


Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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121 FXUS63 KLSX 032317 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 617 PM CDT Thu Apr 3 2025 .KEY MESSAGES... - A Flood Watch remains in effect through Saturday night due to multiple rounds of rain and thunderstorms. The heaviest rainfall is expected Friday afternoon into Friday night. - Severe thunderstorms are possible Friday afternoon and evening, with large hail the primary threat. && .SHORT TERM... (Through Late Friday Night) Issued at 225 PM CDT Thu Apr 3 2025 Widespread rain along with a few embedded thunderstorms has impacted much of the region this afternoon, with exceptions of parts of northeast Missouri and west-central Illinois. This round of rainfall is mainly due to a midlevel disturbance moving through along with enhanced divergence aloft beneath the right-entrance region of a 130+ knot jet streak. The rain should become less widespread later on this evening as the stronger mid/upper level forcing for ascent departs to the northeast. However, did keep some low PoPs (20-40%) going most of the night in parts of southeast Missouri and southwest Illinois closer to the stalled surface frontal boundary. By late Friday morning, another midlevel impulse is expected to move quickly northeast across the mid-Mississippi Valley. More widespread showers and a few rumbles of thunder are forecast. Low-level moisture convergence intensifies by early afternoon, so we are expecting even greater coverage of rain showers along with a better chance of thunderstorms. This activity should be all elevated, but there is enough effective shear for elevated supercells which would be capable of producing large hail. The surface quasi-stationary front is expected to buckle at least slightly northward into the early evening hours. How far north it gets remains a question mark, but may at least approach our far southeast Missouri counties (Reynolds/Iron/Madison). Some of the extended HRRR/RAP and even the new 1200 UTC NAM backed off earlier runs which had the boundary quite a bit further to the northwest. With convection along/north of the boundary and a lack of strong surface cyclogenesis, think that the boundary should stay to our south and east. This is something we will continue to keep a close eye on however as any shift to the northwest would introduce the threat for surface-based convection in parts of southeast Missouri. Surface-based convection would have the threat for damaging winds and tornadoes, especially given very strong low-level shear and helicity in close proximity to the front itself. In terms of the large hail threat, that should be confined to east central and southeast Missouri as well as southwest Illinois. In addition to the severe threat, the best flash threat looks to be Friday evening into the early overnight hours. This is when the low- level moisture convergence maximizes and showers and elevated convection may tend to train over the same areas. The atmosphere will also be primed for efficient, warm rainfall processes due to deep warm cloud depths, tall/thin elevated instability, and anomalous precipitable water values (>1.50" - near 99th percentile of climatology). The axis of this period of heaviest rainfall rates is most likely along and just southeast of I-44 in Missouri and I-55 in Illinois. Rainfall amounts of 2-3+" are likely in this time period, and given fairly saturated soils, this is certainly the period of highest concern regarding flash flooding. Gosselin && .LONG TERM... (Saturday through Next Thursday) Issued at 225 PM CDT Thu Apr 3 2025 (Saturday - Sunday) Rain should continue into early Saturday, mainly in southeast Missouri and southwest Illinois. The good news is that rainfall rates should really be on the downturn as the precipitation looks to be more stratiform vs. convective. Another (weaker) wave moves along the surface front, bringing more widespread stratiform rain a bit further to the north during the day. Saturday`s round of rain looks an awful lot like this afternoon`s. The bad news with that is the same areas that don`t need any more rain will get more rain. The good news is that rainfall rates should be low, with little to no convection expected. One difference is that it will be cooler, with some areas struggling to hit the 50 degree mark. Steady light rain should exit most of the area late Saturday night, and then we watch the parent mid/upper level trough incoming from the west. There is still some uncertainty with the track/strength of this feature, but the operational GFS remains a strong outlier. Given the synoptic setup depicting strong mid/upper level confluence downstream across the Great Lakes, this system really should get sheared apart and weaken. Therefore, the expectation is just for some lingering very light rain to continue across southern portions of the forecast area into the day on Sunday. In the unlikely event the stronger GFS is closer to being correct, a bit more rain would occur with even a chance for some of the rain to mix with wet snow Sunday morning. Sunday will be a pretty raw, chilly day regardless with low clouds and light rain chances. Highs in the upper 40s to low 50s are forecast, with more leeway to be even chillier if the stronger GFS type solution comes to fruition. (Sunday Night - Next Thursday) A very welcome dry and quiet period still looks to be in the offing early next week as mid/upper level flow become northwesterly. There is still a strong signal for a very deep mid/upper level trough to carve out across the Great Lakes early next week, sending our area a shot of some cold air. Probabilities on the LREF for subfreezing low temperatures early next week are very high, on the order 80-95+%. A hard freeze (<=28F) is also likely (50-70%), mainly across northeast Missouri and west-central Illinois. Given the rapid onset of the growing season northward over the past few weeks, these types of low low temperatures could certainly cause damage to sensitive vegetation. Freeze watches/warnings may be needed for early next week (with Tuesday morning likely the coldest) in the days ahead. The next chance of rain comes midweek as a northwest flow disturbance crosses the Mississippi Valley. This system will not have a ton of moisture to work with, and any rain that does fall looks light. In other words, this system should not impact rivers and should allow them to continue to fall through the remainder of the work week. Gosselin && .AVIATION... (For the 00z TAFs through 00z Friday Evening) Issued at 611 PM CDT Thu Apr 3 2025 Showers and an isolated thunderstorm continue to slowly move southeast out of the St. Louis metro terminals this evening. All TAF sites are expected to be clear starting around 04-05Z tonight and continue into the mid-morning period tomorrow. Overnight MVFR ceilings are expected to move into the terminals from a combination of advection from downstream locations and cooling near the surface. By mid-morning tomorrow another round of widespread showers will move into the region and bring reduced visibilities and fuel alternate ceilings into the terminals. These conditions will persist through the end of the TAF period. There is a 60% chance for IFR ceilings tomorrow associated with the heaviest rainfall, mainly south of the terminals. Confidence is these ceilings reaching the mid- Missouri and St. Louis terminals is not high enough to include a FM group at this time. Winds will be light and variable overnight, becoming northeast by morning and increasing in speed to near 10 kts. Delia && .LSX WATCHES/WARNINGS/ADVISORIES... MO...Flood Watch through late Saturday night for Crawford MO-Franklin MO-Iron MO-Jefferson MO-Madison MO-Reynolds MO-Saint Francois MO-Saint Louis City MO-Saint Louis MO-Sainte Genevieve MO-Washington MO. IL...Flood Watch through late Saturday night for Bond IL-Clinton IL- Fayette IL-Madison IL-Marion IL-Monroe IL-Randolph IL-Saint Clair IL-Washington IL. && $$ WFO LSX