Severe Severe weather threat April 17-19

[Webberweather53]

They’re almost definitely not going to back off the day 2 outlook just based on cam evolution at this stage, it would be more of a concern if we were actually on the day of the event and had actual obs to confirm or deny the NAM and hrrr forecasts.

The most recent event is a pretty good example of this, even as the first line broke up overhead in the morning hours with additional prefrontal convection it seemed based on some of the CAMs esp the HRRR that we were done with severe weather, when in reality they did a piss poor job of picking up on the convection that developed near the cold front and moved thru central nc later that night. Severe weather forecasting is a completely different animal than winter weather. In a vast majority of cases you’re forecasting based on large scale conditions at hand and very general CAM evolution until several hours (or less) before the event.

 

[Snowflowxxl]

I'm suprised there is even a Day 1 Enhanced TBH, this looks like a non event

 

[Flurry]

I'm suprised there is even a Day 1 Enhanced TBH, this looks like a non event

It hasn't really gotten started yet

 

[Myfrotho704_]

Looks like most models break the line apart as it gets into central sections of NC. Even the latest NAM is unimpressive. That's not to say that there won't be any wind reports, but I'm starting to wonder if we'll actually see an organized squall line moving through. The RAH AFD is impressive and the expansion of the Enhanced Risk area by SPC is noteworty, but virtually all of the models backed off of the intensity of the line through central NC. It reorganizes east of 95. If this continues, I wouldn't be surprised to see the SPC back off on the Enhanced Risk somewhat in the next Day 2 outlook.

hmmmm, the NAM did the same exact thing with the last event, showed literally no QPF last Sunday and then the hrrr showed a line segment/QLCS making it into NC, and it actually was right, and I barely missed a tornado warning

 

[Rain Cold]

They’re almost definitely not going to back off the day 2 outlook just based on cam evolution at this stage, it would be more of a concern if we were actually on the day of the event and had actual obs to confirm or deny the NAM and hrrr forecasts.

I was looking at the globals too. I expect the mesoscale models to handle this better at this range, but virtually all models weaken the line substantially as they move through, reforming it after. Now, that doesn't mean they're correct, but they are in pretty good agreement right now. Extent and persistence of cloud cover will play a key role. We'll likely have to wait to see how that turns out. Things could go either way at this point, but if we continue to see models backing off, then I expect SPC will trim the outlook.

 

[Rain Cold]

hmmmm, the NAM did the same exact thing with the last event, showed literally no QPF last Sunday and then the hrrr showed a line segment/QLCS making it into NC, and it actually was right, and I barely missed a tornado warning

Yeah, I don't see there being nothing. I don't see there being no severe storms. I'm kind of skeptical about a widespread event through central NC, though.

 

[Webberweather53]

I was looking at the globals too. I expect the mesoscale models to handle this better at this range, but virtually all models weaken the line substantially as they move through, reforming it after. Now, that doesn't mean they're correct, but they are in pretty good agreement right now. Extent and persistence of cloud cover will play a key role. We'll likely have to wait to see how that turns out. Things could go either way at this point, but if we continue to see models backing off, then I expect SPC will trim the outlook.

If the outlook is trimmed it wouldn't be today, at the earliest they'd do it tomorrow when we actually know what the conditions and convective evolution are like. At this stage you should really be mostly focused on the conditions in the CAMs here that lead to convective development & upstream verification today. So far I see no reason to be overly concerned about a forecast bust in either direction here. If you're basing a majority of your forecast on convective evolution on the CAMs more than several hours in advance, you're often going to get burned.

 

[PEA_RIDGE]

I'm telling yall the HRRR has been the best with severe weather this year

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[Myfrotho704_]

Lol it showed what appears to be one of those QLCS spinups right over my house, but has that type of situation throughout the line segment with bow echos and rotation at the inflow notches, i guess the storms is feeling some of those backing sfc winds helping out those inflow notches ? no doubt what the hrrr shows is nasty, I hope it’s wrong but it did decent at showing a the line with the last event 30 hours out, no model is perfect, hrrr can be to aggressive at times but it’s def something to watch

 

[Myfrotho704_]

Also maybe some random mesolow action in that QLCS

 

[Rain Cold]

If the outlook is trimmed it wouldn't be today, at the earliest they'd do it tomorrow when we actually know what the conditions and convective evolution are like. At this stage you should really be mostly focused on the conditions in the CAMs here that lead to convective development & upstream verification today. So far I see no reason to be overly concerned about a forecast bust in either direction here. If you're basing a majority of your forecast on convective evolution on the CAMs more than several hours in advance, you're often going to get burned.

I don't disagree. I certainly am not calling for a bust in either direction, but if I had to make a guess right now, I'd lean away from a widespread severe event in favor of a more typical spring scattering of storms, some of which become severe for a time. That's how this event looks to me. But, as you said, there are things that will matter that we just don't know the evolution of yet. It's wise to wait and see.

 

[Webberweather53]

I was looking at the globals too. I expect the mesoscale models to handle this better at this range, but virtually all models weaken the line substantially as they move through, reforming it after. Now, that doesn't mean they're correct, but they are in pretty good agreement right now. Extent and persistence of cloud cover will play a key role. We'll likely have to wait to see how that turns out. Things could go either way at this point, but if we continue to see models backing off, then I expect SPC will trim the outlook.

If the line actually weakened as it entered central NC there are two primary perspective of looking at how it would affect the forecast. On the plus side, hopefully a weaker line well out ahead of the cold front in addition to pre-frontal convection eats up a lot of CAPE and limits further redevelopment later in the afternoon. On the other hand, the QLCS breaking up would encourage at least initially more isolated, discrete storms which actually elevates the tornado threat, or the QLCS is so weak that there's even less clouds/convection during the day so CAPE is even higher. Personally, I wouldn't be concerned either way yet about the line disintegrating or roaring thru based on what the precipitation output looks like on the models, again you should still really be mainly looking at the conditions that favor or discourage its evolution into central NC.

For sustaining a QLCS &/or large line segment, you need obviously a respectable amount of CAPE (500-1000 j/kg+), strong deep layer shear (preferably more than 30 kts) and lots of low-level SRH (100+ m2/s2 is a good general threshold), all 3 of those primary ingredients appear likely to be present tomorrow and in excess quantities at that. Not to mention there'll be a considerable amount of downdraft cape (DCAPE) thanks to drier air in the mid-levels which increases the threat for damaging straight line winds and may strengthen the low-level rear inflow jet associated w/ the QLCS, which effectively acts to maintain &/or intensity the line segment.

Going with an enhanced risk for most of central NC is likely the right call imo based on the aformentioned conditions at hand.

 

[Webberweather53]

If the line actually weakened as it entered central NC there are two primary perspective of looking at how it would affect the forecast. On the plus side, hopefully a weaker line well out ahead of the cold front in addition to pre-frontal convection eats up a lot of CAPE and limits further redevelopment later in the afternoon. On the other hand, the QLCS breaking up would encourage at least initially more isolated, discrete storms which actually elevates the tornado threat, or the QLCS is so weak that there's even less clouds/convection during the day so CAPE is even higher. Personally, I wouldn't be concerned either way yet about the line disintegrating or roaring thru based on what the precipitation output looks like on the models, again you should still really be mainly looking at the conditions that favor or discourage its evolution into central NC.

For sustaining a QLCS &/or large line segment, you need obviously a respectable amount of CAPE (500-1000 j/kg+), strong deep layer shear (preferably more than 30 kts) and lots of low-level SRH (100+ m2/s2 is a good general threshold), all 3 of those primary ingredients appear likely to be present tomorrow and in excess quantities at that. Not to mention there'll be a considerable amount of downdraft cape (DCAPE) thanks to drier air in the mid-levels which increases the threat for damaging straight line winds and may strengthen the low-level rear inflow jet associated w/ the QLCS, which effectively acts to maintain &/or intensity the line segment.

Going with an enhanced risk for most of central NC is likely the right call imo based on the aformentioned conditions at hand.


View attachment 19146

Here's a forecasting area-averaged sounding over NWS RAH domain from the 12z 3km NAM. The 3km NAM arguably looks even better than the HRRR for maintaining the QLCS & line segment into central NC tomorrow w/ lots of surface-based & MUCAPE, strong low-level & deep layer shear, lots of 0-1 km SRH, in addition to plenty of DCAPE for enhancing downdrafts within the line. Given the strong-low level shear and 0-1 km SRH, there's obviously going to be a tornado threat from embedded, rain-wrapped mesovorts within the line.

 

[Myfrotho704_]

If the line actually weakened as it entered central NC there are two primary perspective of looking at how it would affect the forecast. On the plus side, hopefully a weaker line well out ahead of the cold front in addition to pre-frontal convection eats up a lot of CAPE and limits further redevelopment later in the afternoon. On the other hand, the QLCS breaking up would encourage at least initially more isolated, discrete storms which actually elevates the tornado threat, or the QLCS is so weak that there's even less clouds/convection during the day so CAPE is even higher. Personally, I wouldn't be concerned either way yet about the line disintegrating or roaring thru based on what the precipitation output looks like on the models, again you should still really be mainly looking at the conditions that favor or discourage its evolution into central NC.

For sustaining a QLCS &/or large line segment, you need obviously a respectable amount of CAPE (500-1000 j/kg+), strong deep layer shear (preferably more than 30 kts) and lots of low-level SRH (100+ m2/s2 is a good general threshold), all 3 of those primary ingredients appear likely to be present tomorrow and in excess quantities at that. Not to mention there'll be a considerable amount of downdraft cape (DCAPE) thanks to drier air in the mid-levels which increases the threat for damaging straight line winds and may strengthen the low-level rear inflow jet associated w/ the QLCS, which effectively acts to maintain &/or intensity the line segment.

Going with an enhanced risk for most of central NC is likely the right call imo based on the aformentioned conditions at hand.


View attachment 19146

Dry air aloft is underrated, all the time in the summer when I’m supposed to get “garden variety T-storms” I end up getting a strong downburst becuase of the dry air aloft +high PWAT, high PWATs combined with dry air will create some water loading and negative bouyancy which creates such strong sinking in the downdraft, this combined with some of the wind associated with the LLvL jet, this is why I’m very concerned about wind damage if it was to make it here, it appears the higher PWAT will be more prevalent in eastern NC aswell

 

[Webberweather53]

Dry air aloft is underrated, all the time in the summer when I’m supposed to get “garden variety T-storms” I end up getting a strong downburst becuase of the dry air aloft +high PWAT, high PWATs combined with dry air will create some water loading and negative bouyancy which creates such strong sinking in the downdraft, this combined with some of the wind associated with the LLvL jet, this is why I’m very concerned about wind damage if it was to make it here, it appears the higher PWAT will be more prevalent in eastern NC aswell

Good points about the high PWATs and water loading (which are usually the biggest contributors to downdrafts). This definitely looks like a classic setup for a beefy QLCS pockmarked w/ rain-wrapped tornadoes.

 

[Webberweather53]

Even if there wasn't any convection tomorrow, boundary layer overturning alone would be able to produce 50+ mph (only slightly sub-severe) straight line surface winds given these absurd 850 hpa winds over the Carolinas.

 

[SD]

The most recent event is a pretty good example of this, even as the first line broke up overhead in the morning hours with additional prefrontal convection it seemed based on some of the CAMs esp the HRRR that we were done with severe weather, when in reality they did a piss poor job of picking up on the convection that developed near the cold front and moved thru central nc later that night. Severe weather forecasting is a completely different animal than winter weather. In a vast majority of cases you’re forecasting based on large scale conditions at hand and very general CAM evolution until several hours (or less) before the event.

Agreed the nam and hrrr basically had nothing or few broken showers sunday night.

The nams are probably reacting to that area of rain they develop early in the day across nc which weakens the subsequent development.

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[PEA_RIDGE]

From JP Dice for Alabama

Data is suggesting the higher tornado threat will be south of I-20 this evening combined with damaging straight-line winds. Elsewhere, we can expect a line of storms capable of producing winds over 60mph tracking from west to east beginning at around 5pm.

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[snowlover91]

Even if there wasn't any convection tomorrow, boundary layer overturning alone would be able to produce 50+ mph (only slightly sub-severe) straight line surface winds given these absurd 850 hpa winds over the Carolinas.
View attachment 19148

Eric, I noticed most models in soundings are showing winds in the 60kt range only 1,000 feet off the ground in portions of Eastern NC, some in the 65-70kt range. Apart from convection, is there a way to estimate how efficiently these winds could mix down to the surface in gusts? Here's an example sounding from the 3km NAM. Winds at 1,000 feet are right at 60kts but then at the surface they are sustained below 20kts. I would think in gusts 35-45mph could easily happen without the aid of any convection with a sounding like this?

 

[snowlover91]

And this sounding near the coast from the GFS shows 80kts at 1,000 feet above the ground with 40kts sustained at the surface.. Yikes.