Mid to Late May 2019 Severe Threats

[Quincy]

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.

[JMT417]

A little concerning to me is the SPC already hinting at upgrade to the Day 8 in later forecasts.

[OUGrad05]

13 minutes ago, JMT417 said:

A little concerning to me is the SPC already hinting at upgrade to the Day 8 in later forecasts.

It's certainly interesting D4-D8 that's for sure.  I'm not sure what to make of the D8 language.  Obviously potential is there, but we've all seen systems just go to crap and sometimes evaporate that far out.  This "feels" different given the magnitude of the data supporting the current forecast and wording.  

Right now it's one day at a time for me, I'm not really analyzing models run-by-run at this juncture and don't plan to until we're within 3 or so days of each respective day on the outlook.  Just simply too much movement, too much possibility for significant change that far out.  

I will say I've not seen a pattern like this and models falling in line like this for so many repeated days in a long time, if ever.  A big part of me says the system looks so good right now its bound to downtrend from here.  

Soundings for Mon/Tues are just nuts.

[nrgjeff]

I figure 3-4 chase days during the 4-9 day period. 1-2 days could be broad easy forecasts. 1-2 days may be chase target picker days. Too early to nail down details but here is some day-by-day.

Friday: Looks discrete but could be a bunch of high plains hailers. If 700 mb cools enough and surface winds back enough (both correlated with short-wave) more interesting.

Saturday: Could be sloppy. However overnight mid-range models now favor redevelopment in addition to that slop.

Sunday: Mid Mississippi Valley, nah. Take a rest day.

Monday: Risk of lots of WAA stuff. On the other hand the ceiling could be high. Upper levels WSW instead of SSW. May be a chase day.

Tue/Wed: I doubt both go. However more shortwaves are forecast to eject out. One of those days could be another chase day.

Then the following weekend and perhaps well into the next work week still look active. If SW flow lifts out, I figure the North would offer photogenic less crowded season.

[DanLarsen34]

I’ve been following severe weather quite close since 2012. This sequence feels different. 

I’ve seen the more reserved people I follow pointing out the potential ceiling of this, and going as far to say it could be May ‘03 or ‘08 level.

I hope everyone who chases has great success in the field in coming days. Should be ample opportunities! 

Let’s just hope these tornadoes stay out in the fields of plains away from people.

[weatherextreme]

Reed has commented on this 

 

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Reed Timmer‏Verified account @ReedTimmerAccu May 12

 

SEVERE WEATHER: the end of next week could be the start of one of the more active periods this generation of storm chasers will remember! Reminds me of early May 2003 and late season 2004 with day after day of tornado potential, ski-jump upper-level setups! Starting next Friday

 

[jojo762]

12Z Euro is pretty meh verbatim compared to other runs. Likewise, can't get too bogged down in run-to-run details yet...

[pbrussell]

4 hours ago, OUGrad05 said:

It's certainly interesting D4-D8 that's for sure.  I'm not sure what to make of the D8 language.  Obviously potential is there, but we've all seen systems just go to crap and sometimes evaporate that far out.  This "feels" different given the magnitude of the data supporting the current forecast and wording.  

Right now it's one day at a time for me, I'm not really analyzing models run-by-run at this juncture and don't plan to until we're within 3 or so days of each respective day on the outlook.  Just simply too much movement, too much possibility for significant change that far out.  

I will say I've not seen a pattern like this and models falling in line like this for so many repeated days in a long time, if ever.  A big part of me says the system looks so good right now its bound to downtrend from here.  

Soundings for Mon/Tues are just nuts.

It’s a plains setup, so it will downtrend. 

[MNstorms]

These storms also increase the threat of flooding in areas that are already too wet.

[Weatherdemon]

1 minute ago, Minnesota_storms said:

These storms also increase the threat of flooding in areas that are already too wet.

Certainly not good for NE OK.

Things are under control right now but could quickly escalate if the expected rainfall is realized over the next 6 days.

[Quincy]

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:

1. May 22-29, 2008 – 23 tornadoes/day
2. May 18-25, 2010 – 17 tornadoes/day
3. May 23-28, 2015 – 16 tornadoes/day
4. May 16-30, 2004 – 14 tornadoes/day
5. 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.

[Araqiel]

^That Hudson Bay vortex just will not die will it? Feels like it's been there more often than not for going on two years now.

[WhiteoutWX]

GFS is really trending in the wrong direction for the Fri-Mon period. Went from good, broad WSW flow mostly perpendicular to the dryline over good chase terrain with (implied) discrete activity to now showing a total mess. Fri is a late show after dark, Saturday the wave is poorly timed and plagued by tons of early morning convection as is Monday. Best environments both those days now over eastern OK, Arkansas, or Texas. Really disappointing overall but also all too familiar with how the previous few years have gone. Could still shift back but the trends are concerning is all I’m saying. 

[nrgjeff]

I am about to postpone a chase trip originally planned to start shortly. Previous post explains the issues. Friday the ECMWF does not even have enough moisture. GFS is amped, shocker there. We'll see what the 12Z NAM does when the hi-res goes out to 60 hr. My guess, mainly hailers. 

Quincy is right; there will be tornadoes. However I'm really not interested in the crowds, the MCSs, JIT moisture (Mon) and rising heights midweek.

As for the Hudson Bay low, it becomes a blessing in disguise late May. As Quincy notes it helps to keep the West trough in place. Some NWP also builds an AK ridge in the 11-15 day. Such a blocky pattern could maintain a West trough and SER. How about chase later, farther north, away from crowds, and more photogenic cells?

[cheese007]

When was the last time a promising setup *didn't* fall apart last minute in the plains?

[CheeselandSkies]

DDC/Chapman in 2016, maybe? And even those days were kind of subtle in the medium to long range. My eyes were on that Thursday when the main trough was progged to finally eject into the warm sector. However (surprise surprise) it turned into a VBV-plagued convective crapfest.

[cheese007]

Giant sig severe contour for D3. Never seen on that big 

[nrgjeff]

The DDC Chapman year 2016 was the last trough that worked out. We went days early, anticipating some days before the days. That year MCSs were not shown. Instead the question was initiation under a warm EML. I would much rather have those questions. Pattern recognition works better when mesoscale does not totally wipe out the synoptic setup.

A few days next week remind me of 2016 in between waves with neutral heights. However in 2016 they were not sandwiched between crashing fronts. 2016 showed a good trough early week DDC and then the big one Thursday. Got Woodward the day before DDC. Chapman was on a slightly rising heights day, where the outflow boundary said, we gonna play anyway. Of course the big one Thursday busted. Still the week was a huge success. Some folks got the Spearman tornado the day before Woodward, and a Leoti tornado the day before that.

Best to chase multiple days to increase odds. Too much can go wrong on a single day. 2016 produced Sat-Wed. 5 days in a row! So the main event busted like a 2-round knockout, lol! Next week one should probably be out there even for the crap days. I just can't motivate myself with two MCS days.

[Chinook]

Interesting note: even the 12km NAM (12z, today) has seemingly discrete cells near North Platte NE, on Friday, along with the obvious chance for real supercells there. The 12km models aren't too great at simulating discrete cells.

[cheese007]

D3 enhanced risk with sig hatching including portions of DFW for Saturday

EDIT: Enhanced countour for the 20th including OKC as well

[CheeselandSkies]

4 hours ago, cheese007 said:

D3 enhanced risk with sig hatching including portions of DFW for Saturday

EDIT: Enhanced countour for the 20th including OKC as well

It's almost all for MCS junk.

[SluggerWx]

Interesting analog for W. Kansas near Garden City tomorrow evening (5/3/99 - OKC).

[CheeselandSkies]

If a 5/3/99-like event occurred every time it popped up as an analog on a forecast sounding, every human settlement in Oklahoma and Kansas would have been leveled several times over by now.

Interesting analog for W. Kansas near Garden City tomorrow evening (5/3/99 - OKC).

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[WhiteoutWX]

I think Monday *may* still hold promise across portions of Oklahoma, if convection doesn't get too extensive south of the warm front (big if at this point). Profiles still look pretty good on the GFS along and south of the front with a lot of low-level turning and pretty good clockwise curvature above that. Even the Euro shows a lot of shear with enough instability Monday afternoon to make me think a substantial tornado threat could exist if a few storms can remain discrete. It's very conditional but I wouldn't completely give up on it yet.

Both models show what looks like a pretty major flood threat with tons of QPF over areas that have seen as much as 10 inches of rain in the past 30 days or so. 

[weatherextreme]

The resent update from the SPC upgraded Tornado probabilities for tomorrow 5/17.

 

Went from 5% to

 

MAXIMUM RISK BY HAZARD...
   Tornado:  10% SIG - Enhanced
   Wind:     15%     - Slight
   Hail:     30% SIG - Enhanced

 

SUMMARY...
   Severe storms with very large hail, tornadoes (a few of which could
   be strong) and damaging wind are expected, mainly across parts of
   the Plains from Nebraska south into west Texas. A few severe storms
   with large hail and gusty winds are also possible across southern
   South Dakota, as well as across parts of Ohio through the Middle

 

 

Potential exists for a significant severe event across a portion of
   NE, and a moderate risk will be considered in day 1 updates.

[jojo762]

Still considering anywhere from Nebraska to SW KS for tomorrow. Would prefer SW KS for driving/logistical reasons, but Nebraska looks like the better call. Possibly somewhere near North Platte. Environment near there looks like it should be pretty epic from 22Z to 02Z tomorrow. Main concerns (as has been discussed) will be storm motions oriented in a way that will cause storms to move over the warm-front into a far less favorable environment, in addition to the potential for many storms to form and cause deleterious storm-scale impacts. SW KS has a much higher, albeit conditional, chance at seeing isolated supercells. Complicating factor here appears to be degree of forecast CINh, everything aside from CINh is great, but the cap may ultimately be what does W KS in tomorrow. There may be a relatively small window tomorrow between 22-01Z were CINh is insufficient enough to cause storms to struggle, but latest renditions of the RAP, NAM, et. al, suggest that CINh will really explode shortly after dark,  muting tornado chances. 

12Z Hi-Res CAMs were largely split on what will happen south of the Nebraska/Kansas border and into the TX PH, but fairly broad consensus is that Nebraska will go off. 

[lokee]

nam 3k 6z for 3pm for 5/18

[WhiteoutWX]

Just now, lokee said:

nam 3k 6z for 3pm for 5/18

Unfortunately wind profiles are kinda crap that far west by Saturday afternoon with the 850 winds weakening and veering. Even if instability was able to recover and storms redevelop tomorrow afternoon, don’t expect much in terms of a tornado threat. 

[lokee]

1 hour ago, WhiteoutWX said:

Unfortunately wind profiles are kinda crap that far west by Saturday afternoon with the 850 winds weakening and veering. Even if instability was able to recover and storms redevelop tomorrow afternoon, don’t expect much in terms of a tornado threat. 

I kinda figured they would be hailers. 

[OUGrad05]

Sights turn to Monday/Tues and maybe Thurs at this point.  Watching another system crash and burn on itself is pretty frustrating.  The plus side is we don't really need a major tornado running through the OKC, Tulsa or DFW metros, a few days ago it looked like a couple of these days would be high-end and have major metro areas in the risk zone.  

Monday still holds potential to be high end but the details won't be clear until late Sunday or early Monday morning IMO.