4/21/08 FCST: IA/IL/MO/TX/OK/AR/TN

[Brandon Sullivan]

Models have continued to show a nice severe weather setup over the MS valley from IL through TX/LA area next monday into tuesday.. Although models have been changing with the usual synoptic features, it appears that decent moisture/instability should combine with a nice dynamic environment for a good chase day!

More tonight...

 

[Derek Weston]

Models have really bounced around though. Last GFS stunk. Cautiously optimistic -- but agree an appreciable system will roll through.

 

[Mikey Gribble]

I wondered how long it would take before somebody started this thread up...
Yeah I've been keeping a close eye on this too. Yesterday the GFS, GEFS and Canadian ensembles were actually fairly close on Monday and Tuesday's setup(Canadian went crazy outside of that) as far as upper level features went and I thought the models were actually starting to get a good handle on this, but the GFS has moved more towards the ECMWF with the latest run. Up until today the NCEP models had been fairly consistent, but the latest changes really makes me uncertain on what to expect. If the trough digs farther South and has a slow progression like it did in the earlier GFS/GEFS runs then I think we are in great shape. If the latest GFS and ECMWF trends catch on with the trough elongating (with sort of and east-west axis) as the jet streak rounds the base then I think the tornado potential will drop off a little (for several reasons). Veering low-level winds kill tornado potential quicker than anything else and that's what the latest GFS solution would bring IMO.
The one certainty with this system appeared to be good moisture, but I am even second guessing whether it would get here in time for the first day of this setup with with the faster solutions.
The good news is that all the models have been consistent in developing a fairly strong West coast trough this weekend and there should be very good moisture sitting over the western Gulf and south Texas. Dewpoints over the warm sector in the mid to upper 60's seems very likely by Monday. That is about all we can ask for right now. I am definitely keeping a very close eye on this one.

 

[Mikey Gribble]

With the current model uncertainties this is an extremely tough forecast to say the least. About the only thing you can say with any certainty is that there should be decent moisture over the warm sector by Monday afternoon. The evolution of the upper level trough is going to be critical to the tornado potential on Monday IMO. The latest run of the GFS has veering 850mb winds with a kink in the pressure field over easter Kansas-Missouri. I don't quite understand that, but it's there none the less. Regardless of that Monday still seems like the best day for tornado potential IMO (over Oklahoma at this time), but if the 12Z GFS were to verify that tornado potential would be low. I am seriously confused right now so hopefully the models can get a better handle on this over the next day or two. I posted a forecast here if you're interested http://loadedgunchasing.com/blog.html

 

[Mikey Gribble]

I have been totally lost over the last couple days on this forecast given the inconsistency of the models. I didn't even post a forecast today on my blog because I had no idea what to say. My confidence is still very low on Monday's setup, but I think the models are finally starting to come into better agreement. I want to wait to see the 12Z runs before I do any real forecasting, but I did post a quick update on my blog. The cap seems to be a bit of an issue, but if diurnal convection forms I do think there will be at least a low end tornado threat. There are two places I like right now. The first one is the triple point and the second one is the dryline in northern Texas (Red River area). CAPE and deep layer shear will be quite favorable for supercells if storms fire before dark (which I think they will). Right now the big limiting factor for tornadoes is the relatively weak low-level shear. The lates run of the NAM does have 1km SRH around 150 ahead of the triple point though, which is sufficient for a tornado threat. If the 00Z NAM verified I would target the triple point due to proximity. I also think the tornado potential looks pretty much equal between the Red River area and the triple point so I see no reason to make the 5 hour drive right now. That could certainly change though with the models jumping around. I guess we'll just have to wait and see. I posted a couple forecasts and a brief update here...
http://loadedgunchasing.com/blog.html

 

[Jeff Snyder]

Monday actually looks pretty good to me, actually, and I agree with the 2 general threat areas Mikey addresses above (in fact, we have very good agreement on most counts). The 00z NAM forecast valid for Monday evening indicates a breakable cap ahead of a triple point forecast to be near the central OK/KS border. 850mb temps are a little high for my liking for this time of year along the dryline in OK, but the east east of the triple point appears to be along a 850mb T gradient, so the cap may be less of an issue up there. Current buoy obs (namely, buoy 42002 ESE of BRO) indicate that 70F Tds are finally moving back into the northwestern Gulf, which will be nice to see on land given the abysmally dry low-level moisture sampled on the 00z BRO and CRP soundings today. If we get the moisture return that the models are forecasting (>65F Tds all the way to southwestern IA, with >70F tds from southeastern OK and southward on the NAM, and a tad lower on the GFS), instability should be a-plenty by mid-April standards. A veering and strengthening vertical wind profile looks sufficient to support a supercell threat with any storms that can form in the warm sector Monday afternoon and evening, and a nearly-normal shear vector to dryline orientation would seem to support discrete cells over linear junk.

The NAM and GFS forecasts for Monday evening look an awful lot like the evening analysis from 12 May 04 (analysis maps available HERE). A broad trough to the west, with a cut-off mid-level low N of ND/MT... A nearly-stationary front that stretches from a low in the western OK panhandle to southern KS and northeastward towards western WI, and a nice dryline bulge near and just south of the central portion of the OK/KS border... Model continuity has been poor with all chase setups next week, but there appears, at least, to be some agreement on Monday between the NAM and GFS. Forecasts also point to a threat in southern OK and northern TX, but low-level shear weaker down there (as opposed to the area E of the triple point).

 

[Chris Wilburn]

I agree with Jeff that Monday could be a nice chase day, esp. if the latest 00z run of the NAM was to verify. This is all based off the NAM as I haven't looked at the GFS in regards to this day. I too like the triple point area say south of Wichita near the OK/KS border. I have some concerns over the CAP, but the models have CINH all but eroded at 00z just east of the triple point. Like Jeff mentioned just east of the triple point appears to be on the edge high 850mb temps so this should help. Mid 60 dews and CAPE near 3,000 J/kg should create a highly unstable environment. There will be plenty of moisture and instability so that will not be a problem at all. 0-1 SRH is forecasted to be just over 200 and 0-3 SRH just above 300 which are really nice for rotating supercells. Looking at a forecast sounding for Winfield, KS shows backed surface winds although they are weak. I don't like the veering of the 850mb winds out of the southwest that it shows but that could definitely change for the better. This same sounding shows EHI of 3 and near -5 lift. LFC heights of 1,400 and LCL heights of 1,000 look decent as well. 500mb winds of 40-50kts should make storm motions easy to keep up with. Just for fun the Sign. Torn. Parameter shows 5-7 and Supercell Composite 15-25. I know this is still a few days out, but the latest run of the NAM can give us hope. This all could very well change after the morning run comes out. I agree with what Mikey said in that I was going to post my thoughts on Monday and the middle part of the week, but had no clue what to say. Monday is worth keeping an eye on and it will be interesting to see what SPC says on its Day 3 outlook that should be coming out very shortly.

I forgot to add a comment on your May 12, 2004 thoughts Jeff. This day does look similar to that day after looking over some archived data. I was very new to chasing in 2004 and didn't know much at all so I can't comment on it specifically. I do remember people refering that the CAP may have been a problem this day, but as many of you recall it turned out to be a very nice day with some really tornadoes in the Medicine Lodge area. Hopefully we can get this lucky.

 

[Kenny Drake]

The NAM and GFS forecasts for Monday evening look an awful lot like the evening analysis from 12 May 04 (analysis maps available HERE). A broad trough to the west, with a cut-off mid-level low N of ND/MT... A nearly-stationary front that stretches from a low in the western OK panhandle to southern KS and northeastward towards western WI, and a nice dryline bulge near and just south of the central portion of the OK/KS border... Model continuity has been poor with all chase setups next week, but there appears, at least, to be some agreement on Monday between the NAM and GFS. Forecasts also point to a threat in southern OK and northern TX, but low-level shear weaker down there (as opposed to the area E of the triple point).

From a dynamics standpoint the 12 May 04 comparison fits well. However, I'm not sure we'll be able to get 65-70F deep boundary layer moisture to the triple point in N.OK. That leaves me thinking N.TX/S. OK. However, the dynamics look much more crappy with lack of mid-level forcing. I believe the dry air mass over the SE US will be an issue to the moisture supply. To make matters slightly worse both the NAM and GFS want to veer the 850mb flow westerly in response to an early morning 500 vortmax. The 12 May 04 upper air observations do show an early morning vortmax in the same general region compared to NAM & GFS; but not the westerly 850 flow. The models have not been doing well at all this spring and this forecast seems no different. At this point I think the primary issue (for a tor threat) will be moisture. The early morning s/w will be a key player in the deeper moisture (850mb), along with the pressure/height fall response to the s/w that is progged to be ejected beginning late-afternoon. I agree with Mike as I feel sorrow for the SPC forecaster. I would go "SEE TEXT" or maybe SLIGHT at this point.

 

[Kenny Drake]

I think the 12Z GFS and NAM runs pretty much sums up the lack of run to run continuity and the overall poor handling of this system by the models quite nicely. 12Z GFS still wants to move a fairly significant s/w through the warm sector during the morning hours, while the NAM has backed off. Both models have pushed the system south where now it looks like C.OK is the triple point target. I actually like the models wanting to shift the system south as it will yield shorter distances for the moisture to travel. This seems to be in response to the models wanting to increase CAA across the C. and N. Plains. This is another event where no one knows what to expect until the morning of the event. I still think the warm sector across OK/TX will mix-out hardcore during the afternoon. The models do reflect this nicely (look at OUN 18Z Mon NAM sounding.) If the morning s/w ends up being (a) quicker and/or (b) weaker than the whole setup will likely improve quite a bit. The issue is obviously mid-level subsidence behind the morning's progged s/w. At this time I would not chase, however I'll see how observations look Monday morning to make a final decision.

 

[Darrin Rasberry]

I would wait for a few more minutes to comment on the new Day 2, but I have to leave.

CENTRAL IOWA: Although the GFS is a bit late with the 12z run today, the NAM seems to have backed up the progression of the system a bit. Both show sufficient CAPE and moisture flow, but the new NAM wants to push the temps at 00Z Tuesday a bit higher than the soon to be -12 GFS does (without raising the dewpoints much in return). If the GFS holds to its forecast, as it has been doing over the past day or two, I would not be surprised to see a SLGT reissued over the central Iowa area, perhaps through northern Missouri.

The biggest question I have right now is the massive difference with the much weaker CINH at 00Z Tuesday according to the NAM, versus the strong cap estimated by the (older) GFS at that time.

 

[Mikey Gribble]

This is probably the toughest forecast of the year IMO. Whenever SPC isn't on board it seriously shakes my confidence since those guys obviously know a lot more than I do. I do however think that if we get diurnal convection off the triple point tomorrow it will pose a lower end tornado threat.
There is a significant difference between the NAM and GFS on the placement of the triple point (I haven't had a chance to look over the other models yet so that is all I'm going off of). I don't know enough about meteorology and I haven't spent enough time examining this setup to have an educated opinion on which one is right, but I'm leaning towards the GFS.
Both models have the cap eroding at the triple point between 18Z and 00Z, but neither model has been breaking out convection. I think the cap will be close enough to breaking tomorrow that you just chase and hope for the best.
CAPE/deep layer shear combinations will be quite favorable for supercells if the cap does break. The tornado potential is much trickier. Pretty much all the things I look at for tornado potential are on the low end of the spectrum for tornadoes tomorrow. One thing that bothers me is weak 850mb flow near the triple point. If low-level shear could increase a little bit it would help out a lot IMO. The tipping point for me on tornadoes tomorrow is the moderate to high instability (which can help out a lot on otherwise mediocre setups) and low LCL's. I'm not pumped or anything about the setup, but I do think the chance for tornadoes (if we can get a storm off the triple point with daylight left) is high enough that I'll chase. I'll make the final call on that tomorrow, but as things look now I would target 30 miles East of the triple point by 4PM. I posted a forecast here if you're interested http://loadedgunchasing.com/blog.html

 

[Bill Schintler]

Chase Target for Monday, April 21

Chase target - north:
Richmond, MO (30 miles east of Kansas City).

Chase target - south:
Mountain View, OK (20 miles west of Anadarko).

Timing and storm mode:
In the Midwest, widespread storms should fire by 3 PM CDT, with multicell or linear segments the predominate mode. The majority of these will remain below severe limits. Further south in OK, strong capping will hold off convection until 6 – 7 PM when focus will be provide by a cold front and dryline triple point. This convection will be isolated in nature, with supercells likely. A bust is a possibility in OK.

Synopsis:
WV imagery and ULVL analysis shows a cutoff low centered over the PAC NW with a 100kt H3 streak rounding the trough base over NV. At the SFC, a broad area of low pressure is centered over ERN CO with weak pressure falls noted over CNTRL IA. LLVL moisture remains limited in the Midwest and SRN Great Plains with a narrow axis of 10C H85 moisture nosing into SCNTRL OK on a developing LLJ. The OUN sounding, however, indicates dewpoints of only 0C in this layer. MDLs, most notably the WRF, have initialized as much as 5F too high on SFC dewpoints in many areas. Additionally, temperatures in the H8-H7 layer are verifying a few degrees warmer then MDLs have initialized. Overnight, an extensive area of STCU will develop and surge NWRD as Gulf moisture is advected N. By noon, the WRN edge of this cloud shield will be located along a line from Sioux Falls, SD SW to E of Dodge City, KS; and then E parallel to and just N of the OK/KS border.

Discussion - Midwest:
The intersection of a CF and a subtle N/S-oriented SFC trough or wind shift should provide forcing for storm initiation by early afternoon as a compact mid-level wave E of the primary trough provides additional localized UVM. The SRN extent of this precipitation and convection area will be limited by capping and weaker forcing. Instability will increase early in the day, with MLCAPEs AOA 1000J/kg. However, overall SVR potential may be limited by weak shear in the H8-H5 layer along with linear forcing as the strongest ULVL flow remains N and W of the region until late in the day.

Discussion - OK:
Whether or not convection happens at all in this area will depend on timing of subtle pockets of ULVL forcing, convergence along the DL, and strong SFC heating along and W of the DL. MDL consensus agrees to bring a compact area of UVM through WRN OK between I-40 and the Red River by late afternoon. At the SFC, temperatures will struggle into the mid-80s W of the DL while MDL soundings indicate convective temperatures 5-10F higher then this. All of that said, providing convection occurs at all, it should be limited in coverage. Strong instability and LLVL directional shear with locally backing SFC flow will be sufficient for supercells. SFC dewpoints of 67F and steep mid-level lapse rates should support SBCAPEs AOA 3000J/kg. LCL heights in the 1000-1200m AGL range should limit tornado threat.

-bill
9:15 PM CDT, 04/20/08

 

[Chris Wilburn]

I may be holding out hope until the very end, but at least the tonights 00z run of the NAM has things looking a little better. The triple point in westcentral OK looks nice with little to no CINH left by 00z. The NAM also begins to break out precip. in this area by 00z. Actually the NAM breaks out precip. a tad northeast of the triple point, but at least it breaks precip. out this run. There is a nice dryline buldge on the NAM along the triple point say near Clinton or just north of I40. Forecasted dews of mid to upper 60's and CAPE of 3,000-3,500 J/kg will allow for a very unstable environment. Forecast sounding for this area show no cap at 00z. Sign. Torn. Parameters and Supercell Composites are very high in this area as well. Winds back ahead of the dryline and even show up due easterly on forecast sounding for the region. 850mb winds are southwesterly at 20-30kts which isn't the greatest and 500mb winds forecasted from 30-40kts. Low level shear doesn't appear to be great which may limit tornado potentil. Even though some soundings and models show precip. and no cap near the triple point by 00z you still have to be cautious since previous runs held such a stout cap with no precip. Models are showing just over 4 EHI, 0-1 SRH of just over 150 and 0-3 SRH just over 400. The Lid Strength Index on Earls page shows some eroding of the CAP between 21z and 00z including further down the dryline into TX. This is all based off the 00z run of the NAM. I have not looked at the GFS which I may do here in a little bit. At least things are beginning to look up for tomorrow from what I can see. I guess the biggest concern would be if storms form at all. If storms can form with a couple hours of daylight something interesting may happen. It will be interesting to see what the SPC comes up with, but they know a lot more than I do. As of right now I would say head west of OKC very early in the afternoon and adjust from there.

It should be worth noting that low 60 dews have already made it to SW OK with mid 60 dews almost to the Dallas area and points to the west. There is also a very sharp dryline from Amarillo down to just east of Lubbock. The station in Childress is reporting a dewpoint of 53 and falling off very sharply to a dewpoint of 11 just west of there around 40 miles.

 

[Alex Lamers]

Well this is an interesting situation over OK. SPC mesoanalysis MLCINH/SBCINH both show cap is nearly eroded over I-35 corridor south of Norman to the Red River, but I'm not sure how legitimate that is. It would be really nice to have a sounding right now at...say...ADM.

Looking at OK Mesonet observations, dewpoints are much higher (by ~5F on average) in this area to the south...with wind flow more southeasterly. It seems almost like the dry air is doing a better job of mixing down into the boundary layer further north. Even the surface winds have a bit of a westerly component and dewpoints are in the upper 50s and lower 60s.

If you believe the RUC, the dryline lights up from near Guthrie south along I-35 until the Red River. However, the RUC usually is a bit generous with precipitation. Nevertheless, healthiest looking cu attm over OK are along I-35 in SC OK where a few seem to be bubbling up (as opposed to the flat sparse cu we have in Norman currently).

SPC mesoanalysis also indicates better deep moisture convergence that has persisted the entire afternoon in that area. That being said, better density discontinuity will probably exist further north towards OKC metro late this afternoon and into the evening where the dryline sharpens up more, and could be the part of the dryline with the best forcing eventually.

Winds at this time just seem more convergent to the south over SC OK, and more favorable instability with eroding CINH per SPC mesoanalysis. If the RUC is right...winds in this area stay backed more sse to se into the evening. RUC 00z fcst has 0-1km EHI's over 5 in this area.

Really it all depends on how much the cap has actually eroded. Sometimes the RUC is a little too eager in this regard. Encouraging is the Purcell profile, which shows that westerly winds at 850mb earlier have since come around to the south.

AJL

 

[Tim Marshall]

Yep, I agree with Alex. Things are coming together over southern Oklahoma right now. There appears to be some dewpoint pooling occurring ahead of the dryline bulge. CAPES are 4000 j/kg and climbing. Could be a surprise along or just north of the Red River tonight. I will monitor for now. TM