Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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368 FXUS63 KLSX 211946 AFDLSX Area Forecast Discussion National Weather Service Saint Louis MO 246 PM CDT Fri Jun 21 2024 .KEY MESSAGES... - Above average temperatures will continue through at least Tuesday, and will likely peak on either Monday or Tuesday when at least minor heat impacts remain likely. - Scattered showers and thunderstorms remain likely overnight across at least northeast Missouri and west-central Illinois, with some potential for a few damaging wind gusts and large hail. - Confidence is increasing that another round of showers and thunderstorms will occur sometime between Tuesday and Wednesday, with some modest heat relief following the front. && .SHORT TERM... (Through Late Saturday Night) Issued at 245 PM CDT Fri Jun 21 2024 While our extended period of above average temperatures continues this afternoon, quiet conditions are in place otherwise as high pressure maintains its grip on the region. GOES water vapor imagery reveals a broad upper ridge located across the southeastern and south-central CONUS, with a very subtle impulse moving slowly across Missouri as of 2 PM. This feature may be providing just enough of a boost to allow isolated pulse-type showers to develop across mainly northwest Missouri this afternoon. While a few of these cells may briefly encroach upon central/northeast Missouri through the afternoon, most of our forecast area will remain dry. Should any cells develop locally there is just enough instability present (500- 1000 J/kg ML CAPE) that a stray strike of lightning or two is possible before showers diminish this evening. As for temperatures, so far surface observations indicate that we are heating up at about the same rate as yesterday, and it`s likely that we will once again reach the upper 80s to mid 90s in the next few hours. Meanwhile, dewpoints only in the 60s will keep heat index values from climbing much above actual temperatures, and mitigate the potential for significant heat impacts today. This is also likely to be the case tomorrow as well, although actual temperatures may be a degree or two warmer. Still, this is an improvement from previous forecasts that suggested that tomorrow would clearly be the warmest day of the week, as it now appears that conditions will be on par with today and yesterday. Overnight, a weakening cold front remains likely to move through the area, bringing with it a round of scattered showers and likely a few thunderstorms. While the timing of the front remains very consistent among global models/ensembles and recent CAMs, the coverage of showers in recent CAMs has been somewhat underwhelming thus far. While at least some precipitation chances remain likely in northeast Missouri and west-central Illinois, we have slightly reduced probabilities to account for this trend. As such, the current forecast maintains sub-50% probabilities south of I-70, as confidence low that showers will persist as they move south during the early morning hours Sunday. In spite of the underwhelming precipitation trends in higher- resolution models, a few underlying ingredients for severe weather remain in place. This includes both marginal instability (500-1500 J/kg MUCAPE) and deep layer shear (20-30kt of 0-6km shear), with some enhance low level shear due to a relatively strong low level jet. As such, the potential for a few strong or severe thunderstorms remains extant, although probabilities for this will be higher to our north. Damaging wind gusts associated with some form of linear complex remains the most likely hazard. Considering the expected storm mode, and also the likelihood that storms will be trending downward as they arrive, the potential for hail and tornadoes appears to be quite low at this time. BRC && .LONG TERM... (Sunday through Next Friday) Issued at 245 PM CDT Fri Jun 21 2024 By Sunday afternoon, the previously mentioned cold front will likely settle somewhere across southern Missouri. Exactly how quickly this front moves southwards will have a big impact on both the heat potential and chances for redeveloping thunderstorms Sunday afternoon. A quicker southward progression would usher in a drier and slightly cooler airmass, providing some modest heat relief and pushing thunderstorm chances farther south. Conversely, a slower passage would keep a warm and humid airmass draped across the Ozarks, and leave open the door for a few redeveloping afternoon thunderstorms across the Ozarks. Recent model projections have trended toward the former faster progression, though, and as such both temperature and precipitation chances have been adjusted slightly downward to account for this. By Monday, an upper level ridge of high pressure will rebuild across the western and Central Conus, while southwesterly low level flow develops across the plains and Missouri river valley. This will likely drive a rapid temperature rebound Monday, with ensemble mean 850 mb temperatures climbing to around 23 C (near the 99th percentile of climatology). This is a bit sooner than in previous model projections, as previous iterations indicated that heat would peak on Tuesday. While this may still end up being the case, a faster and deeper shortwave progression Tuesday makes this less likely. In any case, these parameters support surface temperatures climbing well into the mid to upper 90s. This is slightly lower than the operation NBM output, which continues to forecast temperatures reaching 100 degrees both Monday and Tuesday. The NBM has remained the hottest of almost all available guidance in the extended period (3+ days), and considering local climatology, it`s likely overdoing actual temperatures a bit. In spite of this, advisory level heat indices remain likely on Monday at least (100-107), with some limited potential Tuesday as well. These effects may also be amplified slightly by the fact that this will mark at least 5 consecutive days of above average temperatures, although the expected magnitude is not expected to produce more than minor impacts overall. Finally, another cold front is expected to push through the region sometime between Tuesday and Wednesday, and confidence is increasing that this will bring another round of showers and thunderstorms, and likely at least some modest heat relief. While many details remain uncertain, particularly the timing of this front, the potential for strong/severe thunderstorms will also need to be monitored due to the potential for strong instability and at least modest deep layer shear (pending the strength of the passing shortwave). CSU machine- learning guidance also notably highlights this timeframe with rather bullish severe weather probabilities, adding at least some evidence that this will be a day worth paying attention to in future forecasts, at a minimum. BRC && .AVIATION... (For the 18z TAFs through 18z Saturday Afternoon) Issued at 1230 PM CDT Fri Jun 21 2024 VFR conditions are expected to prevail at local terminals through the 18Z TAF cycle, with few exceptions. At COU/JEF, some weak convection has developed in the vicinity this afternoon, but little if any precipitation appears to have yet reached the ground. While a shower or two can`t be ruled out, probabilities remain low (20% or less) that either terminal will be directly impacted, and thus showers were not initially included in the TAF. Otherwise, southwest winds tomorrow afternoon will likely be breezier than today, with additional afternoon VFR cumulus expected as well. BRC && .CLIMATE... Issued at 221 AM CDT Fri Jun 21 2024 With hot temperatures forecast next week, here are some of the daily high temperature records: MON JUN 24 TUE JUN 25 ST LOUIS 102 in 1988 102 in 1954 COLUMBIA 103 in 1988 102 in 1988 QUINCY 99 in 1988 102 in 1931 && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX