3/23/09 FCST: NE, KS, OK

[Greg McLaughlin]

When I saw the GFS 12z output I immediately thought of 3/28/07. I always look at 500mb flow first and then go down from there. I jumped to the 850 chart and noticed the forecast flow from the south. This would limit mid-level directional shear and may promote more of a squall line event. My question is...does anyone remember if the models showed backing low-level from from the southeast and east with the 3/28/07 system, or did it do a poor job of handling the low-level wind fields?

 

[Skip Talbot]

The 500-850mb veering turned out to be greater than the models forecasted for 3/28/07.I remember both 500's and 850's were much more southerly on the models a few days out.

 

[Dick McGowan]

When I saw the GFS 12z output I immediately thought of 3/28/07. I always look at 500mb flow first and then go down from there. I jumped to the 850 chart and noticed the forecast flow from the south. This would limit mid-level directional shear and may promote more of a squall line event. My question is...does anyone remember if the models showed backing low-level from from the southeast and east with the 3/28/07 system, or did it do a poor job of handling the low-level wind fields?

For kicks, I'm looking at the old SPC mesoanalysis archive here, starting on 3/29/07 at 1z for the past 6 hours: http://w1.spc.woc.noaa.gov/exper/ma_archive/

 

[Gabe Garfield]

The direction of the 500/850 mb flow depends greatly on the timing of the shortwave trough. The GFS, you'll notice, ejects the wave early on Monday -- not good. This leads to substantial backing in the mid-upper levels as the wave "swings" through. However, in last night's Euro, the wave is still digging, with an 80-90 kt 500 mb jet max on the backside of the trough. In that scenario, the winds are far more veered in the mid-upper levels.

At this point, I'm betting that the GFS will slow down -- per the usual -- and fall into line with the Euro. If that happens, I think an outbreak of tornadoes is likely. The magnitude of the moisture/instability will determine the intensity of the outbreak.

In my opinion, you don't get much better-looking scenarios than this one. Moisture could be a bit better, but I think that even low 60 Tds will probably be fine for at least a few significant tornadoes, given the extreme shear (c.f. 2/10/09). At the very least, I think we'll see a significant wind event; at most, "the sky's the limit."

 

[Connor McCrorey]

This may just be my noob showing again, but I think people are focusing too much on NE/KS and not enough on TX/OK. If the 12Z GFS verifies (yeah right..) the best 500mb winds will definitely be in N TX, or around the small area of greater instability in SW OK. The 12Z GFS kicked up CAPE a notch from the 0Z, so hopefully that trend will continue. Of course I am slightly biased as there's no way I can make it farther than the OKC area, but I honestly believe N TX/S OK has the best chance for isolated tornadic supercells on Monday.

 

[Rich Thompson]

The moisture may not look that much different this coming Monday compared to 3/28/07, though I still think we may struggle to maintain 60+ Tds given the very shallow depth of the marine layer right now.

The bigger, and potentially more important, differences are a warmer "warm sector" in the 2007 event, plus much slower system movement in 2007. That resulted in a retreating dryline back in 2007. Instead of everything piling up early, a more discrete mode was maintained for longer (storms did eventually line out). Farther south, a lead shortwave trough ejected across the TX Panhandle during the day, supporting warm sector storm development well east of the dryline. All of these factors favored the right convective mode over a large area for many hours. The upcoming forecast looks like slightly less moisture, cooler surface temperatures, and more strongly forced. Not that you can't have a few tornadoes, but I see as many differences as similarities.

Rich T.

 

[Gabe Garfield]

The moisture may not look that much different this coming Monday compared to 3/28/07, though I still think we may struggle to maintain 60+ Tds given the very shallow depth of the marine layer right now.

The bigger, and potentially more important, differences are a warmer "warm sector" in the 2007 event, plus much slower system movement in 2007. That resulted in a retreating dryline back in 2007. Instead of everything piling up early, a more discrete mode was maintained for longer (storms did eventually line out). Farther south, a lead shortwave trough ejected across the TX Panhandle during the day, supporting warm sector storm development well east of the dryline. All of these factors favored the right convective mode over a large area for many hours. The upcoming forecast looks like slightly less moisture, cooler surface temperatures, and more strongly forced. Not that you can't have a few tornadoes, but I see as many differences as similarities.

Rich T.

Rich, do you have access to the Euro surface dewpoint forecast? I'm curious to find out if the Euro is more bullish with moisture return. I would suspect so, given that the faster 500 mb is further south (i.e., increasing downsloping/lee troughing which would lead to a deepening surface low closer to the moisture source).

The other thing is that the jet stream corridor in the GFS solution is fairly narrow, so storms on the south side would have deal with relatively warm 500 mb temps. In the Euro solution, the 500 mb flow is a bit more broad, suggestive of a weaker temperature gradient (c.f. the thermal wind relationship). Thus, cooler 500 mb temps and instability would be greater in the Euro solution than in the GFS solution.

Nevertheless, I think this "just in time" moisture return could be problematic (c.f. Rich's comment about a narrow warm sector). It really depends on whether or not we can manage a truly tropical fetch. At least, it will be better than 3/7!

EDIT: Yikes! I didn't realize how much convective feedback was likely impacting the instability progs for Monday in the GFS. The precip field goes gangbusters by 00z Tues leading to surface temperatures in the low 60s! Yeah, that won't get it done. Yet, the GFS still progs pockets of instability >1000 j/kg. Not too shabby, considering the whole dryline blows up! lol Anyhow, I imagine that the GFS forecast -- in fact -- represents a lower bound for instability on Monday.

 

[Verne Carlson]

Went back to some old NAM models from 3/28/07 and put the current GFS next to them. The similarities are mostly in orientation and locations. The CAPE is going to have to really verify a lot higher as most are saying before we get a repeat. I am loving the negative tilt 500h! We didn't get one in 2008 until May 22.

 

[Jared Farrer]

Latest model runs seem to still be painting a big possible area! Better timing than before. And the system is starting to show up on the medium range models. ECMWF and GFS are still looking decent. I agree with the assement on cape, in that its hard to nail down exactly what we will be dealing with. When this system starts to show up better on medium range I think things could get exciting!

Its funny I heard O'keefe mention 3/28/07 because thats exactly what came to my mind when I saw the runs last night! That ended up being a wedge fest! Dont let it being March throw you off, some years March can really be wild.

 

[Michael O'Keeffe]

The thing I was noticing about surface temps like some people have been mentioning is you can't really get a good indication with the precip. The models realize the precip and temps can be lowered by that and I have noticed especially during the winter the GFS is usually cold bias. All of the NWS offices in the area are calling for low-mid 70s over the region, so that may not turn out to be such a big issue. All of these small scale details are hard to pin point anyway. I say all of us including myself take a deep breath and realize we have a strong system on our hands, but we don't know when, where, or how strong.

I know it can be hard sometimes.

 

[Andrew Stoller]

At this point, I don't know what to think. I'm not even going to focus on moisture return or anything like that this far out, but the backed flow from the surface to 500 mb has me slightly concerned, as the system becomes so dynamic and negatively tilted. That usually leads to all day convective garbage and no insolation at all. OTOH, systems that were as dynamic as this one looks to be with it's associated potential problems, also produced events like the Windsor, CO tornado, with northwest moving supercells!

There looks to be more veering with height further south into southwest OK though, and with that area further away from the surface/upper low center, perhaps it would see less chance of early forcing and precipitation.

 

[V. Gensini]

However even if the GFS verifies I have seen these early season fairly unidirectional setups verify over the Plains, namely 3/28/07, so ya never know.

3/28/07 wasn't that unidirectional!---00Z RUC 850/500mb crossovers:

 

[Derek Weston]

I was worried about moisture/instability on earlier GFS solutions that ejected the upper trough into the Plains on Sunday, but with both models now honing in on Monday, I think another full day of return flow should help mitigate that problem. As for the CAPE values, my completely subjective impression over a few years of following severe weather on the models is that the medium-range GFS will often underestimate the extent and magnitude of instability. The surface dew points on the 00z GFS look nearly comparable to, say, 2007-03-28 (in no way am I suggesting a repeat; just that moisture does not seem like the primary issue, as long as these current slower solutions verify).

As for lacking mid-level winds, I'm not really seeing that either on any of the 00z models. In fact, the wind profiles almost look scary over much of OK/KS/NE. Someone more experienced can correct me if I'm wrong, but it seems to me that if you take current model output of the overall synoptic situation as being fairly close to on-the-mark (which is always a stretch at 144 hrs., of course), the only real question marks standing in the way of a significant event would be timing and storm mode.

I was being picky, admittedly. Interesting point on the GFS this far out... had noticed the differences between the short range models, but I'll have to look for this with the GFS.

With the 500s, just looking at that run... saw pockets of <35kts in areas in areas possibly under the gun for severe weather. Just seemed the mid level support might not arrived in full. (of course, directional shear here was outstanding... and storm motion likely wouldn't be insane -- I'll take that in March)

At the very least... we've got something 'strong' in the offing. And what looks to be unimpeded moisture return off the gulf for at least a few days. Worthy of some excitement.

 

[V. Gensini]

Went back to some old NAM models from 3/28/07 and put the current GFS next to them. The similarities are mostly in orientation and locations. The CAPE is going to have to really verify a lot higher as most are saying before we get a repeat. I am loving the negative tilt 500h! We didn't get one in 2008 until May 22.

CAPE from the morning 12Z models were over forecasted because of moisture. The 12Z NAM/GFS had dews reaching the mid 60's! In reality most Td's in the warm sector were mid 50's. The location of the instability axis was also too far east. The dryline was nearly stationary the entire day and erupted cells in far E CO and W KS, beside the cells near AMA that fired from what seemed to me like a 700mb vort max.

 

[Gabe Garfield]

CAPE from the morning 12Z models were over forecasted because of moisture. The 12Z NAM/GFS had dews reaching the mid 60's! In reality most Td's in the warm sector were mid 50's. The location of the instability axis was also too far east. The dryline was nearly stationary the entire day and erupted cells in far E CO and W KS, beside the cells near AMA that fired from what seemed to me like a 700mb vort max.

Actually, most Tds were in the upper 50s/low 60s, according to http://vortex.plymouth.edu/cgi-bin/...&hh=00&density=&sc=1.0&ge=640x480&id=&zoom=.6

I'm actually surprised -- I seem to remember dewpoints in the mid 60s near Childress that day. In any event, it seems low 60s can get the job done. In fact, one of the worst tornadoes (Woodward, 1947) occurred in an environment characterized by low 60s Tds.