01/02/06: FCST: Ohio Valley and Southward

[nickgrillo]

I believe the first significant severe weather episode of 2006 could be underway for Monday... But let's make this short... A deepening surface cyclone, ahead of an amplifying mid-level shortwave trough, will enter the Ohio valley by the afternoon. Low-level WAA and moisture advection northward (associated with a 30-40kt SW LLJ) will contribute to a swath of 1000-2000 j/kg SBCAPE across the warm sector. Given large-scale ascent impinging the area early, widespread severe thunderstorms could develop by the early afternoon (BL-6km shear is favorable for supercells) across this broad warm sector. While the event is still several days out, this setup does appears favorable for a few strong tornadoes across the lower Ohio and Tennessee vallys

http://www.rap.ucar.edu/weather/model/disp...&hours=hr72hr84 <-- amplifying shortwave

http://www.rap.ucar.edu/weather/model/disp...&hours=hr72hr84 <-- low-level moisture advecting northward

 

[Robert Dewey]

I'm wondering just how south the shortwave will dive. Latest 18Z NAM has it diving even further south (same trend on other models as well). A further south shortwave would mean a much deeper system (for example, the 18Z NAM is about 2MB deeper at the SFC). Depending on how far south it digs, we could really be looking at a powerhouse storm system. Given the trends, my bet is that the best chances for severe weather would be along and south of the Ohio River... Should things level out and / or trend northward, the threat will obviously shift. Either way, this system will be complex. We may be looking at a devastating ice storm here in the Great Lakes with a significant severe weather threat only a couple hundred miles south...

Definitely enough time to watch it evolve...

 

[Brett Adair]

00z NAM continues to raise instability across my region and shear as well. It looks as if 700-850 mb will ensue an extreme dry punch into the region with nice height falls and 7.0+ lapse rates in the 500-850 range. -16 to -19 H5 temperatures will bring an extreme hail threat. Looks alot like my day Tuesday....very good hail day with some tornadoes in these supercells. Looks MDT risk.

 

[nickgrillo]

It's not very often you'll see an SWODY3 with a MDT extending into eastern KY in Janurary.

Well, I am convinced there will be a significant outbreak of severe thunderstorms on Monday (will make it short, I am dead tired)... There has been some pretty good run-to-run consistency with timing of the shortwave and location of the surface cyclone (from both NAM and GFS). Sfc theta-e ridge nosing northward into southcentral Kentucky shortly after noon could be the focus point for a burst of convective initiation... very warm/moist boundary layer with steep 0-2km lapse rates are contributing to SBCAPE of at least 1000j/kg (closer to 2500j/kg further south) all the way into southern Indiana. This is easily gonna be the first big bang of the year, with both adequate deep-layer vertical shear and instability for widespread supercells with at least a few significant tornadoes (particularly from eastern Tennessee on southward).

 

[Brett Adair]

Yes....it looks as if I will be chasing around my area and western GA Monday with Matt Grantham. Shear/Instability look more than sufficient for long tracks supercells with damaging tornadoes down this way. UVV fields indicate this will probably be a discrete event as pockets of deeper vertical motion are established and linear forcing isn't going to be all that wonderful. I look for the MDT to be extended back a little further west in later runs because I believe that the NAM is having feedback issues. I am in the left exit region of the LLJ and on the nose of the ULJ streaks and this model is trying to tell me no precip along the pseudo dryline? Yeah right...east of I-65 in AL is going to be a playground as you head eastward into the ATL metro. I will be planning out my heading tomorrow and will get as I prepare all of the equip. More tomorrow.

 

[George Tincher]

Are my eyes deceiving me or am I really seeing something that one could justifiably call a dryline surging into Tennessee and Alabama come Monday? :shock:

Taking a brief glance at the picture models are painting, I am pretty concerned about C/E Tennessee, extreme ERN Alabama and Georgia. It appears there will be a strong LLJ in place, very adequate CAPE for this time of year (1000+ j/kg over most of the area and higher in some locations), very strong 0-3km SRH and LI's of -2 to -4 being common over most of this area, I can definately see why the SPC decided to go with a MOD for the Day 3 period.

As far as negatives, I don't see too many. But in terms of tornadic activity, it doesn't appear LCL's will be particularly low. In addition, it now appears that much of the area will be capped, whereas previous runs showed little in the way of an inversion. One will have to carefully watch the strength of the forcing vs cap strength as we move closer, but as of right now, I don't think we can count on the cap stiffling out activity.

The area of greatest interest for me Monday will be along and extending about 80 miles E of a line running from Oak Ridge, TN to Chattanooga, TN down to Gadsden, AL. Some major cities included in this area include Knoxville and Atlanta. Considering that temps are forecast to reach the mid to upper 60's for my area Monday, I might even have to expand my "greatest interest" area farther N pending future model runs.

While I won't mention the "O" word just yet, Monday very much looks conducive to a widely scattered outbreak of strong to severe storms capable of producing the full array of severe weather threats.

-George

PS...... if Oklahoma and Texas really are becoming a desert, perhaps KY, TN, MS, AL and GA will become the new Tornado Alley as storms fire off the new SRN Plains generated drylines! Of course I am kidding, so please don't shoot me! :wink:

 

[Bill Hark]

"Fly in the ointment": lack of backed winds


Very impressive setup for January (and chaseable for many since Jan 2 is a holiday) There will be a nice band of instability shooting northward through AL and TN, then GA and eastern TN.
http://www.rap.ucar.edu/weather/model/eta4...hr_sfc_cape.gif
http://www.rap.ucar.edu/weather/model/eta6...hr_sfc_cape.gif
CAPES 1000 to 1500 with storm relative helicities ranging from 150 to 300 m2s-2. There's upper level support with a strong lower level jet. 60 degree dewpoints to the border of Indiana.

Although some are concerned about the cap, I am more concerned about the lack of backed surface winds over most of the area. Per the 12Z Dec 31 Eta, the only are of SE winds are extreme western NC by 12Z Jan 2. This vanishes except for parts of WVa by 00Z Jan 3.

http://www.rap.ucar.edu/weather/model/disp...&hours=hr48hr60

The GFS is similar.
http://www.rap.ucar.edu/weather/model/gfs0...hr_sfc_mslp.gif
http://www.rap.ucar.edu/weather/model/gfs0...hr_sfc_mslp.gif

There will definitely be strong storms and I think this forecast justifies a MDT risk, but there may be less chance of tornadoes. If I lived nearby and had WXWorx turned on, I would definitely chase this event. I live in Richmond and would have to be back at work Tuesday at 7:30AM, therefore a chase is unlikely. Good luck to those who go out!

**EDIT: I changed the links to the 12 Z model runs for Dec 31, but the lack of backing winds is still present!**

Bill Hark
http://www.harkphoto.com

 

[Terry Kollmeyer]

I can't remember when SPC has ever issued a Mod. risk for a Day 3.
Am I forgetting something??


Oh, BTW...Happy New Year!!

 

[Steve_Stuck]

On June 8, SPC put out a moderate risk on day 3.


http://www.spc.noaa.gov/products/outlook/a...50608_1100.html

 

[Scott Olson]

There were I think at least one maybe two times this year where a MDT was issued on the day three. I also believe this is the first year where they changed the rule where MDT's can be issued on Day 3 and High's on Day 2.

I got thinking about Bill's comment about the backed winds as I noted the same thing. Im sure S winds would suffice from a moisture standpoint, based on the location to the GoM but that low level directional shear is a key ingrident in a lot of SE tornado outbreaks. I went backed and looked at a couple and you do have backed winds, in some cases extremely backed (SSE or E). I would think that would be tough on moisture, after all it's pulling it's air from the Atlantic, but they did produce tornadoes. The farther north/slightly less amplified solution isn't helping any. Georgia appears to be looking the best to me despite the backed winds with 40+kts of Effective SRH, decent CAPE. Enough Instability/Lift/Shear to make for an interesting day.

 

[Robert Dewey]

Yeah, I'm not liking this weaker / further north solution on the NAM in terms of severe weather... I also don't like how the system is weakening as it tracks eastward - that's never a good sign.

I agree with Bill and Scott about the backed winds... TN looks like it gets in on the action around 12Z MON - most likely linear stuff... Then later in the day areas further southeast get in on the action. By that time, NAM shows WSW SFC winds, though helicity is still high due to the strong speed shear. In cases like this, helicity can be misleading - you have enormous speed shear, but little directional shear... Unless something changes, I am thinking more of a broken squall line with isolated supercells...

Hopefully the models will start showing some sort of southward digging / intensification so that it holds on to its strength a little longer - but that's just wishcasting at this point.

 

[nickgrillo]

LOL... I'm really not seeing how a slight lack in backed winds will prevent an outbreak in this case. Heck, we had way weaker of a system a few days ago and that did not prevent the 9 reports of tornadoes and widespread damaging winds and large hail all the way into the Ohio Valley. In that case, flow was mostly unidirectional, with only slight veering flow in the boundary layer (but strong speed throughout the column), which helped yield very strong helicity with mostly southwest flow at the sfc.

NAM forecast soundings show pretty decent CAPE profiles across KY at 12z, with a KPAH sounding at 12z showing over 800 j/kg of CAPE by lifting a SB parcel. Despite absolute crap for low-level lapse rates (around moist adiabatic)... Mid-level lapse rates of 7.2c/km and low-level WAA/moist advection will help continue to destabilize boundary layer, with the development of at least moderate instability (>1500 j/kg SBCAPE) by the early afternoon from the Ohio Valley well into the south. Forecast soundings from scross the region show a very favorable environment for tornadic supercells, with a strongly sheared/moist boundary layer by the late morning (at least 50kts of BL-6km vertical shear available). The persistant >40kt SW LLJ will continue the northward advection of deeper moisture, with the sfc 60F isodrosotherm all the way into Indiana by 12z. Still, an outbreak of supercells and tornadoes appears likely from southern Indiana/Ohio well into the south, with early initiation expected, with an increase in storm converage by the mid-afternoon as stronger mid-level ascent sweeps into the region associated with the amplifying shortwave.

 

[Robert Dewey]

Nick, let me clarify...

I'm not saying there won't be an outbreak of severe thunderstorms. What I am trying to say is that I don't think this will be a huge event if the 12Z NAM has its way (meaning we will likely only see isolated tornadoes, most probably F2 or less)... Take a look at the forecast sounding for BHM a F54... There is absolutely NO directional shear to speak of... In fact, the hodograph shows 160 m2/s2 of positive helicitity in the 0-3KM layer, while at PAH at F51 the hodograph shows 120 m2/s2 with -26 m2/s2 of negative helicity - a bit better than further south). Both soundings also show very steep lapse rates, nearly moist adiabatic all the way up, and NO CAP.

Even with a weakening shortwave, that's enough to touch of severe thunderstorms, possibly an outbreak... What I am questioning is the mode. Yes there is a significant amount of speed shear, and supercells will likely be present... But with little in the way of a CAP and a unidirectional profile (again, checkout BHM at F57 and ATL at F57 / F60), I can't help but think squall line.

I am not trying to be a naysayer or underplay the event... I really think there is some potential here, but I think it might be in the form of a broken squall line with isolated supercells. Still a good event in my eyes - I don't need a whole plate full of F4's and F5's to consider it an outbreak.

 

[Michael P. Morris]

I agree with rdewey here as this morning's model runs show weak directional shear in the sfc-700 mb layer. What concerns me the most is the ETA's handling of the development of the surface low, and if the winds have turned to WSWly by the early evening, there is not going to be enough low level shear to support organized storms. Furthermore, the strength of the midlevel vorticity advection will weaken the cap substantially by early afternoon. Gotta go with a squall line on this one, though I wouldn't rule out the possibility of embedded supercell structures given the substantial magnitude of the deep shear vector. I'm visiting my fiancee's family near Atlanta, and though the sight lines are awful in this area, I still will make every attempt to find a high point and get some decent shots.

 

[nickgrillo]

I agree on the chance for more linear activity... But, heck, the prior system (12/28 ) that slammed into the south looked strikingly simular (except this wave is way stronger). A mostly unidirectional profile, with little inhibition... So, I had said to another chaser at the time lines were most likely, with embedded low-level mesocyclones given strong 0-1km helicity. But, what happened? Everything did go off at one time... But, they were mostly discrete, productive tornadic supercells across GA. Lapse rates were quite a bit steeper in the low-levels (sfc-2km lapse rate nearly cooled dry adiabatically) then in the upcoming event, but the two are still pretty simuler.

It could indeed get messy given a weak inversion, but I was just commenting that the lack in backed sfc flow will NOT prevent an outbreak in this case... No biggie :wink: