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5/18/20 - 5/23/20 Upper Low Flooding & Severe Threat

Webberweather53

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Figured it was time to start some sort of thread on this given how impactful this event may be and that we may have several days in a row w/ both a legitimate threat for flash flooding and severe weather (including damaging winds & tornadoes) across a pretty respectable portion of the southeastern US (esp the Carolinas).

While the maximum intensity of the severe &/or rain threat may not be quite up to par with the Easter Weekend tornado outbreak, this event is going to be a marathon instead of a sprint, lasting possibly most, if not basically all of this coming week.
 

Webberweather53

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To give some a sense for just how long this overall event may last, most models hint at precip onset sometime later on Monday into Monday night (ish), and potentially lasting all the way thru next Saturday, basically a 5-6 day event.

1589663906291.png
 

Myfrotho704_

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To give some a sense for just how long this overall event may last, most models hint at precip onset sometime later on Monday into Monday night (ish), and potentially lasting all the way thru next Saturday, basically a 5-6 day event.

View attachment 41542
Been noticing for next Saturday as it exits, we get caught on the SW side of it with NW flow and 40-50kts of 500mb flow, some OPs/ensemble members hint at a more organized severe weather threat with that NW flow, this pattern coming up next week is honestly something else
 

Webberweather53

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Been noticing for next Saturday as it exits, we get caught on the SW side of it with NW flow and 40-50kts of 500mb flow, some OPs/ensemble members hint at a more organized severe weather threat with that NW flow, this pattern coming up next week is honestly something else
Oh yeah, I noticed that as the upper low pulls away and the ridge begins to come back, we may have renewed threats of rain/storms going into the following week! The more water we put into the ground, the more edge we're gonna take off the heat when it does finally come (& of course feedback to creating more storms).
 

Myfrotho704_

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Been noticing for next Saturday as it exits, we get caught on the SW side of it with NW flow and 40-50kts of 500mb flow, some OPs/ensemble members hint at a more organized severe weather threat with that NW flow, this pattern coming up next week is honestly something else
ECMWF likes that day to be Friday as it departs 22BA4065-B57A-4B94-96B9-BE41EF09F0DD.png 405B84CD-EAAB-471A-BB3E-3004DA77DB9E.png
 

Myfrotho704_

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Oh yeah, I noticed that as the upper low pulls away and the ridge begins to come back, we may have renewed threats of rain/storms going into the following week! The more water we put into the ground, the more edge we're gonna take off the heat when it does finally come (& of course feedback to creating more storms).
As we go ahead and if we do get all this rain plus more storms, early this summer we may get away with significant boundary layer mixing which is a cape killer on summer days, kinda neat that this may effect that
 

Myfrotho704_

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This is something to watch, models haven’t been handling this MCV well, as someone said in the comments this could have a effect on the upper level low here
 

Webberweather53

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This is something to watch, models haven’t been handling this MCV well, as someone said in the comments this could have a effect on the upper level low here
This stronger than forecast mesolow creates a southward tug on the parent s/w over Iowa that leads to our eventual upper low, causing it to slow & dig earlier, leading to more rain in the SE US this week

1589665434282.png
 

Webberweather53

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This stronger than forecast mesolow creates a southward tug on the parent s/w over Iowa that leads to our eventual upper low, causing it to slow & dig earlier, leading to more rain in the SE US this week

View attachment 41547
These forecast errors in the GFS have propagated forward to ~36 hours in the forecast period, internal changes mask them thereafter. Also notice the immediate & downstream diabatic ridge from TD One/Arthur is much stronger on more recent GFS forecasts (red shading), this also slows our upper low down and causes it to dig sooner and w/ more vigor, also favoring more rain in the SE US & the Carolinas this week.

I don't think our wet trend on the NWP models is over yet either, especially on the GFS
 

Myfrotho704_

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These forecast errors in the GFS have propagated forward to ~36 hours in the forecast period, internal changes mask them thereafter. Also notice the immediate & downstream diabatic ridge from TD One/Arthur is much stronger on more recent GFS forecasts (red shading), this also slows our upper low down and causes it to dig sooner and w/ more vigor, also favoring more rain in the SE US & the Carolinas this week.

I don't think our wet trend on the NWP models is over yet either, especially on the GFS
This also increases are severe chances each day and allows more warm sector which In turns causes more Convective precip, which In turns causes more rain, yikes
 
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GeorgiaGirl

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I could go for a couple good thunderstorms soon, especially considering that it has been dry. But hopefully the flood threat in the Carolinas doesn't turn out to be as bad as it's being modeled.
 

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This also increases are severe chances each day and allows more warm sector which In turns causes more Convective precip, which I’m turns causes more rain, yikes
Concerning that the models are bringing TD 1 so close as any added rain from that will just add to the totals over eastern NC next week.
 

Shaggy

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The poorly dispersed herd of sheep that is the GEFS is finally starting to get a clue. This wet trend is probably not over yet either.
So as the gefs struggles with the UL and wet scenario why did it do so good sniffing out TD1 over a week ago? Seems strange that the gfs picked up on TD1 so early even before the euro but is now playing catchup to the euro on the rain event. Is it just different biases playing into it?
 

Webberweather53

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So as the gefs struggles with the UL and wet scenario why did it do so good sniffing out TD1 over a week ago? Seems strange that the gfs picked up on TD1 so early even before the euro but is now playing catchup to the euro on the rain event. Is it just different biases playing into it?
The GEFS & GFS have a progressive bias in the mid-latitudes, they'll do anything to merge a s/w into the westerlies, they also don't handle synoptically forced changes from diabatic heating via convection terribly well (often underestimate it)
 

BirdManDoomW

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So we are looking at 5 or 6 days of severe weather chances in NC starting Monday? Did I read that right?
Absolutely not. Overnight Monday (it’s always dark) some severe weather could enter the upstate SC and go north into the piedmont. But after that, I think most of the severe turns to flooding for a few places. By Tuesday I expect most will be locked in clouds until it’s over limiting the severe potential to just isolated through the week.
 

Webberweather53

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I'm assuming this westward trend is what has allowed the gfs to get TD1 as close to the coast as it has?
Yes, that's definitely what's played a role in TD One/Arthur getting closer to the coast in more recent runs, the bigger ramifications from this though are that next week goes from being dry to absolute deluge.
 
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