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Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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381 FXUS63 KLSX 081110 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 610 AM CDT Mon Jul 8 2024 .KEY MESSAGES... - The main concern over the next 2+ days is on impacts associated with the remnants of Tropical Cyclone Beryl. The track and associated heavy rainfall (2-4") has shifted back slightly to the southeast. The heaviest axis is most likely across southeast Missouri and southwest Illinois. A flood watch has been issued for these locations beginning late tonight lasting through Tuesday night. - Main impacts include the potential for heavy rainfall and flash flooding. Concern for aggravated mainstem river flooding on the Mississippi and Missouri Rivers has lessened given the shift to the southeast. - Scattered thunderstorms are expected this afternoon and evening near a cold front. A few of the stronger storms will be capable of producing gusty winds in southeast Missouri and southwest Illinois. && .SHORT TERM... (Through Tuesday Night) Issued at 347 AM CDT Mon Jul 8 2024 (Through This Evening) An ill-defined cold front is currently located across far western Missouri northward into Iowa and Minnesota. This boundary is forecast to slowly move eastward today and serve as a focusing mechanism for the development of scattered showers and thunderstorms late this morning through the early evening hours. The latest HREF shows a 50-60% probability of at least 1000 J/kg of SBCAPE collocated with 25-30+ knots of deep-layer shear in parts of southeast Missouri and southwest Illinois. A few of these storms may become strong to severe this afternoon/early evening, with damaging winds via wet microbursts the main threat. Heavy, efficient downpours are also expected with precipitable water values of 1.75- 2.00" (~95th percentile of climatology). (Late Tonight - Tuesday Night) A decrease in convective coverage is forecast by late this evening as instability wanes. This loss of instability also should close the window on any potential for strong to severe thunderstorms. Attention will then turn to the remnants of Tropical Cyclone Beryl. By late tonight, Beryl is expected to track into far southwest Arkansas per the National Hurricane Center. Widespread rain showers are expected to develop in the wee hours of the morning on Tuesday as strong low-level moisture advection enters southeast Missouri. Broad mid/upper level forcing for ascent is expected to increase on Tuesday as a longwave trough approaches from the west and divergence increases beneath the right-entrance region of a jet streak across the Upper Midwest. This enhanced divergence actually will stop the weakening post-tropical cyclone and may even lead to some weak cyclogenesis. The aforementioned mid/upper level forcing for ascent should act to broaden the precipitation shield further northwest into parts of central and northeast Missouri as well as west-central Illinois. However, the heavier rainfall rates should stay beneath the axis of strong low-level moisture advection which remains just downstream of the track of the 850-hPa cyclone. The track of the low-level cyclone has shifted gradually southeast with each successive model suite over the past 24 hours. This may be at least in part due to a further east track close to the Texas shoreline denoted by last night`s discussion from the National Hurricane Center. The most likely track of Beryl`s remnants is now through the bootheel of Missouri. If this holds, Tropical Storm Claudette (1979) would remain the only storm to pass through the CWA within the month of July. However, this track likely will be close and any deviation to the northwest could clip the far southeastern CWA. Given that the storm should gain more and more extratropical characteristics (interaction with frontal boundary, etc.) as it heads northeastward, the orientation of the heavier rainfall amounts should shift poleward of the track of the surface low. This puts the most likely axis of heaviest rainfall totals (2-4") through parts of southeast Missouri and southwest Illinois. I wouldn`t be surprised to see some isolated totals of 4-5+" within the heavier axis given the infusion of deep, tropical moisture (efficiency) and a lengthy duration of steady rain expected (~18 hours). The probability-matched mean from the HREF, which only ran through 0Z Wednesday, already had some 2.50-3.00+" totals with another 3-6+ hours of rainfall across parts of the area. Given this heavy rainfall threat and the potential for flash flooding, went ahead and issued a flood watch for parts of southeast Missouri and southwest Illinois. The threat is more conditional a bit further northwest (including parts of east-central Missouri into south-central Illinois). The shift of the track to the southeast has dropped rainfall totals a bit in these regions, and the gradient on the northwestern edge in reality likely will be tighter than currently forecast. These locations are very saturated though, so any shift back to the northwest could cause hydrological problems. If that shift were to occur back to the northwest, then the flood watch would need to be expanded. For this initial issuance however, focused on the areas where there was the most confidence in significant (2-4") rainfall totals. Another potential concern is for isolated tornadoes. This threat is more likely to our southeast given the track of the system, but a shift back to the northwest could put parts of south-central Illinois in play for Tuesday afternoon. Temperatures on Tuesday will be quite cool for early/mid July given the widespread clouds and persistent rainfall. Coolest values are expected to be in parts of southeast Missouri where low 70s are forecast. Locations in northeast Missouri and west-central Illinois should be the warmest, with highs nearing the 80 degree mark. Gosselin && .LONG TERM... (Wednesday through Sunday) Issued at 347 AM CDT Mon Jul 8 2024 (Wednesday - Sunday) Seasonably cool conditions are forecast on Wednesday as the remnants of Beryl move off into the Ohio Valley/lower Great Lakes. A cloudy start to the day for some is expected to transition to more sunshine by the afternoon. High temperatures in the low to mid 80s are expected. Model guidance is hinting at a weak midlevel shortwave approaching from the northwest. This feature could bring some low chances of showers and thunderstorms (likely diurnally driven) to portions of the area. While nearly all members from the grand ensemble have this feature, there is some discrepancies with the strength and placement Thursday-Friday. A stronger, further southwest placement would likely yield higher coverage of showers and thunderstorms each afternoon/evening along with cooler temperatures. A weaker, further northeast location of the midlevel shortwave in terms of sensible weather impacts likely would lead to dry conditions along with temperatures warming back to near or slightly above normal. Ensemble guidance is in good agreement that mid/upper level ridging in the intermountain west will begin to move eastward into the central Plains. This is expected to result in dry weather along with above normal temperatures next weekend (highs in the 90s). GOSSELIN && .AVIATION... (For the 12z TAFs through 12z Tuesday Morning) Issued at 605 AM CDT Mon Jul 8 2024 Isolated to scattered showers along with a few thunderstorms are the concern through this evening. Activity has weakened overall the past few hours with not much in the way of lightning. Therefore, elected to go with a VCSH instead of a VCTS at terminals for this morning. Better chances of thunder exist this afternoon/early evening for the metro terminals. More widespread rain and a few rumbles of thunder associated with the remnants of Tropical Cyclone Beryl will begin to move north in central Missouri early Tuesday morning followed by metro St. Louis terminals. Gosselin && .HYDROLOGY... Issued at 354 AM CDT Mon Jul 8 2024 Isolated flash flooding is possible in parts of southeast Missouri and southwest Illinois late tonight through Tuesday night due to persistent and efficient rainfall rates. Antecedent conditions are not as favorable as further northwest as these locations largely missed out on the heavier rainfall earlier this month. However, there may be at least pockets of very heavy tropical rainfall that overwhelms 3 or 6-hour flash flood guidance (2-3" and 3-4+" respectively). In addition to the potential flash flood threat in parts of southeast Missouri and southwest Illinois late tonight through Tuesday night, river flooding is also a concern. If the heavier axis of rainfall does end up in these locations, both the mainstem Missouri and Mississippi Rivers likely will not be impacted too much. Any subsequent crest on the Missouri River is very likely (>90%) to be below the previous crest on the latest HEFS guidance. For the Mississippi River, most of the river is at or very near crest. Some points may see their crests come up slightly, but not significantly unless the track of the heavier rainfall axis were to end up further northwest than currently forecast. The exception may be Chester, which has a ~20% chance of moderate flooding over the next week. Basins that are most likely to be more directly impacted by the remnants of Beryl include the Meramec, Big, Bourbeuse, and Black River basins. Probabilities for minor flooding on the Meramec and its tributaries are not particularly high due to slightly below normal flows. However, if the heavy axis of rainfall sets up over any of these basins, minor flooding odds would increase sharply. The Black River at Annapolis has the best chance of seeing minor flooding given the placement of the heavier rainfall axis. Gosselin && .LSX WATCHES/WARNINGS/ADVISORIES... MO...Flood Watch from late tonight through Wednesday morning for Crawford MO-Iron MO-Jefferson MO-Madison MO-Reynolds MO- Saint Francois MO-Sainte Genevieve MO-Washington MO. IL...Flood Watch from late tonight through Wednesday morning for Bond IL-Clinton IL-Fayette IL-Marion IL-Monroe IL-Randolph IL- Saint Clair IL-Washington IL. && $$ WFO LSX