Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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258 FXUS63 KLSX 280809 AFDLSX Area Forecast Discussion National Weather Service Saint Louis MO 309 AM CDT Tue May 28 2024 .KEY MESSAGES... - A few showers and a thunderstorm or two will be possible across northern Missouri and west-central Illinois this afternoon. Thunderstorms are not expected to be strong or severe, and most areas will remain dry today. - Dry, mild conditions persist tomorrow and Thursday. - The chance for rain and a few thunderstorms returns late Thursday through Saturday, but the chance for strong or severe storms remains low. && .SHORT TERM... (Through Late Wednesday Afternoon) Issued at 257 AM CDT Tue May 28 2024 Benign conditions exist throughout the area early this morning as deep northwesterly flow prevails. Water vapor imagery reveals a broad upper trough located across the Great Lakes, with an embedded mesoscale vorticity maximum nearing the greater Chicago area and another weaker vort max on its heels in Minnesota. Locally, dry air aloft has allowed skies to largely clear, and with some hints of valley fog beginning to form across Ozark river valleys where decoupling as allowed surface winds to weaken. This patchy fog will likely expand slightly through the night, but persistent southwesterly breezes outside of protected valleys will likely prevent this from becoming widespread. During the day tomorrow, the trailing shortwave described above will pivot into the Great Lakes region along the path of its predecessor, which will tighten the pressure gradient at the surface and usher in a reinforcing weak cold front. While this feature will be working with a modified airmass and marginal moisture, some shallow convection will be possible due to a combination of cooling aloft and deep mixing. Thermodynamic profiles suggest shallow convection (equilibrium levels of only 15kft) and limited instability (maximum of 1000 J/kg, but more likely 250-500 J/kg), which will limit the ceiling. However, there will be sufficient effective shear within the storm depth to allow showers to persist and perhaps support a weak thunderstorm or two. These isolated showers are not likely as strong as yesterday`s activity, though, and most would likely remain north of I-70. Meanwhile, deep mixing will also support efficient momentum transport, leading to noticeably gusty surface winds in the afternoon. This will be especially noticeable across Illinois and northern Missouri, where gusts to 25 mph will not be out of the question. Otherwise, expect comfortable temperatures and humidity, with highs in the mid 70s to low 80s. Overnight, a surface ridge of high pressure will move into the area, bringing with it cooler, drier air and weakening winds. Some patchy shallow fog may redevelop in low-lying areas, but this isn`t expected to be widespread. High pressure and dry air will continue to dominate area weather Wednesday and Thursday as well, with mild temperatures generally in the 70s to near 80 and mostly light winds. BRC && .LONG TERM... (Wednesday Night through Monday) Issued at 257 AM CDT Tue May 28 2024 A relatively active pattern is likely to return to the region by the end of the work week, as a slow-moving shortwave attempts to push through a blocking ridge aloft. As this shortwave and its attendant surface low moves east, a compact low-level jet streak will transport moisture northward, although model projections suggest that warm temperatures aloft and weak lapse rates will limit the potential for noteworthy instability. When coupled with only modest wind shear aloft, the potential for strong or severe thunderstorms appears to be low at this time. However, relatively high confidence exists that at least some precipitation will develop between Thursday night and Saturday afternoon, likely due to a combination of modest warm air advection and elevated instability. This high forecast confidence may not be immediately apparent in the gridded forecast, but this is because the bulk of the uncertainty is in the timing and precipitation amounts. The primary driving feature will be up against a blocking ridge aloft, and this limits the predictability of such a feature. This is evident in ensemble members and cluster analysis, which depict this feature and precipitation timing with a high degree of variance. This explains why the current gridded forecast features 48 hours of widespread 30 to 50% precipitation chances throughout the period, but the chance for at least some rain falling sometime during this timeframe is much higher. As we head into next week, zonal flow aloft is expected to return with gradually building heights aloft, along with the potential for additional passing shortwaves and associated precipitation. Temperatures, moisture content, and instability are also expected to be on the rise, increasing the potential for thunderstorms as shortwaves pass through the ambient flow. However, the timing of these features remains highly uncertain, and the northerly displacement of stronger winds aloft will likely limit available wind shear, and may inhibit the potential for stronger thunderstorms. Likewise, CIPS and CSU guidance also do not show a strong signal for such storms in the extended period, which appears reasonable based on current available data. BRC && .AVIATION... (For the 06z TAFs through 06z Tuesday Night) Issued at 1031 PM CDT Mon May 27 2024 Dry, VFR conditions will prevail at the terminals through the valid TAF period. Aside from a low threat for thunderstorms across west-central Illinois that may impact KUIN, precipitation is not forecast. Winds will remain generally northerly through Tuesday night, with increasing gusts during the day Tuesday. The gusts will diminish after sunset when diurnal heating ends. MRB && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX