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Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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014 FXUS63 KLSX 181058 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 558 AM CDT Thu Jul 18 2024 .KEY MESSAGES... - Dry weather with mild, below average temperatures will continue today through Saturday. - A gradual transition to a more humid and unsettled pattern will occur early next week, with shower and thunderstorm chances chances building into the middle of the week. && .SHORT TERM... (Through Late Friday Afternoon) Issued at 210 AM CDT Thu Jul 18 2024 High pressure has settled into the region today behind yesterday`s cold front, with northwest flow aloft and an abnormally dry and cool airmass for mid-July. Yesterday evening soundings from upstream in Illinois revealed PWATs well below 1 inch (.88 inches), which is below the 25th percentile of climatology for this time of year. This applies to temperatures as well, as observed temperatures today fell 3 to 5 degrees short of average, and are expected to dip even further to around 5 to 8 degrees below average. The core of this airmass has since moved even farther south and into our area, where it will remain until late in the weekend. As such, expect several days of mild and dry weather between today and Saturday, with high temperatures in the upper 70s to low 80s, and morning low temperatures in the upper 50s to low 60s. While there may be some variation in temperatures, confidence is high that readings will be below normal over those three days. BRC && .LONG TERM... (Friday Night through Wednesday) Issued at 210 AM CDT Thu Jul 18 2024 Sunday through the middle of next week, the primary driver of local weather will be a slowly approaching broad upper trough / cutoff low from the central Plains. Ensemble guidance is in decent agreement regarding this feature, surprisingly so considering that it is weakly forced and features like this typically have a low degree of predictability. In any case, as this feature very slowly moves across the central Plains, the dry air currently in place will gradually erode and precipitation chances will likewise slowly increase. At this stage, there remain many unresolved mesoscale features that will dictate the specific day-to-day details. However, it will likely take some time for the airmass to recover in the wake of our currently dry air. This feature is not currently forecast to arrive with strong dynamics that would help advect moisture northward rapidly, so this is likely to be a gradual process. This is borne out in the ensembles as well, with ensemble mean PWAT values gradually increasing to near climatological averages sometime between Monday and Wednesday. Keeping in mind that we are very close to what is typically the climatological maximum of PWAT values, this also aligns with gradually increasing precipitation probabilities which peak between Tuesday and Wednesday and both the LREF and NBM. Also noted in the ensemble data is a time-lag of some higher outliers in precipitation to the Wednesday-Thursday timeframe, which may be a reflection of the fact that PWAT and instability projections continue to increase during this timeframe. This introduces the potential for more variable convection but with a higher ceiling for more intense rain rates/totals. Either way, chances for showers and thunderstorms will exist between Sunday and Thursday, with current probabilities peaking Tuesday/Wednesday. Chances for strong or severe storms remain low considering the modest dynamics, but this may slowly increase as instability builds toward the middle of the week. Meanwhile, the persistent presence of the upper low is also likely to suppress temperatures heading into early next week, with values remaining near to slightly below normal favored to continue. BRC && .AVIATION... (For the 12z TAFs through 12z Friday Morning) Issued at 553 AM CDT Thu Jul 18 2024 VFR conditions are expected for the vast majority of the 12 TAF cycle. Some very shallow ground fog may be briefly possible early this morning at fog prone terminals, specifically SUS, but this is not expected to be impactful. Otherwise, light winds and VFR ceilings will prevail through the day, with some afternoon cumulus development likely. A similar potential for fog will exist late tonight and early tomorrow morning at SUS/JEF/CPS, but again is not expected to be impactful and is not included in this iteration of the TAF. BRC && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX