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Tallis Rockwell

Severe Weather for 4/19-4/23

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Decent weekend ahead.

  
  ...Synopsis...
   A progressive shortwave trough will move from the southern Plains
   into the lower MS Valley during the day, and will continue to the
   GA/SC/NC coast by Monday morning. The entire southeastern region
   will be under the influence of strong westerly winds aloft which
   will strengthen as the upper trough approaches. Southwesterly 850 mb
   winds will increase in response to the wave, reaching 50+ kt by 00Z
   from MS eastward to the coast.

   At the surface, low pressure will develop near the ArkLaTex by late
   afternoon as a warm front lifts north across the southeastern
   states, oriented roughly from southern AR into northern MS, AL, and
   GA at 18Z. Meanwhile, a cold front/dryline will develop over east
   TX. Across the warm sector, rich moisture with 65-70 F dewpoints
   will result in moderate instability. The low, likely associated with
   widespread convection, will translate eastward along the synoptic
   front overnight, reaching NC by 12Z Monday.

   ...Southeastern states...
   Substantial instability will already be in place Sunday morning from
   TX into the lower MS Valley, with rapid destabilization expected
   behind the warm front farther east into GA. Midlevel lapse rates
   aloft will be modestly steep, averaging 6.5 to 7.0 C/km. Early day
   storms related to warm advection will be present over AL and GA, and
   they will likely be elevated with perhaps a hail threat initially.
   As the air mass warms, a wind or tornado threat could develop in the
   vicinity of this activity as it continues east.

   To the west, storms are likely to be severe by 18Z near the Sabine
   Valley/ArkLaTex. Here, long hodographs but modest low-level SRH may
   result in cells capable of very large hail, and perhaps an eventual
   MCS with damaging-wind threat. A severe wind corridor may thus
   develop during the day across northern MS, AL, and GA. 

   There is also a threat of daytime supercells, possibly tornadic, as
   the small capping inversion is eroded by early afternoon. During the
   evening and overnight, low-level shear will increase further, and
   models suggest southern AL into GA may be a favored area for
   tornadoes. Bows or isolated supercells are possible into SC early
   Monday morning as the relatively cooler air mass is destroyed by the
   warm front.

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You may wan to edit the image, repost image, something like that.

Global models generally have a low pressure near Texarkana on Sunday, 18z, with a 500mb shortwave axis west of Dallas, moving eastward quickly. The low pressure will move toward Mississippi.

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I

On 4/16/2020 at 8:00 PM, Chinook said:

You may wan to edit the image, repost image, something like that.

Global models generally have a low pressure near Texarkana on Sunday, 18z, with a 500mb shortwave axis west of Dallas, moving eastward quickly. The low pressure will move toward Mississippi.

Can't believe I missed that crap.

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9 hours ago, BrandonC_TX said:

Interesting enough SPC includes a significant hail risk today (4/19) for half of the DFW Metroplex, primarily Dallas and Collin counties eastwards.

Got 1/2" here. Wasn't looking like much at all this morning had some isolated showers and storms that ended up weakening then around 10am storms redeveloped quickly along western fringe of metroplex. Looks like it was subtle convergence boundary aloft tapping into a very unstable airmass with those steep lapse rates.

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Already a large area with greater than 2500 J/Kg of MLCAPE south of the warm front.  Definitely a volatile environment if surface winds are not veered and storms can form.

17z Mesoanalysis.PNG

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As of 3pm Central the cell south of Houston was getting scraggly on radar. Good it missed/weakened around Angelton. Lake Jackson is south, but close, and Freeport is safely south. Praying it does not cycle and get its act together going into Galveston. South wind not particularly strong could help mitigate another tightening. 

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15% hatched Day 3 Slight risk for DFW down through central Texas.  SPC currently anticipates veer-back-veer wind profiles over the highest risk area, with large hail as the primary risk.

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SPC going with a Day 2 ENH now for SE OK and E TX into AR and LA. Even a small 10% Hatched Tornado risk ahead of the dryline. As mentioned in their discussion, a lot of uncertainty remains regarding capping and potential early day convection, but wind fields look impressive enough if we can get some discrete cells along the dryline in the afternoon. 

 

day2probotlk_1730_torn_prt.gif

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What is interesting about DFW-area tornado history seems to be that several of our worst outbreaks in recent memory have occurred on more conditional days that are not a moderate or high risk.  Seeing something like a 10% hatched risk over DFW, as such, definitely gets my attention.  December 26, 2015 was only an enhanced risk day (10%, not hatched), and the October 20, 2019 tornadoes occurred largely within a slight risk (there was an ENH further northeast).  The April 3, 2012 tornado outbreak occurred near and within an enhanced-equivalent (10%, not hatched, considered a slight risk at the time) tornado risk, but an outflow boundary helped to back the winds that day.  I believe that our last high risk was back in April 2007, and it largely busted.

I am in no way saying that tomorrow will be like 12/26/15, 10/20/19, or 4/3/12. But there is the low probability that such an outcome could occur; there have also been hatched risk days that have failed to produce tornadoes (or failed to verify at that risk level, such as what happened to the MDT for tornadoes in the southeast on 4/19/20). Any conditional tornado event can produce tornadoes, but the risk is fortunately conditional.  Several of the worst DFW tornadoes in recent memory have occurred on otherwise-conditional days, however, and mesoscale factors were largely to blame (as on 4/3/12 and I believe on 10/20/19 as well).  Of course, we could see a downgrade or upgrade for Day 1 as the risk becomes more clear in the hours to come.

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20 minutes ago, BrandonC_TX said:

What is interesting about DFW-area tornado history seems to be that several of our worst outbreaks in recent memory have occurred on more conditional days that are not a moderate or high risk

It caught my attention too, because I do remember the outlooks on April 3, 2012 + Dec. 26, 2015 + Oct. 20, 2019. 

20 minutes ago, BrandonC_TX said:

But there is the low probability that such an outcome could occur; there have also been hatched risk days that have failed to produce tornadoes (or failed to verify at that risk level, such as what happened to the MDT for tornadoes in the southeast on 4/19/20)

Stuff like that also came to mind, stuff like May 18, 2019 + some other day in June last year that I can't remember + Jan. 10, 2020 (another time in late-April of 2016, where I believe we even had a PDS Watch in the area). In those cases, the events either busted (in our area) or were just not as severe as forecasted (again, in our area). Of course, mesoscale details on the days of these events had a lot to do with either increasing or decreasing the threat. April 29, 2017 also came to mind where the cap broke over to our east and things went nuts in Van Zandt county especially.

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I think a portion of whatever makes or breaks tomorrow’s event will come down to if and where outflow boundaries set up as a result of tonight’s convection in Oklahoma. That, and the obvious question of where or if the cap breaks.

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Definitely, if there are any outflow boundaries, they could set up localized areas of higher risk, but it won't matter if the cap doesn't break. I'm no expert though, and I really don't know what to think about the setup overall. The Weather Channel app is telling me there's about a 90% chance of storms tomorrow, as is also suggested by one future radar website I pulled up. However, the convection-allowing models I looked at on pivotal weather for the most part show little convective activity over here, with a couple exceptions, but nothing particularly concerning (in the DFW area). The ingredients do look to be in place though, so I am just not sure.

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hrrr_2020042118_027_KOKC.png

Some pretty potent soundings on the 18z HRRR along the I-35 corridor tomorrow, all the way up to the triple point which comes pretty close to OKC metro. Great low level lapse rates, low LCLs, plenty of shear, critical angles near 90 degrees. Not much more you could look for on a sounding really.

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12 minutes ago, TexMexWx said:

Definitely, if there are any outflow boundaries, they could set up localized areas of higher risk, but it won't matter if the cap doesn't break. I'm no expert though, and I really don't know what to think about the setup overall. The Weather Channel app is telling me there's about a 90% chance of storms tomorrow, as is also suggested by one future radar website I pulled up. However, the convection-allowing models I looked at on pivotal weather for the most part show little convective activity over here, with a couple exceptions, but nothing particularly concerning (in the DFW area). The ingredients do look to be in place though, so I am just not sure.

This strikes me as one of those events that’s so common around NTX where the models push the dry line too far east and then slowly pull it back west over the 24hours leading up to the event. I suspect the spc and nws have been holding the same opinion, given that the risk area since 2 days ago has been outlined further west than models often depicted. But that doesn’t say much about how severe these will be or not. Definitely have to acknowledge the conditionality of this setup. 

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18 minutes ago, Chreeyiss said:

This strikes me as one of those events that’s so common around NTX where the models push the dry line too far east and then slowly pull it back west over the 24hours leading up to the event.

I haven't been looking at models for very long, in fact not at all until recently. A bit of a newbie, haha. If that's part of the thinking here then I see it a lot more clearly.

18 minutes ago, Chreeyiss said:

But that doesn’t say much about how severe these will be or not. Definitely have to acknowledge the conditionality of this setup. 

Yup. Seems like there are various scenarios possible as to how it unfolds, with nothing being a given.

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Finally we are seeing some of the CAMs show storms in various areas of eastern Texas and Oklahoma. I was really wondering about that. Up until this morning, the models have been showing so much CAPE and so few storms. There should be wind convergence near the dryline.

bF9dq9m.png

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18 minutes ago, Snowstorm920 said:

A bit of veer back in the profile otherwise this is an ominous sounding over DFW tomorrow evening. Taken from the 3km NAM with a supercell forming

The solution on the 18z NAM3k seems to hint at the possibility of a localized outbreak in/near DFW; I definitely do not want to see that solution verify.  The earlier 12z run pops the storms further east, though.  The 18z HRRR solution fails to initiate convection over DFW/North Texas in the afternoon, but could spell trouble up near Oklahoma City with storms firing on the triple point.

EDIT: That trend towards afternoon storms is concerning to say the least.  We will have to see how the observations play out tomorrow.

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I'm getting increasingly concerned about the I-35 corridor from Fort Worth up to OKC. 21z RAP shows this area (OKC in particular) in an extremely favorable area of ascent (left exit jet region) during peak instability + dryline initiation. Just seems like an event that could "overperform" compared to current expectations with a rogue supercell or two going gangbusters. Low level lapse rates are formidable on HRRR and RAP forecast soundings as well. IDK

 

 

 

300wh.conus.png

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FWD seems to think the highest severe threat is a touch west of where SPC has it. Makes a big difference for DFW

IMG_20200421_195204.jpg

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