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Severe Weather Threat 4/28-4/30

US_Highway15 said:
Dont think there will be a high risk in the morning day 2, but depending on the morning models, a high risk could be issued for the 1 PM update.
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Agreed. It's a possibility. I'd put it at like a 5%-10% for the first D2 outlook and a 25% for the 2nd. Honestly depends on the models. If the 06z and 12z are similar to the 00z then I can just picture the genuinely terrifying forecast soundings that are found.
 
Schoeppeya said:
HRRR depicting a very high end environment at 0z Monday night. Wowza

ETA:. This is a crazy sounding forecast near the iowa/minnesota border. Cape >3k, 0-3km SRH>500, LCL's under 300, and 3Cape of 189 is nuts.
View attachment 40404
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Another small thing to mention is how this isn’t contaminated. This is definitely the highest level of threat this portion of the country has seen in quite awhile, that being northern IA, MN, WI. This would absolutely be an environment for EF3+ tornadoes.
 
Right move to keep it where it’s at. Need more of a discrete cell look to maybe expand or further upgrade. Still seeing more of that linear punch of winds keeping the convection linear. Models are also still showing some disagreement. However, still a very significant day developing for Monday.
 

Detailed Outlook​

SPC AC 270559
Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 1259 AM CDT Sun Apr 27 2025
Valid 281200Z - 291200Z

THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS FROM CENTRAL IOWA INTO SOUTHEAST MINNESOTA AND WESTERN WISCONSIN​

### SUMMARY
An outbreak of severe weather remains likely across portions of the Upper Midwest, centered on Monday afternoon and evening. Very large hail, strong tornadoes, and damaging winds are expected.

Synopsis​

A strong (~75 knot) mid-level jet streak will advance from the central Plains to the Upper Midwest on Monday. As this occurs, a surface low will deepen across the Upper Midwest with an eastward advancing dryline to its south and a cold front to the west-southwest. A warm front will extend east of this surface low and move rapidly north through the day, spreading mid 60s dewpoints across much of the Upper Midwest.

MN/IA/WI​

Thunderstorms will likely be ongoing at the beginning of the period north of a warm front across Minnesota and Wisconsin amid moderate isentropic ascent. Isolated large hail will be possible from these storms. A surface low will be located somewhere near eastern South Dakota and will start to advance east during the morning. This surface low is expected to deepen somewhat through the day (perhaps to the low 990s mb range) as it moves toward the Twin Cities. There is a consistent signal for thunderstorms to develop along the cold front as it intersects the surface low/dryline in west-central Minnesota Monday afternoon. After some initial hail threat, these storms will likely become quickly linear along the front with an increasing severe wind threat. Strong low-level shear will also favor some line-embedded tornadoes, particularly if there area any areas within the line which become more favorably oriented to the low-level shear vector.
Additional thunderstorms are forecast to develop along the dryline from southern Minnesota to west-central Iowa during the afternoon on Monday. 00Z CAM guidance is not overly bullish with convective development south of the MN/IA border, likely due to relatively weak convergence along the dryline in western Iowa from most high resolution guidance. However, the lack of convection from much of the guidance does not seem realistic considering the overall pattern. ECMWF/GFS/NAM are fairly consistent with a deepening surface low across Minnesota and moderate to strong height falls across the dryline. The southern extent of the previous Day 3 moderate risk corresponds well to the southern extent of the more favorable height falls, and convective precipitation signal from the ECMWF/GFS. Therefore, no adjustments to the southern extent of the moderate risk were deemed necessary. However the moderate risk was expanded slightly westward to account for the expectation for earlier convective initiation (19-21Z) than shown by most guidance. Given very strong effective shear and over 3000 J/kg MLCAPE, expect any supercells which develop to become quickly severe with a threat for all severe weather hazards.
As the surface low across Minnesota deepens the low-level jet is also forecast to strengthen across Iowa, Minnesota and Wisconsin. This will elongate low-level hodographs and support an increasing tornado threat during the evening. STP values of 3 to 5 are common across much of the warm sector ahead of anticipated storms with some guidance indicating values of 7 to 10. Therefore, any mature, discrete supercells which can form and maintain within the open warm sector on Monday late afternoon/evening will pose a threat for strong to potentially intense tornadoes.

Eastern Kansas and northern Missouri to West Texas​

Overall forcing will be weaker farther south along the dryline, but subtle height falls across a mostly uncapped warm sector could result in isolated to scattered supercell development Monday afternoon. A secondary area of higher severe weather probabilities may exist from portions of West Texas into southwest Oklahoma where the nocturnally strengthening low-level jet could support scattered supercell development Monday evening. However, weak height rises cast some doubt on storm coverage during the evening, precluding higher probabilities at this time.
..Bentley.. 04/27/2025
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full discussion
 
Don't buy the CAMS showing little in the way of initiation at all. The nose of the 300mb jet streak is ejecting right over the dryline and warm sector, and is fast moving too. Unrealistic to have nothing shown on the models. They will probably catch up eventually.
 
UK_EF4 said:
Don't buy the CAMS showing little in the way of initiation at all. The nose of the 300mb jet streak is ejecting right over the dryline and warm sector, and is fast moving too. Unrealistic to have nothing shown on the models. They will probably catch up eventually.
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Was gonna say. Literally nothing is showing on HRRR/RRFS in Iowa which is truly bizarre. If anything the NAMNST is the one thats showing the most initiation. Cannot help but think that's waving confidence in terms of mentioning any higher probabilities.
 
Des Moines IA AFD:

Still expecting a potential outbreak of severe storms Monday but
questions remain as to the extent over our area.

Monday is certainly a challenge as for the forecast with some
preferred areas of higher likelihood of convection and some that are
more uncertain. As the sfc low over north central Nebraska deepens
from 06z Monday to far northeast South Dakota through 18z, strong
south southwest flow at H850 will drive moisture into Iowa and
states north. At this time, there is little sfc convergence over our
area. A pre-frontal trough/dryline in eastern Nebraska lags the main
H850 moisture surge at this time, but eventually moves east into
western Iowa around 00z Monday. Meanwhile, a separate prefrontal
trough develops around I35 around 18z and moves to near/east of US63
by 00z. It appears that scattered, very strong convection may
develop along this trough during the mid to late afternoon with
another area of convection near the intersection of the main dryline
and the approaching cold front in far northwestern Iowa by mid to
late afternoon. This area especially in the north, seems to be an
area of concern throughout the afternoon as the instability/shear
parameter space and the most convergence should be supportive of
more widespread, strong to intense convection. This southern edge of
this area is debatable, but appears to be bound by a position near
US20/US30 or somewhere in between. There should be rapid storm
motion and potential for all modes of severe storms in this area. If
the southern portion of the dryline has enough surface convergence
to eventually fire convection, it is more likely to occur from 21z
through 06z as better sfc convergence, higher shear aloft and
increasing low level jet can help to keep the southern portion of
the line going into the mid to late evening hours over southern
Iowa/northern Missouri and portions of far eastern Iowa. Soundings
indicate that the boundary layer in the south along the trailing
dryline will cool enough after 8 pm that most storms will be
elevated with instability lessening the remainder of the evening.
Model forecasts suggest that most of the storms will be out of the
area around midnight to 1 am. The greatest coverage of storms looks
more likely in the north with more scattered nature in the south.
There may be a gap of sorts in south central Iowa, but this remains
uncertain depending on how much of this area can realize surface
convergence once the storms get going. The convective outlook for
Day2 is quite broad and encompassing for these reasons and may still
see some adjustments in the next 24 hours regarding coverage/risk.
The potential for a scattered intense storms in the south still
exists in the late afternoon and early evening, but is more
conditional as it looks right now. Thus the greater uncertainty, but
looking at soundings, it`s hard to not include the potential for
some higher end, strong to intense storms into south
of the US20/US30 corridor in Iowa as well.
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The models do actually have a reason for no initiation, as all of them show a stout inversion just above the 850mb layer.

Although, all models show this inversion eroding away at around 7pm Monday.

More interestingly as to why models hesitate at showing initiation is the PBL wind fields, particularly the surface winds.

Both global and cam models show slightly different but consequently substantial differences in surface wind vectors.

On one side, you have models showing the surface wind vector moving parallel to the LLJ, which creates directional shear, which is not favorable for tornadoes. What this also does is weaken surface convergence along the dryline, alongside denying any chances of confluence banding potential in the OWS.
1745752707336.png
On the other side, you have models showing surface wind vectors oriented more perpendicular to the LLJ, which is significantly more favorable for tornadoes and of course induces surface convergence and increases the chances of potential confluence banding in the OWS.

Which is why the WRF models and the NAM have convective initiation along the dryline, and even model confluence banding which also initiation convection in the OWS.
1745753166046.png
Obviously however, in the bigger picture (synopticly speaking) there’s no reason why convection initiation shouldn’t happen.

Particularly regarding height falls, all models show strong forcing over the area of interest, (the moderate risk which the SPC highlighted) this forcing can be shown below.

The mid level cyclonic vorticity and corresponding geopotential height falls create a density/pressure difference in the horizontal (aka: forcing), which maximizes right over the area of interest.
1745753579609.png1745753616060.png
This example below is a good representation of forcing, and matches perfectly with the graphics right above.
(Ignore the “jargon”) and focus on how the warm moist air advection uplifts the tropopause and the cold dry air on the backside of the Jet streak lowers it. This causes a misalignment in the gradient of pressure and gradient of density of a fluid (in this cause the fluid is the atmosphere) which induces quasi geostrophic accent.

In simple terms, this QG accent is the forcing and is a huge contributor to convective initiation in extra-tropical cyclones.
1745754649756.jpeg
This is why the SPC is calling bull s*&t on the no convection scenario most models are showing, because it’s simply not a synoptically realistic scenario. Obviously this is just one factor and hopefully I explained this well enough so it’s actually helpful to you guys.
 
CheeselandSkies said:
12Z HRRR continues with no dryline convection whatsoever in Iowa or southern MN through 01Z Tuesday.

Dunno if I've ever seen SPC explicitly say "The CAMs suck and we're throwing them all out" in so many words before.
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I mean, neither have I but to be fair, when the vast majority of things say convection should occur but the CAMS arent showing it, I can completely understand.

One thing I am a little worried about is the CAMS actually correctly modeling the environment. If that happened then I wouldn't be shocked to see supercells popping up along the confluence bands and throughout the sector as a whole. Even if we get a linear solution, the parameters from 21z to 03z are enough that strong to intense tornadoes could occur in the line itself.

I'm not buying the no convection solution, but also not buying the liniar one because the capping might make up for the strong forcing. If anything I excpect a discrete mode initially and then it transitions to semi discrete. With a line forming as the cold front catched up to it after the main event.
 
I will say, some of the CAM solutions have fairly veered low-level winds along the dryline which would reduce convergence. The thing is, we aren't usually dependent on dryline convergence to initiate storms in this region the way the Plains are, because setups are more strongly forced and we're further from the EML source region.
 
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