


Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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375 FXUS63 KLSX 011940 AFDLSX Area Forecast Discussion National Weather Service Saint Louis MO 240 PM CDT Tue Apr 1 2025 .KEY MESSAGES... - Strong to severe thunderstorms are possible after midnight tonight, primarily across parts of central and northeast Missouri. Severe thunderstorms are more likely on Wednesday afternoon, especially in southeast Missouri and southwest Illinois. - Periods of heavy rain are likely Thursday through Saturday. The threat of moderate to major river flooding is becoming more likely across parts of southeast Missouri and southern Illinois. Significant flash flooding will also be possible in these areas. A flood watch has been issued Thursday - Saturday. - Gusty southerly winds are forecast on Wednesday afternoon, with peak gusts of 40-45 mph expected. && .SHORT TERM... (Through Late Wednesday Afternoon) Issued at 230 PM CDT Tue Apr 1 2025 A very busy and complex forecast exists over the next 24-36 hours, and really beyond. A subtle surface warm front will push north of the area during the early overnight hours. Some scattered showers and thunderstorms are expected, mainly in northeast Missouri and west-central Illinois. Some small hail is possible, but there doesn`t appear to be enough instability for marginally severe (~1" in diameter) hail. There should then be a several hour break, with dry weather then for most of the rest of the night. A line of thunderstorms is forecast then to move eastward into northeast Missouri just before 1200 UTC. These storms should be on a weakening trend, due to timing (less instability) and working into a less favorable environment overall. Storms also should be elevated in nature, making it more difficult to mix severe winds down to the surface unless they remain more organized/robust than currently anticipated. Marginally severe hail is the main threat with any deeper cores, followed by damaging wind gusts to 60 mph. The expectation is for continued slow, but steady weakening of these storms as they gradually push eastward. However, I do think they will not completely dissipate due to a continued strong (50+ knot) low-level jet. The minimum in activity is likely to be around noon, as a vorticity maxima ejects into the Upper Midwest and its influences leave our area. Convective coverage should increase again by early afternoon as cooling midlevel temperatures and slightly stronger low-level moisture convergence combine to develop thunderstorms. There is some capping in place and not exactly the strongest low-level convergence along the Pacific cold front, nor any pronounced mid/upper level convergence. All that being said, all CAMs and deterministic guidance show a nice uptick in coverage of storms. The cooling temperatures aloft, low-level moisture convergence, and increasing diurnal instability however should be enough even given the limitations detailed above. Storms will likely get going near the I-44/I-55 corridors in Missouri and Illinois respectively by early afternoon. The exact axis isn`t a certainty however, and will be dependent on how far east the morning convection gets before nearly dissipating. If storms get further east, the area threatened by severe weather Wednesday afternoon will be mostly confined to parts of southeast Missouri and southwest Illinois. However, storms weakening quicker and further to the west would lead to a larger footprint where severe weather may threaten. Very strong (50-70 knots) deep-layer shear remains a given, and at least pockets of moderate (750-1000 J/kg) surface-based instability are likely. This is of course a very favorable thermodynamic environment for organized strong to severe thunderstorms. While at least some severe weather in southeast Missouri and southwest Illinois is all but certain, there remains low confidence in how significant the severe weather will be in our area. There are a lot of similarities to Sunday`s event, though with higher magnitudes of both deep-layer and low-level shear. Forecast hodographs are mostly straight, with near unidirectional flow in the lowest 3 km. There is quite a bit of speed shear however in the low levels, with 0-1 km shear expected to be around 20-25 knots. Straight hodographs, with even some anticyclonic curvature aloft, suggests the potential for splitting supercells, with perhaps even a few dominant left movers. The deep-layer shear vector is also nearly parallel to the frontal boundary, which means updrafts may tend to get cut off by the progression of the actual front itself. Right- movers would have a better probability of deviating off of the front, and would have the highest probability of being severe, and potentially with very large (2+") hail and tornadoes. Large hail looks to be the main threat overall, though the magnitude of the 0-1 km shear is certainly enough for tornadic potential, especially if there is a remnant outflow boundary around (which could locally enhance SRH). Tornadic potential is limited by relatively high LCLs and lack of greater cyclonic curvature evident on hodographs (low SRH). The parameter space for tornadoes (and some potentially strong/EF2+) does improve later on in time Wednesday evening, but storms are expected to be out of our area around or just after 0000 UTC Thursday. Gosselin && .LONG TERM... (Wednesday Night through Next Tuesday) Issued at 230 PM CDT Tue Apr 1 2025 (Wednesday Night - Saturday Night) A relative quiet period for the area looks in store for most of Wednesday night as storms exit the area near or just after 0000 UTC Thursday and the Pacific cold front progresses southeast of the area. This front will get hung up and become quasi-stationary Thursday morning. Another midlevel shortwave and some increasing low- level moisture convergence should spread in some widespread rain (and perhaps a few rumbles of thunder) to much of the region on Thursday. This round of rain does not look particularly heavy, with only modest low-level moisture convergence and very limited instability aloft. However, some moderate rain could fall in parts of southeast Missouri and southwest Illinois during this time period. Given what should be more saturated soils after Wednesday`s rain, will go ahead and start a flood watch Thursday morning. Another relative "lull" is expected Thursday night, though continued showers and elevated convection are expected in/near the stalled frontal boundary. This means parts of southeast Missouri and southwest Illinois may continue to see showers and thunderstorms, further saturating soils and sending creeks/streams upwards. The time period of biggest concern is Friday night through Saturday afternoon. This is when deterministic guidance shows the strongest low-level moisture convergence, with stronger mid/upper level forcing for ascent downstream of the approaching shortwave. In addition, the strongest moisture advection is nearly stationary, suggesting a very prolonged period of potentially heavy rainfall. Very strong integrated water vapor transport (>95th percentile), deep warm cloud depths, and anomalous precipitable water values (1.50"+ - >99th percentile of climatology), and tall/skinny elevated CAPE all suggest efficient rainfall processes. This is a long way of saying there is increasing concern for potentially historic rainfall amounts over a 3-5 day time period in our southeast Missouri counties. Probabilities for at least 5" of rain by Saturday night are 70-80% from the LREF, with a 20-30% chance of at least 8". If there is a lot of training between Friday night and Saturday, I would not be surprised to see some 12-15+" totals somewhere. The most favored axis for the heaviest rainfall amounts is just to our southeast, but it is close and a slight shift northwest could bring these types of amounts into parts of Reynolds/Iron/Madison counties in a worst case scenario. Very significant flash flooding is possible, with widespread moderate to major flooding along rivers and streams in southeast Missouri including the Black, St. Francois, and Meramec basins. Some light stratiform rain is expected Saturday afternoon/night (and possibly into Sunday morning) with the main shortwave. This feature gets sheared out however due to strong mid/upper level confluence across the Upper Midwest/Great Lakes. This type of rain won`t exactly be helping things, but should be light and devoid of convection. In other words, this rain won`t make things worse and should have little/no impact on river crests. Temperatures by this time will also cool into the 40s for a lot of the area, so a pretty chilly rain looks in store for early April. (Sunday - Next Tuesday) A welcomed quiet period of weather still is in the offing to end the weekend heading into next work week. Both the GEFS/EPS agree that anomalous mid/upper level troughing is expected to dig into the Great Lakes, with 850-hPa temperature anomalies of -8 to -12C. The ECMWF/EPS is more noteworthy with this push of cold, and would suggest not only a freeze but a hard freeze (<=28F). The LREF probabilities for subfreezing low temperatures early next week are 50-80%. On the cold end scenario (EPS), chances peak at about 50% of a hard freeze (again, <= 28F) in parts of northeast Missouri and west-central Illinois. Given the quick progression of the growing season to the north, it appears freeze products may be in the offing early next week. The good news? Dry weather is expected, which will start to allow our soils to dry out slightly and rivers to fall. Even beyond the day 7 forecast, dry weather is very likely to continue through the end of next work week. Gosselin && .AVIATION... (For the 18z TAFs through 18z Wednesday Afternoon) Issued at 1148 AM CDT Tue Apr 1 2025 Breezy southeasterly winds are expected to continue this afternoon/evening, veering more to the south by late tonight into Wednesday morning. Scattered showers and some weak thunderstorms may impact KUIN shortly after midnight. Have a PROB30 group for this potential, and if a thunderstorm does directly impact the terminal, a brief period of IFR visibilities are possible in downpours. A broken line of thunderstorms is forecast to move through much of the area on Wednesday. For central Missouri and KUIN terminals, the best chance of thunderstorms is early to mid morning. IFR visibilities and low MVFR or IFR ceilings is possible, along with some potential for gusty winds. For metro St. Louis, the more likely time is early/mid afternoon, just after the end of this forecast period. Gosselin && .LSX WATCHES/WARNINGS/ADVISORIES... MO...Wind Advisory from noon to 7 PM CDT Wednesday for Franklin MO- Iron MO-Jefferson MO-Knox MO-Lewis MO-Lincoln MO-Madison MO- Marion MO-Pike MO-Ralls MO-Reynolds MO-Saint Charles MO- Saint Francois MO-Saint Louis City MO-Saint Louis MO-Sainte Genevieve MO-Warren MO-Washington MO. Flood Watch from Thursday morning through late Saturday night for Crawford MO-Iron MO-Jefferson MO-Madison MO-Reynolds MO- Saint Francois MO-Sainte Genevieve MO-Washington MO. IL...Wind Advisory from noon to 7 PM CDT Wednesday for Adams IL-Bond IL-Brown IL-Calhoun IL-Clinton IL-Fayette IL-Greene IL- Jersey IL-Macoupin IL-Madison IL-Marion IL-Monroe IL- Montgomery IL-Pike IL-Randolph IL-Saint Clair IL-Washington IL. Flood Watch from Thursday morning through late Saturday night for Bond IL-Clinton IL-Fayette IL-Marion IL-Monroe IL- Randolph IL-Saint Clair IL-Washington IL. && $$ WFO LSX