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NorthGeorgiaWX

Saturday, April 11

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Good morning!

You'd never know by looking at this map that a severe weather outbreak was coming. 

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Let's look at all the SPC probability images. The "Enhanced" Risk area has been expanded slightly to include more of NE Georgia

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And the SPC Day 2 Severe Weather Outlook Discussion

Quote

Day 2 Convective Outlook  
NWS Storm Prediction Center Norman OK
1259 AM CDT Sat Apr 11 2020

   Valid 121200Z - 131200Z

   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS FOR CENTRAL/NORTHERN LOUISIANA...SOUTHEAST ARKANSAS...MUCH OF
   MISSISSIPPI...WESTERN/CENTRAL ALABAMA.....
.

   ...SUMMARY...
   An outbreak of severe thunderstorms appears likely Sunday into Sunday night, with the greatest threat expected from Louisiana through much of the Southeast and Tennessee Valley. Strong tornadoes, potentially widespread damaging winds, and large hail are all possible.

   ...Synopsis...
   The ejecting shortwave trough initially over the southern High Plains on Sunday morning is forecast to move quickly eastward to the lower MS Valley by early evening, and then accelerate northeastward toward the Ohio Valley late Sunday night into Monday morning, as it becomes absorbed within an amplifying longwave trough that will  encompass nearly all the CONUS by 12Z Monday. In conjunction with the ejecting trough, a broad surface low centered over the central/southern Plains on Sunday morning will move eastward to the mid-MS Valley by 00Z Monday, and then move northeastward and rapidly intensify into an intense cyclone over the lower Great Lakes by 12Z Monday. A warm front will surge northward ahead of the low across the lower MS Valley and Southeast, while a strong cold front will move southward through much of the Plains in the wake of the  departing cyclone. 

   ...East TX northeastward through much of the Southeast and TN Valley...
   One or more clusters of deep convection will likely be ongoing at 12Z Sunday morning somewhere over east TX and potentially into portions of the lower MS Valley. The intensity and areal extent of any such clusters remain uncertain, but ample shear and instability will favor a threat of hail and damaging wind with any organized convection at the start of the period. Some tornado threat will also be present Sunday morning with any semi-discrete storms that begin to interact with the richer low-level moisture in the vicinity of the warm front. As this convection spreads northeastward,  intensification is possible into portions of the ArkLAMiss region, with an increasing tornado threat in late morning/early afternoon
with any surface-based storms, given rapidly increasing low-level moisture and shear. North of the warm front, evolution into a QLCS will be possible, with a corresponding risk of damaging wind into portions of the TN Valley. 

   Meanwhile, further south, moderate to locally strong instability is forecast to develop along/south of the warm frontal position, which will be modulated by the impact of outflow from any early convection described above. Mid-level flow will increase to 70-100 kt as a south-southwesterly low-level jet intensifies into the 40-60 kt range. These wind profiles combined with ample instability (MLCAPE of 1500-3000 J/kg) will support the potential for intense supercells. Any surface-based initiation along and east of a pseudo-dryline moving into western LA by late afternoon could evolve into one or more long-tracked supercells capable of producing strong tornadoes, large hail, and damaging wind gusts. The extent of  development within the warm sector remains somewhat uncertain, given the presence of a capping inversion and generally subtle foci for  initiation. 

   While the conditional risk of all severe hazards will be quite high if supercells develop, uncertainty remains regarding how convection will evolve from the morning into the afternoon. Any remnant outflow related to early convection will determine the northern extent of the higher-end tornado potential, and some guidance suggests the potential for elevated convection within a mid-level moist plume across the warm sector during the afternoon, which could either dampen the severe potential, or evolve into surface-based convection with a substantial severe threat. Given these factors, there is too much uncertainty to upgrade the ongoing outlook at this time. 

   Evolution into more of QLCS is suggested by most guidance by Sunday evening, which would pose an increasing threat of widespread damaging winds and a few tornadoes across much of AL into western/central GA through the overnight hours. Higher wind probabilities may be needed in subsequent outlooks if confidence in this scenario grows. 

     ...TN Valley into the OH Valley -- Sunday night...
   Substantial uncertainty remains regarding the potential for destabilization from northern portions of the TN Valley into the OH Valley, due to the potential for widespread convection to the south of this area. However, rapidly strengthening wind fields in advance of the deepening cyclone will support the potential for damaging wind and perhaps a tornado risk by Sunday evening should even modest destabilization occur, as strong convection attempts to move in from the southwest in tandem with the deepening cyclone. 

   ... Eastern Georgia into the Carolinas/Mid Atlantic...
   Substantial low-level moistening is expected over eastern GA into the Carolinas/Mid Atlantic through the period. There is a nonzero risk of organized convection along/north of the warm front during the day into the evening, which would pose some risk of locally damaging wind or perhaps a tornado, but confidence in this scenario is low at this time. A more likely scenario is for widespread upstream convection to evolve into multiple clusters or a QLCS and move into this region sometime early Monday morning. Intense wind profiles will support a risk of widespread damaging wind and a few tornadoes, given sufficient instability. 

   The magnitude and coverage of the severe threat in this region will be determined in part by how fast organized convection approaches from the west. If convection accelerates and arrives faster that current guidance would indicate, then there is less time for low-level moistening and destabilization, and the magnitude and northward-extent of the threat may be limited. If convection does not arrive until very late in the period, then a more substantial severe threat could evolve. If some of the slower guidance turns out to be accurate, then the primary severe threat in this region may not come until the D3/Monday period. Probabilities may need to be increased in this area once the details come into better focus.

 

I am including the Birmingham forecast discussion, because if you look at heirs and look at Atlanta's you'd wonder if we were even looking at the same system.

Quote

Sunday

Impacts:
Severe weather is likely to occur across the Deep South on Sunday. Tornadoes, large hail, and damaging winds are the primary threats. Some tornadoes may be strong/long-lived.

Timing:
Multiple waves of storms could occur between 1pm and 2am.

A highly dynamic system is expected to move through the Plains on Sunday. The warm front is still along the Gulf Coast Sunday morning and will lift northward as we get into the afternoon hours. Along and behind the warm front, a very moist/unstable air mass builds in. Instability increases to 1500-2000+ J/kg with dewpoints in the upper 60s to near 70s. I expect showers and thunderstorms to develop along the warm front as it lifts northward. This could turn into the initial wave of severe weather, though most of this is expected to be elevated making the main threat damaging winds and hail. However, we could see some of these storms become surface-based and present a tornado threat given the high shear in place. This is why our threat timing could be as early as 1pm.

Through the afternoon into early evening hours, the environment becomes even more unstable as the warm/moist flow from the Gulf continues. Mid-level flow from the southwest could help set up a capping inversion around 850mb since the source region for this air is coming out of Southern TX. This would prevent widespread showers/thunderstorm activity for the afternoon; however, a pre- frontal shortwave is expected to eject from the base of the deepening trough and lift through our area. This occurs around the same time as both the upper and lower level jets begin nosing into Central AL. This could allow for enough lift to break through the capping inversion - if this happens, we could see scattered, more
discrete thunderstorms. These thunderstorms, if they are able to develop, could become strong supercells fairly quickly in the rich/highly sheared environment. CAPE values could still be in the 1500-2000J/kg range with 0-1km and 0-3km SRH approaching or exceeding 500. This could be the most dangerous time with this system in terms of both tornado and hail threats. It also happens to be the most uncertain because it all depends on the shortwave impulse and jets providing enough forcing, plus any daytime heating, to help break through the capping inversion.

Late into the evening and into the overnight hours, a line of storms moves across the MS River and into the northern portions of Mississippi. This line will be most closely associated with a developing dryline as the main cold front still hangs back by a few hours. These storms are forecast to move through Central AL and could turn into a QLCS as instability and shear are expected to remain abundant across the area. Therefore, we could see another threat of tornadoes, damaging winds, and hail through at least 2 am, which is why the ending time to our threats is so much later.

I think it's likely that the exact timing of threats could change in the 24 hours or so as we lead up to the storm system and our convective- allowing models start to resolve the upstream characteristics of this system a little better.

Looking at the severe parameters for the NAM 3km, and while the colors are pretty, the values are not. I have two loops and several images. 

Significant Tornado Parameter

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Supercell Composite

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The rest of the images are for 2 AM Monday. Yea... not good timing.nam-nest-conus-georgia-shear_0v6000m-6757600.thumb.png.8719f995b2cd3e7c61251a292329901d.png

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I'll have more as the day goes on, keep checking back with me!

Have a great Saturday!

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Ugh. I don't like the way the red in the SPC graphic is inching closer to us...

 

:classic_blink:

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Is it just me or does the 06zNAM look SLIGHTLY better this morning? Not perfect but not QUITE as scary and strong? I know ONE model, one run, but searching for silver linings anywhere lol

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Any chance is bad, but I like how most of Georgia has only a 5% Chance on the Tornado Probability graphic. Obviously I wish it was lower, but if I have learned one thing from here, and life in general, you have to look at the positives in these sort of situations.

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I asked this yesterday, but an not sure it was seen, so:

So, I have a question about fronts. I know that, if it is hot in an area, and a cold front comes through, storms can happen, and vise versa, but what if an area is cold and a front comes through that makes things colder, or a warm front comes through an already warm area and makes it hotter? Do storms still happen, or no, due to the current temperature, and the temps the front brings, being similar?

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