Hi all,
Ok – with an active upper trough forecast to move across the US later this week I figured it was time for some warm-up forecasting given that some of us will be heading over relatively soon to start chasing. I’ve just been having a look at some charts for this coming Thursday/Friday’s event in the US. (Sorry bit distracted – laughing hysterically at Thank God You’re Here – first episode of the US version).
Now that we are inside the 84hr mark, there are a few more model options to have a peek at. Moisture has backed off a bit since the last time I checked (this time yesterday or so). I’m going to focus mostly on Thursday as I think Friday is an easier day to forecast – there are more variables which may or may not come off on Thursday so I think its worth a closer look.
SPC have recognized the potential for scattered severe convection late on Thursday evening but they also comment (in their Day 4-8 outlook) that the main show will be Friday (as noted in the previous paragraph).
ETA/NAM (what is it officially called?) only goes out to 84hrs and the action time for Thursday is in the 72hr forecast period (so a fair way out still). Note: the following comments are based on the 00z output which is for 7pm Thursday. This model shows a 991mb surface low sitting over north eastern New Mexico with the dryline extending along the eastern boundary of the Texas panhandle from about Laverne, OK down to Childress, TX and down to around San Angelo, TX.
A weak-moderate frontal system passing over the western plains at the moment is set to clear most of the moisture out of the area until southerly winds set up ahead of the significant upper trough associated with the Thurs/Fri system. The 65f DP line sits right along the TX coastline by 7pm Wednesday night and the southerly return flow looks to commence in the early hours of Thursday morning as the LLJ starts to set up. DP’s of 50f (10C) nosing up towards the south eastern TX panhandle with the 60f (16C) line sitting in southern TX around Austin. Both GFS and ETA/NAM have this moisture rapidly moving northwards during the day (although note that GFS has the dryline sitting slightly further east and moves the whole system through faster than the ETA/NAM) and by 7pm, the 60f line is sitting just N of the Red River (ie the southern border of TX and OK) – actually, GFS has 65f (18C) DP’s sitting over the Red River into south central Oklahoma. This is thanks to the southerly low level jet (LLJ) of about 40knts transporting this moisture northwards.
The surface temps look to be just nudging into the 80f (27C) mark so we are looking at temp/dp spreads of about 20f (11C) which is an arbitrary line in the sand for tornadic/non-tornadic storms (due to the relatively high bases of storms resulting from such spreads – the lower the LCL, typically the more likely for tornadic activity). The other thing to note about the surface progs is the further increase in moisture overnight Thursday. By 7am Friday morning, DPs in the southern plains (ie across most of OK) are looking at being pretty well into the low 60’s (16-17C)- its actually probable that this will happen later on Thursday evening which could/will help lower LCL’s as the surface temperature drops as well…interesting [Smile] .
There will be quite a steady cap in place on Thursday with the 850 temps across the area being relatively warm. GFS indicates they could be into the 18-19C range, whereas the ETA/NAM drops back to 16C. This could play a bit of a part in making or breaking the day (pun somewhat intended). Forcing along the dryline doesn’t look overly strong and with the main upper level feature is sitting quite a way back to the west on Thursday evening, a cap on the stronger side of these figures could see one of those lovely clear sky busts. Interestingly enough though, both GFS and ETA/NAM are progging a weak upper level short wave trough ahead of the main upper level low to move over the SW Texas panhandle. This feature is barely noticeable on these models but its there and could be enough to help break the cap *late* in the afternoon. Furthermore both models are indicating precipitation along the Red River east of Childress, TX and both models erode all the convective inhibition (CIN) over the south eastern TX panhandle and eastwards along the Red River. Given the above, I think if 850 temps remain under 17C, there will be a show.
Instability indices are showing up some reasonable numbers with CAPE in the 1500-1750j/kg range (with a tiny patch of higher CAPE – getting up to around 2000j/kg) and LI’s are around -6C or so in far SW Oklahoma/SE Tx Panhandle and eastwards along the Red River. Shear is quite nice with ETA/NAM suggesting a 30-40knt S’ly LLJ (850mb), 40-50knt SSW-SW’ly winds at 700mb, 60-70knt SW’ly mid-level jet (500mb) and a 90-100knt upper level jet (200mb) – GFS has each level about 10knts higher and the upper levels have a slightly more westerly component. ETA/NAM also shows a 20knt SSE’ly at the surface over the region. Certainly nothing to sneeze and should storms develop, they would have a decent chance at becoming supercellular.
Given all of the above, I think there is a reasonable chance of some nice isolated convection along the dryline on Thursday. My current thoughts would put me somewhere S of the Red River (trying to make the most of the moisture coming up from the south) near the SE corner of the TX panhandle. With GFS and ETA/NAM differing slightly on the position of the dryline, I’d probably set up somewhere like Vernon or Quanah along Hwy 287. The problem with picking a target close to the Red River is that there is limited locations to cross over into Oklahoma should the need arise. Assuming storms initiate somewhere between Vernon and Childress (TX), the shear profile indicates that storms are going to initially move in a NE’ly direction and anything that really gets going is probably going to move ENE and eventually cross over the Red River into southern Oklahoma. So in picking Vernon and Quanah as starting points, both have options of roosting north across the Red River into southern OK should things initiate further N or should storms develop and move towards the Red River.
I’m probably going to have time to refine this forecast on Wednesday night. This will be much more useful as it will allow us to look at the surface observations to see how accurate the models are in forecasting the moisture return (which could play quite a big part in whether storms develop and whether there will be tornado potential…if the DP’s can get up into the low 60’s, there would be at least a chance of a tornado in the early evening hours as surface temps drop slightly and the LCL gets lower). With 27/17 and -15C at 500mb and the shear like it is, we’d all be pretty happy with this set up at home [Smile] .
Cheers,
Macca
