroughly.

 

[JamesCaruso]

Excellent and helpful clarification, thanks Jeff!!!


Sent from my iPhone using Tapatalk