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Severe Threat May 15-16, 2025

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Upgrade indeed.

"
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER MUCH OF
WISCONSIN...

...SUMMARY...
Scattered severe thunderstorms are likely across parts of the
western Great Lakes states and into parts of the Midwest Thursday
afternoon and evening. Very large hail and a few tornadoes may
occur. A more isolated/conditional severe threat will extend
southwestward from western Kentucky into Arkansas late.

...Synopsis...
An upper low will deepen over the northern Plains as a midlevel wave
ejects northeastward toward the upper MS Valley. A surface low will
also deepen over ND/MN, with occlusion. Extending southeast from the
low will be a warm front, which will rapidly jump into northern MN
and WI by late afternoon, and through much of Lower MI during the
evening. Southeast winds near this boundary will bring mid to
perhaps upper 60s F dewpoints northwestward, resulting in areas of
strong instability. Given the fast motion of the shortwave trough
and strongly veering winds in the low-levels, the primary severe
risk area may be relatively narrow.

The main front/dryline will stall near the OH River or Mid MS Valley
and southwestward toward the Ozarks. This corridor, primarily later
in the evening, will likely foster isolated severe storms as well.

Elsewhere, a lingering upper trough will exit the Northeast, and
warm advection on the backside of this feature may support isolated
strong to severe storms from parts of PA into the Mid Atlantic
during the day.

...Upper MS Valley/Great Lakes/Midwest...
Models are in good agreement depicting a strong convective signal
near the MN/WI border by 20Z or so, with increasing coverage points
east through evening. Forecast soundings across WI and vicinity
depict steep lapse rates aloft and hodographs favoring supercells. A
strong tornado cannot be ruled out given the very favorable
thermodynamics, including a slight capping inversion atop the moist
boundary layer in combination with 200-300 m2/s2 ESRH.

Southeast of the Enhanced Risk area into IL and IN, storm coverage
looks to be more isolated. However, any supercells in this area will
have the same significant hail and tornado potential as points
north. This activity would likely wait until late in the day/after
21Z and closer to 00Z when heating is maximized to break the cap.

Nocturnal severe storms will also be likely across much of Lower MI,
northern IN and OH as activity moves in from the west. Supercells
producing large hail again appear likely.

...OH Valley into AR Late...
The aforementioned stalled boundary will be situated beneath
persistent southwest flow at 850 mb Thursday evening and overnight.
Robust moisture/high theta-e will be in place from eastern OK into
the OH Valley, and scattered storms are likely to develop overnight.
This area is a bit more conditional in terms of coverage of severe,
but clearly large hail will be possible. A brief tornado may occur
as well alth"ough hodographs will be primarily straight by this time.
 
US_Highway15 said:
View attachment 41399View attachment 41400
Upgrade indeed.

"
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER MUCH OF
WISCONSIN...

...SUMMARY...
Scattered severe thunderstorms are likely across parts of the
western Great Lakes states and into parts of the Midwest Thursday
afternoon and evening. Very large hail and a few tornadoes may
occur. A more isolated/conditional severe threat will extend
southwestward from western Kentucky into Arkansas late.

...Synopsis...
An upper low will deepen over the northern Plains as a midlevel wave
ejects northeastward toward the upper MS Valley. A surface low will
also deepen over ND/MN, with occlusion. Extending southeast from the
low will be a warm front, which will rapidly jump into northern MN
and WI by late afternoon, and through much of Lower MI during the
evening. Southeast winds near this boundary will bring mid to
perhaps upper 60s F dewpoints northwestward, resulting in areas of
strong instability. Given the fast motion of the shortwave trough
and strongly veering winds in the low-levels, the primary severe
risk area may be relatively narrow.

The main front/dryline will stall near the OH River or Mid MS Valley
and southwestward toward the Ozarks. This corridor, primarily later
in the evening, will likely foster isolated severe storms as well.

Elsewhere, a lingering upper trough will exit the Northeast, and
warm advection on the backside of this feature may support isolated
strong to severe storms from parts of PA into the Mid Atlantic
during the day.

...Upper MS Valley/Great Lakes/Midwest...
Models are in good agreement depicting a strong convective signal
near the MN/WI border by 20Z or so, with increasing coverage points
east through evening. Forecast soundings across WI and vicinity
depict steep lapse rates aloft and hodographs favoring supercells. A
strong tornado cannot be ruled out given the very favorable
thermodynamics, including a slight capping inversion atop the moist
boundary layer in combination with 200-300 m2/s2 ESRH.

Southeast of the Enhanced Risk area into IL and IN, storm coverage
looks to be more isolated. However, any supercells in this area will
have the same significant hail and tornado potential as points
north. This activity would likely wait until late in the day/after
21Z and closer to 00Z when heating is maximized to break the cap.

Nocturnal severe storms will also be likely across much of Lower MI,
northern IN and OH as activity moves in from the west. Supercells
producing large hail again appear likely.

...OH Valley into AR Late...
The aforementioned stalled boundary will be situated beneath
persistent southwest flow at 850 mb Thursday evening and overnight.
Robust moisture/high theta-e will be in place from eastern OK into
the OH Valley, and scattered storms are likely to develop overnight.
This area is a bit more conditional in terms of coverage of severe,
but clearly large hail will be possible. A brief tornado may occur
as well alth"ough hodographs will be primarily straight by this time.
Click to expand...
If models come into any agreement, you would have to think that the ENH gets moved down south into parts of Michigan and Indiana.
 
CODNEXLAB-FORECAST-2025051318-NAMNST-MW-con-1kmhel-47-60-250.gif
CODNEXLAB-FORECAST-2025051318-NAMNST-MW-prec-radar-47-60-250.gif
I really believe that the tornado threat Thursday is gonna be maximized around Wisconsin and portions of northern IN and southern MI. The one saving grace right now is that the latest NAMNST run shows the threat in IN/MI being more linear, while Wisconsin gets a more isolated supercell threat. I honestly do not buy the linear mode that the NAMNST is spewing out to be honest due to the CINH values.
 
US_Highway15 said:
New NAM run shows a very volatile atmosphere near the Northern IN/MI border. Dangerous because the NAM is finally starting to show supercells around this area.
Click to expand...

SHEESH. that's nuts. Does moisture from lake Michigan help erode the cap?

Here's soundings from the bottom two supercells. The one over lake Michigan has no cap, and the one just to the south has significantly more dry air aloft

1747170161882.png1747170144817.png1747170305335.png

Also don't love this sounding I pulled from directly over Chicago. There's that May 3rd '99 OKC analog again. Hopefully storms don't fire any earlier than expected.

1747170507264.png
 
Grand Poo Bah said:
SHEESH. that's nuts. Does moisture from lake Michigan help erode the cap?

Here's soundings from the bottom two supercells. The one over lake Michigan has no cap, and the one just to the south has significantly more dry air aloft

View attachment 41414View attachment 41413View attachment 41415

Also don't love this sounding I pulled from directly over Chicago. There's that May 3rd '99 OKC analog again. Hopefully storms don't fire any earlier than expected.

View attachment 41416
Click to expand...
Just read through ILX's 4 PM ET AFD and they were talking directly about the dry air and how that might keep storms at bay, including the tornado threat (previous 12Z model runs weren't showing wind profiles backing to the SE), but these soundings show the complete opposite, plus the textbook hodographs.

AFD:
"Again though, there are uncertainty
regarding surface winds near the front and the tornado threat
will be higher if they can remain backed (SE). The shear profile
relative to the boundary favor a discrete or semi-discrete
mode, and the dry air may also help keep mode rather discrete."
 
Thursday is concerning because the risk area includes some very large population centers (Chicago being the most obvious, but also Milwaukee, Indianapolis, Detroit, Cincy, Grand Rapids).

You never like to see PDS TOR soundings or intense helicity updrafts in the Chicago metro 2 days out, but the NAM is showing exactly that.
 
Re: dry air, that is definitely my number one concern with Thursday. The NAM/NAM 3 km tend to under-mix in the boundary layer, which often leads to unrealistically high dewpoints and unrealistically low temperatures. The elevated mixed layer/capping preventing initiation until later and sunshine could result in mixing that may lead to unfavorable LCLs/tendency for veered surface winds further south on Thursday.
 
andyhb said:
Re: dry air, that is definitely my number one concern with Thursday. The NAM/NAM 3 km tend to under-mix in the boundary layer, which often leads to unrealistically high dewpoints and unrealistically low temperatures. The elevated mixed layer/capping preventing initiation until later and sunshine could result in mixing that may lead to unfavorable LCLs/tendency for veered surface winds further south on Thursday.
Click to expand...
Is there any particular models that show realistic boundary layer (or dry air) numbers? HRRR, RAP, GFS?
 
US_Highway15 said:
Is there any particular models that show realistic boundary layer (or dry air) numbers? HRRR, RAP, GFS?
Click to expand...
HRRR will tend to be over-mixed (too dry). There are no models that do it perfectly, but you also have to consider the setup. If there is more of a likelihood for extensive sunshine/more mixing/temperatures getting into the upper 80s (like Thursday further south), lean towards the drier models.
 
lake.effect said:
Thursday is concerning because the risk area includes some very large population centers (Chicago being the most obvious, but also Milwaukee, Indianapolis, Detroit, Cincy, Grand Rapids).

You never like to see PDS TOR soundings or intense helicity updrafts in the Chicago metro 2 days out, but the NAM is showing exactly that.
Click to expand...

Very true. 31 million people under the gun for significant severe weather.

1747171754208.png
 
andyhb said:
HRRR will tend to be over-mixed (too dry). There are no models that do it perfectly, but you also have to consider the setup. If there is more of a likelihood for extensive sunshine/more mixing/temperatures getting into the upper 80s (like Thursday further south), lean towards the drier models.
Click to expand...
Yes, but we are now further into the year now, and alr have several areas getting higher temps, my area has highs of 85 on Thursday and I am on the outside edge of the risk, also, do we know if morning convection is going to form, cuz wasn't that one of the fail modes for the 28th event?
 
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