Howdy /r/stormchasing! I'm gonna be going out for the first chase of the season tomorrow, and I always write out a forecast before I go out, so I thought I'd share it with you guys. I'm going to be focusing on Colorado, since that's where I'll be heading out.

Larger-scale stuff

Models are in good agreement with the general forecast setup. The classic omega-block pattern currently in place over the continental US will finally begin to break down by tomorrow afternoon. The nose of a strong 100 kt jet streak will nose its way into central Colorado Saturday morning. This will provide strong synoptic-scale lift over the Colorado Front Range out to the Wyoming and Nebraska borders. By late morning to around noon, strong south-southeasterly 500mb flow and east-southeasterly 700mb flow should result in favorably shaped hodographs for supercells throughout northeastern Colorado.

At the surface, easterly winds associated with a strengthening surface low should bring 45-50F dewpoints as far west as the foothills. These dewpoints in combination with temperatures near or above 70F will be sufficient for MLCAPEs greater than 1000 J/kg in the I25-I76 triangle (I know there's a word for that region but I can't recall it!) by 18z (noon local time), and from 1500-2500 by 21z.

Target 1: I-25 between Denver and Cheyenne

The high CAPEs in combination with lift from the aforementioned jet streak and a strong upslope component to the wind from a strengthening lee cyclone should be enough to ensure that numerous thunderstorms should be underway by early afternoon along the I-25 corridor where the lift will be maximized, perhaps even by late morning. Shear and CAPE should be more than sufficient for supercells, and by 21z the strengthening 700mb flow should result in nicely curved hodographs supportive of tornadic supercells. The storm motions of right-moving supercells will unusually be to the north-northwest, so storms which form in the Denver area will threaten areas like Fort Collins and Cheyenne by the early afternoon.

Later in the afternoon, though CAPE and low-level shear may continue to increase, the jet-level winds will break down and become chaotic as the jet streak moves east, resulting in messy hodographs and a probable end to the significant severe threat along the I-25 corridor by 22z (4pm local). Even before that, there is a question as to how isolated the storms can remain. Without a clear boundary on which to form, storms will probably form on local terrain features where the upslope and upper level forcing combine to overcome the morning Convective Inhibition. Or perhaps they will form in the late morning on the high terrain south of Denver and propogate northward and become surface-based in the metro area towards noon. Aside: I think it's worth noting that several NCAR ensemble members from last night's run and some earlier 4K NAM runs support the later scenario, but this evening's model runs seem to have abandoned that possibility (except the latest HRRR run apparently). Regardless of where they form, there is a possibility that the strong forcing will make for a messy cluster of splitting storms all colliding with each other. If one storm can become dominant, however, a significant tornado is not out of the realm of possibility: The NAM and the RAP both show some pretty serious low-level shear and curved hodographs in this region. I can't get vertical soundings for the other hi-res models (4K NAM, HRRR) but they all show areas of SCP above 5 and STP above 2 along the I25 corridor.

Target 2: Palmer Divide east of Denver, northeast to the Nebraska border

Further east, the situation is a little bit less clear, but mostly in the fine details. A surface low will be set up somewhere in the vicinity of metro Denver. As it intensifies, as it pulls in surface moisture from the east, it will also be pulling in drier air from the southwest, sharpening up a dry-line oriented roughly ESE-WNW somewhere in northeastern Colorado. The evening model runs are still coming in as I write this, but the major uncertainty in where this dryline sets up seems mainly connected to the strength of a southward moisture surge from a plains cold front, in combination with the outflow from the thunderstorms it is triggering this evening across Wyoming, South Dakota, and Nebraska. The latest MPAS run keeps the dryline fairly far south even through 00z, keeping most of northeastern Colorado in the sector of favorable CAPE through sunset. Some earlier runs of various models had the dryline shooting much further north, though as I'm writing this the evening models all seem to be coming around to the MPAS solution.

Upper level winds are also a bit of a wildcard. Shear vectors/storm motions will be roughly perpendicular to the boundary, so storms should be able to stay isolated fairly well. And with strong forcing and shear and high cape and moisture convergence, storms should mature quickly; the NCAR ensemble has a few members popping off really beefy rotating updrafts after 21z. However, that same strong forcing may eventually make things cluster up after 00z even with the favorable shear, and the same problem of the upper-level flow breaking down as the jet streak moves east will be present. Currently it seems as if the favorable flow will remain just south and east of the favorable CAPE, but just a small shift southward in the dryline or northward in the jet streak could mean a very favorable tornadic environment in far northeast Colorado from 00z-02z.

Chase strategy

My strategy for this chase is more complicated than usual. The early threat along I25 seems to be the most consistently promising across models and runs, but the short window of favorable shear/instability combination (roughly 18z-21z) is bit offputting, not to mention the proximity to dense population centers. Further east and southeast along the dryline, convection looks to initiate later (closer to 20-21z), but will have a longer window of favorable shear. So my plan is to set up on I25 just north of the Denver metro area, and see where initiation occurs. If by 20z (2pm local) nothing looks favorable, or if storms pop off early on the Palmer divide, I'm gonna shoot east and try to catch the secondary target by 22z.

All storms should be slow moving (20-30 kts) and with low PWATS and strong upper-level winds with linear shear I'm thinking the supercells will be classic to maybe even LP, so the chase itself should be pretty mellow. It'll just be a matter of catching the right storm and seeing what happens!

PS

If anyone's curious where these neat graphics come from, most of them (the operational models, NAM, GFS, etc) come from the awesome College of DuPage Weather Site. The MPAS spring experiment graphics can be found here, and the NCAR WRF ensemble is here.

PPS*

I just remembered why I hate doing these things. In the time it took me to write all this out a whole new set of model runs has come out for me to digest...and I still have the morning runs when I wake up!

Edit 1: Why am I still awake?

I always have trouble sleeping before a chase it seems. The 06z run of pretty much every model are emphasizing the risk to the southeast of Denver between 18z-20z, so I may change my initial target to be east of Denver rather than north. It appears the low will solidify further east than earlier runs were indicating (perhaps a bit of a DCVZ influence??) , and a very decent tongue of surface moisture in northeasterly or even northerly flow will lead to enhanced shear and CAPE in that area.

Edit 2: Morning update

Some subtle changes in the morning models:

• The late-morning push of moisture south around the backside of the developing surface low looks even further south than it did in my first update: CAPEs over 1000 could push as far south as Colorado Springs before the boundary starts sharpening and moving north.. I'll definitely be aiming east of Denver rather than north to begin my chase, maybe even southeast depending on where the first cells go up.
• Possibly related to the first point: the dryline now looks like it will take longer to sharpen up, and will start further south and oriented more NW-SE than last night's forecast. This looks to extend the region of favorable shear/instability overlap near the KS-CO border along and north of I70 at and after 00Z. Looks very promising for the secondary target as well!

Post-mortem edit: a semi-successful chase

Everything panned out pretty much as anticipated. Started southeast of Denver and followed the early storms north to I76, missed the tornadoes around Wiggins due to lack of road options and not wanting to get cut off from our further east play. Saw some pretty intense rotation for a bit but it never really threatened to touch down. Stayed on the tail end of those storms instead of breaking to far eastern Colorado, which of course turned out to be a mistake as those eastern storms produced that spectacular series of tornadoes near Wray, CO. Still, we saw some gorgeous storm structures, a rotating wall cloud or three, and even managed to see the Wray tornado(es?) from our vantage point 25-30 miles to the WSW. In retrospect, our storm probably wasn't producing because it was rooted along the dryline, and was mixing too much dry air into itself even though it had access to plenty of CAPE and low-level shear. Another lesson learned the hard way!