Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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919 FXUS63 KLSX 240349 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1049 PM CDT Sun Jun 23 2024 .KEY MESSAGES... - Heat, and more noticeably, humidity will begin to build back into the region on Monday. Heat index values around 105F are forecast in parts of central Missouri Monday afternoon, with the dangerous heat likely expanding eastward on Tuesday. - A cold front is expected to move through late Tuesday night. This front will provide a good chance for widespread showers and thunderstorms as well as a break from the heat (highs closer to normal). Some of the storms may be strong to severe Tuesday night, particularly in parts of northeast Missouri and west-central Illinois. - Heat and humidity could return heading into next weekend, but at this early point in time, the magnitude does not look as strong as early this week. In addition, there is a fair amount of uncertainty as to how hot daytime temperatures will be due to multiple chances for showers and thunderstorms. && .SHORT TERM... (Through Late Monday Night) Issued at 139 PM CDT Sun Jun 23 2024 A clear, calm night is expected across the area as a weak ridge of surface high pressure slowly slides eastward across the Mississippi Valley. These conditions (combined with persistent dry low-level advection during the day today) are very favorable for radiational cooling. Have therefore stayed close to the 25th percentile of available model guidance for low temperatures tonight, ranging from the mid to upper 60s. The exception is in/around the urban heat island of metropolitan St. Louis, which should only drop back into the low 70s. The most intense heat so far this summer is on track to advect into portions of the area on Monday from the southwest. Increasing low- level warm air advection along with a clear sky and building mid/upper level heights all point to a jump in surface temperatures. In addition, humidity will be on the increase along/behind a subtle surface trough axis. Dewpoints are expected to pool near this boundary as it heads eastward through the day. While there is more confidence in high temperatures tomorrow (90s - with mid to potentially upper 90s in parts of central Missouri), the dewpoint forecast is less certain. A few factors that give me pause in widespread low 70s dewpoints include surface winds increasing out of the south (downsloping off of the Ozark Plateau), ongoing dryness this month (dry soils), and MOS dewpoints (which tend to do verify pretty well) being a bit cooler. Because of these concerns, did lower dewpoints ever so slightly across parts of central Missouri. This did not change peak heat index values much though given similar high temperatures. Peak heat index values of around 105F are forecast in parts of central Missouri where a heat advisory remains in effect. Warmer low temperatures are forecast Monday night due to increasing southerly flow. Lows in the 70s are expected areawide, generally about 7-10 degrees above normal for the date. Gosselin && .LONG TERM... (Tuesday through Next Sunday) Issued at 139 PM CDT Sun Jun 23 2024 (Tuesday - Tuesday Night) Heat and humidity remains the focus on Tuesday, but unlike Monday, there is an additional complicating factor. There remains a signal, albeit weak, for a cluster of thunderstorms Tuesday morning exiting the mid-Missouri Valley. The EPS remains the most bullish with keeping convection going into at least northwestern sections of the area by late Tuesday morning, with around 30% of its members showing a wetting rain (>=0.10") ending at 1800 UTC. However, probabilities from the GEFS (<10%) and the CMCE (10-20%) are quite a bit lower. Any cluster(s) of thunderstorms though could provide enough debris clouds to hamper solar insolation a bit and cool down temperatures so we will have to keep an eye on this possibility. In the absence of any direct or indirect effects however of this convection, the heat (mid to upper 90s) should spread north and east compared to Monday. Somewhat similar to Monday, there is more confidence in high temperatures compared to dewpoints. There is concern that dewpoints forecast by most model guidance is too high Tuesday afternoon, particularly in parts of central, east central, and southeast Missouri. With a wind direction out of the southwest, these winds will be able to downslope off of the Ozark Plateau lowering dewpoints. The ongoing dryness of June and lower values on the MET/MAV MOS also cast doubt on widespread low 70s dewpoints. That being said, it will still be hot and humid. Even lowering dewpoints 1-2F still yields peak heat index values of ~105-107F over most of the area (exceptions being northeast Missouri and west central and south central Illinois). A heat advisory likely will be needed, but given this is still 48 hours away and there is at least a low probability of convection yielding lower high temperatures, elected to hold off on any heat headlines for Tuesday. Attention for Tuesday night turns to the increasing chances of showers and thunderstorms along/ahead of a cold front. The timing of this front has been consistent, passing through late Tuesday night into Wednesday morning. This front should have more to work with compared to the one which passed through last night. Namely, stronger surface convergence along the boundary, better mid/upper level support with the shortwave moving through, and increasing low- level moisture convergence Tuesday night. PoPs have been raised about an additional 10% compared to the previous forecast, with chances of showers and thunderstorms centered on late Tuesday night in the 60-80% range. Probabilities on the LREF for a more substantial rain (>=0.25") are also trending upwards. Chances are now 40-70% between Tuesday night and Wednesday morning of at least a quarter inch and 25-50% for 0.50" of rain. Of course, with convection, there likely will swaths with substantially more/less rainfall, but the good news is that the strength of the forcing mechanisms suggest that convection should be widespread. There will also be a threat for organized strong/severe convection. While deep- layer shear is likely to be weak (~25 knots), better forcing for ascent and high instability/CAPE (LREF probabilities of 3000+ J/kg 60-80%) suggest that multicellular clusters should be able to amalgamate cold pools that would be prone to produce strong-severe wind damage. This threat is most likely in northern sections of the forecast area through the early overnight hours. Some type of weakening trend is likely late Tuesday night further south as instability gradually wanes. (Wednesday - Thursday Night) All signs continue to point toward a break in the heat (and eventually humidity) for the middle of the week behind the cold front. Cooler temperatures are forecast to arrive first on Wednesday (mid to upper 80s), though cooler readings are possible if the front is faster or convection maintains itself through the morning hours. The high humidity levels however are likely to maintain themselves across the area, with a very gradual drop of dewpoints behind the cold front (similar to last night/this morning`s fropa). Thursday`s highs may be slightly cooler, but more importantly, be accompanied by much more comfortable humidity levels. Ensemble guidance has trended noticeably cooler with the air mass coming in behind the cold front, with 850-hPa temperature anomalies some 1-2+C cooler than yesterday`s guidance. This has led to a decrease in the high temperature forecast on both Wednesday and Thursday, with additional cooling possible if these trends continue. (Friday - Next Sunday) Forecast confidence decreases late week into the following weekend, namely due to uncertainty with the position and strength of a mid/upper level ridge centered across the southern CONUS. A slight majority of guidance is weak enough and just far enough south to put our area on the northern periphery/"ring of fire" region where multiple shortwaves moving through could initiate one or multiple rounds of convection. While low temperatures will assuredly be above to well-above normal, there is a lot of uncertainty with high temperatures during this time period. Interquartile ranges on the NBM are generally in the 6-10F range, but the cold tails/10th percentiles are several degrees cooler. For now, leaned toward the more conservative scenario (temperatures not getting impacted by thunderstorms) with highs getting well back into the 90s Friday and Saturday, but those temperatures could come down several degrees if confidence in the overall pattern and the potential for thunderstorms increases. Gosselin && .AVIATION... (For the 06z TAFs through 06z Monday Night) Issued at 1049 PM CDT Sun Jun 23 2024 VFR conditions persist under clear to mostly clear skies through the period. Light and variable surface winds turn southerly and increase by mid-morning Monday, but largely remain at or below 10 knots. Maples && .CLIMATE... Issued at 247 AM CDT Sun Jun 23 2024 With hot temperatures forecast, 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...Heat Advisory from noon to 8 PM CDT Monday for Boone MO-Cole MO- Moniteau MO. IL...None. && $$ WFO LSX