Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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842 FXUS63 KLSX 051944 AFDLSX Area Forecast Discussion National Weather Service Saint Louis MO 244 PM CDT Fri Jul 5 2024 .KEY MESSAGES... - Near to slightly above average temperatures are forecast this weekend with lower humidity. - The chance of showers and thunderstorms will return on Sunday afternoon and continue through Tuesday, with the greatest chances Sunday night and Monday. - High temperatures will hover near normal much of next week. && .SHORT TEthe RM... (Through Late Saturday Night) Issued at 230 PM CDT Fri Jul 5 2024 We will have a respite from the active weather we experienced earlier this week as we head into the weekend. The upper low/trof currently extending through the upper-mid MS Valley will gradually weaken and depart east-northeast into southeast Canada and the Othe hio Valley over the next 36 hours. As a result, weak post-frontal surface high pressure will dominate the region resulting in tranquil, less humid, and slightly cooler temperatures compared to earlier this week. Increasing broad longwave trofing across the central U.S. heading into Sunday night will be the harbinger of a return to a bit more active pattern. The first sign of this is the potential for residual showers associated with a decaying Plains MCS to move into NE MO late Saturday night. I`m not super excited about the potential and we have no POPs at this time, but we`ll need to keep an eye on the development of weak low-level WAA and moisture return attendant with the eastern periphery of a southwesterly LLJ on the backside of the retreating anticyclone. && .LONG TERM... (Sunday through Next Friday) Issued at 230 PM CDT Fri Jul 5 2024 Renewed deepening of the upper trof across he Plains Sunday- Sunday night will result in backing mid-upper flow and allow for embedded vort maxes to begin impacting the area. The accompanying retreat of the lower tropospheric anticyclone will bring a decided return of low-level southerly flow and accompanying WAA/warmer temperatures, along with increasing moisture/instability. I`m not sure we will see much activity during the daylight Sunday, as other than the possibility of a residual convective boundary across central/northeast MO, there are a lack of forcing mechanisms. However on Sunday night, the threat of showers and thunderstorms should increase significantly across the western CWA associated with a lead vort max within the southwest flow aloft, an advancing cold front across western MO, and forcing/moisture transport via a southwesterly LLJ. The slowly progressive positively-tilted upper trof will advance from the Plains into the mid-upper MS Valley Monday into Tuesday. The combination of ascent associated with a series of vort maxes in the southwest flow ahead of the trof axis and the advancing cold front itself interacting with a warm, moist and unstable air mass should bring scattered-widespread showers and thunderstorms Monday into Monday night. There is general agreement in the evolution of the large scale pattern Tuesday through Wednesday with the ensemble means and cluster analysis of the LREF depicting broad upper trofing from eastern Canada southeastwarthe d into the mid MS Valley and a strong upper high centered over the Intermountain West. However there are some subtleties which could impact the forecast, namely the speed of the frontal progression through the area on Tuesday and the longevity of attendant precipitation chances. The other item is the remnants of TC Beryl. The superensemble mean track and greatest clustering keeps the curving track of Beryl south of the bootheel around midweek with mainly increasing mid-high level moisture within the southwest flow aloft impacting SEMO and SRN IL. The result is a larger footprint of POPs across the southeast half of the CWA Tuesday and low POPs the confined to far southeast MO and southern IL Tuesday night-Wednesday. During the later part of next week the central U.S. is dominated by very weak mid-upper flow sandwiched between ridges centered in the western Atlantic and Intermountain West. While not resolved well in the ensembles, there are signs in the deterministic solutions of an upper low floating around somewhere within the aforementioned weakness in the flow across the central U.S.. Models typically have low skill with these types of features in this kind of flow regime, and if present it could end up bringing low precipitation chances. Otherwise, following the Tuesday cold fropa, forecast temperatures are forecast to hover within a few degrees of normal, so no extremes. Confidence in this is greater than average with the NBM max temperature distribution showing a small interquartile range Tuesday-Friday. Glass && .AVIATION... (For the 18z TAFs through 18z Saturday Afternoon) Issued at 1150 AM CDT Fri Jul 5 2024 VFR flight conditions are expected through the forecast period. Diurnal cumulus is expected this afternoon with scattered high clouds tonight. Northwest surface winds will gust to 20+ kts this afternoon and diminish early this evening. Glass && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX