Severe WX Severe Weather Threat 4/5/17-4/6/17

[GTWXAlum]

It seems like with the potential of this event increasing over the past couple of days a separate thread would be a good idea. I didn't see this posted anywhere else, but here's an excerpt from Matt's AFD from Birmingham:

"Substantial instability could develop Wednesday afternoon, and
this system will have to be watched very closely. The latest run
of the ECMWF has trended toward less influence of the subtropical
jet with a stronger polar jet streak, leading to a stronger
surface low and better response from low-level wind fields. If
this trend continues, we could be looking at a serious severe
weather event due to the amount of potential instability
(2000-3000 J/kg). For now, only a low confidence risk of hail and
damaging winds will be introduced into the HWO."

 

[Equus]

I do wonder why the SPC elected to keep Wednesday as a sub-15% day on the most recent 4-8 given the concern shown by BMX and models trending towards a more significant event shaping up. Suspect they'll bring it in on the next one.

Something about Wednesdays in April just makes the atmosphere angry, I swear.

 

[ARCC]

Looks like it could be the first legit outbreak threat of the season. Euro shows a large broad based trough with SW winds at 850mb in a highly unstable warm sector.

 

[Kory]

I do wonder why the SPC elected to keep Wednesday as a sub-15% day on the most recent 4-8 given the concern shown by BMX and models trending towards a more significant event shaping up. Suspect they'll bring it in on the next one.

Something about Wednesdays in April just makes the atmosphere angry, I swear.

The SPC has been verifying horribly with their outlooks as of late. Like horribly.

 

[Kory]

Looks like it could be the first legit outbreak threat of the season. Euro shows a large broad based trough with SW winds at 850mb in a highly unstable warm sector.

We're talking like >3000 J/KG CAPE and a morning cap that will allow that to build....

 

[warneagle]

Yeah this has the makings of a higher end event for sure...

 

[ARCC]

We're talking like >3000 J/KG CAPE and a morning cap that will allow that to build....

Even the GFS which is not as favorable as the Euro is putting out crazy numbers. The sigsvr index is near 80,000 around BHM on 4/6 at 0z.

Time to turn on the "Fred" signal.

 

[Richardjacks]

The 18z gfs looks slightly more like the Euro but it continues to show quite the amount of CAPE building northward. Also a fairly compact upper system that doesn't look too strung out or messy promoting an uncontaminated warm sector. The base is a bit sharper than the Euro..so I am not sure long-tracked super cells are as likely with the GFS. Either way- one to watch for sure.

 

[Fred Gossage]

Both the GFS and Euro are still in "trend mode" from what I've seen at least the past 24 hours... They're both trending toward less STJ influence and a more organized polar jet disturbance. They're trending toward higher instability and low-level moisture further north. They're trending toward stronger wind profiles. (The Euro currently has a face value 3 km EHI of up to 9, 1 km EHI of 4+). They're both trending stronger with the surface low. I'm not ready to bite and publicly jump up and down yelling "outbreak", but I think this is the first legitimate shot Dixie Alley has had for a higher end event since January, or before....

 

[andyhb]

Figured I'd drop in to the new-look board for the first time. The mid level synoptics here are, to put it lightly, rather alarming. A 100 kt WSW/SW 500 mb jet streak looks to be rounding the base of this trough on Wednesday afternoon, leading to very strong deep layer shear over the warm sector. In addition to that, we have an impressive pool of mid level lapse rates looking to be advected right over the warm sector, with >7 C/km over basically all of Dixie (and approaching 8 C/km in spots on the GFS, which may be underdone). The fact that moisture is likely to be on the low end the day prior over the Plains only increases my concern for Wednesday, since it means there is less likely to be bigger issues from antecedent convection.

I mention all of this since I use a top to bottom approach when evaluating these things especially in the mid range (after evaluating the basic stuff like "is there enough moisture"). This one looks troublesome in multiple areas further up in the troposphere, and thus one would expect the low levels to follow by some point.

Should probably mention the East Coast could see significant problems on Thursday as well.

 

[GTWXAlum]

When the usually conservative GFS spits out numbers like this for the 0-3km EHI (valid 0z Thursday), its raising my eyebrows to say the least

 

[Fred Gossage]

And the Euro has a max over north central AL at that time that is about 3 times higher... Even its 0-1 km EHI is up to 4... with the low-level mass fields still trending toward a more organized and stronger solution.

 

[Richardjacks]

check out how fast the front is moving on the 18z gfs...wonder if it will undercut storms...

 

[Fred Gossage]

check out how fast the front is moving on the 18z gfs...wonder if it will undercut storms...

It's sharper with the trough than the Euro, but it moves the front faster. You'd expect the front to move faster with less of a parallel component aloft. Something's not quite right there...

 

[Richardjacks]

I agree...moves too fast for the trough

 

[GTWXAlum]

0z GFS shows a a lot of coastal convection Wednesday which if true would largely mitigate the threat

 

[Fred Gossage]

0z GFS shows a a lot of coastal convection Wednesday which if true would largely mitigate the threat

It's not so much that alone, as much as the operational GFS is faster (faster than the NAM, the last Euro run, the last set of even its own ensembles, even the Canadian ensembles, the UKMET, etc)... and the actual front itself gets to there that by midday. That's FROPA related storms. If you look back at 12Z Wed down there, it's very quiet.

 

[GTWXAlum]

It's not so much that alone, as much as the operational GFS is faster (faster than the NAM, the last Euro run, the last set of even its own ensembles, even the Canadian ensembles, the UKMET, etc)... and the actual front itself gets to there that by midday. That's FROPA related storms. If you look back at 12Z Wed down there, it's very quiet.

It'll be interesting to see if the ensembles/other 0z guidance speed things up as well or if the GFS op is on an island

 

[Taylor Campbell]

The next run of the NAM will have a forecast out to 1pm central time on Wednesday. The 12z will forecast out to 7pm.

 

[Fred Gossage]

The 00Z NAM and 00Z UKMET (which does about as well with synoptics as the Euro) say a slower solution is right. We'll see. The op GFS has been the faster model on other runs too... even of its own ensembles. Pretty much, every scrap of guidance that's not the op GFS... would have to speed up for the op GFS to be right with its timing.