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FXUS63 KLSX 250818 AFDLSX

Area Forecast Discussion National Weather Service Saint Louis MO 318 AM CDT Thu Apr 25 2024

KEY MESSAGES

- A multi-day severe weather period is just beginning across the central United States. While we are confident that severe weather will occur regionally every day from today through Sunday, when and whether this threat will exist in our forecast area is more uncertain. Most days we will be on the periphery of the greater threat area, with the greater threats to the west.

- Sunday evening represents the greatest severe thunderstorm threat for the entirety of our forecast area as a cold front shifts the threat area eastward.



SHORT TERM
(Through Late Friday Afternoon)
Issued at 316 AM CDT Thu Apr 25 2024

While high pressure moves east through southern Canada today turning our winds to the southeast, our eyes begin to look to the west as Gulf of Mexico moisture begins to move north through the Plains.
This is all in response to a broader trough developing across the southwestern US leading to troughing in the lee of the Rockies and a persistent southerly flow east of it pulling in the low level moisture. Thunderstorms today are expected to form along the leading edge of this moisture across Kansas and into southwest Missouri where a broader convective complex is likely to develop during the afternoon and track southeast toward Arkansas. While this does represent the opening round of a multi-day period of severe thunderstorms, we have strong confidence that this threat remains to our southwest today. Low level moisture, and thus the available instability, does not make much progress eastward today, but we are likely to see increased cloud cover blowing off from this activity in a general WNW flow aloft. This will lead to slightly cooler temperatures today especially in central Missouri where clouds will be thickest. It's also in central Missouri where we have the best chance of seeing some rain along the periphery of the broader convective complex. Although the stratiform rain region may clip our area, we expect thunder to be pretty limited as instability decreases rapidly to the east.

While the convective complex drops southeast into Arkansas this evening, the first prominent shortwave trough moves across the central Rockies and into the Plains tonight. In response to this, a southwesterly low level jet will begin to push the moisture northeastward into our area. The core of this moisture surge will be at about 850MB where a warm front serves as a focus for elevated showers and thunderstorms to form and push northeast through much of our area. Guidance suggests this doesn't begin until early Friday morning and only exits our forecast area around midday. Given this scenario the greatest threat from any thunderstorms would be hail since they are being forced by elevated moisture advection. Forecast soundings remain rather modest when it comes to the instability, with around 500-700 MUCAPE to work with. While not a lot, this could also be an underestimation by the courser resolution long range guidance. Even so, the high end potential here should be marginally severe hail.

As the shortwave trough continues to push off to the northeast and the warm front does so as well, we'll open ourselves up fully into the warm sector on Friday. Dewpoints will rise into the 60s setting a base for a moist, unstable air mass that lasts until Sunday. In the wake of the Friday morning trough, we're likely to see this air mass capped by a temperature inversion aloft. This combined with the lack of a clear focusing boundary to help initiate additional thunderstorms will make it difficult to see another round of thunderstorms forming in our area Friday afternoon. Further to the west along the dryline there is a comparatively greater chance for thunderstorm initiation Friday afternoon especially further north closer to the front and surface low. The best chance for severe weather in our area Friday evening will likely be a result of the propagation of dryline storms eastward. Even that will be difficult with storm motions likely more northerly than easterly. That said, if we do get storms in our area they will be in an environment favorable for supercells which would have a risk for large hail, tornadoes, and damaging winds. Confidence in this occurring this far east has decreased relative to prior forecasts and this is reflected in the new Day2 SPC outlook which has shifted the slight risk area westward.

Kimble

LONG TERM
(Friday Night through Wednesday)
Issued at 316 AM CDT Thu Apr 25 2024

Shortwave ridging in the wake of Friday's trough will reinforce the capping inversion across our area. While we will likely see our most extreme levels of surface based CAPE developing as a result of the continued moist air mass and strong heating (warmest day of the forecast), it is less likely that we will be able to realize that potential energy during the day on Saturday. Further west, though, the next trough will be moving across the Rockies and into the Plains setting up much more likely initiation of severe thunderstorms along the dryline in Kansas and Oklahoma and northeastward along the remnant front into Iowa. So once again the focus area for severe thunderstorms will be to our west and northwest on Saturday. That said, the instability and shear environment will be quite strong on Saturday, so any storms which are able to make their way into our area will have a good environment to work with. This is more likely late in the evening into the overnight when storms will have likely formed into clusters and pose a greater risk for damaging winds or QLCS tornadoes.
There's a lot more uncertainty on this happening as it will depend on the evolution of convection forming to our west. If our area does see severe weather Saturday night it will likely be in central or northeast Missouri.

As the trough moves slowly NNE on Sunday it will have the effect of pushing the dryline and cold front further to the east setting the stage for the next round of thunderstorm initiation in the warm sector. This is when the main severe weather threat area shifts into our area representing our greatest overall threat of this entire period. Confidence has grown relative to prior forecasts in the area of thunderstorm initiation being in or near our forecast area likely along a prefrontal trough or remnant convective outflow from Saturday night's storms. With the upper trough axis approaching we'll also have added support aloft and strong shear. The greatest uncertainty will be on the degree of destabilization in the wake of Saturday night's convection, but even that uncertainty is more on whether we will see the more extreme levels available on Saturday.
We are confident we will see at least enough instability for thunderstorms to form once again initially in the form of supercells merging into a broader convective complex with time. All severe weather hazards will be possible including tornadoes, large hail, and damaging winds. In addition there is at least some potential for training storms to produce locally heavy rainfall.

As the trough makes progress eastward it will push a cold front through our area Sunday night into Monday, pushing away the moist and unstable air mass. The air behind the front will not be particularly cold, though, as ridging aloft quickly builds back in behind it. Temperatures next week remain warm, although greater uncertainty exists in the overall upper air flow pattern and when rain chances might return.

Kimble

AVIATION
(For the 06z TAFs through 06z Thursday Night)
Issued at 954 PM CDT Wed Apr 24 2024

VFR flight conditions are expected to prevail at least through early to mid afternoon on Wednesday across the area. Lower clouds will build into central and parts of northeast Missouri late Wednesday afternoon and evening, along with widely scattered showers, and perhaps a thunderstorm or two. Additionally, southeast flow will likely become gusty in central and northeast Missouri during the afternoon and evening. The rest of the area should remain in VFR category through the remainder of the period.

Carney

LSX WATCHES/WARNINGS/ADVISORIES
MO...None.
IL...None.

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