Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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656 FXUS63 KLSX 171721 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1221 PM CDT Tue Sep 17 2024 .KEY MESSAGES... - Warm and dry conditions continue through at least Thursday. - A few opportunities for rain begin late this week into this weekend. && .SHORT TERM... (Through Thursday) Issued at 315 AM CDT Tue Sep 17 2024 A fairly stagnant flow pattern continues across the country today. A deep trough continues across the western US with downstream ridging over Texas and the Southern Plains and continuing into the Northeast US. A cut off low is swirling westward into the southern Appalachians, undercutting the persistent ridge over the Northeast. The relative ridging in between the two troughs promotes subsidence across our area today and for the next several days, suppressing any attempts at convection. Meanwhile the surface high associated with the Northeast ridge is rather expansive, extending westward through the Great Lakes. It`s this surface high which has brought in the drier air with dewpoints falling into the 50s on a light easterly breeze becoming southeasterly with time. With dry air and a mostly clear sky today, we`ll see temperatures warming back up into the mid to upper 80s to near 90 degrees, perhaps a degree or two warmer than yesterday. Nighttime lows will be cooler, dropping into the 50s in the cooler valley locations thanks to that drier air. Temperatures warm ever so slightly Wednesday into Thursday, with more and more 90 degree readings likely. This is in part due to the expanding influence of the Southern Plains ridge and more southerly low level flow. Kimble && .LONG TERM... (Thursday Night through Sunday) Issued at 315 AM CDT Tue Sep 17 2024 As we get toward the end of the week and the weekend, our eyes turn more to the west for our potential weather makers. Guidance is coming into better agreement that the Southern Plains ridge becomes firmly established this week while a deep trough digs over the Southwest. This puts our area along the northern periphery of the ridge. Any waves triggering thunderstorms to our west would have a tendency to move eastward toward our area. The first wave to bring rain chances to our region is the trough currently over the Great Basin. While the core of this trough tracks northeast through the Northern Rockies and into Canada, a portion of this wave pushes eastward around the top of the ridge with a surface cold front pushing eastward as well. As moisture gets pulled northward ahead of this wave, thunderstorms are expected to form late Thursday into Thursday evening across Iowa and northwest Missouri. These may track into our region Thursday night into Friday morning before dissipating. With the better forcing to the north, the best rain chances will be in the northern half of our forecast area, mainly northeast MO and western IL. Convection is likely to at least be elevated by the time it reaches our area, although it will also likely be outrunning the instability as well, leading to a dissipating trend. This idea is consistent with the new Day3 Outlook from SPC which focuses the severe weather threat to the north (where wind shear is stronger) and west (where surface based instability is accessible). The main impact for us will be the chance for much needed rainfall. The surface cold front likely stalls or washes out near the area as ridging takes back over behind the departing trough. The front will serve as a focus for additional thunderstorms Friday into Friday night, although this is more likely to our west across Kansas or Nebraska where better moisture will be pushing northward ahead of the deep trough over the Southwest. What this trough looks like when it eventually heads out into the Plains and where it tracks varies considerably among the guidance. If it gets absorbed into the northern stream and takes a northeast path, much of the energy will be steered around our area. If at least a portion of the trough takes a slower eastward trek over the top of the ridge it would represent a better opportunity for rainfall for our region in the Sunday/Monday time period. The uncertainty in the flow pattern causes the rain chances in our forecast to be more broadly distributed through the weekend. The temperature forecast is also more uncertain as warmer temperatures (near 90) would be likely if the ridge holds on longer while clouds and rain could prevent those warmer temperatures from occurring. While there is uncertainty in how the trough ejects and the ridge breaks down this weekend, guidance does agree that we will transition to at least a period of troughing early next week. A cold front brings an end to the warm temperatures and shifts us back toward more typical temperatures for this time of year. Kimble && .AVIATION... (For the 18z TAFs through 18z Wednesday Afternoon) Issued at 1218 PM CDT Tue Sep 17 2024 Confidence is high in dry and VFR flight conditions at all local terminals through the forecast period. There is a low chance for fog tonight, but it is currently expected to develop south of the local terminals. Elmore && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX