Sunrise 5:41 AM   Sunset 8:12 PM Moonrise 4:36 PM   Moonset 2:58 AM

  Print   HELP   Reset   Save   Recall  New

FXUS63 KLSX 200844 AFDLSX

Area Forecast Discussion National Weather Service Saint Louis MO 344 AM CDT Mon May 20 2024

KEY MESSAGES

- Isolated to scattered showers and thunderstorms are expected to develop this afternoon, mainly in parts of east central Missouri and south-central Illinois. A few of these storms may be strong to severe with damaging winds and hail up to the size of quarters between 2 and 8 PM.

- The main focus for this forecast package is on convective trends Tuesday through Wednesday. The best chance for strong to severe thunderstorms continues to be Tuesday evening, mainly in parts of central and northeast Missouri and west-central Illinois.
Another round of severe weather is possible Wednesday afternoon in parts of southeast Missouri and southwest Illinois, but is more conditional.

- Summerlike temperatures will persist through Tuesday. Highs on Tuesday afternoon are expected to be in the upper 80s to low 90s and may break the daily record high in St. Louis (93F - 1941).



SHORT TERM
(Through Late Tuesday Afternoon)
Issued at 339 AM CDT Mon May 20 2024

Earlier strong to severe convection in western Missouri has rapidly weakened over the past several hours, with only some isolated to widely scattered showers remaining. These showers may move through parts of central and northeast Missouri as well as west- central Illinois before completely dissipating this morning. Of more interest is for the potential for additional thunderstorms this afternoon. The best chances (30-40%) are in parts of east- central Missouri and south-central Illinois, roughly between 2-8 PM. This is where the combination of increasing diurnal instability, a weak surface convergence zone, and a possible remnant MCV (from ongoing convection in NE OK) overlaps this afternoon. A few of these storms may be strong to severe, with damaging winds and hail up to the size of quarters both possible.
Convective mode will likely be a mixed mode of pulse thunderstorms along with multicellular clusters with the aforementioned MCV potentially aiding in a bit more wind around the 700 hPa and increasing effective shear more into the ~25-30 knot range. The atmosphere should also be at least moderately unstable as there should be partial sunshine due to breaks in the convective debris clouds. The 0Z HRRR has probabilities for at least 1000 J/kg of CAPE in the 70-90% range, but does sharply drop to around 10-20% for at least 2000 J/kg at 2100 UTC. This is quite a bit less instability compared to yesterday, due to a combination of cooler surface temperatures (more cloud cover) as well as weaker midlevel lapse rates. However, the slightly higher deep-layer shear and subtle, but relevant forcing mechanisms (surface convergence zone, MCV) may be able to compensate.

Any thunderstorms should gradually exit parts of west central and south-central Illinois by early this evening, ending any severe threat. The expectation is for mainly dry weather through the remainder of the night with stronger forcing for ascent well to our northwest across the mid-Missouri and Upper Mississippi Valleys. Did keep some slight chance PoPs (~20%) into the early overnight hours in central and northeast Missouri as well as west-central Illinois where there is broad, weak low-level moisture convergence and some diffluent flow aloft.

Tuesday is expected to be dry across the entire area, at least through the afternoon hours. Very warm conditions are expected, with highs reaching into the upper 80s to low 90s. These values were near the 90th percentile of available model guidance due to strong low-level warm air advection and plenty of sunshine. The warmest conditions are likely to be within metro St. Louis where the high may threaten the daily record for the date (93F - 1941).

Gosselin

LONG TERM
(Tuesday Night through Sunday)
Issued at 339 AM CDT Mon May 20 2024

(Tuesday Evening - Wednesday Night)

There has been very little/no change with the expectations for Tuesday evening. This continues to be the period of highest confidence with respect to chances for severe weather.

Convective initiation is expected by mid afternoon along either the synoptic cold front or a prefrontal trough near the Missouri-Kansas border. These storms likely will initially be discrete/supercellular given about a 30-45 degree angle of the shear vector off of the initiating boundary. Given the strong mid/upper level forcing for ascent associated with a negatively-tilted shortwave, storms should quickly transition to a line/QLCS before reaching parts of central and northeast Missouri. The line/QLCS is likely to arrive between 6- 8PM in these locations and move east/southeast in time. The line is likely to weaken as it does move to the east/southeast as our area becomes further and further removed from the stronger forcing for ascent. In addition, instability gradually abates nocturnally and deep-layer shear slackens a bit with southeastward extent. There is uncertainty as to exactly when and how fast this downturn in convective organization occurs, but the best guess is for the best temporal-spatial window for widespread severe weather to be 2300- 0300 UTC in parts of central and northeast Missouri as well as west- central Illinois. Damaging winds should be the main threat, though isolated tornadoes are also possible within any portions of the QLCS that are able to surge out more to the east/northeast. Otherwise, at least a large portion of the line may be close to parallel to the 0- 3km shear vector which is forecast to be around 35 knots.

Convective trends/evolution become more ambiguous heading into Wednesday. At least some thunderstorms should be ongoing across the area Wednesday morning, but given the factors mentioned in the paragraph above, the expectation is for much less in the way of organization by late Tuesday night/Wednesday morning. The surface cold front is expected to lag a bit back to the northwest, with its location likely near the I-44/I-70 corridors in Missouri and Illinois respectively by 1800 UTC. If early morning convection doesn't impact atmospheric recovery for the afternoon, this would put a good portion of southeast Missouri and southwest Illinois in the threat zone for severe weather. The threat however remains conditional, owing to some lingering uncertainty with the exact location of the cold front mid afternoon on Wednesday, along with the degree of instability that will be available for storms.

(Thursday - Sunday)

A brief break in the more active weather pattern is possible on Thursday, though the far southern CWA will at least have a lingering threat for showers and non-severe thunderstorms. This is because the cold front is expected to stall somewhere south of the CWA, but exactly where is unknown. Given the expectation for a well-organized MCS to eventually form though Wednesday night, the effective front likely will be across the mid south. If this indeed occurs, PoPs on Thursday may need to be reduced and shifted further to the south than is currently forecast.

Models are in pretty good agreement that a midlevel shortwave trough will move through the mid south Thursday night and interact with the stalled frontal boundary to yield greater coverage of showers and thunderstorms. Once again, the best chances are in parts of southeast Missouri and southwest Illinois. Similar concerns as Thursday exist for Thursday night though. If the front is far enough south, much of the area may be dry and PoPs (40-60%) may be a bit on the aggressive side of the envelope.

Ensemble spread has decreased heading into the holiday weekend with a better consensus that the mid-Mississippi Valley will be beneath quasi-zonal to southwest flow aloft. None of the 500-hPa height clusters show mid/upper level ridging like was shown yesterday. What does this mean in terms of sensible weather? Basically, it looks like the area will have multiple rounds of showers and thunderstorms possible through the upcoming holiday weekend. Timing exactly when/where is impossible this far out, but if you have outdoor plans, stay tuned for future forecast updates. Daytime temperatures also won't be as anomalously warm due to plenty of clouds/precipitation chances with highs mostly in the low to mid 80s. These values also are a bit skewed toward the higher end of the spectrum, as widespread convection occurring during the day could really limit the temperature climb.

Gosselin

AVIATION
(For the 06z TAFs through 06z Monday Night)
Issued at 1043 PM CDT Sun May 19 2024

Primary concern for the 06Z TAF period will be the potential for scattered showers and thunderstorms, both near the beginning of the period and again during the later afternoon and evening.
Confidence is growing that at least some showers and thunderstorms will reach COU/JEF near 06Z, with gusty winds and visibility reductions possible for a brief period. Some shower activity and perhaps a thunderstorm or two will also be possible at UIN overnight, but are not expected to reach St. Louis until during the day.

After a lull in showers during the mid morning and early afternoon, redeveloping scattered thunderstorms will be possible during the late afternoon and evening. Confidence is low regarding exactly when and where this activity will develop, but there is a chance for this activity at all of our local terminals.

Ceilings are likely to remain mostly VFR, but dips to MVFR will be possible at times as showers/thunderstorms move through the area.

BRC

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

Full Color Weather Map

Link to Loop


Other links: GEOS-West     Contential US     Full GOES-East

St. Louis, MO,