Let's start with Saturday, the map below shows a few key features that will dictate the Go Zone at 00Z Sunday (Saturday evening). First, let me state that we're only considering conditions during normal business hours (i.e. before sunset). The impressive trough currently in the west will crawl east so that by 00Z the best forcing is just coming off the Front range. There's also a decent upper-level ridge positioned just east of the Mississippi. The sfc low is around Sterling, CO with a diffuse occluded dryline/warm front boundary extending towards Hastings, NE where the triple point is positioned. My concerns are:
1) Forcing too far away to initiate storms before sunset (or even twilight, ahem, Squall Line).
2) Models have large discrepancy in precip output. In fact, only two models generate precip before 00Z: the GFS and the NMM. The NMM's precip is in areas with dewpoint depressions of 10 C and higher, making this a good shot for isolated, great looking supercells, but not a good tornadic threat. The GFS's precip output basically summarizes my best guess for a pre-sunset Go Zone. Initiation would be ~6pm with any storms that do go quickly becoming tornadic. However, the chance of precip, based on the GFS ensembles and SREF is <25%, making me guesstimate a pre-sunset tornado threat of 2-5%. My over/under for daytime tornadoes Saturday is 1.5.
Now moving on to Sunday afternoon: much more interesting the past few runs. All models have been slowly the progression of the low, now the position at 00Z Monday is near Albert Lea, MN. There is a high probability of elevated nighttime convection Saturday night, which may actually provide a few strong tornadoes initially as the cells are still surface-based, say 9-11pm in western Iowa? This elevated convection will very slowly crawl east because the upper-wave is moving NNE making it difficult to sustain any MCS that move towards the east. South of the residual cloud cover from the morning MCS should see ample heating, with a nicely veering wind profile, good speed shear, good BL moisture and forcing coming into the area by 00Z. Initiation of discrete supercells should be earlier than Sunday, say 4pm with my Go Zone being MSP south to Tripoli, Iowa. Because the frontal boundary will be crawling along, we may see a prolonged discrete threat. Over-under for daytime tornadoes is 15.
Wow, Dima way to jump in and put up your first "Go Zone" foreacst on the blog. I would like to first say, I think your brief synoptic analysis is pretty spot on and am inclined to agree with where you put your two "Go Zones" for best tornado chances, right?
ReplyDeleteHowever, I would like to speak up a bit about your Sat concerns and add some more details for Super Sunday. First I don't think your model output precip arguement holds as we all know the GFS, NMM, RUC, WRF are not explicit cloud (convective) resolving models and are thus parameterized. I think it's extremely hard to justify using model precip output as a measure for finding/determining discrete convective activity.
Second, I want state that I agree with your first concern dealing with upper level (synoptic forcing). Even more so, I am worried about the ~850mb EML (CAP) over most of the warm sector. Again though, sub model gridscale forcing (mesoscale) will be ongoing in your go zone region starting in the afternoon on sat. ie bulging dryline and the strong southerlies feeding into the warm front forced ascent which should be enough to initiate severe convection (Can any of the convection go surface based though?). Like you said if surface based storms fire they will quickly go tornadic. I am not buying the dewpoint depression arugement in this case as I think downdrafts will be strong enough to help overcome higher bases (worked out well for me last time ;) ). That is why I will take the over on your o/u (all I need is one great storm and the thousands of chasers will do the rest, :) )
Now for Super Sunday!!!
ReplyDeleteI like the idea of the models slowing down for several reasons. First there could end up being an extra 6-12hrs of moisture transport into the mid/upper Mississippi River Valley. Second, timing of upper level support would coincide with prime chasing hours. Third, ECMWF and GFS are both taking the 300-500mb trough to a negative tilt by 12z Monday. If there are signs that will happen Sunday evening, then upperlevel forcing will go into "beast mode" and LARGE tornado outbreak potential will be possible.
Wanted to note: Storms that form in your go zone will be moving awefully fast (>50 mph). Meaning that many of the chasers will become "the chased". Might be more of a reason to stay on the southern area of your Go Zone not to mention there will be more mositure and cape to work with.
Finally so its documented for our bet:
ReplyDeleteSunset times at various cities for Sat. (will basically have until 02Z since I get the extra hour of twilight)
K.C.-7:48pm -> 8:48pm
Omaha- 7:56pm -> 8:56pm
Hastings-8:05pm -> 9:05pm
Sioux City-8:00pm -> 9:00pm
St. Paul - 7:50pm -> 8:50pm
Another thing to consider which Dima touched on briefly is that there is a REAL threat of discrete supercells capable of producing tornadoes Sunday night. Biggest reason? Good hodographs last through 6z with SW winds aloft which is less favorable for upscale growth...Just something to watch out for, especially portions of Ill and WI as the system slowly moves easy.
ReplyDeleteSquall line - I believe that the hi-resolution NMM and ARW are explicit convection (cloud-resolving). The ARW has no precip before sunset anywhere in the area. The NMM has stuff between Wichita and KS City, but temps are forecasted near 90F with dewpoints of upper 50's - too much of a spread. Recall that in your Cromwell tornado, the dewpoint-depressions were 15f at most.
ReplyDeleteOCD-I think you have me there, I did look it up and both the NMM and ARW are explicit cloud-resolving model versions of the WRF. So my mention of NMM was off. That beings said you stated you based your best forecast off of GFS precip output. Second I stated "I think it's extremely hard to justify using model precip output as a measure for finding/determining discrete convective activity." I said that statement refering to all models (cloud resolving and non-cloud resolving, Gregs NMM included, lol. I should have been more clear as it looks like a direct statement to GFS,WRF,ECMWF. I know we love to use the HRRR reflectivity output, but again its a secondary tool that gets used as a "another opinion". The other chase if I would have listened to the HRRR and not mine/your analysis I would have stayed in K.C./St. Joesph, MO with nothing to show.
ReplyDeleteI have no intentions of hanging in Central/Southern KS to chase the dryline here. I was going to be farther north. I agree 40 Td depressions is ludacris to chase with. However, given the strong low and midlevel level shear and steep lapse rates, storms that fire on the Northern Dryline will be spinning and will have extremely strong downdrafts, which you may recall is probably the most important mesoscale gamma feature for getting funnels to the ground. I know Trip Dawg is preaching this these days.
Remember we are trying to make ourselves better and joking/poking kind of just comes with the territory. :)