Day 7
Although all reports may not yet be in, SPC is showing that no tornadoes were reported from the storms over south western Texas yesterday (not sure about the Mexican storm yet although it is highly likely that it was tornadic). So even though we missed a lot of the action and probably some great structure in Texas, we didn’t miss out on tornadoes.
Today was a designated driving day to get into a good position for tomorrow (more on that below). We left San Marcos (between San Antonio and Austin, Texas) at about 10am and basically drove all day and we have stopped for the night in Wichita, Kansas (yep – we drove through a lot of Texas and ALL of Oklahoma today – about 580mi (925km).
Just a little bit of info on today…the “action area” of the upper trough from yesterday was still hanging around eastern Texas into Louisiana and Arkansas and the centre of the weakening upper level circulation was sitting over central Oklahoma and moving eastwards. A considerably moist boundary layer was present over the eastern half of Texas and there was still some reasonable (but weakening) shear. LI’s were around -7 to -8 over south eastern Texas and getting up to about -4 to -5 over north eastern Texas so there was still quite a bit of instability around. A few isolated storms (possibly short-lived supercells) developed near DFW late this afternoon but we had already left by that stage. Two or three more lines of storms developed over far eastern Texas, Louisiana and Arkansas and these had plenty of storm warnings on them but that area isn’t the greatest to chase and it was very linear. We basically decided to make today a “relaxing driving day” as opposed to a full on chase day followed by a 6 hour drive after chasing and then another 6-8 hour drive tomorrow to the target area. From what we saw on radar today, this probably was a decent move as I don’t think we missed out on too much over Texas.
We did run into a line of storms which formed along some sort of boundary near Oklahoma City. Hail of up to 4cm was reported from these storms quite close to where we were but these storms were relatively high based. Given that they were located near the centre of the cold pool, the updrafts were quite crisp. We also got a fairly AWESOME sunset here in Wichita tonight. It was nice to have a “day off” today (even though we saw plenty of storms on the drive N). We have plenty of time to watch things begin to evolve for tomorrow.
Ok – now for tomorrow. The first thing to note is a significant upper level trough digging into the central high plains tonight and tomorrow. This upper level feature is forecast to stall over the high plains for the next 4 days (Fri – Mon) with GFS finally pushing it eastwards by day 5 (to the eastern parts of the plains states) and then into the Mississippi River valley area on Day 6 in a weaker state. This finally sees a ridge push into the plains, giving us some time to recover from this somewhat hectic but at the same time very welcome start to our trip. If this upper level scenario comes off, we will have had significant chase days on 9 of the first 10 possible chase days.
So…this upper level trough – the timing of its arrival tomorrow and its impact on surface features is still in question with GFS and NAM varying somewhat as to the outcome for tomorrow’s convection. It is currently having an effect on NE’rn Colorado and eastern Wyoming where storms developed along the front range today. This evening, a well mixed boundary layer sees dew points in the low 60’s (16-17C) across all of eastern Oklahoma and Kansas with a warm front currently moving slowly northwards through central Kansas. Higher DP’s (65-70f’s) are present over central Texas at the moment. Both GFS and NAM are showing slight increases in moisture levels tomorrow so that DP’s should be around the mid 60’s (18-19C) by prime time.
With the approaching upper trough, a dryline is progged to set up across south western Nebraska extending down through western Kansas, western Oklahoma and into west north central Texas. Significant pressure and temperature gradients between the upper level trough and the developing upper level ridge over the great lakes/missisippi river vally region is set to cause some very nice shear late tomorrow (and even more insane on Saturday). Sharpening of the dryline in response to the approaching upper trough is going to result in a low level jet (LLJ) of 30-35knts (at 850mb) from the SE. This is being overlayed by strongly diverging winds at 700 and 500mb’s and also in the jet stream (lets say 250mb). 700mb winds are SSW’ly at 40knts, 500mb winds are SW’ly at 45knts at 250mb winds are SW”ly at 70ktns. Instability is not an issue with CAPE’s extending from 2000j/kg over south western Nebraska up to 4500j/kg over Texas and LI’s of -4 to -7.
So, we have shear, we have instability…what is left in our equation? Trigger. We have one of those too…the convergence on the dry line. So we should have some freaking awesome storms right? Well…yes…BUT…there are two other things we need to consider. These two things are where GFS and NAM have some interesting differences (interesting given we are only 24hrs out). The two things we need to look at (which are linked) are the cap and focus points for convective initiation given the varying strength of the cap along various parts of the dryline. GFS has some very warm 850mb temps sitting over the dryline from Texas up into central Kansas. 850mb temps of 24C extend all the way up to northern Kansas (lets say Hays) but north of there, the cap weakens to 21C and then 18C in quite a short distance. A quick look at surface RH (basically the DP), GFS has a dryline bulge right near Hays. A bulge in the dryline is often the focus of convection and frequently results in any supercells becoming tornadic. The reason for the focus of convection is that it acts as a convergence zone for moisture and as a result is often the place where a strong cap can be broken. The reason for supercells becoming tornadic is that the bulge in the dryline causes the surface winds to curve around into the corner created by the bulge, giving the low level flow a more easterly component and increasing the SRH in the vicinity of any storms that develop in this area. (Feel free to correct me if this is not totally right or if there is a better way to describe this). So GFS has the bulge sitting up on or just north of I-70 (which runs E-W across Kansas about 2/3rds of the way up). The place to be in this situation would be to sit just NE of the dryline bulge and wait for convection to develop. It is not uncommon for supercells to fire south of the dryline bulge too if the cap can be broken and when this does happen, these supercells tend to be very isolated and photogenic. If the GFS scenario comes off, I have significant doubts as to whether any convection will initiate south of the bulge with the very warm 850mb temperatures (ie 24C+). Some chasers are convinced that the significant lift associated with the approaching upper trough will be enough to break the cap but I made a rule when I was here last time (after 3 clear sky busts) that I wouldn’t chase an area which had 850mb temps of >20C. Tomorrow will be no different. The NAM scenario is similar to the extent that it also has a dryline bulge however, it situates it about 60-70mi further S near Great Bend, Kansas (almost right in the middle of Kansas). This results in the main area of convergence being in a higher moisture area and thus instability is quite a bit higher (2000-2500j/kg with GFS compared to 3500-4000j/kg with NAM). The main difference arises with the 850mb temperatures extending S of the dryline being in the 20-22C range. This is more likely to be breakable than the GFS scenario. If this was to come off, a few isolated very strong supercells would probably develop S of the bulge in southern Kansas into central western Oklahoma.
Given the combination of shear and instability tomorrow, I think we can expect to see some very strong supercells develop with tornadoes quite likely with any supercells which form (whether in the bulge or S of there). Very large hail is also likely (3-4 inches/6-10cm) as are damaging winds. SPC has currently only gone for a slight risk due to the uncertainty regarding the cap strength (and thus how widespread storms will be tomorrow), however, I think we will see an upgrade to a moderate risk at least for central/north central Kansas and probably southern Nebraska for tomorrow, possibly extending quite a way to the east (moreso N of I-70). Convection should evolve into an MSC in the late evening as the LLJ (850mb winds) strengthens futher to 50-60ktns. This should move eastwards across northern Kansas and southern Nebraska overnight tomorrow and through Saturday morning. The likely development of this MSC and the potential for this to have severe weather all night across the border regions of Kansas and Nebraska should only strengthen the case for a moderate risk to be issued for tomorrow.
At the moment, there is really no way of telling which of the two scenarios is going to be in place by prime time tomorrow afternoon, however, either way, we will most likely be playing the dryline bulge (about 95% sure). This is going to be the best chance of getting convection to initiate before dark and also the best chance of tornadoes (there is a decent chance the dryline could fire further S after dark as the lower levels cool and the cap weakens…this could be nasty if this happens given the strengthening LLJ…).
We will check data tomorrow morning (we have to check out of here by about 10-11am) and it should be clearer by then where exactly this bulge is going to be. Every chaser and his/her dog will be out an about tomorrow and Saturday as this is looking to be one of the best set ups for the year so far. We are expecting significant chaser convergence in the vicinity of the dryline bulge tomorrow although the chance of more isolated convection further S along the dryline will draw some chasers away from the crowds. We should have time to post an update tomorrow morning.
For now…we just sit and wait.
Macca & Chris
