3/30/08 FCST: OK/TX

[matt patterson]

Im kind of surprised that this has not been brought up yet. Sunday looks to be pretty interesting with good instability and good wind shear setting the stage for supercells across a large part of Central OK and NC Tx. Cape values look to spike just south of the OKC area aoa 2500 and the upper support should be in place per the 12z/3-25 runs on both the GFS and NAM. Still 55-60 hrs out but definately something worth keeping an eye on.

 

[BrandonWhittington]

I have been watching this for past couple of days. Indeed the setup is interesting. Im just really waiting to see how much moisture we can actually get up here in this neck of the woods. There looks to be a triple point develop in NW Oklahoma. I would like to see the dryline the Eastern panhandles. Everything else looks good, temps look good, and heck, we have enough shear to have supercells. SPC even metioned that there could be a tornado threat...We will see as everything else unfolds. Again, the only thing im concerned with is moisture return. Last year models had a bad time forecasting the dryline. The dryline every event was 50-80 miles further west that depicted.....We will see....

 

[Jonathan Whitehead]

Well I didnt find out about this till this morning in my METR classes. So my weekend has really been thrown to hell (see ST Forum main page at bottom).
Anyways thanks to my OWL shift I was able to go through and dissect Sunday as much as I could this far out with 12Z model runs. So far it looks to be a good setup although the main jet streak will be well in Northern Plains, still enough winds at 500 (35-45kts give or take 5kts depending on which model you look at). 850 winds will be veered across OK at about 30-35 kts across mainly areas along and east of I-44.
Overall one thing I have noticed is the GFS is much more aggressive then the NAM over the next 54 hours. GFS is warmer on sfc temps and aggressive in moisture return, although the NAM comes in line by midday Sunday with the dews in the way of location. Most of OK will see dews 60-66. CAPE looks to be moderate with both models showing 2500-3000 in mainly western and SW OK. The NAM helicity shows 300-400m^2/s^2. I did not look at SREF/EUKMET models at all.

So overall it looks to be respectable setup. Def one to keep an eye on. This far out its tough to look at CAP strength since that is always impt. So with modest but sufficient shear in place id put Sun at a MDT risk with a 10% tornado all areas west of I-35 in OK.
Will wait and look at models again tomorrow evening (18Z runs).

NOTE: below are OUN soundings/hodographs for the forecasted period.

 

[Bill Tabor]

The region from CDS to SPS seems somewhat favorable for supercells / torns on Sunday during the day, but per the latest NAM run (12z) I see no sign of SPC's mentioned mid level wave until Monday. Likely this will rely on daytime heating and sfc convergence to break the cap. Good news though without the strong mid levels perhaps the storms will be moving slowly (assuming they breakout without a wave).

 

[Ilya Neyman]

The NAM shows a wave crossing OK Sunday evening-night, along with a precip. bullseye. Both the NAM and GFS show 700mb omegas approaching SW OK Sunday afternoon-evening, before spreading NE overnight.

It's anybody's guess right now but based on this and consistency so far I would say Sunday looks at least respectable. Most likely timeframe looks to be LATE Sunday afternoon-evening across western OK. If the wave does come through and convection would be sustained, a substantial severe threat could continue well into the night affecting a larger area to the E and NE.

Chris, I agree with your fcstd SPC progs. I'd say MDT risk could be in store, and would not be surprised with a 15% tor, maybe even hatched.

Plenty of time for this to change. One thing that's important will be movement of the cold front and where it eventually stalls, before possibly retreating.

Ample moisture, great shear and a splitting jetstream all point toward Sunday severe.

 

[Kenny Drake]

The region from CDS to SPS seems somewhat favorable for supercells / torns on Sunday during the day, but per the latest NAM run (12z) I see no sign of SPC's mentioned mid level wave until Monday. Likely this will rely on daytime heating and sfc convergence to break the cap. Good news though without the strong mid levels perhaps the storms will be moving slowly (assuming they breakout without a wave).

I can make out a weak s/w at 500mb per 500mb Vorticity output from the 12Z NAM. It doesn't show well before 12Z Mon but the 12Z Mon forecasted 500mb vorticity shows a vorticity maximum in NE Oklahoma. Prior to the 12Z Mon Forecast there is a slightly enhanced area of vorticity that moves NE across OK to the forecasted position at 12Z. The GFS is also picking up on this feature but is ever so slightly slower. I like the GFS forecast better as it puts the associated vorticity advection right over the dryline at about 6-7pm. Moisture will be richer than last night. The cold intrusion associated with that wave is not going to make it to the Gulf. Winds have already backed to the south across the northern plains as the sfc high moves east. The weaker s/w's ahead of the main system (across the SW on Sunday) aided by WAA should provide elevated precipitation Saturday into Sunday morning. Generally, this helps by adding moisture to the surface (the magnitude of this effect is arguable.) My main concern on Sunday will be mid-level clouds associated with s/w's embedded within the larger wave preventing initiation except where forcing is strongest. If this happens I'll be playing the triple point and possibly backing south depending on the CAP'ing situation. Environmental wind velocities look favorable for cells wanting to move right. Perhaps we'll see a monster right mover playing the warm-front with initiation along the triple point? This is 2 days out and I'm dreaming but this setup looks reasonable for it. Now, the weakness of the surface low is concerning me a little but we'll have to wait and see how the mid-levels pan out before determing what happens in more detail at the surface. I'd say at this point anywhere from NW TX to NW OK looks favorable for a triple point location late in the day on Sunday. It really depends on what the stronger s/w to the north does with regard to location and timing. The GFS and NAM generally agree on this vortmax being over Idaho by 00Z Monday (per 12Z Fri runs).

 

[Bill Tabor]

I don't see anything on the NAM 500 vort for OK Sunday until 12z Monday morn.
http://www.rap.ucar.edu/weather/model/displayMod.php?var=eta_500_vrt&hours=hr48hr60hr72

How do you see a wave crossing? Perhaps you have a higher res map? Maybe COD's is better. I see a little bit of wave action on the GFS which actually I didn't look at before - but it seems fairly weak. Not sure if those small impulses would be enough to do much lift and mixing wise to break the cap - although it is interesting that the NAM mysteriously builds a small vort center in extreme ne OK by 12 Mon. Smoke and mirrors I suppose how it gets it there - but yeah somehow it sneaks it in by Monday morning. I was really looking for a bit more significant wave such as is just to the west of OK by 12 Mon. A night time wave doesn't do a lot for me chasing during the day Sunday.

EDIT: Ok, just looked at COD...apparently it does show quite a bit more detail, and has more time slices available to view as well as compared to UCAR. I see what you guys are talking about now. Looks like it is enough for precip too - at least particularly by 6z Monday. Hmm... seems I may be better switching to COD for primary viewing.

 

[Kenny Drake]

I don't see anything on the NAM 500 vort for OK Sunday until 12z Monday morn.
http://www.rap.ucar.edu/weather/model/displayMod.php?var=eta_500_vrt&hours=hr48hr60hr72

How do you see a wave crossing? Perhaps you have a higher res map? Maybe COD's is better. I see a little bit of wave action on the GFS which actually I didn't look at before - but it seems fairly weak. Not sure if those small impulses would be enough to do much lift and mixing wise to break the cap - although it is interesting that the NAM mysteriously builds a small vort center in extreme ne OK by 12 Mon. Smoke and mirrors I suppose how it gets it there - but yeah somehow it sneaks it in by Monday morning. I was really looking for a bit more significant wave such as is just to the west of OK by 12 Mon. A night time wave doesn't do a lot for me chasing during the day Sunday.

EDIT: Ok, just looked at COD...apparently it does show quite a bit more detail, and has more time slices available to view as well as compared to UCAR. I see what you guys are talking about now. Looks like it is enough for precip too - at least particularly by 6z Monday. Hmm... seems I may be better switching to COD for primary viewing.

That link you posted shows 12Z Sunday. I don't want to see a vortmax with associated vorticity advection over the initiation region before afternoon since we want the CAPE to fatten up via sfc heating. This scenario would add negative vorticity advection over Oklahoma during the afternoon hours as the vortmax is right over head. However, as I mentioned I am hoping that the NAM is wrong with the speed of the s/w in question (hence why I liked the GFS, which is slower with it.) The NAM does pick up on the s/w at 00Z in a rather favorable area:
http://www.rap.ucar.edu/weather/model/eta60hr_500_vrt.gif (image will change in a few hours when the 00Z Sat runs come out.) Note the +vorticity over eastern NM. That puts + vorticity advection over the convective region ahead of the dryline (yay). This feature could end up being the key player with regard to initiation south of the warm/stationary front.

 

[Rich Thompson]

I think there are two key factors in a scenario like that forecast for Sunday evening: 1) initiation of a few discrete cells near/before dark, and 2) influence of the larger scale (four corners) wave on the low-level flow after 00z. *If* the low-level flow can ramp up in the 00-03z time frame in response to height falls over the high Plains, you might be able to get one of the familiar supercell/tornado clusters after dark with the surviving diurnal convection. Legitimate (mean) mid 60s dewpoints in the boundary layer would help to maintain instability well after dark.

The scenario is somewhat similar to Greensburg (5/4/07), Attica (5/12/04), Hoisington (April 2000?), OKC (5/9/03) and April '02 near Woodward, though it's earlier in the season and we won't be quite as unstable. I'm not specifically expecting a repeat of those previous events, but don't be shocked if we see a handful of tornadoes Sunday night with 1-2 supercells near the stalled boundary.

Rich T.

 

[Jim Bishop]

That link you posted shows 12Z Sunday. I don't want to see a vortmax with associated vorticity advection over the initiation region before afternoon since we want the CAPE to fatten up via sfc heating. This scenario would add negative vorticity advection over Oklahoma during the afternoon hours as the vortmax is right over head. However, as I mentioned I am hoping that the NAM is wrong with the speed of the s/w in question (hence why I liked the GFS, which is slower with it.) The NAM does pick up on the s/w at 00Z in a rather favorable area:
http://www.rap.ucar.edu/weather/model/eta60hr_500_vrt.gif (image will change in a few hours when the 00Z Sat runs come out.) Note the +vorticity over eastern NM. That puts + vorticity advection over the convective region ahead of the dryline (yay). This feature could end up being the key player with regard to initiation south of the warm/stationary front.

I see exactly what you are referring to. But that tiny vort max is likely not going to be enough to bring ample lift along the dryline for convective initation. The two main problems here are the veered 850mb flow during the day, owing to a stout CAP. The WRF & GFS suggest the 850mb flow will remain veered (southwesterly) until nearly 00Z, when they back to the south.

With a strong CAP and 850mb flow not forecast to be favorable for good moisture until early in the evening, you need the main vort max to eject. So the second problem is it doesn't looke like that will eject until 12Z Monday. So I think we're left with a greatly sheared but CAPPED warm sector ahead of a dryline with good instability, though it's conditional whether the CAP can break from daytime heating and dryline mixing alone.

Anyways my two cents.

 

[Kenny Drake]

I see exactly what you are referring to. But that tiny vort max is likely not going to be enough to bring ample lift along the dryline for convective initation. The two main problems here are the veered 850mb flow during the day, owing to a stout CAP. The WRF & GFS suggest the 850mb flow will remain veered (southwesterly) until nearly 00Z, when they back to the south.

With a strong CAP and 850mb flow not forecast to be favorable for good moisture until early in the evening, you need the main vort max to eject. So the second problem is it doesn't looke like that will eject until 12Z Monday. So I think we're left with a greatly sheared but CAPPED warm sector ahead of a dryline with good instability, though it's conditional whether the CAP can break from daytime heating and dryline mixing alone.

Anyways my two cents.

I agree it all comes down to the strength of the CAP. However, I think the CAP will be broken along the triple point. The real question is whether we'll get enough daytime heating and/or ~800mb cooling to break the CAP to the south along the dryline. I do not think moisture will be a problem. The Gulf has had the same air mass over the area the entire week and I do not see a strong sfc high across the SE. The juice will be there, we'll just need daytime heating. NAM forecast shows near saturation at 850mb ahead of the dryline from the pseudo-warm front to the Gulf. GFS agrees and also shows a nice dry punch just behind the forecasted triple pt location.

 

[Jim Bishop]

I agree it all comes down to the strength of the CAP. However, I think the CAP will be broken along the triple point. The real question is whether we'll get enough daytime heating and/or ~800mb cooling to break the CAP to the south along the dryline. I do not think moisture will be a problem. The Gulf has had the same air mass over the area the entire week and I do not see a strong sfc high across the SE. The juice will be there, we'll just need daytime heating. NAM forecast shows near saturation at 850mb ahead of the dryline from the pseudo-warm front to the Gulf. GFS agrees and also shows a nice dry punch just behind the forecasted triple pt location.

I think our disagreement is a matter of timing and nothing else. You're saying the CAP will be broken along the triple point in the late afternoon/early evening. What I'm saying is the probability of the CAP breaking before dark is quite low since the main upper support won't be there until after dark along with the fact that those 850mb winds won't back to the south until around 00z. However once the CAP does break things look quite favorable for supercells and tornadoes.

Of course if the models are wrong with the timing of the shortwave and it speeds up significantly then western OK will light up Sunday afternoon. Of course usually the models are too aggressive and the shortwaves slow down a bit as we get closer to the day.

 

[Jeff Snyder]

Sunday looks to be a "day before the day" event, to which others have eluded. Anything in the way appreciable mid-level forcing will likely remain absent on Sunday, and my biggest concern at this time pertains to convective initiation. As the trough continues to shift east through the western US Sunday afternoon, pressure falls in the immediate lee of the Rockies looks like it will give us problems in terms of low-level convergence along the dryline. Initiation looks most probable along the stationary front (woo hoo -- I'm labeling it a stationary front since the last model solutions don't show it advancing nearly as much as some previous runs showed) as the LLJ cranks up near sunset. The NAM is showing a pretty nice dryline bulge pointing right towards Altus and Erik by 00z, so that may give us a bit of hope. With the deep-layer shear vector normal to the dryline, and with some CINH in place in an area of relatively weak convergence, discrete supercells appears as though it'll be the favored mode Wednesday evening.

I also agree with Rich T. in that this setup looks similar to a few others in that daytime initiation looks questionable, but conditions look very ripe for a significant tornadic supercell threat just afternoon sunset if convective initiation occurs along the dryline. This morning's 12z NAM run does not show QPF along the dryline, though it does show precip near the front from northwestern OK southcentral and eastern KS.

EDIT: I wrongly attributed a comment to Jim LaDue -- I meant attribute it to Rich T. Sorry for any confusion, Jim!

 

[jladue]

Jeff, I guess I better post something for you to agree with. Yes if the forcing aloft is delayed and frontal forcing requires a little help from the low-level jet then nocturnal supercells are most likely. Perhaps we'll get a couple cells to fire on the triple point before dark. I certainly think at this time it's worth setting up wherever the point happens to lie.

 

[Kenny Drake]

For what it is worth the 00Z NAM and the 12Z NAM for Saturday are consistent in having the environment uncapped @ KOUN by 00Z. The ONLY thing that doesn't look too good on it is the lack of a 50knt LLJ.

http://beta.wxcaster.com/cgi-bin/parse_skewt_trace.cgi?model=NAM&fcsthr=036&STATIONID=_KOUN

The NAM decouples the BL by 3Z:
http://beta.wxcaster.com/cgi-bin/parse_skewt_trace.cgi?model=NAM&fcsthr=036&STATIONID=_KOUN

Which I agree with since the strong 850mb jet is not there. I think there will be 2-3hrs just before and after sunset tomorrow for a significant supercell threat. Regardless, it looks like I will be going out since it is so close to home, and... ITS SUNDAY!!!