Latest on Wednesday Severe Threat

[jkt21787]

Still looks like Wednesday could be quite an active day across the southern plains, mid south, and lower mississippi valley. Strong cold front will race east with a 80kt jet at mid levels, 60 kts at low levels. Very high shear in all levels will be present, values well over 400 m/s2 in some places. Also some hints of at least marginal instability across the red river valley northeast into Arkansas.

SPC has day three outlook with a slight risk, and a 25% severe risk across most of AR, NE TX and E OK, with the same area hatched in for a higher probability of significant severe weather.

If (a big if) instability can develop sufficiently across this area wednesday, supercells could become likely and rapidly rotate under the strong wind shear with a threat of tornadoes. Even if instability remains meager, a powerful squall line with damaging and perhaps destructive winds will move east containing bow echoes and a threat of isolated tornadoes along the cold front.

Noticed a quote from NWS Memphis this afternoon...

HELICITES ARE QUITE HIGH WITH SOME ETA SOUNDINGS OVER 1000 M2/S2.

1000 m/s2?! Is that even possible? I would think that shear that strong could tear apart any developing updrafts and actually limit the threat, although I am far from an expert.

The squall line will continue east thursday with a remaining threat of damaging winds in central and east TN, AL, GA, and the Western Carolinas.

The bottom line is: People in this area need to closely pay attention to developing storms over the next 48-72 hours and listen for later outlooks, watches, and discussions from the SPC and warnings and forecasts from the NWS.

Heavy rainfall and localized flooding could also be an issue, especially near the already flooded Ohio and Mid Mississippi Rivers. Other streams and creeks could easily see quick rises with heavy downpours.

Any comments are welcomed.

[NCSUWX23]

Id love for some sort of severe weather in this area with such a dramatic cool down on its way. It would definately make a statement and show that the spring like weather is all but a memory from here on out. I do not believe we will though this far east as usually the mountains tear them apart and then there most likely wont be enough sun to provide enough instability.

[jkt21787]

Oftentimes, the Appalachains will take apart any storms moving into NE GA and the Carolinas. Looks like some of the action could reach western portions of the state, but the severe potential is very questionable. Not to mention I believe the front is also supposed to slow as it moves east.

Sun and resulting instability will be a problem for all of the area. Still think that even little instability won't severely hamper the squall line threat (just the supercell/tornado threat). I do think back to the Late novemeber outbreak in LA/MS/TX where there was also a lack of instability but still dozens of tornadoes with several injuriesand a few deaths. Doubt that this will be a repeat of that, but it should be watched.

[wxguy25]

Still looks like Wednesday could be quite an active day across the southern plains, mid south, and lower mississippi valley. Strong cold front will race east with a 80kt jet at mid levels, 60 kts at low levels. Very high shear in all levels will be present, values well over 400 m/s2 in some places. Also some hints of at least marginal instability across the red river valley northeast into Arkansas.

SPC has day three outlook with a slight risk, and a 25% severe risk across most of AR, NE TX and E OK, with the same area hatched in for a higher probability of significant severe weather.

If (a big if) instability can develop sufficiently across this area wednesday, supercells could become likely and rapidly rotate under the strong wind shear with a threat of tornadoes. Even if instability remains meager, a powerful squall line with damaging and perhaps destructive winds will move east containing bow echoes and a threat of isolated tornadoes along the cold front.

Noticed a quote from NWS Memphis this afternoon...

HELICITES ARE QUITE HIGH WITH SOME ETA SOUNDINGS OVER 1000 M2/S2.

1000 m/s2?! Is that even possible? I would think that shear that strong could tear apart any developing updrafts and actually limit the threat, although I am far from an expert.

The squall line will continue east thursday with a remaining threat of damaging winds in central and east TN, AL, GA, and the Western Carolinas.

The bottom line is: People in this area need to closely pay attention to developing storms over the next 48-72 hours and listen for later outlooks, watches, and discussions from the SPC and warnings and forecasts from the NWS.

Heavy rainfall and localized flooding could also be an issue, especially near the already flooded Ohio and Mid Mississippi Rivers. Other streams and creeks could easily see quick rises with heavy downpours.

Any comments are welcomed.

Is it possible to have Helicities that high? Sure they can.

w/ the strong LLJ in place across the region providing decent inflow, and plenty of speed and directional shear ---- helicity values will be extremely high.

Normally helicity values of > 500 m^2/s^2 (Meters Squared per Second Squared thunderstorms will readily rotate.

COMBINE this w/ CAPE of just say 1000J you would be talking about an EHI value of ~3.12 (this number would suggest to me that there COULD be a few F2s or F3s).

It’s more complicated than this, since the overall thermodynamic environment will dictate the potential for severe wx; but the EHI can give you an idea of where tornadoes are most likely based on the combination of the best shear and instability at a given time.

You Noted:

1000 m/s2?! Is that even possible? I would think that shear that strong could tear apart any developing updrafts and actually limit the threat, although I am far from an expert.

This is true to an extent and the Bulk Richardson index deals w/ the relationship between the shear magnitude as compared to instability. When you have Very strong (speed) shear but weak instability (BRN generally < 10) the shear may overwhelm the updraft, thusly making it unsteady.

When you have a balance between these two parameters then the environment becomes favorable for tornadoes.

BRN is not one of my favorite tornado forecasting b/c there are many different values that you could plug into the BRN equation for the Shear and instability terms and have a result which gives you a BRN number favorable for tornadoes even in spite of the fact that the ACTUAL shear and instability magnitudes independent of one another DO NOT favor tornadoes. E.g., a weakly sheared and slightly unstable environment COULD produce a favorable Richardson number leading one to falsely believe that there is a tangible tornado threat.

Also the BRN number in NO way guarantees that convection is going to develop on a given day. Actually one could successfully argue that this is a CRITIAL weakness of many Severe weather indices. Convection will ONLY develop if there is a forcing mechanism present for convective initiation. Let’s say for example that you have an environment characterized by favorable deep layer shear and instability near the Theta-e ridge. BUT at the same time there is a strong CAP over the region w/ the aforesaid instability/shear parameters and high Theta-e, low level convergence or frontal boundary is non existent, jet dynamics are unfavorable and so on. Convection will NOT develop if there is no forcing mechanism.

[wxguy25]

it doesn't matter if you have CAPE of 10,000 J/kg. No lift = no convection.

[jkt21787]

Thanks for the great answer wxguy.

I personally never look too much at BRN, as you said I have seen it in favorable ranges many times with little or no major threat actually existing. 1000 m/s2 of shear could sure be trouble if there is that balance with instability, it remains to be seen I guess.

Additionally, you mention the cap scenario, and I was wondering about that. Some offices are mentioning at least a weak cap until forcing along the front can break it. This in itself would reduce the supercell potential. Do you think the cap will be a big factor Wednesday? And how much would any sun help to break it?

Thanks again.

[wxguy25]

Thanks for the great answer wxguy.

I personally never look too much at BRN, as you said I have seen it in favorable ranges many times with little or no major threat actually existing. 1000 m/s2 of shear could sure be trouble if there is that balance with instability, it remains to be seen I guess.

Additionally, you mention the cap scenario, and I was wondering about that. Some offices are mentioning at least a weak cap until forcing along the front can break it. This in itself would reduce the supercell potential. Do you think the cap will be a big factor Wednesday? And how much would any sun help to break it?

Thanks again.

As long as the forced ascent is strong so that a parcel can continue to rise to its LFC in spite of the negative energy in place---Convective initiation will take place. ALSO remember that a CAP can cause an increase in thermodynamic instability below it. A weak CAP can prevent a mass blowup if you will of convection, allowing for more discrete cells to form and tap the available instability and shear. These are situations where one can get some really nasty tornadic supercells.

BTW good job w/ your first post.

[jkt21787]

Thanks for your answer and the kind words about my post

Truly, I always love looking at severe weather threats moreso than anything else, mostly because that is what most big events around here are.

I never really got into looking into the winter forecasting aspect of things (NAO, PNA and all that stuff) until I came here. I always just prayed for snow, and not for ice And as far as hurricanes, same goes true, especially until this whopper of a year.

The SPC day 2 outlook should be intresting. I personally think they may initially hold off on a moderate risk because of uncertainity with instability and such, but they have surprised me before. Nevertheless, if the models hold, I think some part of the Red River Valley and Mid-South will ultimately end up with a moderate risk day (at least wrt wind potential).

[jeff]

Wind energy and linear frontal forcing look good for a fast moving squall line with widespread damaging winds. Any supercells will likely be limited to the areas just ahead of the squall line and over N LA into SW ARK.

It should be noted that breaks in the overcast may be enough to weaken the cap along with short wave cooling to extend the severe threat back SW toward KCLL.

[jkt21787]

Wind energy and linear frontal forcing look good for a fast moving squall line with widespread damaging winds. Any supercells will likely be limited to the areas just ahead of the squall line and over N LA into SW ARK.

It should be noted that breaks in the overcast may be enough to weaken the cap along with short wave cooling to extend the severe threat back SW toward KCLL.

Did forget to mention N LA in my first post, they are definitely in the action as well.

Squall line with widespread damaging winds is the main threat as it stands now. The supercell threat may well be unkwown until the exact synoptic pattern develops on Wednesday, although my best guess would be in the areas you mentioned as well.

Tornado threat will exist in the squall line as well, with spinups possible in bows, and perhaps more sustained and damaging tornadoes in broken areas of the line that could take on supercell characteristics for a time. Had a situation like that back in Nov 2001, very much the same setup. Had a long track tornado across NW MS into a SE Memphis suburb causing major damage and several deaths along the path unfortunately.

BTW, the NOV outbreak last year I referred to had a total of 66 tornadoes accord. to SPC. Again a situation where there was a lack of instability somewhat.

[jkt21787]

ALHurricane, I noticed you were on a minute ago.

What are your latest thoughts on the severe threat?
Thanks.

Intresting quote from Tulsa this evening...

THE 00Z ETA CONTINUES ITS TREND OF ERODING THE COLD AIR LATER
TUESDAY NIGHT WITH LEE SIDE CYCLOGENESIS IN SOUTHEAST COLORADO. WE
SUSPECT THAT THE UKMET MAY BE MORE CORRECT IN SUPPRESSING THE
SURFACE DEVELOPMENT TO THE SOUTH AND THUS KEEPING IT WEAKER. SHOULD
THIS INDEED BE THE CASE...THE CHANCES OF SEVERE WEATHER WEDNESDAY
WOULD BE QUITE MINIMAL EXCEPT PERHAPS IN FAR SOUTHEAST OKLAHOMA.
TEMPERATURES WEDNESDAY WOULD ALSO MUCH COOLER AT LEAST IN PARTS OF
NORTHEAST OKLAHOMA IF THIS VERIFIES. WILL LET THE NEXT SHIFT TAKE A
CLOSER LOOK AT THIS.

[jkt21787]

Any word on the 00Z models and tulsa's comments. I noticed that in another thread that the models ran without mexican obs, would this have an effect in regards to this?

[ALhurricane]

What I gather from Tulsa's discussion is that the UKMET develops the surface low further south than the ETA. This has a significant effect on severe weather possibilities for their CWA as it would mean the difference between being in the warm sector or the cool sector. For points further south and east, that won't be a problem. It will definitely be warm sectored.

I've actually liked the handling of the ETA so far and have leaned towards it. New ETA showing enough instability to cause a significant damaging wind threat. Supercell development ahead of the line will likely be limited due to a cap around 650-700 mb. GFS seems a bit too fast...so I think the ETA timing is a lot better.

[jkt21787]

OK AL, Thanks a lot.

I thought that is what Tulsa was meaning tonight. Certainly looks like we will be in the warm sector, already in somewhat, the temp is now in the 60s, dewpoint is now nearing 60, and winds are southerly and starting to gust a bit. At least its getting rid of the pesky fog and drizzle in place all day.

As you said, squall line event looks most likely as of now. Would you be concerned with any supercell formation within broken areas of the line or just ahead of it as the cap breaks locally increasing tornado potential? Also what's your thinking on sun for Wednesday, looks minimal as of now, but it will have a big impact on the threat.

[ALhurricane]

Memphis area definitely to become warm sectored. That fog was a forecast pest today.

Embedded supercells within the line are a definite possibility. Sun on Wednesday looks minimal, but there may be some breaks which will only help to worsen the situation. Mid level cap that will be in place will have to be broken by dynamic forcing...and that will occur with the line.

[jkt21787]

Thanks again as always AL!

We will be in the warm sector, which is much different than in the past threats from late NOV and early DEC, when the warm front stayed south in North MS.

The embedded supercell threat will need to be watched closely. The supercell I mentioned from NOV 2001 formed about 20 or 30 miles ahead of the main squall line, that may also happen. Had a similar situation back last May, when several supercells formed just ahead of the main line right over Memphis.

SPC day 2 out in a few hours...