4/21/07 FCST: OK / KS / TX / NE

[Jim Bishop]

I like what I'm seeing south of I-40 in the Texas Panhandle. The latest WRF has more veered (southwest) 500mb winds, which tells me the storms will be moving more northeasterly, rathern than northerly. The shear supports tornadic supercells, and I'd expect to see them. It's a long drive from Houston, especially when trying to recover from an ear infection. But it looks too good to pass up, especially since it's on a Saturday.

 

[Kenny Drake]

Things are still looking good for supercells tomorrow. LCLs with respect to marginal moisture return are still an issue with regards to the tornadic potential, but nonetheless these supes will be very chaser friendly not to mention it is on a Saturday. I may or may not be going out. I have a Dynamics exam on Wednesday and Monday and Tuesday look interesting across the south central plains with all likelihood of seeing much better dewpoints. Latest surface obs have revealed the PGF kicking in beginning to advect in a modifying air mass from the SE states. Dewpoints at the leading edge of this inflow are in the low 50s in east texas (where the air has not advected over the gulf). What is more important however is the inflow as it makes its way to the south texas coast which in turn should be forced northwards for tomorrows event. Dewpoints in southern Texas right now are in the low to mid 60s. This is lower than ideal however topographical effects (all the recent rain in Texas) may help retain dewpoints as the air is advected northwards into the TX Panhandle and northwards where dewpoints are currently in the upper 50s. This will definitely be doable for tomorrows setup as long as temps can stay in the upper 60s to low 70s. I think forcing along the front will be plenty to overcome any negative buoyancy resulting in upper 60 to low 70 temps.

 

[Donald Giuliano]

I think there's always one storm that comes through every year where I get waaaayyy too excited about things waaaayyy too far in advance. This one seems to fit the bill for me, hopefully I've gotten it out of my system for the rest of the year.

My primary concern Saturday with regards to squall line vs. supercell mode has to do with when the low-level shear will kick in, and whether or not it will be sufficiently backed relative to the upper-level flow. Regarding the latter, the surface/850mb low has a lobe extending to the northeast, unlike March 28, when it was oriented N/S along the lee side of the Rockies. This yields less backed flow in the low-levels north of the panhandles and a more linear shear profile with height, which given the meridional nature of the upper-level flow, will simply not work.

Between the KS border and I-40, the shear situation improves, but the timing is questionable, as initiation seems to be occurring before the low-level winds are able to back sufficiently given the SSW flow aloft. However, the storm motions in the region may be easterly enough for storms to be able to sustain themselves as separate entities long enough for the backing to kick in (which never happens further north).

South of I-40, things look more favorable for discrete activity, but south of Lubbock capping looks to be an issue. A notable difference between Saturday vs. 4/6/2001 is that the upper-level flow was much more backed further south.

However, there are many more similarities than differences between Saturday and 4/6/2001. And putting those things aside, just look at the progression of the 500mb trough axis from 00Z to 06Z. It travels from approx. El Paso, TX, to Altus, OK, in 6 hours! Given that there is about 500 miles of distance between those two points, that's an average speed of over 80mph. Sounds like things will certainly be going ridiculously linear at some point. As things stand now, I think there may be a narrow window of opportunity between 23Z and 02Z. Is it worth driving to Paducah? I don't know, I'll have to make that determination Sat. morning. Maybe it would be worth it just to get absolutely blasted by a derecho. I was out chasing on 4/6/2001, and I wouldn't mind seeing another 100mph squall line--I've never seen anything move so fast in my life.

 

[Jim Bishop]

I think there's always one storm that comes through every year where I get waaaayyy too excited about things waaaayyy too far in advance. This one seems to fit the bill for me, hopefully I've gotten it out of my system for the rest of the year.

My primary concern Saturday with regards to squall line vs. supercell mode has to do with when the low-level shear will kick in, and whether or not it will be sufficiently backed relative to the upper-level flow. Regarding the latter, the surface/850mb low has a lobe extending to the northeast, unlike March 28, when it was oriented N/S along the lee side of the Rockies. This yields less backed flow in the low-levels north of the panhandles and a more linear shear profile with height, which given the meridional nature of the upper-level flow, will simply not work.

Between the KS border and I-40, the shear situation improves, but the timing is questionable, as initiation seems to be occurring before the low-level winds are able to back sufficiently given the SSW flow aloft. However, the storm motions in the region may be easterly enough for storms to be able to sustain themselves as separate entities long enough for the backing to kick in (which never happens further north).

South of I-40, things look more favorable for discrete activity, but south of Lubbock capping looks to be an issue. A notable difference between Saturday vs. 4/6/2001 is that the upper-level flow was much more backed further south.

However, there are many more similarities than differences between Saturday and 4/6/2001. And putting those things aside, just look at the progression of the 500mb trough axis from 00Z to 06Z. It travels from approx. El Paso, TX, to Altus, OK, in 6 hours! Given that there is about 500 miles of distance between those two points, that's an average speed of over 80mph. Sounds like things will certainly be going ridiculously linear at some point. As things stand now, I think there may be a narrow window of opportunity between 23Z and 02Z. Is it worth driving to Paducah? I don't know, I'll have to make that determination Sat. morning. Maybe it would be worth it just to get absolutely blasted by a derecho. I was out chasing on 4/6/2001, and I wouldn't mind seeing another 100mph squall line--I've never seen anything move so fast in my life.

I think you hit the nail on the head. I think things will become linear rapidly basically north of I-40, approximately what you just mentioned.

South of I-40 is where the tornadic supercells will be where the mid level flow is more veered. But yes, the upper trough is very intense and seems to eccelerate after 00z, which concerns mean with respect to how long the storms in the Lubbock/Plainview region will remain descrete. The stronger cap will help. Either way we are looking at a transition into one mean squall line later in the evening! Hopefully NOT sooner than later.

 

[Bob Schafer]

However, there are many more similarities than differences between Saturday and 4/6/2001. And putting those things aside, just look at the progression of the 500mb trough axis from 00Z to 06Z. It travels from approx. El Paso, TX, to Altus, OK, in 6 hours! Given that there is about 500 miles of distance between those two points, that's an average speed of over 80mph.

I don't know. If you look at the 500mb avort maxes, the progression is from about Roswell NM to Perryton TX. While that may not be a valid comparison, that's about 300 miles, i.e. ~50mph.

http://www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/nam/12/images/nam_500_036m.gif
http://www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/nam/12/images/nam_500_042m.gif

Then, consider that it seems many chasers are talking about chasing storms in the S PH in the expectation of more easterly storm motions, and there's not that problem.

Go here:

http://weather.cod.edu/fsound/

Choose ETA, 36 hour, and 33 lat, 102 lon, and you get storm motion of 253/36.

 

[Kenny Drake]

Alright, in general things are looking great tomorrow for convection. There will be plenty of convergence along a sharp dryline extending from the TX panhandle northwards through the KS/CO border. Given this, and the precip forecasts by both GFS and NAM, there will be plenty of convection. Now my worries.
(1) Temp/Td spread. Dewpoints will likely be in the upper 50s with the possibility of some isolated low 60s. Temperatures are forecasted to be in the mid to upper 70s. This will yield a spread of approx 15-20F. LCLs will be fairly high, so if/when I go out tomorrow I will be being close attention to the LCLs.
(2) Wind profiles. Even with SE winds at the surface ahead of the dryline, other heights will be less than favorable. 850mb is forecasted by both NAM and GFS to generally be out of the S to SSW along the dryline in TX/OK Panhandle. 500mb winds will be out of the SW. There is turning with height, however I would like to see a little more than that. In fact, NAM forecast soundings for Guymon throughout the day show the strongest winds at the 850mb level. By 18Z winds actually back with height from 850 to 500mb:
http://beta.wxcaster.com/cgi-bin/parse_skewt_trace.cgi?model=NAM&fcsthr=024&STATIONID=_KGUY

Wind profiles are slightly better farther south, however capping will become a concern. Forecast soundings for Childress at 18Z show a fairly significant cap between 850 and 750mb. GFS is progging a much stronger CAP than the NAM.

My initial target will be somewhere along I-40 just east of Amarillo. I'm anticipating a quick turn towards a linear MCS so playing the tail end charlie seems the best for my location (so I'm hoping I have to move south, where wind profiles will be more favorable). For those across the northern plains, the warm front looks much more promising with very low T/Td spreads and more veering with height owing to being much farther downstream from the 500mb low. I would not be surprised if the SPC upgraded portions of SD to a MOD risk for tomorrow.

I'm still trying to decide in my head whether I want to go out tomorrow or not. I'd like to think there will be better chase days before finals, but finals are in a couple of weeks. Mon/Tue next week look more interesting but again the distance is too far considering I have classes both days until late in the afternoon.

 

[Chris Wilburn]

This mornings run of the NAM had to be at least very encouraging to most of us. A small swath of around 2,500 J/kg near Lubbock and just north of Lubbock and near 1,500 J/kg all along the dryline to the north into Kansas. Dewpoints still forecasted in the 55-60 range which should do, esp. if we can get upper 50's in some places. Speed shear looks really good, but directional shear could be better in my opinion and is less than I like to see for tornadic supercells. I guess like others I have fears that storms may go linear before storms can really get isolated and organized. I wish the 500mb winds would veer a little more, but I guess you take what you can get. I still see some potential for tornadoes for sure, but for some reason I can't get over the somewhat lack of directional shear. I know a lack of one thing can be overcome by other things like dewpoints and CAPE so we shall see. I am still unsure if I will chase or not. We have a small group leaving at 5am so we should be to a fairly nice target area by noon or 1pm. I need to make a decision quick as I need to be up by at least 4am. I have been going nuts trying to decide whether to go or not so somebody give me a very good reason to go one way or another lol

 

[Matt Chatelain]

0z wrf has very nice dew points for tomorrow with upper 50s to lower 60s just ahead of the insanely sharp dryline and all the way up into Kansas. Surface winds appear to be less backed but should be sufficient with any right moving supercell. Still thinking that storms will have greater chance of being descrete south of I-40, and the new wrf has an even stronger cap south towards Lubbock. We plan on targeting somewhere in the area between plainview and lubbock. Hoping to see lots of happy storm chasers tomorrow.

 

[Bill Schintler]

Chase target for Saturday, April 21

Chase target:
Hayes Center, NE (40 miles SW of North Platte).

Timing:
Storm initiation 4 PM CDT.

Storm mode:
Supercell storms producing a full spectrum of severe weather, including isolated tornadoes.

Synopsis:
100kt H3 streak was rounding the base of WRN CONUS trough this evening while a strengthening 50kt LLJ was aiding in the NWRD transport of moisture. Area soundings (DDC, AMA, LBF, OUN) generally indicate rich moisture from the SFC through 850mb. Meanwhile, ABQ and EPZ soundings each exhibit dry-adiabatic lapse rates from the SFC to 500mb, and this airmass is advecting N and E and will contribute towards an EML on Saturday. There’s still some uncertainty regarding model performance, although the NAM and GFS have come into good agreement concerning the unfolding of events Saturday. The NGM appears to be too fast with the progression of the SFC low while the latest run of the UKMET has come into better agreement with the NAM and GFS, the salient feature being the track and strength of the SFC low. The overall trend has been to slow or even retrograde the low track, as well to strengthen it through 00Z.

Discussion:
While severe weather appears likely over a broad area Saturday, the most favorable combination of 0-3km hodograph curvature, 0-6 km shear, instability, and moderate LCL levels may be found just E of a slowly advancing DL and NE of strengthening low pressure over NE CO. In this area, backing of SFC flow should be enhanced by the topography which will also serve to aid in pooling of SFC moisture, much as was the case Friday. Storms should rapidly fire by late afternoon as large scale assent commences in association with the left-exit region of a 70kt H5 max while H7 temperatures rapidly cool with the advancing trough. The forcing in NEB is not linear, unlike areas to the south. In the TX panhandle northward towards WRN KS, very strong UVVs will occur along a north-south oriented line and when combined with unidirectional shear a linear storm motion may be favored there. H7-H5 lapse rates of nearly 8 C/km along with SFC dewpoints of 55F with a 50 mb deep moist layer will contribute towards MLCAPEs of 2000J/kg and LIs to -7C. The aforementioned backed SFC flow and resulting low-level hodograph curvatures in addition to cloud base heights of 1000m AGL may result in an enhanced risk of isolated tornadoes.

10:30 PM CDT, 04/20/07
[FONT=&quot]- bill[/FONT]

 

[Billy Griffin]

I've never been one to be accused of being overly-optimistic, but I just took a glance at the latest WV imagery and I am more than impressed! That low is currently centered over what appears to be the San Diego, CA area, so I'm curious how much of an impact this will have on the more-southern ends of the risk area... namely, the TX panhandle. It appears to be digging in quite well, and I can't see it making that much of a northerly turn in < 24 hours.

My initial target is basically the same as it was three days ago - near Amarillo. Although current thinking is I might drop south a bit. AMA will be a good place to be by noon.

Based off model data, improved moisture, and other parameters resembling widespread outbreaks similar to this one, I'm going to go out on a limb here and believe that by the 11am or 3pm day 1 outlook, parts of the TX panhandle may be upgraded to HIGH risk! Limited convection has occurred thus far, which will aid in keeping available daytime heating an opportunity throughout most of the day. Way out on the limb with no safety net, but I now feel there is an increasing possibility of long-lived, strong tornadoes over this region. I wouldn't be surprised to see cells lined up from Lubbock to Guymon. If ( IF ) they can just stay discrete for an hour or two, this will be a very productive day for many chasers!

 

[Ray Walker]

Billy I agree that this day has the potential to be upgraded albeit very slight possiblity but it is there. However for this to be a big outbreak then several key ingredients have to line up just right and one is that storms WILL have to STAY discrete for much longer than one or two hours. More like 3 to 5 hours. I do think that from the Oklahoma panhandle region all the way down into SW Texas or SE New Mexico that the possiblity is there for tornadic supercells to just come unzipped down the VERY sharp dryline but I am also kind of concerned not really about a rappid transistion to squal line because I do not think this is likely but more about storm line segments or clusters to form. That is not a bad think in a strongly sheared environment like this but it would keep the tornado OUTBREAK possiblities to a minimum. I think after the first hour or so of initiation we will be able to tell what will happen.

 

[Jeff Snyder]

My current thoughts remain similar to my previous thoughts, as well as to others' thoughts. I still like the area south of I40, east of I27. The 00z NAM continues to show strong pre-dryline UVVs from 850-700mb, similar to what we saw on 3-28. Model forecasts show very strong, very linear forcing along the dryline by 0z, so I'm really hanging my hat on the forecast area of weaker UVVs east of the dryline.

NAM forecast hodographs show strong counterclockwise curvature in the 1.5-4km layer, some of it caused by the lack of directional shear (with strong speed shear) above ~1.5km, and some of it caused by backing winds with height in the 3km-6km layer. I have always had contempt for this type of wind profile (sort of S-shaped), and I've never found success on days with that degree of counterclockwise curvature (those cells that remain discrete tend to have problems with sustaining cyclonic circulation, IME). The forecast hodographs look a little better south of I40, though even those show some counterclockwise curvature in the low-mid levels. Others have talked about storm mode in terms of marginal boundary-layer RH (potential for moderately strong cold pools) and shear-vector orientation relative to forcing boundary, so I won't say much about those.

I'm also a little concerned about the speed at which the vort max and midlevel trough axis zooms by. Between 00z and 06z Sun, the trough axis appears pivot northeastward and move rapid east-northeastward. At 00z, the leading edge of the vort max is near the I27 corridor; by 06z, the leading edge of that same vort max is not too far west of I35. Though this actually helps to veer the 0-6km shear vector with time across the northern and central TX panhandle (and some pts northward), it also helps usher the dryline rapidly eastward. Approximating the dryline movement around 30mph (~170 miles in 6 hours), it seems likely that storms moving NNE N of I40 may stay on the dryline the entire time, meaning that initial storms will persist in an elongated, linear area of strong forcing for a large period of time (spells squall line to me). Farther south, where storm motion should be more to the NE around 40-45kts, the easterly component of motion (or the component of motion normal to the dryline) should be positive, so the storms appear to have a better chance of moving off the dryline S of I40.

My prelim target is shared by several -- the area between Plainview and Turkey looks like a good starting point. I hope we can get some initiation ahead of the dryline, since I don't like the strength of UVVs on the model forecast charts along the dryline. Just my 2 cents.

FWIW, I don't expect a HIGH tomorrow, based largely on storm mode and moisture / LCL concerns.

 

[Billy Griffin]

Absolutely Ray & Jeff, and that's why it wouldn't surprise me to SPC sit on a MDT risk on the initial Day One, but upgrade as the day progresses. And your statement is right on the money... if those storms stay discrete and produce, it could be a great day. If they cluster or go linear too quickly... well, then we'll all be trying to outrun the squall line! I guess it's almost impossible to predict exactly what will happen and we just wait until it does. Either way, I'm just happy to be back out chasing after a two-year respite. Look forward to getting some good HD video with my new cameras and I'll be streaming video back to KOMO-4 news in Seattle. They're excited, as this is the first time they've tried anything like this.

 

[Ray Walker]

Billy the thing that I really find interesting is that on 3/28 a lot of chasers stayed home because they thought the same thing as many are saying right now, STORMS WILL GO LINEAR. Well they sure didnt and a significant tornadic event unfolded. I think tomarrow is going to be a big day even if a squal line forms.

 

[Donald Giuliano]

My current thoughts remain similar to my previous thoughts, as well as to others' thoughts. I still like the area south of I40, east of I27. The 00z NAM continues to show strong pre-dryline UVVs from 850-700mb, similar to what we saw on 3-28. Model forecasts show very strong, very linear forcing along the dryline by 0z, so I'm really hanging my hat on the forecast area of weaker UVVs east of the dryline.

....

I'm also a little concerned about the speed at which the vort max and midlevel trough axis zooms by. Between 00z and 06z Sun, the trough axis appears pivot northeastward and move rapid east-northeastward. At 00z, the leading edge of the vort max is near the I27 corridor; by 06z, the leading edge of that same vort max is not too far west of I35. Though this actually helps to veer the 0-6km shear vector with time across the northern and central TX panhandle (and some pts northward), it also helps usher the dryline rapidly eastward. Approximating the dryline movement around 30mph (~170 miles in 6 hours), it seems likely that storms moving NNE N of I40 may stay on the dryline the entire time, meaning that initial storms will persist in an elongated, linear area of strong forcing for a large period of time (spells squall line to me). Farther south, where storm motion should be more to the NE around 40-45kts, the easterly component of motion (or the component of motion normal to the dryline) should be positive, so the storms appear to have a better chance of moving off the dryline S of I40.

I have to agree with you here. I too have noticed that the models are hinting at the possibility for initiation out ahead of the dryline, and if we see that along about 23-00Z when the shear is forecast to get going I'd be pretty excited about those storms. And as far as the speed of the trough axis is concerned, I've seen plenty of situations where the models don't blast the boundary out fast enough, but had I looked at the 500mb forecast and lined up the surface features with that, I'd have realized what would happen. One caveat, though: I think this method works better during peak heating when you have deep, strong mixing behind the dryline, whereas later in the day and after dark it's easier for the mid-level trough to outrun the surface features as the boundary layer behind the dryline decouples and the downward transfer of momentum ceases. On the other hand, linear convection could allow this momentum transfer to continue, and thus keep the upper-level features coupled with the surface features well into the night. Or who knows, maybe something else entirely will influence the situation in a surprising way. What will actually happen? Only time will tell.

Isn't meteorology awesome? I love this field, so many questions to ask, so much to learn!