May 17-20 Severe Weather Threats

[Quincy]

As we approach the climatological peak of the severe weather season, the threats for severe storms should continue in various forms through the balance of this week.

Tuesday: The fringe of enhanced northwest flow across the Missouri Valley/Midwest may glance appreciable instability across the Central Plains vicinity. Scattered to numerous strong thunderstorms are anticipated, along with at least isolated severe thunderstorm activity. 

Wednesday: As the upper level flow pattern resets, a belt of enhanced winds should eject from the Northern Rockies toward the central/northern Plains. Only modest instability is expected across the northern tier, as well as over the High Plains. Regardless, a few strong to severe storms appear possible in both regimes. While somewhat greater instability becomes displaced to the east, a more appreciable severe threat may evolve across the eastern fringes of this sub-forum, before moving towards the Ohio/Tennessee valleys.

Thursday/Friday: A more expansive and potentially active severe weather setup could evolve across the Central U.S.. Details may still change, but seasonably strong upper levels winds are progged to eject from the Rockies across the plains and toward the Great Lakes/Ohio Valley by the start of the weekend. While the trajectory of the ejection is not classic for mid/late May, (trough appears more elongated and progressive) it should spawn at least scattered severe thunderstorm activity.

Beyond Friday, there are signs of another pattern reset, which could result in a few days of less active than usual conditions, from this weekend into early next week. 

[MNstorms]

Tomorrow looks like it might have a small window.

[Chinook]

6 hours ago, MNstorms said:

Tomorrow looks like it might have a small window.

I think the SPC could go with marginal or slight in Minnesota tomorrow, as you discussed. As for Thursday evening, there will be excellent severe weather parameters near the low pressure and warm front near Minneapolis and La Crosse WI. I think the SPC's day-2 discussion will be considering an enhanced risk near there.

[Quincy]

A pair of intense bow echoes are moving across central/eastern Kansas tonight. There have already been several measured wind gusts to 75+ MPH. 

[Chinook]

I don't think I've ever used GRLevel2 to make a 2D vertical slice of base velocity quite like this. This is at 0323z when the bowing squall line was at Topeka. There were 75kt winds detected at some low altitudes south of Topeka.

[Quincy]

Today looks interesting across Minnesota, but I’m not sure that the moisture and wind profiles will support anything more than brief/transient supercells. Upper level support is there, but some veering low level flow will likely limit SRH. CAMs show a broken line of marginally organized storms, which seems realistic given the environment.

Thursday has bigger potential. SPC is playing it a bit conservatively for now. The CIPS analogs show a very strong signal for severe thunderstorms around the northern Iowa/southern Minnesota area, which lines up with the jet axis and expected warm front placement. 

[Chinook]

19 hours ago, Quincy said:

Today looks interesting across Minnesota, but I’m not sure that the moisture and wind profiles will support anything more than brief/transient supercells. Upper level support is there, but some veering low level flow will likely limit SRH. CAMs show a broken line of marginally organized storms, which seems realistic given the environment.

Thursday has bigger potential. SPC is playing it a bit conservatively for now. The CIPS analogs show a very strong signal for severe thunderstorms around the northern Iowa/southern Minnesota area, which lines up with the jet axis and expected warm front placement. 

possibly useless fact: CIPS analog #5 (today's 12z run) has 5/31/98, which was a derecho from Wisconsin to Michigan, with severe weather reports in Ohio, Pennsylvania, New York, and Massachusetts on 6/1/98.

[MNstorms]

NAM 3k is going all out. HRRR still mixing dew points like usual. 

 

[Quincy]

The main thing that worries me is a series of perturbations that swing through the Upper Midwest around late morning/midday. That could overturn the environment and disrupt return moisture flow. Hence why some CAMs only manage to initiate 1-2 storms right before 00z.

With that said, the model, ensemble and analog consensus continues to show a potentially volatile environment along the warm front near MSP toward southwestern Wisconsin by peak heating. There are even some signs that a broad area of conditionally favorable instability/shear could evolve over much of Iowa and southern Minnesota across the warm sector by 00z. This is why some CAMs are rather aggressive with a broken line of supercells erupting by early evening near the IA/MN border. 

[Quincy]

SPC maintains hail/wind-driven enhanced risk for northeastern Iowa into southeastern Minnesota and west-central Wisconsin. They did bump tornado probabilities back down to 5%.

With that said, morning trends are favorable if you want severe thunderstorms. CAMs show less early day convection disrupting the environment near a warm front in southern Minnesota. 14z observations show dew points already in the mid to upper 60s across southwestern Iowa. SPC mentioned possible moisture field disruptions from an MCV over Missouri, but it looks like moisture recovery is well underway.

Confidence is highest in convective initiation by mid to late afternoon around south-central/southeastern Minnesota. Initial storm mode should be discrete or at least semi-discrete. Storms that track near the warm front, especially as the low level jet strengthens this evening, may pose the greatest tornado threat. 

[MNstorms]

 Probability of Watch Issuance...95 percent

   SUMMARY...Supercells are expected to develop this afternoon along a
   warm front near the Iowa/Minnesota border. These storms would be
   capable of all severe hazards, including tornadoes and very large
   hail. A tornado watch is likely within the next 2 hours.

   DISCUSSION...A warm front continues to lift northward near the
   Iowa/Minnesota border. Dewpoints near the boundary are generally in
   the low 60s F. A narrow corridor of low/mid 60s F dewpoints exists
   in southwestern Iowa with dewpoints in parts of central/eastern Iowa
   mixing out into the upper 50s F. With an MCV currently in
   southern/central Missouri, the degree and quality of moisture return
   is uncertain with eastward extent. The KOAX/KFSD VWP have sampled
   the 35-40 kts 850 mb winds that are helping to increase low-level
   shear in the vicinity of the warm front. Objective mesoanalysis is
   showing near 400 m2/s2 effective SRH right along the boundary in
   northwest Iowa. Storms that form along and interact favorably with
   the boundary will be capable of all severe hazards, including
   tornadoes and very large hail. Should any storms move north of the
   boundary, the tornado threat would be less, but large/very-large
   hail would remain possible. Though low-level shear/SRH would suggest
   the potential for a strong tornado, the continued decrease in
   dewpoints within western/central Iowa makes this a low-probability
   scenario. A tornado watch is likely within the next 2 hours.

[Quincy]

Even though moisture is mixing out across parts of Iowa, moisture pooling along the warm front should allow for a narrow corridor of favorable low-level moisture. Most prominently in southeastern Minnesota. Keep in mind that the warm front will probably still nudge slightly farther north.

Storm motion should allow for any discrete cells to stay near the warm front, at least early in the convective cycle. Given the orientation of the moisture, parallel to expected storm motion, there could be a scenario where one or two supercells can persist for a couple of hours, before storm interactions and/or nocturnal stabilization kicks in. 

[MNstorms]

I would think the watch would come any minute now with cells popping and cumulus clouds for awhile now.

[MNstorms]

And there it is. 4th tornado watch is the 2nd most since at least 2006 for me and it's only May. 

[Quincy]

Current storms look to be just north of the warm front. Large hodographs, but better low level thermodynamics are close to the IA/MN border. 

 

[MNstorms]

I just had nickel size hail.

[MNstorms]

oof

[Chinook]

multiple areas that could be tornadoes near western Saint Louis

edit: there may be tornado debris at Ladue/ Kirkwood? I have not heard any spotter reports.

[Calderon]

6 minutes ago, Chinook said:

multiple areas that could be tornadoes near western Saint Louis

edit: there may be tornado debris at Ladue/ Kirkwood? I have not heard any spotter reports.

Radar confirmed in Ladue. A lot, and I mean A LOT, of very large, expensive homes there and so many trees. 

[Chinook]

check out this tweet if you would like to see the full loop of this newly-released radar data from 2013

[Powerball]

 

[Powerball]

59 minutes ago, Powerball said:

 

Welp, that's looking like a big fat fail right now...

[Quincy]

Glad I didn’t second guess my decision to chase up north the past few days. Just not sure if there’s going to be anything worth chasing tomorrow. 

[Powerball]

Models continue to struggle with convection this morning, as the radar remains quiet this morning acroas Southern OK and North TX.

This afternoon could get interesting. The same models have been indicating a line of storms developing mid/late afternoon along the front as it eases SE. Most of them hold off on organization until just SE of DFW, although initiation slightly earlier than projected would make a big difference for impacts in DFW proper. This is certainly a plausible solution as the cap should be non-existent by midday.

If that happens, soundings are certainly supportive of damaging downburst winds and flash flooding.