January 2nd 2006 - Outbreak Discussion / Review

[Robert Dewey]

I was just reviewing some data for this event (I didn't really follow it in realtime)...

The area near ATL was primed for severe weather... If one followed the RUC closely, it actually predicted things quite well up to 3 hours in advance. I know following the models so closely isn't the best way to forecast, but I find the RUC usually does a pretty good job at forecast convective initiation...

The WV loop showed the dryslot overhead, but as the upper level low continued eastward some mid level moisture streamed into the area. Also, vertical velocity through the profile (which were initially negative due to the descent behind the mornings convection) became positive in association with the cold front and strong PVA. In addition, very strong divergence aloft at 250MB probably aided in the creation of relatively intense vertical velocity... All of this resulted in a slightly cooler/moister profile (particularly from 850MB and above) with low and mid level lapse rates of over 8.1 C/KM, CAPE values of nearly 2000J/KG, and a CINH of only -10 J/KG.

Winds also backed at the SFC to SSE, allowing for localized helicity values of nearly 300M2/S2 - most of which was in the 0-2KM layer. The VIS image did show a very nice Cu field across western GA... But further east, low level clouds were quite visible... Soundings over that region indicated things were VERY moist up through about 900-850MB, with LCL's of only 500-600FT (thus the low cloud base seen on the tower cam).

What is interesting is the previous RUC nailed the development of thunderstorms on it's QPF fields with the 18Z run (3 hours earlier)...

...

Sorry if the images load too slow, they are located on a home server and the upload speeds are capped. Feel free to steal my images.

NATIONAL 1KM NEXRAD WITH 18Z RUC QPF FIELDS (QPF BETWEEN 21Z AND 00Z)

NEXRAD 2101Z
NEXRAD 2121Z
NEXRAD 2141Z
NEXRAD 2201Z

VIS 1902Z
VIS 2115Z
WV 2115Z

All RUC data from the 18Z run, several hours prior to initiation of supercells.

RUC 3HR QPF (18Z F06)
250MB Jet / Divergence
500MB ABS Vort.
500 Vort. Adv.
700VV
BL RH
CAPE / CIN

 

[Nic Wilson]

Thanks for posting all of the post-case information Robert. I am visiting my parents in the northeast suburbs of Atlanta currently, so I followed the event closely the entire day -- although I didn't go out chasing because it was my birthday and am recovering from foot surgery.

The shear and helicity parameters were excellent yesterday across a large swath of Georgia. Additionally, the mid-level lapse rates were very impressive via the 18 UTC FFC sounding. However, a majority of the Atlanta area was trapped in grunge following the morning quasi-MCS which limited the lower-level instability, and in turn negated tornadic potential. Overall, the Atlanta area dodged a bullet because this event could've been much worse...thankfully a few hail reports and isolated microbursts were the only reprecussions. Local television was very reluctant to even cut into the college bowl games ongoing at the same time, so for their sake I'm glad it turned out to largely be a non-event.

 

[nickgrillo]

Cool maps, Rob...

I'm glad to see my northern focus played out like I had forecasted... Convection began to initiate near the MO bootheel (and eastward) as a band of ascent moved into the warm sector before Midnight... Then, stronger convection developed along the sfc trough a couple hours later (as the mid-level shortwave impinged the area ahead of the sfc trof) and quickly rooted in the boundary layer... One storm in particular began to show a very strong low-level couplet, and passed just to the north of St. Louis shortly around 09:00 UTC and 2 tornado reports (possible tornado-related damage on one report). Other supercells developed to the south and began to exhibit low-level mesocyclones and got TOR-warned, though no other tornadoes were reportred.

<img src=http://mtarchive.geol.iastate.edu/2006/01/02/pix/sfc/theta_e/2006010220_theta_e.gif>

The sfc trof continued to lead the convection east through the morning. A couple of discrete and semi-discrete tornadic supercells formed just ahead of the main line, and strengthened as they move into a sfc theta-e ridge and associated locally higher sfc/near-sfc based instability (with CAPEs at least 1000j/kg near the storms) and strongly backed sfc flow (which locally enhanced low-level shear) but began to fall apart less than a couple hours after initiation as the main line swallowed them (before the line quickly diminshed as CINH increased and boundary layer decoupled through nocturnal cooling) as the best deep-layer forcing exited the region. 7 tornadoes were reported in central KY from these storms.

The supercells in GA looked pretty intense... As you mentioned, instability was quite strong in the region (owed to rapid cooling with height, very warm/moist boundary layer) and along with significant deep shear, it let these storms go pretty insane for a while. All in all, it really wasn't as big as I first expected... But, it was still a cool day nonetheless.

 

[Mike Johnston]

Good observations. The main NAM models were certainly not depicting backed winds at the surface for areas south of the immediate path of the surface low, while daytime surface observations and the RUC updates indicated a fairly broad area of backed surface winds in the warm sector. Although the surface low gradually weakened on its eastward path, it seemed to remain a just a little deeper for a little longer than forecast. In addition, that ever-elusive mesolow (see speculation on the forecast thread) did indeed develop in the western Carolinas. All in all, I agree the RUC performed its job very well on this event.

One of the interesting and prominent features of this event was the in-situ wedge over the lee side of the Appalachians. If you look at the SPC storm reports graph, you easily see the wedge. With the prior overnight and morning convection, the warm front/wedge provided conditions for a broad overrun of moisture over Georgia with the corresponding precip, cloud cover and subsidence much of the day. In the morning version of the convective outlook, SPC had clearly modified its moderate risk area for this condition. Now, did the wedge serve mainly to enhance the severe convection which did transpire by providing a necessary surface boundary/lifting mechanism? Or, did the wedge serve as a more or less limiting factor to what otherwise might have been an even more serious outbreak? Not sure, but I speculate the latter may be the case. Witness some of the severe reports through the extreme eastern coastal plain (on the other side of the wedge) and radar observations of convection over adjacent coastal waters. As it was, there was a brief, couple-hour window where available instability met with sufficient helicity, etc. However, absent the in-situ wedge, I can't help but speculate the ideal factors would have been juxtaposed for a greater length of time resulting in a more widespread and serious outbreak than was the case.

 

[Robert Dewey]

I think the trough of low theta-e's / cooler temperatures actually played a role in backing the SFC winds across areas of GA. If you notice further south, away from the low theta-e trough, winds are SSW/SW. Looking at SFC obs for that time, MSLP is a bit higher in the low theta-e region. Since pressure flows from high to low, there would have been some northeasterly component added to the SFC wind in northern GA, so winds ended up more southeasterly (or in some cases easterly) - basically following the isobars.

SFC plot (21Z) - http://69.14.190.10/images/case/sfc_obs_21z.gif
http://69.14.190.10/images/case/sfc_data.gif

 

[Jason McKittrick]

These images aren't helping my SDS. It seems so close, touchdown in St. Louis a couple of days ago, yet we are most certainly atleast 3 months from anything chasable on the plains. Was there anybody chasing that stuff down in Georgia?

 

[Brett Adair]

These images aren't helping my SDS. It seems so close, touchdown in St. Louis a couple of days ago, yet we are most certainly atleast 3 months from anything chasable on the plains. Was there anybody chasing that stuff down in Georgia?

Yes...I was. Bummed out as the supercell outran me though. Yet I still was able to get some hellacious supercell structure photographs.

http://alstormtrackers.blogspot.com