Severe WX January 10-12, 2020 Severe Weather Forecast Thread

[Taylor Campbell]

As long as the parameters are met season wouldn't seem to be a big issue; with near 100kt 700mb winds and unseasonably high dew points I'd say we've got a good shot at hitting derecho qualifications somewhere.

On that note, if those parameters stay intact I fully expect a 45/hatch moderate risk for wind on Saturday. A derecho forecast usually calls for probs that high, though any uncertainty that a derecho may not develop would probably hold it at 30. The only real question at this point is the possibility for discrete development which might also up the tornado probs. Given the prolific tornado production of QLCS with hodos like are being modeled, even as high as a 10-15% would probably be a safe bet at the heart of the QLCS regardless of supercell development.

As I look at it today, I find it very hard to go with anything less than a moderate risk. This is looking like a classic mean one.

 

[Equus]

Yep yep, if warm sector starts looking more supercell-friendly, perhaps even a MDT for both wind and tornadoes. Have seen setups with far less scary dynamics go MDT after all

 

[Richardjacks]

We aren't going to know the true supercell risk until Saturday morning, too many if's at play such as timing of the pivot and quality of warm sector, but the QLCS/derecho risk is quite high, regardless.

 

[warneagle]

Serial derechos, those that usually are generated by a squall line, are more common when jet dynamics are intense, e.g. in the cold season and early spring.

Yeah, warm season derechos are more of an upper Midwest phenomenon rather than a southern one. You’re much more likely to get sufficient dynamic support for a derecho event in the south in the cool season.

 

[Richardjacks]

The 18z nam is a little slower allowing slightly higher tds further north. Will have to watch to see if this is a trend. Dynamics and shear are just crazy.

 

[KoD]

NWS Huntsville AFD said "forecast severe weather parameters are astounding"

Will stay with a more consistent ECMWF overall system position location, which is slower than the faster GFS. But am going a bit weaker with the system like the GFS, as the ECMWF could bring a higher severe threat to the area if its forecast holds. In either case and accounting for timing differences, forecast severe weather parameters are astounding, especially for the winter, and would be really high in the spring. 850mb winds from the NAM/ Canadian/ECMWF were up into the 70-80kt range during Sat, yielding storm relative helicity values of 400 to over 800m/s at times. Dewpoint values were in the low/mid 60s for much of Sat, with highs later that day nearing 70. But CAPE values were a bit lower into the 50-150 J/kg range, at times above 300 (especially from the Canadian). Thus, all modes of severe weather are possible - especially damaging winds and tornadoes. A negatively tilted upper trough in conjunction with the deepening surface low could allow discrete convective cells with tornado production possible. A quasi- linear convective system/squall line like feature the models were hinting at would be another area with tornado development.

 

[CheeselandSkies]

NWS Huntsville AFD said "forecast severe weather parameters are astounding"

Interesting...there's certainly some level of threat there but I haven't seen anything I would call astounding yet, and the model agreement isn't great.

 

[Kory]

Interesting...there's certainly some level of threat there but I haven't seen anything I would call astounding yet, and the model agreement isn't great.

When you have 0-1KM helicity values in excess of 400 m2/s2 and 0-3KM values in excess of 600 m2/s2 across two states, that’s astounding. Some of the shear values are just absolutely bonkers. But, this may be a case where we will need sufficient stretching of the updrafts...I.e., tall CAPE profiles.

 

[ARCC]

Interesting...there's certainly some level of threat there but I haven't seen anything I would call astounding yet, and the model agreement isn't great.

Im curious why no one is mentioning the pitiful lapse rates on every model.

 

[Kory]

Im curious why no one is mentioning the pitiful lapse rates.

I don’t know if I’d consider 6.0-6.5 C/KM lapse rates pitiful (those are mid level of course). I think the surface-3KM lapse rates need some work, but a later arrival would certainly aid in low level destabilization.

 

[Richardjacks]

Im curious why no one is mentioning the pitiful lapse rates on every model.

It is assumed that low level temps will be higher than progged which will bolster the lapse rates...in this type of scenarios that assumption works out unless the warm sector gets contaminated

 

[ARCC]

I don’t know if I’d consider 6.0-6.5 C/KM lapse rates pitiful (those are mid level of course). I think the surface-3KM lapse rates need some work, but a later arrival would certainly aid in low level destabilization.

12z Euro in central MS ahead of the line. Needs a ton of work.

 

[ARCC]

It is assumed that low level temps will be higher than progged which will bolster the lapse rates...in this type of scenarios that assumption works out unless the warm sector gets contaminated

NAM doesn’t show any contamination, but largely the same except for the higher mid level temps. 0-3km cape is decently better.

 

[Richardjacks]

after looking at a few soundings, I am beginning to wonder if there may a weak cap in the 850/900mb area Saturday am for west Alabama ahead of the line, this would not be good.

 

[NorthGaWeather]

LR's are marginal in that sounding. However, CAPE is very sufficient.

 

[Richardjacks]

LR's are marginal in that sounding. However, CAPE is very sufficient.

yes and the vertical wind profile is basically perfect for rotating storms, even low level backing.

 

[ARCC]

LR's are marginal in that sounding. However, CAPE is very sufficient.

Yes and no. It is sufficient for winter time severe storms, but the issue is the majority of it is off the surface due to the cap. If storms remain elevated the threat from both wind and tornadoes drops significantly.

 

[NorthGaWeather]

yes and the vertical wind profile is basically perfect for rotating storms, even low level backing.

Exactly. That's a very impressive wind profile.

 

[Kory]

Yes and no. It is sufficient for winter time severe storms, but the issue is the majority of it is off the surface due to the cap. If storms remain elevated the threat from both wind and tornadoes drops significantly.

I don’t think surface temps should be taken verbatim when we’ve seen them verify 3-5+ degrees warmer than the past couple events at this range on the models.

There is going to be some serious WAA going on Saturday as long as there isn’t junk convection in the warm sector.

 

[NorthGaWeather]

Yes and no. It is sufficient for winter time storms, but the issue is the majority of it is off the surface due to the cap. If storms remain elevated the threat from both wind and tornadoes drops significantly.

There is an area of weak inhibition, however I don't see much on that sounding that would prevent a severe QLCS. WAA will be very strong and likely will change the near-surface parameters.