Sunrise 6:08 AM   Sunset 8:03 PM Moonrise 1:42 AM   Moonset 11:33 AM

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FXUS63 KLSX 011947 AFDLSX

Area Forecast Discussion National Weather Service Saint Louis MO 247 PM CDT Wed May 1 2024

KEY MESSAGES

- An active pattern will promote multiple chances for showers and thunderstorms throughout the next week, though a majority of this time will be dry.

- The threat for strong to marginally-severe thunderstorms remains in place tomorrow, and is expanded a bit further east compared to earlier thinking. Confidence remains low (less than 30%) that any severe weather occurs, but large hail to 1 inch and damaging wind to 60mph would be the threats if storms can become strong enough.

SHORT TERM
(Through Late Thursday Night)
Issued at 246 PM CDT Wed May 1 2024

A weak cold front associated with a shortwave aloft dropped south this morning across the region, but with little in the way of cold air aloft, a wind shift was the only sensible weather associated with it. The region is more broadly situated under weak ridging between two waves, one to our northwest and one to our southeast. The front will begin to inch northward late this afternoon as the shortwave exits and a surface low in the central Plains slowly tracks northeast. While surface-based instability is currently in the 1500-2000 J/kg range at least, convergence along the front is meager at best. With little in the way of upper- level lift or forcing to support deeper, more organized convection, weak isolated to widely-scattered thunderstorms are still forecast this afternoon and early evening.

The front advances north with greater speed overnight when the low- level jet intensifies, but by that point all indications are that the jet (and any low-level forcing for thunderstorms to persist)
shunts far north of the region into the Upper Mississippi Valley. As such, while a rogue thunderstorm or two cannot be ruled out overnight in northern Missouri, the region will stay almost completely dry. 850mb/925mb warm air advection intensifies overnight in northern Missouri, but we're unable to realize much of that warmth at the surface. By the time we can mix more deeply on Friday, the warm advection becomes far more muted. While southerly surface winds and some degree of insolation will help keep temperatures above normal (mid-70s to mid-80s from northwest to southeast), more extreme values will stay out of our reach.

Convection overnight in the central Plains will eject into northwest Missouri, gradually tracking east and weakening by it arrives in our northern forecast area around sunrise. How widespread any convective debris will be may have some impact on our instability later in the afternoon. Nevertheless, instability redevelops ahead of a slowly- approaching, weak surface cold front from the west. Aloft, a modest shortwave embedded in the upper-level ridge will also drift into the Mid-Mississippi Valley during the late morning. With some degree of mid-level ascent, albeit weak, in place, a more tangible threat for strong to marginally-severe thunderstorms across most of the forecast area during the afternoon and very early evening. Barring a robust cold pool from the morning convection eliminating instability, which is highly unlikely (less than 10% chance), MLCAPE values are forecast to jump into the 750-1250 J/kg range across the region. Particular focus for the most appreciable uncapped instability is in central/northeast Missouri and west-central Illinois, but areas as far east as the St. Louis metropolitan area cannot be ruled out for a stronger thunderstorm. The one noteworthy limit to the severe weather potential is a lack of deep-layer shear.
If convection manages to sustain itself, large hail to 1 inch and damaging wind to 60mph would be the predominant threats. By sunset, instability will wane and without a low-level jet to promote continued forcing, thunderstorm strength will drop considerably.
High rain chances (upwards of 80%) continue overnight, but these may be a bit too high given the very weak forcing available.

MRB

LONG TERM
(Friday through Next Wednesday)
Issued at 246 PM CDT Wed May 1 2024

The weak cold front barely makes it through the forecast area on Friday, and the continued unimpressive forcing combined with gradual height rises aloft will allow only weak showers and isolated thunderstorms to develop early Friday. The more noteworthy impact from the boundary will be the relatively-cooler (but still near- normal) air it brings in its wake. Cool easterly winds usher in enough dry air to keep most of the region rain-free from Friday afternoon to Saturday morning before another wave glances the region. This is the first of a series of weak waves that threaten the region, with a cutoff low stalled over the northern High Plains and ridging over the southeast CONUS promoting generally southwest flow aloft. With subtle timing and amplitude differences leading to large impacts on sensible weather, there's not a lot of confidence in the exact timing of any rain chances from Friday through the weekend. However, the forcing looks consistently weak and transient, and the region can expect plenty of dry time until early next week.
Any rain that does fall will not have immediate impacts on the rivers at this stage, though heavier rain across the Missouri River Basin may eventually lead to secondary rises.

By Sunday, global-scale ensembles converge on a trough deepening once more in the Intermountain West, promoting broad, diffluent southwest flow aloft and setting up another potentially-active weather pattern in the central CONUS. At the same time, more pronounced southerly flow in the boundary layer and resultant warm air advection will bolster temperatures back into the low/mid-80s yet again across most of the region through early next week. Exactly how warm we get will come down to rain timing and cloud cover, which isn't all that certain at this point. While the synoptic-scale pattern will be more favorable for stronger thunderstorms to develop early next week, the devil is always in the details and there are far too many differences in the ensemble guidance to be confident in much beyond rain chances at that stage.

MRB

AVIATION
(For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1215 PM CDT Wed May 1 2024

VFR conditions will prevail at the terminals through Thursday afternoon. A weak cold front that passed through the region this morning will lift back north gradually this afternoon and evening, sending the current northerly winds back out of the south.
Convergence along the front is very weak, and despite sufficient instability for convection, this coupled with a lack of upper- level forcing will result in isolated to widely- scattered storms at best. There is little confidence in direct terminal impact (even "vicinity" wording") and as such thunder was kept out of the TAFs.

There will be another threat for showers and thunderstorms tomorrow morning and afternoon from west to east, but the threat is outside of the current TAF period (except KSTL, where VCSH was added at the very end of the TAF to suggest that potential).

MRB

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

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