04/07/08 FCST: KS/OK/TX

[Kenny Drake]

Well, since the previous forecast thread is not following the guidelines I'll create one for Monday.

GFS and NAM are in general agreement with digging a fairly significant 500mb wave into the Rockies during the day and ejecting the associated s/w towards the 00Z timeframe. Rapid pressure/ht falls should occur in response beginning overnight Sunday into Monday morning. The obvious concern will be the amount and depth of moisture return at the low-levels ahead of the system due to a s/w progged to move across the northern plains the day before (and current sfc obs show a cold front headed for the Gulf of Mexico in TX). The strength and location of the more northerly (and earlier) s/w will have major implications on the low-level features for the FCST. The 12Z Friday NAM looks weaker and more northerly than the 00Z Friday (12Z isn't out yet) GFS with regard to the 500mb s/w. Regardless, I do not think the timeframe prior to the arrival of the s/w and mid-level forcing will allow much in the way of moisture return north of Kansas. If the s/w takes a more southerly track (like the 00Z Fri GFS suggests), then parts of S. KS/OK/TX look prime for convection Monday afternoon into evening. If the NAM verifies, mid-level forcing will be weaker across the aforementioned region yielding a stronger CAP due to less favorable mid-level dynamics. However, the NAM does pick up on a 500mb s/w hugging the US/MEX border with several weaker s/w's being ejected out ahead of it. Mid-level response suggests ageostrophic divergence ahead of the 500mb jet max associated with these smaller s/w's being ejected. This would put parts of the area of interest under an area of enhanced lift at mid-levels.

Just for comparison the 00Z Fri UKMET does pick up on the s/w with vortmax over the CA coast at 00Z Monday, almost 2 states west of where the GFS and NAM are progging it to be at the same time. 00Z Fri ECMWF has the 500mb s/w progged to be near 40 lat and near 100W long, which would be fairly in line with the 00Z Fri GFS. Ensembles generally agree with the GFS/ECMWF solution.

This is definitely a day to watch and could be a precursor to the major event expected towards the middle of next week.

 

[Michael O'Keeffe]

I like the both the looks of the GFS and WRF this afternoon. The 12z WRF is showing dews in the 60s with a nice tongue of 65+ streching up I35 toward OKC. The dryline looks nice and sharp in SC into C Oklahoma down into Texas. SBCAPE is exceeding 3500 j/kg in parts of C Oklahoma. Though the low level jet is meager as well as the 500mb winds they are both out of reasonable directions at least. The GFS is showing a much stronger setup in terms of placing a nice triple point in the Oklahoma Panhandle region as well as a widespread LLJ of 30-40knts out of the SE. So both are in genral agreement that Oklahoma may once again see storms and MAYBE tornadoes though every setup produces everything but tornadoes it seems.

 

[Mikey Gribble]

The big difference between this and the last couple setups is backing low-level winds, which was the killer for tornadoes on the last two IMO. I'm a little worried about moisture return and as mentioned above 850mb winds are fairly weak. Aside from that everything looks great. I am a huge fan of good directional shear and this system certainly has that going for it. If we can get dewpoints in the 65 degree range and 850mb winds increase slightly then IMO the environment ahead of the dryline and North of the Red River will be quite favorable for strong tornadoes. I posted a forecast here http://loadedgunchasing.com/blog.html

 

[Ilya Neyman]

Monday's definitely on the drawing board for possible severe and all signs point to Oklahoma.

Latest 00z NAM is coming around to earlier GFS solutions in depicting a stronger, deeper and more negatively tilted shortwave ejecting out of the western trough.

The differences between this morning's 12z and tonight's 00z runs are quite significant.

For one, an initial shortwave impulse progged to lift out over the northern plains is now forecast to do just that - lift out. The 12z solution gave that system a stronger 300mb jet streak located over NE, KS, IA, MO..as well as a less-amplified and weaker mean jet on its heels over the Great Basin.

12 hours later guidance indicates the MN system to eject toward the Great Lakes as an open wave with the jet streak now progged from IA into NW IL.

Meanwhile, 300 mb winds are substantially stronger, broader and further south with the next system. For example, the earlier run progged 65-70kt flow over S. AZ by 12z Monday morning. That's now up to 90 kts, with a shortwave ridge across CO indicative of a digging system.

At 850mb earlier WSW-SW flow is now progged out of the S and strengthening with time.

Not surprisingly a richer moist sector is fcst to develop NW across OK with low-mid 60 dewpoints returning by afternoon ahead of a sharp and bulging dryline. CAPE's are now forecast in the 3,000+ range.

All this says nothing yet, as model consistency and variability has been anything but great this season but I expect a lot more posts on this setup in the hours to come!

Ilya Neyman

 

[Dave Holder]

As Ilya said, the new 0z WRF is trending more towards the GFS and shows significant improvement in the chances for large hoses.

My previous concerns about surfaced-based initiation has been somewhat diminished, with an uncapped 3000+ J/kg environment to work with at 0z. I think that convergence along a somewhat diffuse dryline might be able to do the trick.

I'm really impressed with directional shear as Mikey Gribble mentioned. I also believe that the lacking of this component blocked any tornadic activity from occurring the last few chases.

The details will emerge with subsequent model runs, but if this setup holds together, we could be in for an active day Monday!

 

[Brett Roberts]

The parameters depicted on the 00z NAM are almost remarkable for much of western and central OK south of I-40. Obviously such details are unimportant this far out, but just for reference, we're talking 0-3 km EHI values in excess of 8 along the I-44 corridor from OKC to SPS, a result of 3000-4000 J/kg SBCAPE and beautiful, strongly curved hodographs (excellent directional and moderate speed shear). With a sharp dryline bulge near SPS-LAW and minimal CINH by 00z, a great chase day would likely be in the offing taking this run literally.

As Ilya correctly pointed out, and we've all seen quite well this past week, models have been anything but consistent lately. With such small spacing between waves in this pattern (one currently over the Rockies progged to move into MN/WI by Monday; the Monday trough; and then the big show for mid-late week following on its heels), I guess small differences in each wave's strength are significantly changing things each run as new upper-air data becomes available. It's hard not to get excited when you see runs like this, though, especially if the 00z GFS comes in singing the same tune. I noticed earlier that the UKMET has shown a much stronger trough and excellent chase setup for Monday on its past two runs, even as the American models faltered, so let's hope the NAM depictions from last night and this morning were aberrations rather than the other way around.

 

[Mikey Gribble]

Everything still looks on track for Monday. If storms can form along the dryline a localized tornado outbreak is likely IMO. The highlight of this setup is definitely the excellent low-level directional shear. I would like to see SR inflow winds a little higher, but with 20-25kts at 850mb we should be fine. My concern right now is getting a storm off the dryline at a reasonable time. If we get good insolation I don't think we'll have a problem. Assuming storms do fire along the dryline any storms North of the Red River will be capable of strong tornadoes. I don't think this is a high end tornado threat, but it is at the upper end of moderate if things work out. Unless SPC is seeing something I'm missing, I would expect them to go moderate on the next outlook. My target right now is Clinton. I posted a forecast here http://loadedgunchasing.com/blog.html

 

[Eddie Natenberg]

I usually don't post much but...........

This is one of those rare forecasts where you have to do a double take at the model output. If the strength of the cold air advection behind the trough is properly forecasted then the shortwaves emanating from the trough axis will able to propagate faster through the zonal flow. This is the tricky part of the forecast. If weaker cold air advection occurs we could see a forecast like we did before where we have height falls farther downstream of the trough axis elongating the low to the NE. Either way, I am very impressed at the 48-54 hour forecasted hodographs, looks very unusual, you usually don't see perfect three quarter circle hodos (I want to save it and hang it on the wall). The 850 winds are very telling as to the magnitude of the upstream forcing. The strongest velocities are centered right around 700 mb, would make it a low tropospheric forcing event, keeping the boundary layer from decoupling.

Again I am very impressed. If the forecast stays "CONSISTENT" I can see a strong moderate to high risk coming up very soon (IMO only). But with the problems discussed above I would say this is not an easy forecast for the NAM WRF especially with sparse upstream obs. Now that I've pushed that button, you can all watch my forecast bust.

Now quietly and orderly proceed practicing your tornado precautions.
http://beta.wxcaster.com/cgi-bin/parse_skewt_trace.cgi?model=NAM&fcsthr=054&STATIONID=KOUN

 

[Chris Wilburn]

It is nice for the NAM to come around and for it to generally agree with the GFS is very nice as well. I will not go too far into details, but there will be plenty of moisture 60-65 tds, near 3,000 CAPE and nice 0-1 and 0-3 helicity values with forecasted nice hodographs mentioned above. I too like the low level wind shear from the surface to 850mb and forecasted 40-5-kt 500mb winds should make for very chaseable storm motions. I agree with the Clinton target. There are good road option to the east and west with I-40 and there is a really good north and south road as well. Hopefully the models don't take a dive again. I guess it is just wait and see until then. I agree with the Moderate risk assumptions, esp. on tomorrows day 2 if the 12z models runs look just as nice. They may very well go moderate on todays day 2 as well.

 

[Brandon Lawson]

I'm a bit surprised at the slight risk issued by the SPC for Monday. Their discussion has no negatives, and at the same time, does not even hint at the possibility of an outbreak. Of course, this year the models have gone through some very sudden changes within the two day time frame. But at least this time around the GFS has been showing this for a while.

If there was one downside to the severe potential on Monday, it would be the possibility of a lack of moisture return (but even the models do not suggest this); and perhaps this is what the SPC is taking into consideration.

The European has just come out and it too agrees with the GFS and the NAM.
As the evening nears and temperatures drop, LCL's will lower and any storms that can fire before dark will move into a very tornado friendly environment.

 

[Mikey Gribble]

I am pretty sure SPC is backing off a bit because of some questions on moisture return, questions on storms firing along the dryline, and because of the low coverage of the threat coming off the dryline. What the NAM is showing right now on moisture (low 60's) is adequate, but if that drops it could pose a bit of a problem. Right now you have to go to the buoys 200 miles off the Texas coast to hit the mid 60's. The NAM has a moisture jump overnight on Sunday, but there isn't much in the way of low-level winds to advect that North. That being said, adequate moisture return is my biggest worry right now. I would be a lot more comfortable if there was a strong LLJ that tapped into the gulf on Sunday night. I still think SPC is being a little too gun shy on this. Especially since if we do get good moisture up into Oklahoma there is going to be a chance of strong tornadoes. Yeah it's a conditional threat, but the degree of that threat warrants a higher risk IMO.

 

[John Cameron]

The low level flow that was missing on the 3rd will be there on the 7th. How could you not like this in OK/TX?

EDIT: I posted this before I looked at the 300mb flow...not great , but lets hope its just enough to get things goin'.

 

[Andrea Griffa]

I'm taking a speedy look to WRF and all seems very good: I'm speaking about a very defined dry line with backing low level winds in front of a low pressure. Not bad the veering low level jet with 35-40kts that should lead to a very high storm relative helicity available to be ingested by updrafts that will form.
I like the mid level flow as well, not too strong to have too fast moving supercells but right to tilt the updrafts of the storms.
Only a thing: here we go with the never ending dilemma of stormchasing: triple point or dry line?

 

[Jeff Snyder]

Well, I've tried to restrain myself from looking at this too much given the amount of work I need to do and given the rather poor model verification the past few chasing events. However, for the first time in a while, the situation is actually looking better and better on subsequent model runs, which I'll take any day over "well, it doesn't look as good now as it did yesterday". So, on we go...

Even though mid- and upper-level flow won't be particularly strong, it does appear that we will have, as others have noted, backed low-level flow in advance of a shortwave trough forecast out into the Plains overnight Monday and Tuesday. If we see sufficient moisture return (a significant 'if'), heating is forecast to yield moderate instability (CAPE 1500-2500 j/kg) by late afternoon east of the dryline and south of the warm front across much of western and central OK and southward into northern Texas. There won't be much in the way of mid-level support (i.e. any strong vort max is largely missing), so we'll likely have to rely on dryline circulation and enhanced convergence near the triple point (likely somewhere in NW OK) for initiation. The SErly surface flow veers very nicely to westerly in the mid-levels, creating 45-55kt 0-6km shear across the OK part of the warm sector. In addition, model forecast hodographs from the 12z NAM show fantastic shear profiles along the warm front... For example, check out the END forecast sounding/hodograph valid 00z tomorrow evening, and notice that the shape is similar to what some modelers use to initialize "idealized" model simulation (i.e. almost a perfect 1/2 circle). The forecast sounding/hodograph valid at 3z (just change the address on that link to reflect the 39 hour forecast) shows ~250 0-1km SRH, with sufficient CAPE and sufficiently-low CINH (if the sfc temp doesn't drop quite as much as forecast) for a mature supercell to remain surface-based in such an environment. If things set up as the NAM and GFS indicate (as have for the past couple of runs), I think that there is a threat for significant tornadic supercells in parts of Oklahoma and perhaps far northern Texas given initiation.

For the first time in a little while, we should see relatively backed 850mb flow (not just backed flow at 0z after being veered all day, though 850mb flow in northern TX looks to stay veered a bit longer than my liking) and shear profiles strengthening through the day, two things that I like to see on chase days. One of the only things that I don't like to see, and the biggest potential primary fly-in-the-ointment tomorrow (IMO), is the "just in time" moisture. The GFS and NAM continue to indicate > 60F tds in the warm sector tomorrow (and the NAM shows >65F from I40 southward), but they also show questionably rapid moisture return tomorrow morning. For example, the NAM has the 60F isodrosotherm moving from Denton TX to Watonga OK between 12z and 18z. This equates to a 175 mile movement in 6 hours, or an average speed of about 30 mph. The interesting part of this, however, is the sfc winds are forecast to only be 15-20mph. This implies that moisture is being supplied from the sfc (evapotranspiration?) or higher moisture is being mixed down from above. The later is possible (and much more likely in a backed flow regime than a veered flow regime), but even the 850mb Tds jump quite a bit between 12z and 18z. The 12z BRO sounding shows a relatively deep moist layer (100+ mb; though not too impressive in magnitude of moisture), though CRP is quite a bit more shallow (50mb). The 12z NAM 12 hour forecasts indicate >60F tds in southeastern TX and >65F tds along the entirety of the TX Gulf coast, and I'm worried that we won't see that given the current observations (particularly across southeastern TX, where Tds are in the 55-57F range and sfc winds are only 5-10 mph). Given forecasts of 850mb flow of only 10-20 kts across southeastern TX through the day and night, I'm really not sure how we're going to see much above 61F in the warm sector tomorrow in OK. Since the moisture needs to move northward ~450-500 miles in 24 hours, I don't see how 10-20kt flow will be sufficient. Granted, the low-level flow strengthens quite a bit tomorrow, but will it be enough to pull the juice northward after the meager advection in southeastern TX overnight? I'm really not a fan of "just in time" moisture events, largely because I'm been burned far more times than I've been rewarded in such situations. If only we were going to have a strong southerly or southeasterly LLJ in southern TX tonight...

Wrapping this up... I think the threat is highly conditional, with the most significant point of failure being insufficient moisture return. If we can indeed get Tds >62F across Oklahoma tomorrow, I think the threat for a siggy tor is mentionable considering the shear profiles and amount of instability forecast. I'm glad to see the upgrade in probs in the latest Day 2, and I expect to see another increase if moisture return is as progged. My current target would be near the triple point. Forecast storm motion may be favorable for a storm to ride the warm front, which would further increase tornado potential. IMO, this looks like the best setup of the year so far across the southern plains (at least 24-30 hours before the event).

 

[David Drummond]

One thing to note Jeff, looking at the VWPs on the radar sites down I35, and looking at all the CURRENT surface obs, there are already 55 degree dewpoints nearly to the Red River here at 3 pm. I've been watching them on a slow but steady climb up through the DFW area today. Seems to be a jet, albeit weak, already in place along the I35/I45 corridor right now. This may be shallow, I'll be interested to see the balloon reports this evening. It's something anyway...

One thing I am really liking seeing is the vis sat today. Skies are virtually CLOUD FREE across TX and OK...something we haven't had the pleasure of in these past events prior to the event this year. Even though the recent cold air intrusions, this is helping warm the whole air mass up nicely today, with temps near 80 even here in West Texas under north surface winds we are in the mid/upper 70s. Not a hint of any mid/upper level moisture coming up out of the southwest to "cloudy up" things, I hope that trend continues in to tomorrow.