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Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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972 FXUS63 KLSX 161206 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 706 AM CDT Tue Jul 16 2024 .KEY MESSAGES... - A round of thunderstorms may bring locally heavy downpours and gusty winds through the morning. While some localized instances of both are possible this morning, this is not expected to be widespread. - While isolated to widely scattered showers may linger through the day, more significant storms are more likely this evening and overnight, mainly south of I-70. Heavy rain with localized flash flooding is the most likely hazard, and a Flood Watch has been issued for 7 PM tonight through 7 AM tomorrow. Isolated strong to severe wind gusts will also be possible. - While dangerous heat may linger across the Ozarks this afternoon, a cold front will bring heat relief and even well below average temperatures Wednesday through the end of the week. && .SHORT TERM... (Through Late Wednesday Afternoon) Issued at 350 AM CDT Tue Jul 16 2024 The primary item of interest in the immediate term continues to be a slow-moving and expansive line of thunderstorms draped across central Illinois, where it has finally begun to backbuild into eastern portions of our forecast area as of 3:30 am. Within the core of this line of storms, training storms with high rain rates have produced exceptionally high rainfall and prompted flash flood warnings. However, these more significant impacts have eluded our forecast area thus far, and it appears somewhat unlikely that we will observe impacts of this magnitude locally this morning, although some locally heavy rain and flooding will be possible. Another possibility is for redevelopment farther to the west across northeast and central Missouri. There is some moisture pooling ahead of an advancing cold front in these areas, and CAMs have somewhat inconsistently produced some redeveloping storms in these areas as a modest low level jet intensifies over the next few hours. So far this has not materialized, and latest trends have been to back off from this development. There is also not much in the way of kinematic support for this, aside from modest convergence along the nose of the aforementioned low level jet. In any case, whatever storms do manage to develop (including those that are ongoing) will do so within a moisture-rich (2 - 2.25 inch PWAT) and unstable environment (MUCAPE 2500-3500 J/kg), albeit with only marginal wind shear (30-35kt effective shear). As such, storms are likely to be capable of producing heavy rain rates, and even some localized training given the orientation of the LLJ. The primary question at this point, though is whether enough convection can develop at this point to tap into the thermodynamic environment. At this stage, confidence is low that this will materialize, but it can`t be completely ruled out just yet. During the day today, both a composite outflow boundary and cold front will slide southward through the area, the speed of which may be augmented by lingering thunderstorms from this morning should they materialize. There is quite a bit of variability in progression of the outflow boundary in particular, and this is clearly shown by large ensemble temperature spreads. Recent CAMs are also very inconsistent with developing convection and its outflow, and this feature and has recently swung dramatically from widespread convection and outflow (cool), to almost no new convection and a much slower and weaker boundary (hot). It is possible that this front will push far enough south by mid afternoon to significantly limit our heat potential across the Ozarks, but this remains highly uncertain and will depend on how convection develops this morning. On the other hand, more limited convection and weaker outflow may speed up the redevelopment of storms later this afternoon and evening, which has some support in the latest runs of the highly variable HRRR. Regarding the latter, it still appears that the cold front will sink into our southern areas by this evening, and the combination of forcing from a subtle shortwave and strengthening low-level jet will trigger another round of thunderstorms. This signal has been a bit more consistent than tonight`s activity, although the timing continues to shift. Latest trends have been to speed up its development into this evening, and we will have to watch closely in the event that this timing needs to be sped up to this afternoon. Whenever storms to manage to develop though, storm motions parallel to the stalled cold front will support training thunderstorms, and a moisture-rich environment will once again support high rain rates. While there remains some timing/location uncertainty, we have high enough confidence to issue a Flash Flood Watch from 7 pm tonight to 7 am tomorrow for parts of the Ozarks and southern Illinois, which is where we think the greatest likelihood of training thunderstorms exists. Still, adjustments in both time and space may be needed, particularly if storms develop earlier in the day or farther to the north. While our current deterministic QPF forecast QPF is underwhelming, high resolution ensemble forecast (HREF) data supports narrow corridors of 2 to 4 inch rainfall totals, with a few pockets as high as 5 to 6 inches also present. This aligns with the forecast environment, even if it remains somewhat uncertain exactly when and where it will occur. Meanwhile, the potential for other severe thunderstorm hazards appears less likely due to steadily weakening upper level support and more marginal instability during the evening. Still, we can`t rule out some isolated strong wind gusts this evening, particularly with any bowing segments that manage to develop. The potential for severe storms may also be slightly higher if storms develop earlier in the afternoon when both instability and bulk wind shear are likely to be maximized. Otherwise, storms are likely to diminish sometime Wednesday morning, and the cold front/outflow will continue to push south of the area. By Wednesday afternoon, some lingering post-frontal instability may support isolated shower/thunderstorm development across mainly the Ozarks and southern Illinois, but confidence is low regarding this potential. Most of the area is likely to remain dry Wednesday, with a significant cooldown into the low 80s expected as well. BRC && .LONG TERM... (Wednesday Night through Monday) Issued at 350 AM CDT Tue Jul 16 2024 The long-term discussion will be mercifully much shorter, as the pattern will shift dramatically to one featuring high pressure and well below average temperatures Thursday through at least the end of the weekend. This trend will begin Wednesday as previously discussed, but from Thursday onward northwest flow aloft and a large area of surface high pressure will sink into the area, ushering in significant negative temperature anomalies. In fact, both the NAEFS and ECMWF ensemble mean 850mb temperatures drop to below the 3rd percentile by Thursday, and negative anomalies persist through the end of the weekend. This is highly likely to support a prolonged period of below average temperatures, and is corroborated by an extended period of narrow forecast temperature spreads in the upper 70s to low 80s that persists through the end of the weekend. Meanwhile, this pattern also overwhelmingly favors dry conditions from Thursday through the end of the weekend, with precipitation chances drifting upwards early next week as the pattern finally shows some signs of shifting. BRC && .AVIATION... (For the 12z TAFs through 12z Wednesday Morning) Issued at 658 AM CDT Tue Jul 16 2024 Primary concern during the 12Z TAF will be the potential for multiple rounds of thunderstorms. The first round is ongoing and primarily impacting the St. Louis metro area terminals, with lightning and heavy rain as the primary hazards. Visibility reductions and ceiling reductions will be possible periodically for a few hours this morning, but storms are likely to move south and diminish prior to noon. After a lull in the early afternoon, another round of thunderstorms will be possible in the evening, which may impact I-70 corridor terminals. Confidence is low regarding whether these storms will develop this far north. Finally, lowering ceilings are likely this evening and overnight as a cold front moves through the area, potentially reducing ceilings to MVFR levels or possibly lower at I-70 terminals. At UIN, most of these impacts are likely to remain south of the terminal today, but a stray shower or thunderstorm can`t be ruled out. BRC && .LSX WATCHES/WARNINGS/ADVISORIES... MO...Flood Watch from this evening through Wednesday morning for Crawford MO-Iron MO-Madison MO-Reynolds MO-Saint Francois MO-Sainte Genevieve MO-Washington MO. Heat Advisory until 8 PM CDT this evening for Crawford MO-Iron MO-Madison MO-Reynolds MO-Saint Francois MO-Sainte Genevieve MO-Washington MO. IL...Flood Watch from this evening through Wednesday morning for Marion IL-Randolph IL-Washington IL. Heat Advisory until 8 PM CDT this evening for Randolph IL. && $$ WFO LSX