Historic Tornado Outbreak April 27, 2011

[cyclone77]

Unless something changes I'm out for Monday. There's just too much precip/clouds in the warm sector north of the Arkansas border which will severely limit instability. Down in Arkansas this shouldn't be an issue. The storms that blow up down there will likely pivot northeastward into southern and eastern Missouri, and the southern Illinois after dark.

[TheWeatherPimp]

Based off the 12z GFS, Tuesday could be a very significant severe weather day with a very large risk area

[Chicago Storm]

First look off of the SPC 4km WRF for tomorrow evening...

[TheWeatherPimp]

Both the 12z NAM & GFS portray a significant threat of tornadoes Wednesday Evening across Indiana/Ohio/Kentucky.

[Calderon]

of course the one cell in west TN has to be right over Jackson...

[hm8]

No change to the Day 2 outlook...

[Fred Gossage]

Both the 12z NAM & GFS portray a significant threat of tornadoes Wednesday Evening across Indiana/Ohio/Kentucky.

And further south as well... from late morning or midday onward.

[TheWeatherPimp]

Wow on the 12z GFS for Tuesday

[CUmet]

I realize that this is the unreliable NAM at 78-84 hours out, but for Wednesday it has a 100 kt isotach at 500 mb overlapping the warm sector. I honestly cannot recall seeing that ever happening in any model run of a potential severe weather event over at least the past half-decade, and there have only been a select few severe events in the past that have featured this (Palm Sunday 1965, Super Outbreak 1974 come to mind). This is not to say that I *expect* an event of this magnitude, but there is a massive amount of potential here.

[TheWeatherPimp]

Tuesday and Wednesday look to have some amazing potential with all ingredients coming together for what could be a multi-day regional outbreak of severe weather and tornadoes. Obviously some fine details to work out and with this much precipitation occurring along the frontal boundary, it will be difficult to pinpoint some of the mesoscale features until we get into the more near to short term, but as CUmet stated, all kinds of potential here.

[CUmet]

12z NAM coming a little more NW for Mondays event..but cloudy/rainy warm sector..still cape around 1500

looks decent

perhaps keep this thread for monday..but then spin one off for tuesday and/or weds?

also nam a little more se for weds at 12z

I was actually thinking of keeping this thread for Tuesday-Wednesday, and spinning off a new thread for the Monday event since the Tues-Wed event looks like the main show. The discussion in this thread has been mostly about the Tue-Wed threat thus far too. Honestly, we'll probably end up having 3 separate threads for the 3 different days, given the likely magnitude of the event as a whole, so should we just start that up now?

[TheWeatherPimp]

SREF Sig Tornado Param really shooting up now for Tuesday and Wednesday

[ksstormhunter]

Just pushed out my 19Z Severe Weather analysis. Right now showing AR through Eastern OK to NE TX for the next 24 hours with the highest instability areas. DeQueen, AR-McAlster, OK,-Dallas, TX to Little Rock, AR in the highest threar for the next 24 hours. Surface CAPE at this hour is 2100 (j/kg), 0-3KM SRH is around 288 m/s, and 0-1KM Shear running about 40 knots. Main threat EF2 type tornadoes, convective gusts around 70 knots, and hail up to 1.5" in diameter. Uploaded analysis http://smartwxmodel.net/severe.pdf'>http://smartwxmodel.net/severe.pdf and city specific forecasts http://smartwxmodel.net

[Fred Gossage]

Just pushed out my 19Z Severe Weather analysis. Right now showing AR through Eastern OK to NE TX for the next 24 hours with the highest instability areas. DeQueen, AR-McAlster, OK,-Dallas, TX to Little Rock, AR in the highest threar for the next 24 hours. Surface CAPE at this hour is 2100 (j/kg), 0-3KM SRH is around 288 m/s, and 0-1KM Shear running about 40 knots. Main threat EF2 type tornadoes, convective gusts around 70 knots, and hail up to 1.5" in diameter. Uploaded analysis http://smartwxmodel.net/severe.pdf and city specific forecasts http://smartwxmodel.net

I can tell you right now that there are many many cases in history.... especially east of the Plains... where numbers like that have supported F3-F5 damage. It's a really bad idea to try to tie EF-Scale to parameter thresholds like that. You can have numbers larger than that.... and other things prevent tornadoes completely.... or have parameters lower than that.... and have F4-F5 damage.

[ksstormhunter]

I can tell you right now that there are many many cases in history.... especially east of the Plains... where numbers like that have supported F3-F5 damage. It's a really bad idea to try to tie EF-Scale to parameter thresholds like that.

Sorry not trying to tie those specific values to that type of Torndoes, those are just the sampling of values I put in the discussion, My outputs for potential tornado type tie into more than those I actually use more then just the instability indicies, I use 87 instability values and surface observation parameters, and upper air weather parameters.

[baroclinic_instability]

Tuesday and Wednesday look to have some amazing potential with all ingredients coming together for what could be a multi-day regional outbreak of severe weather and tornadoes. Obviously some fine details to work out and with this much precipitation occurring along the frontal boundary, it will be difficult to pinpoint some of the mesoscale features until we get into the more near to short term, but as CUmet stated, all kinds of potential here.

I was actually thinking of keeping this thread for Tuesday-Wednesday, and spinning off a new thread for the Monday event since the Tues-Wed event looks like the main show. The discussion in this thread has been mostly about the Tue-Wed threat thus far too. Honestly, we'll probably end up having 3 separate threads for the 3 different days, given the likely magnitude of the event as a whole, so should we just start that up now?

For now I am ok keeping all three in one. Monday-Tue-Wed will likely be three moderates in a row, possibly a high on Wed at some point. All three days have regions with significant potential although Tue/Wed look to be potentially very dangerous.

[janetjanet998]

For now I am ok keeping all three in one. Monday-Tue-Wed will likely be three moderates in a row, possibly a high on Wed at some point. All three days have regions with significant potential although Tue/Wed look to be potentially very dangerous.

I just started a new thread for monday as you were typing that.. I don';t care..delete it if you wish

http://www.americanwx.com/bb/index.php/topic/17617-monday-april-25th/

[CUmet]

I just started a new thread for monday as you were typing that.. I don';t care..delete it if you wish

I think the spinoff is fine, otherwise it's going to be one massive superthread especially given the magnitude of the event. We can keep this thread around for general thoughts on the Tuesday-Wednesday period. As we get closer, we can split off another thread for Tuesday and Wednesday each as necessary.

[baroclinic_instability]

NAM it putting out some sort of freakish 90 KT winds below 800 hpa on Wednesday.

[Superstorm93]

SREF is looking pretty intense.

[thewxmann]

I was actually thinking of keeping this thread for Tuesday-Wednesday, and spinning off a new thread for the Monday event since the Tues-Wed event looks like the main show. The discussion in this thread has been mostly about the Tue-Wed threat thus far too. Honestly, we'll probably end up having 3 separate threads for the 3 different days, given the likely magnitude of the event as a whole, so should we just start that up now?

I don't know if I'm missing something, but I think all three days are equally dangerous at this point, and that any one of the three days could end up being the "lesser" day. Especially since the GFS/NAM trended stronger with the lead system and more delayed with the "main" system, just as the lead energy came onshore this morning.

[Indystorm]

I think a met commented earlier that it is very rare to see such intensity on a NAM precip display such as this.

[CUmet]

I don't know if I'm missing something, but I think all three days are equally dangerous at this point, and that any one of the three days could end up being the "lesser" day. Especially since the GFS/NAM trended stronger with the lead system and more delayed with the "main" system, just as the lead energy came onshore this morning.

I don't think we disagree very much at all. Honestly, I think all three days will be significant tornado outbreaks. I think Tuesday and Wednesday both have a higher ceiling than Monday. Tuesday probably has the highest "bust" potential due to the slightly-off timing of the second disturbance.

[baroclinic_instability]

I don't know if I'm missing something, but I think all three days are equally dangerous at this point, and that any one of the three days could end up being the "lesser" day. Especially since the GFS/NAM trended stronger with the lead system and more delayed with the "main" system, just as the lead energy came onshore this morning.

I don't think we disagree very much at all. Honestly, I think all three days will be significant tornado outbreaks. I think Tuesday and Wednesday both have a higher ceiling than Monday. Tuesday probably has the highest "bust" potential due to the slightly-off timing of the second disturbance.

Totally agree. The first ejecting wave Monday night keeps trending deeper with faster height falls overspreading the warm sector. For whatever reason it seems some are dismissing Monday night when in reality it is looking like a highly active evening/overnight. Tuesday, looks interesting as CUmet stated, as the guidance has been trending towards a very shallow upper tropospheric wave ejecting ahead of the main anomaly. Slightly more zonal than a day before with the atmospheric response being faster/earlier height falls Tuesday evening and slightly less amplified/SW flow.

Shows up nice on PV maps.

And the corresponding height field response with greater mid level height falls/cooling than previously projected over much of Ark with initiation in the late afternoon and through the earlier evening.

18Z GFS at 60 hours:

12Z GFS yesterday at 12Z:

By Wednesday, it is anyones best guess--but the consequence of that low amplitude leading wave Tuesday and a more zonal and less amplified flow in the overall trough means Wednesday may have a slightly lower ceiling than before although still very impressive. I agree that all three days have nearly equal potential now.

[hoosierwx]

I have never been on the coast waiting for a hurricane to come in but with the continuing flooding and the impending severe weather it seems to have that feel to it.

[thewxmann]

I don't think we disagree very much at all. Honestly, I think all three days will be significant tornado outbreaks. I think Tuesday and Wednesday both have a higher ceiling than Monday. Tuesday probably has the highest "bust" potential due to the slightly-off timing of the second disturbance.

I agree that Tuesday has the highest bust potential, but IMO Monday has the better likelihood of discrete tornadic supercells, as the forcing might be too linear on Wed (this, of course, still up for grabs, depending on how the energy ultimately evolves and amplifies).

All else equal though, yes Tues and Wed are likely to be more widespread. Still lots of uncertainty on the tornado side of things, however.

[baroclinic_instability]

Much easier to see the differences since yesterday in guidance doing it this way.

[Fred Gossage]

By Wednesday, it is anyones best guess--but the consequence of that low amplitude leading wave Tuesday and a more zonal and less amplified flow in the overall trough means Wednesday may have a slightly lower ceiling than before although still very impressive. I agree that all three days have nearly equal potential now.

Just be careful here. Once you get east of the Plains, and especially east of the MS River... into an enviroment that often has deeper boundary layer moisture and is further away from the elevated mixed layer/capping source... these lower amplitude flow events can often be associated with some of our most gruesome tornado events in history. Too much forcing over here, and we either line out or get grungy... very fast. I could go through records and hit you with a VERY long list of dates associated with violent Dixie Alley outbreaks with a less amplied, broader-based trough scenario... and in several cases... a positive tilt configuration.

[baroclinic_instability]

Just be careful here. Once you get east of the Plains, and especially east of the MS River... into an enviroment that often has deeper boundary layer moisture and is further away from the elevated mixed layer/capping source... these lower amplitude flow events can often be associated with some of our most gruesome tornado events in history. Too much forcing over here, and we either line out or get grungy... very fast. I could go through records and hit you with a VERY long list of dates associated with violent Dixie Alley outbreaks with a less amplied, broader-based trough scenario... and in several cases... a positive tilt configuration.

No I agree--but earlier guidance was much more energetic with the leading anomaly with more backed low level wind fields Wednesday. The flow aloft here may very well be even stronger than initially progged with this deamplifying trough--I was just thinking the ceiling is still slightly lower than earlier progged but still highly impressive with widespread significant potential.

[Fred Gossage]

No I agree--but earlier guidance was much more energetic with the leading anomaly with more backed low level wind fields Wednesday. The flow aloft here may very well be even stronger than initially progged with this deamplifying trough--I was just thinking the ceiling is still slightly lower than earlier progged but still highly impressive with widespread significant potential.

Yesterday's runs... while having a very impressive synoptic system... were very forced in a linear fashion for Wednesday. Upward velocities were extremely strong and with a very linear configuration. This, combined with the low-level jet axis being just ahead of the surface boundary on yesterday's runs... restricting the low-level convergence to just ahead of the surface cold front... really screamed a linear storm mode. With this lower amplitude trough configuration, we have trended toward a situation where low-level convergence associated with the western side of the low-level jet is spaced significantly ahead of the surface front... and the upward velocities on the synoptic scale are also more spaced out and subtle. Also, while this won't really back the surface winds toward a southeasterly direction... we're seeing the 850-mb winds trend more SSW to SW, overtop the due south surface winds... which is a more classic configuration for our big tornado days in this part of the country. We often don't see events with a SE sfc flow work out because of the trajectories of the low-level air mass. In fact, all four of Alabama's F5 tornadoes in history happened with surface winds ranging from due S to SW... and an 850-mb flow out of the SSW to SW. While the synoptic system isn't quite as strong on today's model runs... we've stepped away from more of a forced, linear storm mode... and toward more subtle forcing overtop a weakly capped LLJ axis with significant instability. Any veering of the 850-mb jet toward the SSW to SW is going to rapidly ramp up 0-1 km storm relative helicity values... and 0-3 km values are already progged to be sky high. My main concern with Wednesday down here is actually that we may have a little bit of a problem with LCL heights if we reach some of the progged surface temperatures shown in some of the model guidance.

Edited for typo.