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FXUS63 KLSX 051906 AFDLSX

Area Forecast Discussion National Weather Service Saint Louis MO 206 PM CDT Sun May 5 2024

KEY MESSAGES

- A few more opportunities for thunderstorms and severe weather are coming up. The two main threat periods locally are late Monday night and Wednesday.

- A stretch of cooler and drier weather is expected later this week.

SHORT TERM
(Through Late Monday Night)
Issued at 205 PM CDT Sun May 5 2024

High pressure over the Great Lakes has brought briefly cooler and drier weather into our area today. While larger scale ridging is moving through the Northern Plains, a southern stream shortwave trough is exiting slowly out of the Southern Plains and toward the lower Mississippi Valley today. Showers and thunderstorms have been widespread across Oklahoma and Arkansas, moving into southwest Missouri. Thunderstorms are generally on the leading edge of the broader precipitation area as the low level moisture rotating into the system undercuts some drier air (steeper lapse rates) aloft, but overall instability remains weak so strong or severe storms are not expected. We do expect at least some of this activity to cross through the southern portion of our forecast area this evening and into the overnight, although it will be weakening as it very slowly moves through. It's not clear it will make it all the way to St Louis for example. But at least some remnant clouds will. These clouds and any remnant showers will finally dry up and disperse Monday morning. Expect a significantly warmer day on Monday especially if more sun develops.

A trough moves through the northern Rockies on Monday, nudging the ridge eastward toward the Great Lakes. As it does so, a renewed surge of moisture will make its way north through the Great Plains on Monday just as the trough begins to move out as well. This will set up a renewed round of severe thunderstorms over the central and southern Plains on Monday with the focus for this activity remaining to our west during the day. It is expected that the storms that form in the Plains will develop into a broader convective line and march east during the overnight hours Monday night into early Tuesday morning. This is when we'll see the impacts locally, mainly after midnight Monday night through about dawn on Tuesday. Given the expected evolution, the primary severe weather threat will be damaging winds. However, strong low level shear does exist during this time period so there will be at least some threat for embedded tornadoes in portions of the line which can become oriented perpendicular to the low level shear vectors.
The overall strength of the convective complex is expected to be weakening as it tracks out of western Missouri and through our area, so we're not sure just how strong or widespread the severe weather threats in our area are. This is reflected in the SPC outlook for Monday which decreases the threat level with eastward extent.

Although overall rain amounts Monday night are not expected to be particularly excessive, river levels remain high across the region and the additional rainfall will lead to renewed rises. Additional river flooding may result.

Kimble

LONG TERM
(Tuesday through Next Sunday)
Issued at 205 PM CDT Sun May 5 2024

Monday night's thunderstorm complex will move east Tuesday morning and lay down an effective cold front across southeast Missouri.
Some models indicate renewed convection may form along this front Tuesday afternoon, but this will most likely be outside of our forecast area, focused more in the Ohio River Valley. Winds aloft remain quite strong out of the WSW so storms that form along and ahead of the front will be able to take advantage of strong shear to become organized and pose a threat for mainly large hail and damaging winds. But again, this threat will likely be southeast of our forecast area. Subsidence in the wake of the departed trough should lead to dry conditions after the morning rain. Temperatures are forecast to be even warmer, mostly in the 80s, but it could be cooler if clouds linger through the day.

The next trough organizes over the northern Rockies on Tuesday and moves east on Wednesday. This will result in another surge of moisture northward ahead of it and another round of potential severe thunderstorms. The threat area for this round looks more centered on our forecast area on Wednesday as the timing of the trough aligns well with peak heating. What's unclear at this point is where the remnant frontal boundary will be thus setting the bounds of the warm sector and primary severe threat area. Most likely the warm front will shift north into at least northern Missouri putting the bulk of our area squarely within the warm sector where strong instability could develop in an environment with strong shear as well. Thus thunderstorms that form during the afternoon Wednesday would likely take the form of supercells with large hail, damaging winds, and tornadoes all possible. Some models indicate that more widespread convection will develop late Tuesday night into Wednesday morning as the moisture pushes back to the north. This represents another source of uncertainty for the severe weather threat Wednesday as morning convection (with a hail threat) could disturb the destabilization in the warm sector and reduce the threat later in the afternoon.

As the trough moves east, a cold front will push south behind it, ending the thunderstorm threat Wednesday night. Another secondary front drops down from the north on Thursday reinforcing a drier and cooler air mass for the rest of the week. Models begin to diverge more in the handling of the overall upper air pattern this weekend. The primary differences revolve around another trough dropping south through the western Great Lakes (how strong will it be?) and the remaining piece of the western trough over the southern Rockies. The result is more uncertainty in the temperature forecast this weekend as a deeper trough would result in cooler temperatures while a weaker or more progressive one would open us back up to ridging and warmer temperatures. Either way, though, Gulf of Mexico moisture remains suppressed either to the Gulf Coast or even further south to the Yucatan. So we expect we will have a relatively prolonged break from the severe weather episodes we have been seeing lately.

Kimble

AVIATION
(For the 18z TAFs through 18z Monday Afternoon)
Issued at 1223 PM CDT Sun May 5 2024

Very light winds today with mostly VFR conditions. Main area of concern is rain and lower ceilings moving in from the south. This is most likely to affect the central Missouri terminals this evening and overnight into tomorrow morning, but may impact the St Louis area as well. MVFR is likely, but IFR is also possible.
Conditions improve during the day on Monday.

Kimble

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

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