Severe Event March 25th 2021

[mob1]

18Z NAM extends a very significant tornado threat well North of it's previous run. 

[andyhb]

Yeah that's a pretty decided shift on the NAM towards a more organized surface low and slower progression, and that has been taking place since last night.

[Bob's Burgers]

That is a beastly run of the NAM

[mob1]

3K NAM also looks far better (for severe potential) for northern AL/MS and well into TN. Meso models are definitely trending towards a deeper low and better moisture surge. 

[Bob's Burgers]

A high risk looks practical after seeing these 18z runs.

[Snowstorm920]

1 minute ago, mob1 said:

3K NAM also looks far better (for severe potential) for northern AL/MS and well into TN. Meso models are definitely trending towards a deeper low and better moisture surge. 

Its still pretty bare with deep convection in Mississippi and Alabama. Looking at some soundings in the warm sector I'm not sure why that is either. There is an EML and associated CAP in the afternoon but nothing that would hamper convection that much.

[mob1]

5 minutes ago, Snowstorm920 said:

Its still pretty bare with deep convection in Mississippi and Alabama. Looking at some soundings in the warm sector I'm not sure why that is either. There is an EML and associated CAP in the afternoon but nothing that would hamper convection that much.

I'm not really sure either. Updraft Helicity looks somewhat improved over 12Z, especially for TN. Overall it still looks fairly devoid of deep surface based convection. 

[ATDoel]

3 minutes ago, mob1 said:

I'm not really sure either. Updraft Helicity looks somewhat improved over 12Z, especially for TN. Overall it still looks fairly devoid of deep surface based convection. 

JPeters earlier mentioned a pocket of warm dry air at around 850mb inhibiting convective growth, looks like it's still there on the 18z.

[jpeters3]

2 minutes ago, ATDoel said:

JPeters earlier mentioned a pocket of warm dry air at around 850mb inhibiting convective growth, looks like it's still there on the 18z.

This is my guess as to why the NAM nest has less warm sector convection.  The warm air does not directly cap things, but could hinder storm growth through entrainment.  It's just a guess though.

[CheeselandSkies]

@Fred Gossage just made a great point on another forum, which is that on the 18Z HRRR, you can watch the "junk" updrafts die out with time as the supercells mature and are able to dominate the environment. It was not portraying this evolution leading up to last Wednesday.

[jpeters3]

6 minutes ago, CheeselandSkies said:

@Fred Gossage just made a great point on another forum, which is that on the 18Z HRRR, you can watch the "junk" updrafts die out with time as the supercells mature and are able to dominate the environment. It was not portraying this evolution leading up to last Wednesday.

I wouldn't lend much credence into these small details.  3 km models cannot properly resolve supercells, much less smaller nonsupercellular updrafts.  For reference, supercell updrafts are typically 5-10 km across.  Nonsupercells are 1-2 km across.

So what the model is portraying on the convective scale is a bastardization of the real atmosphere.

[Eskimo Joe]

1 hour ago, forkyfork said:

how is this not going to result in convection everywhere

Stop posting facts. You'll scare off the danger noodles.

[ATDoel]

11 minutes ago, Eskimo Joe said:

Stop posting facts. You'll scare off the danger noodles.

This is probably a dumb question, but it looks like the lack of cap, forecasted for this event, also caused a lot of the junk convection last week, which prohibited super cells from really getting started and why we "only" had 20 something odd EF2s and weaker tornadoes.

Unless I'm mistaken, this forecast has higher amounts of shear, moisture, cape, etc.  Even if we get a bunch of junk convection again this go around, is it not fairly logical to think that there's a good chance we have a similar outbreak as last week except, well, with more and stronger... everything?

[mob1]

2 minutes ago, ATDoel said:

This is probably a dumb question, but it looks like the lack of cap, forecasted for this event, also caused a lot of the junk convection last week, which prohibited super cells from really getting started and why we "only" had 20 something odd EF2s and weaker tornadoes.

Unless I'm mistaken, this forecast has higher amounts of shear, moisture, cape, etc.  Even if we get a bunch of junk convection again this go around, is it not fairly logical to think that there's a good chance we have a similar outbreak as last week except, well, with more and stronger... everything?

It's really hard to equate how different events will be affected by junk-vection and/or messy storm mode. With that said, it's definitely possible to have a fairly significant event even with very little spacing between storms, just not the violent tornadoes you get with high caliber outbreaks. 

 

One other notable difference between this event is also the LLJ, which is already quite impressive even early in the afternoon. Like is always the case though, you always have to temper expectations a little bit when there's basically no cap. 

[jpeters3]

9 minutes ago, ATDoel said:

This is probably a dumb question, but it looks like the lack of cap, forecasted for this event, also caused a lot of the junk convection last week, which prohibited super cells from really getting started and why we "only" had 20 something odd EF2s and weaker tornadoes.

Unless I'm mistaken, this forecast has higher amounts of shear, moisture, cape, etc.  Even if we get a bunch of junk convection again this go around, is it not fairly logical to think that there's a good chance we have a similar outbreak as last week except, well, with more and stronger... everything?

I think it's reasonable to assume that we'll see something *at least on par with* last week in terms of # of tornadoes and coverage.  It's hard to imagine that not happening given the more impressive parameter space.

So what we all are unsure of is whether it will get even worse than last week's bar.

For the record, last week actually had quite a few discrete supercells.  They just didn't consistently produce long track tornadoes.

[jojo762]

NAM NEST is on an island of its own wrt convective evolution across MS/AL... I suppose it should not be totally ignored, but outliers ought to be given minimal preference in a forecast.

Latest NAM is very impressive. As has been discussed, it expands the area of higher-end tornado potential further north. CAMs generally show a consistent convective evolution (some supercells, some mixed-mode) up and down most of the cold-front/pre-frontal trough so such a situation could prove to be quite problematic if this were to materialize.

18z HRRR generally agreed with the NAM in a larger warm-sector (though, perhaps not as broad as the NAM shows) so that's something.

**If the warm sector were to surge that far north by mid-afternoon, with the same thermodynamics as currently forecast, it would not be a stretch to say this could be a historic day, as forecast soundings across most of the progressively expanding warm sector would support high-impact tornadoes.

If this trend continues the ENH and MDT risks would need to be expanded significantly.

[CryHavoc]

1 hour ago, mob1 said:

It's really hard to equate how different events will be affected by junk-vection and/or messy storm mode. With that said, it's definitely possible to have a fairly significant event even with very little spacing between storms, just not the violent tornadoes you get with high caliber outbreaks. 

 

One other notable difference between this event is also the LLJ, which is already quite impressive even early in the afternoon. Like is always the case though, you always have to temper expectations a little bit when there's basically no cap. 

Probably the most incredible thing, at least from my personal standpoint, about 4/27 was how many well-defined, sustained hooks we saw flaring up in the atmosphere in a relatively small amount of space.  At one point it seemed like every single cell that even started to grow was immediately isolated and spinning, even if they had a very potent neighboring cell just 5-10 miles away from them.  I'm not a Met, so perhaps this wasn't as surprising to the pros in this forum/elsewhere, but I had never seen storms that close together be able to fuel themselves so powerfully, and wasn't even aware that parameters could exist that would permit so many mature supercells to develop so close together.

It was hard to believe that we had so many tornadoes on the ground in such a confined area compared to the 74 super outbreak -- although you could argue there may have been unreported tornadoes from that day, it was harrowing to see an entire section of a state covered in red tornado warnings with multiple tornado emergencies being sent out at the same time.

[Calderon]

HUN straight up issued tornado emergencies versus tornado warnings for a period on 4/27.

Also, literally every single thing was rotating that day. Tony Lyza posted something a few months back where multiple rain showers were rotating moderately before they even had achieved 40dBz returns.


.

[jojo762]

Unsurprisingly, WFOs JAN, BMX, and HUN all referenced strong, long-track tornadoes in their afternoon AFDs.

What kind of made me laugh a bit was the casualness of the MEG AFD considering half of their AFD currently has MDT risk caliber tornado probabilities.

Quote

Storms will be ongoing Thursday morning as the warm front
continues north. This will allow for some destabilization across
the MS Valley as more Gulf moisture starts to advect into the
region. The surface low pressure system will continue northeast
during the day. Increasing LL moisture and diurnal heating will
support ~1500 J/kg of MLCAPE Thursday afternoon. Surface
instability, a strengthening LLJ ~50kt coupled with strong wind
profiles from the surface to the mid- levels will support strong
to severe thunderstorms across the Mid-South on Thursday ahead of
a surface cold front. All modes of severe weather will be possible
including a heavy rainfall threat.

 

[Indystorm]

Surface low continues to deepen in old Mexico just south of the big bend.  Now down to 996 mb.

[DanLarsen34]

13 minutes ago, jojo762 said:

Unsurprisingly, WFOs JAN, BMX, and HUN all referenced strong, long-track tornadoes in their afternoon AFDs.

What kind of made me laugh a bit was the casualness of the MEG AFD considering half of their AFD currently has MDT risk caliber tornado probabilities.

 

That’s frankly alarming given the high-end threat that their area may face tomorrow. 

[MUWX]

1 minute ago, DanLarsen34 said:

That’s frankly alarming given the high-end threat that their area may face tomorrow. 

I’m not really sure what impact that has one the general public

[hlcater]

MEG sucks. 

[Quincy]

Texas doing the day before the day things...

[jojo762]

7 minutes ago, Quincy said:

Texas doing the day before the day things...

MY GPTTC/hypothetical target was Jonesboro, TX... Feeling pretty good about it 

[Dustin]

29 minutes ago, MUWX said:

I’m not really sure what impact that has one the general public

I wonder how many broadcast mets read the AFDs?  There could be some downstream impact on public perception. 

[MUWX]

12 minutes ago, Dustin said:

I wonder how many broadcast mets read the AFDs?  There could be some downstream impact on public perception. 

That’s true. 
 

As for the HRRR, it came in pretty substantially different. Doesn’t look nearly as bad as the 18z HRRR imo 

[Santa Clause]

1 minute ago, MUWX said:

That’s true. 
 

As for the HRRR, it came in pretty substantially different. Doesn’t look nearly as bad as the 18z HRRR imo 

Agreed on HRRR

[jojo762]

00z HRRR is a touch more tame than the 18z run was... But still shows many discrete/semi-discrete supercells across AL and MS during the afternoon and evening -- toward evening it really starts to light up KY/TN with a broken line of what I would assume would be a mixed-bag of supercells and clusters, given the forecast parameter space across both these area i'm not exactly sure it is going to matter how "discrete" convection can stay.

[Bob's Burgers]

This hrrr is definitely weaker.