Crashing Dryline?

[Lanny Dean]

I should've been more clear about that. During the daytime (when diabatic and subsequent sensible heating are strong), the dryline moves by mixing drier air down from a loft rather than by advection of dry air. However, at night when sensible heat flux weakens significantly or becomes negative, you don't really have that turbulence to drive continued mixing. Also the cT air mass behind the dryline tends to have lower pressure than the mT airmass ahead of the dryline, so there is a pressure gradient directed towards the cT air mass and little to no vertical mixing to counter that. Thus, the westward propagation during the evening is generally due to advection.

Thank you for the clarification, Jeff - that was my question and point. True westward moving DLs by definition and theory, would not be considered mixing IMO but rather advection.(keeping in mind that I struggled with two years of physics classes! )
After reading through the thread and trying to remember *some* of what I had "learned" in those hated physics classes, I had some questions regarding this. First of all, I know the thread is regarding a "crashing dyline" and although I have never really seen a DL "crash" in a west ward movement - I thought this was probably the best thread to discuss the topic.

I have never personally witnessed the dryline move westward during the daytime, however. If you see it and it's near the caprock, I would suspect topography may be playing a mesoscale role in causing that, but I'm not totally sure what that could be.

I have been lucky enough to witness a westward moving DL during late afternoon many times over the years but one particular event really comes to mind: May 25, 2010. AKA the Bartlett CO event. Unlike my scenario described above, this event produced significant severe weather: multiple landspouts and a true supercelluar tornado. The tornado was not overly strong (EF-1) but the event is very interesting and I believe adds to this discussion.

The surface features at 23z clearly showed a stationary front draped from sse of KPUX to ese of KGLD. The associated dryline was analyzed draped south from the stationary frontal boundary through eastern Kiowa County CO into Bacca County as can be seen in the attached MCD.
Sitting just off of HWY 160 on KS/CO border with the tour guests, we had been observing a classic LP supercell that *appeared* to be moving in a westward fashion. A quick look at radar trends and metar/ASOS data revealed that indeed we had a northwesterly moving supercell. A deeper look also revealed that the DL was moving west as well, however the frontal boundary remained stationary. Relocation to the intersection of HWY 160 and CR 56 at roughly 7:50PM MDT. From this position we observed a small funnel and noticed full condensation at roughly 7:55pm. The tornado took on various shapes and ultimately became a classic cone that was also moving west northwest. I will refer you to Pueblo's write up with a few stills from Roger Hill as mine just do not compare:

http://www.crh.noaa.gov/news/display_cmsstory.php?wfo=pub&storyid=52912&source=2

Obviously what I find so interesting about this event is the westward movement of the DL , the associated westward movement of the supecell in regards to tornadogenesis and of course, the westward movement of the tornado. I might add that I did not expect this supercell to produce due to the westward movement, mediocre helecity values and what I thought were "sub-par" CAPE values. In fact, we discussed leaving the storm early with the tour guests in the mood for dinner but opted to stay "another 10 minutes just in case"

Understanding that an eastward moving DL is the result of mixing - I am not so sure and don't' believe that the above event is would be due to such. By definition, the above westward movement would be induced by advection not mixing per se. Thoughts? Ideas?

 

[Jeff Duda]

I obtained the level II data from that day to see in detail what you are talking about, Lanny. I see where that storm started moving northwest when it was producing that tornado. That motion has to be related to storm interactions. For at least 1.5 hours before the turn, that storm is moving very slowly northeast. It makes the turn after weakening and in the vicinity of other storms that go up along what appears to be an OFB. My guess is the fledgling storm locks onto this boundary (otherwise it would've certainly died) which is drifting northward. Hell, it probably gets its vorticity to produce that tornado from the OFB! There's certainly nothing on the synoptic scale to explain that sudden change in the movement of that storm.

As to the westward movement of the dryline: if that is what you meant, then I have seen that on a few occasions. I believe the boundary is moving westward due to advection in response to outflow from nearby storms. Perhaps on days where the air aloft in the "mixing zone" (just above the PBL top) isn't super dry or the heating just isn't strong enough for mixing to counteract advection from storms, evaporatively cooled air from nearby thunderstorms can shove the dryline backward. Also, since this was late in the day, sensible heating was certainly much weaker so there wasn't as much "forcing" to keep the dryline propagating eastward anyway. This isn't really westward propagation due to some larger-scale process going on (as I had presumed...my bad), but just one air mass taking over another, just due to the presence of thunderstorms. Take away those storms and I doubt you see the dryline move westward during the day.

 

[Lanny Dean]

Thanks for the in depth reply, Jeff, and for looking at the level 2 data. It's been a long while but if I remember correctly, I wasn't sure if that was an OFB or the actual DL. I *think* (I should say at the time, I thought it was the DL itself) Either way, the event made an impression on me and I felt it was a decent "case type study" for this particular conversation. Thanks again for looking into this!

 

[NealRasmussen]

To me, there are two types of drylines. The 'real' DL that rolls downslope from the lee of the
Rockies. And then then there's the Surface Trough that everyone now calls a DL.

A real DL is marked by a rapid shift from the moist sector to dry air. You can see 65° dew point in
one county, 40° in the next one west, and 20 in
the next. Winds will go from the moist sector,
generally S or SE, to due W. Often very strong.

A surface trough is more a linear area than a boundary. Winds gradually veer with distance
west. Dew points gradually fall off. This can
create problems, as a strong cell can be diluted
by turkey towers firing west of it. The moisture convergence is spread east to west, not a tight
south to northish line.

I have seen quite a lot less true DLs it seems since about 2000. Just seems to this old chaser they
get more rare. Usually now a wishy washy surface
trough.

And of course there's the hybred. The surface trough that wants to be a DL, and the DL that
wants to be a surface trough.