Quincy

It's still early to call boom or bust. The linear mode (especially up north) advertised on some models might be right. Remember in 2017 when a few of the high shear/high CAPE setups in May across Oklahoma failed to meet expectations? Just because there's substantial instability with strong wind shear does not equal a supercell tornado outbreak. That 5/18/17 tornado-driven high risk busted hard, especially when CAMs were in good agreement about messy storm modes. There was a moderate risk later in the month that was very junky from eastern Oklahoma into the Ozarks. With that said, this event has a higher ceiling than most we've seen over the past 4-5 years across the Southern Plains. Aside from storm mode, I'd also be concerned about a mass of convection moving from the panhandles into Oklahoma during the morning. This is advertised on the GFS/Euro/RGEM and to a lesser extent, then 3km NAM. I have the most confidence in the dryline lighting up with a potentially significant tornado threat. The instability progs ahead of the dryline are pretty ridiculous, on the order of 3,000-4,000+ J/kg MLCAPE by peak heating and that's across relatively high terrain, in the Lubbock-Midland-Fort Stockton zone. Speed shear is substantial and directional shear is good with plenty of veering with height. There is a southwesterly component to the upper level winds and although the angles with respect to the dryline probably won't be 90 degrees, it will still get things done with S to SE near-surface winds. The biggest question mark is up north, in my view, from the panhandles/northwest Texas into western/central Oklahoma. That's where the parameter space is maxed out, but where there could also be convective overturning earlier in the day. Another scenario is a reinforced boundary being left somewhere between the Texas panhandle and the Red River Valley from morning convection, but it's early to speculate where that might setup, if such a boundary does get laid down.

Let's look at the 00z sounding from DDC, a lucky RAOB very close to where a tornadic supercell formed just a short time later. Note the enlarged low-level hodograph. The vast majority of the SRH was contained in the lowest 0-1km. Even though wind fields look unidirectional above 700mb, it's really all about the lower level wind fields. 32 knots of 0-1km shear and >300 m2/s2 0-1km SRH and I'm sure that only increased leading up to the cyclic supercell. Of course, there was large buoyancy, relatively steep lapse rates and more than adequate deep layer shear as well. A faint convective signal along the dryline in the models can end up going big one way or the other (blue sky bust vs. intense supercell). In this case, there was more than ample large scale forcing and substantial height falls impinging on western Kansas was enough. I will admit that I wasn't overly focused on this event since I was not chasing, but I would have favored the southern target (Southwest Texas) due to the questions regarding convective initiation.

To recap the next several days with some recent model data... Today: Severe storms are likely across the central High Plains, eastward across the central Plains this evening. A few other isolated storms can't be ruled out to the south, particularly over Southwest Texas. Tomorrow: Severe storms seem probable in at least two regimes. The first will be with an MCS that could rejuvenate later in the day from Northeast Texas into the Arkansas/Missouri vicinity, or evolve into a broken line of storms. The second area will be in the wake of the MCS, assuming there is adequate recovery, from Oklahoma into parts of Kansas. This is still somewhat conditional, but odds appear to be increasing a bit for at least isolated storm development near/just west of I-35. Sunday: Looks like a relatively quiet day as storm threats largely shift east of this sub-forum. For anyone out chasing, it may be a good day to relax and get ready for the coming week... Monday: Not quite sure what to make of this day, just yet, as confidence is somewhat low. There does remain at least the potential for a significant event, particularly across the Southern Plains vicinity. Even if isolated storms aren't dominant, there should be some severe weather. Tuesday: The severe threat will likely shift northeast toward the lower to mid-Missouri Valley. Wednesday into next weekend: The synoptic pattern should remain favorable for steady bouts of severe thunderstorm activity around the central U.S. General ridging across the Southeast is modeled with favorable upper level flow continuing to eject from the Four Corners region, eastward across the Plains. It's not clear if there will be a higher-end severe threat, but at the very least, multiple severe storm days are likely, possibly with no single day that goes quietly. The operational 00z Euro shows another seasonably strong surface low ejecting from Colorado into the central Plains around late-week, but that could change. It still looks like an at or above average stretch of severe thunderstorm activity across the Plains vicinity through the next 7-10 days.

The 12z model suite is trending toward less convective overturning across Oklahoma tomorrow morning, allowing for the atmosphere to at least partially recover by peak heating Saturday. A consensus of model solutions bring a sub-1000mb low into northwestern Oklahoma between 21z SAT to 00z SUN. Ahead of a sharpening dryline, the SREF/NAM/HRRR all show mid to upper 60s dew-points recovering in central Oklahoma with dews around 70F in south-central Oklahoma by 00z. Models also show less veering of the low-level jet, suggesting that wind profiles will remain favorable for supercells, assuming that early day convection is not overly abundant, in order to disrupt the kinematic fields. If there is a silver lining in a relative lack of convection overnight (NW Texas/western Oklahoma), it is that it could bode well for a more robust severe threat across the risk area tomorrow.

I would always advise caution with getting too specific about convective evolution, especially beyond days 2-3. Regardless, the synoptic pattern looks, at a minimum, favorable for at or above average tornado activity across the Plains through the next 7-10 days. Will there be a high risk, outbreak, or photogenic tornadoes? Who knows. That will come down to mesoscale details and other things that will usually not be clear until much closer to a specific day. If you wanted a favorable pattern for mid to late May tornadoes across the Plains, that's what most ensembles and medium range models are pointing at. Here is a look at the latest CPC day 6-10 hemispheric pattern with analogs, as well as the GEFS day 8-12 mean 500mb height anomaly forecast: Not only does the GEFS show this, but the Euro weeklies and EPS are in fair agreement as well. This pattern, one with troughing across the West and ridging over the East, will support severe thunderstorm activity, possibly significant, across the Plains. The model agreement for this weekend is very good for the big picture and there is reasonable agreement that the general pattern persists through much of next week. Of course, that doesn't mean that every event will perform on the higher end of the spectrum and it doesn't mean that mesoscale details can't put a damper on what appears to be a potentially significant event. It also does not mean that every single day between May 17-25 is going to be a day with a lot of tornadoes. I will say that if you look at the top 5 extended periods (5 days or longer) with well above average tornado counts in the Plains since 2003, you will see that the signal is there. The CPC analog data is also popping 5/23/08, which was near the beginning of the historic stretch in 2008. One thing you want to see is negative height anomalies across eastern Canada, otherwise there is reason to believe that the pattern will break down, allowing for eastern U.S. ridging to move offshore. For the record, here are the top 5 (2000s) stretches for Plains tornadoes in mid/late May that I identified: May 22-29, 2008 – 23 tornadoes/day May 18-25, 2010 – 17 tornadoes/day May 23-28, 2015 – 16 tornadoes/day May 16-30, 2004 – 14 tornadoes/day May 21-30, 2016 – 12 tornadoes/day The climatological average is approximately five tornadoes per day across the Plains in mid to late May. Read more.

As has been discussed in the medium/long range threat, there is a strong signal for a synoptic weather pattern that will be favorable for severe weather across the central United States over the next week or two. Based on (multi model/ensemble progs) the positioning of western U.S. troughing, presence of ridging across the southeastern U.S., favorable upper level wind profiles and poleward moisture transport, the period beginning on May 17th and lasting for potentially 7-10 days or more, will likely feature at or above average severe thunderstorm activity around the Plains and adjacent portions of the central states. I'll leave more specific discussion of individual days to the replies, but the first in a series of embedded shortwave troughs should impinge on the High Plains vicinity by Friday. This trough is forecast to rotate around the base of a broader trough through this weekend, before reloading. Another trough, potentially more significant, is then progged to approach the High Plains early next week. Ensemble guidance, including the GEFS, Euro ensemble and Euro weeklies suggest that the overall pattern will continue to more or less repeat itself through much of next week (May 19-25). It is unclear how long the pattern may continue, but the weeklies suggest that the pattern may turn more zonal (at least as reflected in the mean upper level pattern) toward the final 3-5 days of May. Another consideration in the pattern is the signal for blocking across eastern Canada. This feature is what can help keep the pattern "blocked" up, meaning that ridging remains in place over the Southeast, while general troughing continues across the western half of the CONUS. If there were less downstream blocking, one might expect the pattern to be short-lived, but this is what makes this upcoming period potentially long. The model data also shows seasonably strong upper level winds streaming east across the Pacific into the West, which is another signal that makes this upcoming pattern intriguing. I'll reply back in a bit with some comparisons of this upcoming pattern to previous historical stretches with above average tornado activity in the Plains in mid/late May.

Tomorrow is a good example of why it's usually not a good idea to be cautious about getting too specific with tornado threats more than a day or two out. Especially when convection the morning of is going to, potentially, play a major role in late-day redevelopment of storms. There are exceptions to this rule, when the large scale signal is very clear, but this is not one of those cases. It was clear by yesterday than an MCS was going to alter tomorrow's "threat" around peak heating and the signal was there as early as Sunday within some of the models.

Some footage of a tornado I witnessed in Clay County, TX this afternoon, about four miles northwest of Petrolia: Prior to this tornado, there was a narrow stovepipe tornado just north of Dean, TX. The tornadic supercell cycled and either briefly produced two tornadoes at the same time (not completely conclusive) or at the very least two tornadoes, overall. The second was the more dominant one, which bounced up and down for roughly 15-20 minutes,

SRH was more impressive (than forecast for tomorrow) over a large chunk of Louisiana on 4/2/17. That being said, each outbreak is different.

In my mind, storm mergers/interaction are the only thing that lead me to be a bit hesitant to put this in 4/28/14 territory. Good discussion with Lyza on Twitter. Low level shear is a bit more impressive tomorrow, but I’d counter that with somewhat less impressive lapse rates/instability and similar to slightly less deep layer shear. Profiles are also rather saturated this go around. High res guidance show a lot of convection, possibly early in the event, which may be a limiting factor. Numerous tornadoes appear likely, along with at least a few EF-2+. If storms are not too cluttered or in close proximity along the prefrontal trough, then you’ll get several long-track, tornadic supercells. Don’t overlook the warm sector east into Mississippi as well before 00z. Deep layer wind profiles are even more impressive there, suggesting you may see long residence time if you manage to pop a sustained supercell or two. Wind profiles become more meridional farther west into East Texas, but a QLCS tornado threat may still evolve. Fortunately the tornado threat seems limited north of the AR/LA border due to a mass of convection and weak buoyancy. Each event is different. A great parameter space isn’t always a slam-dunk outbreak, but odds favor an outbreak tomorrow for a variety of reasons.

Wind profiles in the vicinity of the risk area strongly favor supercells (WSW to W in the upper levels and S to SE near the surface) and mid-level lapse rates will be quite steep. The biggest issue is boundary layer moisture, as even the most robust HRRR-based forecast soundings show relatively large T/Td spreads from the surface, right up to the mid-levels. Given the wind profiles, last-minute arrival of marginal moisture and lingering SBCIN along the dryline (-100 to -200 J/kg at 00z Thursday), it is likely that any storms that do manage to form will probably remain fairly isolated. The issue is that the instability axis will be relatively narrow and storms will have a very short window to be near-surface based. Otherwise, the supercell/hail threat should predominantly from elevated storms. Forcing and upper level support look good as well, as the shortwave orientation and arrival time bode well for storm initiation shortly after peak heating. While the setup could yield a tornado, if everything goes just right, the main story should be hail. The HREF/HRRR UH tracks are almost certainly indicative of hail, rather than low-level rotation, given such steep mid-level lapse rates and far from ideal boundary layer moisture content.

The LLJ progged by the HRRR also borders on uncharted territory (reference sounding climo for BNA/ILN), solidly 60-80 knots by 18z on the nose of the forecast instability axis. It’s probably a good thing there won’t be much instability to work with...

The issue with a slowing/stalling hurricane is going to be upwelling, plus the water depth gets shallower closer to the coast. Major hurricanes have been known to have sometimes wild fluctuations in intensity (ERC, as one example), but it's hard to see this thing strengthening much, if at all, beyond Wednesday.

It's really hard to put this into context since there hasn't been a storm that has evolved anywhere near what's expected with Florence. You can look at Emily in 1993, as that was a category 3 hurricane that slowly curved out to sea, but came close to an Outer Banks landfall. Flooding would be even worse this time around and over a more populated area, while rainfall amounts would be substantially greater. You can't really make the comparisons to Hugo and Isabel, since those storms tracked steadily northwest after making landfall. Storms like Floyd and Irene were relatively fast movers as well, so it's safe to say that the area hasn't really experienced anything like what's about to come, even in the "best case scenario" of a weakening storm staying just offshore. Even without a direct landfall, most of the Outer Banks are going to be heavily inundated, if not completely underwater due to a prolonged easterly fetch and increasing size of the storm. A stall and/or a drift to the W/SW would cause major to catastrophic impacts to larger population centers, like Wilmington and Jacksonville, and quite possibly Myrtle Beach as well.

Objectively, this was one of the most volatile environments of 2018, it's just interesting how none of the CAMs picked up on convection in that area. Regardless, with the amount of forcing coming in from the west, a rapidly increasing low-level jet and an extreme CAPE/high shear environment, any cell that could intensify and remain isolated was going to have potential to produce significant severe.

Low-level shear may be on the lower end of the spectrum, but with 5000 J/kg MLCAPE, 50-60kts of deep layer shear and >100 m2/s2 0-1km SRH, the downgrade seems a bit odd. It doesn't help that convection allowing models have been inconsistent for the eastern portion of the threat zone, but the potential for intense supercells seems unusually high (given local climatology) for eastern Montana into western North Dakota. I'd think very large to possibly giant hail may be the biggest threat, but even with modest low-level shear, the thermodynamic profile alone coupled with more than adequate deep layer shear suggests an "enhanced" tornado threat.

Looks like a do over for the last North Dakota moderate risk, plus this is a bit farther south. While this has early day convection like that day, today's morning convection is exiting the area along a warm front and should have little to no negative affect on the environment, based on timing and observational tends. Cautiously optimistic that there could be a few tornadoes today and possibly a strong one given the degree of instability coupled with substantial deep layer shear. Storms interacting with the outflow reinforced warm frontal boundary in western to central ND should pose the greatest supercell tornado risk. The threat for a potentially significant MCS increases tonight in the same general area.

Thursday looks interesting in the northern Plains, but can someone else spare me the agony and discuss all the caveats/red flags?

In all honesty, the moderate seemed a bit overdone. It's a very complex setup and it doesn't take much to throw a wrench in higher-end potential, especially with a seasonally anomalous setup so far south. On the bright side, drought areas have and should continue to get drenched with much needed rainfall.

Outflow is surging south across the Texas panhandle and should reach I-40 in the Amarillo vicinity shortly. Would have to guess that most areas east of US-83 are game over for anything significant. It's unclear if the panhandle outflow will be detrimental, period, or if the atmosphere can recover and use the boundary as a focal point late this afternoon.

You do realize tomorrow was upgraded to a moderate risk? Granted it's not looking like an outbreak of discrete storms, but significant severe seems likely given the degree of shear and instability expected.

The low-level jet signal is pretty intense given the latitude this late in the season. Yes, there's some convective feedback, but LLJ is progged to reach 50 knots by 03z (near the solstice, that's only an hour after sunset around here) all the way down into Oklahoma with 50-60+ kts in Kansas. Have to imagine a big wind producing convective system evolves late, but earlier day is more of a question mark. It may be a convective mess with the degree of instability with storms just firing all over the place. Who knows. I can't say I've chased long enough to see a similar setup south of I-70 in late June. It will be interesting, but this year especially has taught me to not get too excited about a forecast two or more days out. The fact alone that we're seeing noteworthy upper level flow this far south for 2-3 days is intriguing. Beyond this weekend, such flow (probably even stronger) looks to return for much of next week from the northern Plains to Upper Midwest/Great Lakes. The severe season is not entirely over... yet...

The season is cooked for anywhere near/south of I-70. Mega death ridge reigns for the next 7-10 days, but the pattern could favor some AOA severe activity from Idaho/Montana/Wyoming into the Dakotas. The week two period favors more Great Lakes troughing, so that would tend to suggest an MCS/derecho pattern from the Upper Midwest into the Lakes vicinity.

There's pretty solid reliability in the end of May. Even in a bad year, odds favor that more than half (sometimes all) of the days should feature at least something worth chasing. This year has been abysmal for tornadoes, but there was the Wyoming event yesterday and several other days have featured supercells and "interesting" storm structure. The window during which one chooses to go on a chasecation does seem a bit risky. If you go before the final week of May, odds increase that there will be several down days and possibly a busted period. Go before May 15th and you're doomed, unless you get really lucky. Wait until June and while the ceiling may be higher for at least one or two higher-end days, there's also increased odds that there will be down time. I guess if tornadoes are all that you're after, it's a bit risky. At least it has been this year and last year as well. Unless we're really moving into a new pattern (I doubt it), late May is king and produces well in the vast majority of years.

Today's northeastern Colorado is a fairly typical 10% day in late May. (Could be big or could be slop) Shear is there and so is moisture, but lingering clouds and concerns about storm mode/mergers suggest this could just as easily flop as perform. With that said, climatologically favored upslope trajectories with ample moisture suggest that there probably will be at least a few tornadoes. Will they be seen or last longer than a couple of minutes? We'll see.