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FXUS63 KLSX 021028 AFDLSX

Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 528 AM CDT Sun Jun 2 2024

KEY MESSAGES

- Dense fog will continue to threaten parts of northeast and central Missouri, as well as west-central Illinois, through 10am. A Dense Fog Advisory remains in effect until then.

- After a low (15-30%) chance for showers and thunderstorms across northern Missouri today, a more widespread threat for scattered thunderstorms materializes Monday. A few of these thunderstorms could produce large hail and damaging wind, but most will be sub- severe.

- A pattern change still looks all but certain starting late Tuesday that will usher in somewhat less-active weather by late in the week.

SHORT TERM
(Through Late Monday Afternoon)
Issued at 302 AM CDT Sun Jun 2 2024

Weak high surface pressure is settled across the Upper Mississippi Valley in the wake of a slow-moving surface low and attendant weak mid-level wave. In between ongoing low stratus across most of Illinois and incoming cirrus from remnant convection across the Plains, clear skies and light winds allowed for fog to develop in parts of central/northeast Missouri and far west-central Illinois overnight. This threat for fog, which will be dense in spots, will persist until diurnal heating can allow for humidity to decrease sufficiently. A Dense Fog Advisory will be in effect until 10am, but given we are approaching the summer solstice and the sun angle is high, fog will likely dissipate before that. As the day progresses, high-resolution guidance hints at an MCV or remnant outflow boundary from the convection currently in the Plains drifting through northern Missouri. Provided instability can recover this afternoon with warmer temperatures returning, and the low-level lift is sufficient, isolated to widely-scattered showers and thunderstorms are possible in that area. Other than heavy downpours and a few lightning strikes, no impacts are expected from the convection.

Nocturnal convection in the central Plains will enter western Missouri overnight Sunday into Monday morning, but all indications suggest that this activity will weaken as it approaches us and interacts with more stable air. Anything more impressive will more likely follow along the nose of the low-level jet into Iowa. That said, warm and humid southwest low-level flow will allow for instability to build through the morning on Monday. With the possibility of an outflow boundary existing in the region from the earlier thunderstorms and most deterministic guidance developing a weak mid-level shortwave in the quasi-zonal flow aloft, the potential for at least scattered thunderstorms exists for much of Monday. During peak heating, when instability will maximize, a few thunderstorms may strengthen and become marginally-severe. While deep-layer shear is uniformly unimpressive (15-25kts) among deterministic guidance, weakly-capped MLCAPE values of 1500-2000 J/kg in this shear parameter space would threaten a few strong thunderstorms with large hail and damaging wind across the region.
If there is a remnant boundary in the region to focus strong convection, the severe weather threat may be locally-higher around that feature. That said, I have no confidence in that feature existing, much less where it would be. The SPC Day 2 Convective Outlook was expanded north across the forecast area, and we've begun messaging the potential for isolated severe thunderstorms on Monday.

MRB

LONG TERM
(Monday Night through Saturday)
Issued at 302 AM CDT Sun Jun 2 2024

By late Monday night, a shortwave will dig across the northern Rockies and track east across the northern CONUS. This wave, and the surface low/attending cold front that it develops, will put the wheels in motion for a change in the upper-level flow pattern and sensible weather. Before that, however, another warm and humid day will be in store for the Mid-Mississippi Valley on Tuesday ahead of the cold front. While capping appears stronger on Tuesday and deep- layer shear is a bit weaker, we may see a similar setup as we do on Monday. There are some deterministic models that even develop a shortwave ahead of the more impressive mid-level wave Tuesday afternoon, which would provide synoptic-scale ascent for stronger thunderstorms. That said, there is no concrete signal that all parameters overlap for severe thunderstorms just yet. The SPC Day 3 Convective Outlook keeps any threat for severe weather to the northwest of the forecast area, but that may change as time goes on.

The first hint of the pattern change arrives late Tuesday night into Wednesday, when ensemble and deterministic guidance generally agree the main cold front traverses the region. This will be our best potential for widespread rain in the forecast period, but the time at which it's currently forecast to pass will likely preclude it from a strong to severe thunderstorm risk. That said, the front won't have much cold air behind it with most guidance suggesting 850mb warm-air advection persisting behind it. However, it will scour out low-level moisture and lead to more comfortable conditions across the region. The ridge-trough pattern that establishes aloft would be supportive of another shortwave and weak cold front dropping south amidst the cyclonic flow, bringing a low threat for rain Thursday. While some deterministic guidance suggests this, the NBM and most ensembles do not. Rain chances within similar cyclonic flow patterns are often tricky to catch this far in advance amidst global-scale guidance, so I wouldn't count on a completely dry Thursday just yet. That caveat goes on through Friday as well: amidst cyclonic flow aloft, we may see diurnal showers along and east of the Mississippi River that are not currently reflected in the forecast. However, this hinges greatly on the uncertain location and amplitude of the eastern trough, which led to leaving the NBM PoPs alone through the extended period.

High temperatures will gradually cool day-to-day with northwest flow remaining established aloft into the weekend, but exactly how cool we get is also tied to how the upper-level flow pattern evolves. The current forecast does not cool daily highs notably until Saturday, but some ensemble solutions suggest we may see cooler weather as early as Thursday. Low (15-35%) rain chances return to the region by Saturday, owing to a potential weakening in the western upper-level ridge and longwave troughing taking hold of the CONUS. If this occurs, the near-zonal flow aloft would threaten a low risk for precipitation. The preponderance of guidance show a more amplified ridge-trough pattern that keeps the region dry. Again, the NBM PoPs were used in this late stage of the forecast, which adequately hints at the first scenario described.

MRB

AVIATION
(For the 12z TAFs through 12z Monday Morning)
Issued at 522 AM CDT Sun Jun 2 2024

While low stratus managed to avoid the TAF sites overnight, fog varying from MVFR to LIFR is impacting terminals in the river valleys and KUIN at this hour. Meanwhile, cirrus from convection currently firing in the central Plains is overhead to varying degrees across the region. As the sun rises this morning, fog will be quick to dissipate and VFR conditions will prevail at the terminals through the TAF period. There is a low threat for thunderstorms (less than 20% chance) at the central Missouri terminals and KUIN, but the high uncertainty surrounding thunderstorm development and placement precluded mention of any precipitation from the official forecast. Weak, variable winds amidst a weak surface high will become southerly from west to east this afternoon and evening. Thunderstorm chances in the region increase late Monday morning at the earliest, beyond the valid TAF period at all sites except for KSTL.

MRB

LSX WATCHES/WARNINGS/ADVISORIES
MO...Dense Fog Advisory until 10 AM CDT this morning for Audrain MO- Knox MO-Lewis MO-Marion MO-Monroe MO-Pike MO-Ralls MO- Shelby MO.

IL...Dense Fog Advisory until 10 AM CDT this morning for Adams IL- Pike IL.

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