872 FXUS63 KLSX 070528 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1128 PM CST Sun Jan 6 2019 .UPDATE... Issued at 850 PM CST Sun Jan 6 2019 Adjustments to rain timing and increasing concerns on high winds for some areas on Monday. New model runs continue to confirm what they were trending with earlier in the day, in delaying the timing of the rain chances for tonight and Monday. Outside of any random sprinkles falling from a mid cloud deck (higher than 8000ft), onset of the best rain chances look to be delayed until after midnight for areas north and west of STL metro, and until after sunrise Monday for STL metro and areas to the south and east. Rainfall amounts look light, so despite the high PoPs, QPF is expected to be less than a tenth of an inch for most areas. Rain should exit by midday Monday for most areas save southwest IL. Clouds will also clear quickly after rain exit, and deepening mixing layer thanks to SW surface winds should be able to tap into the western edge of the leftover low level jet as it slowly drifts eastward. The strength of this low level jet will be near 40kts when it begins to be tapped into during the late morning and midday hours for northeast and central MO and west-central IL, and offers a brief window for gusts to 40mph for many of these areas, which is just below Wind Advisory criteria. The oncoming shift overnight will need to evaluate for Advisory potential. The timing, strength, and placement of the low level jet for later in the day does not favor these high wind gusts as much for STL metro and areas to the S and E. Either way, clearing and a stout W-SW surface flow will result in decent warming potential for many areas, well into the 60s--still well below record values (see CLIMATE section below). TES && .SHORT TERM... (Through Late Monday Afternoon) Issued at 340 PM CST Sun Jan 6 2019 Mid/high level clouds have been increasing in coverage and thickness today ahead of an approaching low pressure system. Despite the increasing cloud cover, the influence of a ridge axis aloft as well as steady warm air advection and the passage of a warm front have allowed temperatures to reach the mid-50s to lower 60s by early afternoon across all but the northernmost part of the CWA. These temperatures are 20-25+ degrees above normal for this time of year. The low pressure system will continue moving through the region tonight and tomorrow, placing the LSX CWA beneath a broad region of DCVA located downstream of the upper trough axis. Ahead of the approaching upper low, an increasing H85 jet will transport moisture northeastward through the region tonight and tomorrow. At the surface, a cold front will sharpen across the central CONUS tonight before slowly moving through MO/IL tomorrow. BUFKIT soundings show that an initially dry air mass below 500-600 hPa gradually moistens with time. Finally, models depict steady isentropic ascent along the 300-315 K surfaces in the presence of ample moisture, and the overall area of ascent moves across the region tonight and tomorrow. Taken together, this suggests that mid/high clouds will continue to thicken late this afternoon and tonight (well ahead of the cold front) due to isentropic ascent, which is consistent with satellite imagery as well as the increasing mid-level echo noted across MO/IL on composite radar imagery over the last few hours. Once the column reaches saturation, further lift will result in a broad corridor of rain which moves across the region from late tonight through tomorrow afternoon. Rain will end during the day tomorrow as the H85 jet shifts eastward and strong isentropic downglide occurs near and behind the cold front. Wind speeds and gusts will increase as the pressure gradient tightens, but the strongest gusts will probably occur near and shortly after the cold frontal passage, especially after 18z. Immediately behind the front, model guidance depicts nearly unidirectional winds from the surface to the frontal inversion aloft, which is conducive for mixing those stronger wind gusts down to the surface. At this time, it does not appear that the winds will be strong enough to require any headlines. Tonight's overnight lows will be noticeably warmer than last night due to thick cloud cover, steady warm air advection, and periods of precipitation. With a warm start to the day, highs should easily reach the 60s across most of the area tomorrow, which would be 20-30 degrees warmer than average for this time of year. The area of greatest uncertainty for tomorrow's high temperatures is across central MO. Depending on how quickly the clouds clear out behind the departing low pressure system, the current forecast may not be warm enough for the western part of the CWA. Kanofsky .LONG TERM... (Monday Night through Next Sunday) Issued at 340 PM CST Sun Jan 6 2019 Conditions will be rather benign through much of the period. Most sensible weather steers around the immediate forecast area with limited opportunities for measurable precipitation. Temperatures will remain above normal Tuesday with forecast highs ranging from the upper 40s to lower 50s. However, a cold front will extend south of a shortwave, which pushes west to east through the southern Great Lakes Tuesday. The front has no moisture to work with and passes through dry, but will be effective in advecting colder air into the region late Tuesday into Wednesday. Synoptic troughing over the eastern CONUS will then couple with an amplified ridge over the Western U.S., setting up west to northwest flow aloft through the Midwest. Surface high pressure builds in from the northern plains through the midweek period, resulting in dry but cool conditions through Thursday. Given the trajectory of the colder H8 air, expect slightly below normal temperatures over Illinois to near normal temperatures over central Missouri. There will be plenty of sunshine Wednesday, possibly into early Thursday. Highs will generally range from the lower 30s to around 40 degrees. The only mention to precipitation comes late in the week. deterministic spread lends less confidence in any one solution. Despite this, chances are low for precip with general trends showing surface high pressure weakening over the Midwest. An upper shortwave pushes southeast through northwest flow aloft, while low pressure pushes east to west through the gulf states Friday into Saturday. Each feature stays just far enough to the north and south (respectively) of the area with limited moisture depth and dry air to overcome. Clouds continue to linger into Saturday with another upper level wave dropping down from the north during the afternoon. This looks to pass through dry as well with the main impact being a reinforcement in north and northwest flow. Temperatures are near or slightly above normal Saturday just ahead of this feature. Maples && .AVIATION... (For the 06z TAFs through 06z Monday Night) Issued at 1128 PM CST Sun Jan 6 2019 Rain chances will be on the increase overnight from west to east, beginning with UIN and COU over the next few hours, and then at STL metro sites after sunrise Monday morning. In the meantime, a SW low level jet 0f 45-50kts has developed and resulting in LLWS at all TAF sites, ending from west to east overnight. Rain and lower ceilings in the MVFR category should occur nearly concurrently later tonight and Monday morning before clearing occurs. SE surface winds 10-15kts with modest gusts at times will gradually veer SW by Monday. Gusts look to increase substantially at UIN and COU, where 30-35kts possible by midday and early afternoon. Considered IFR category at UIN but for now left as MVFR. SPECIFICS FOR KSTL: Timing for rain still looks set for mid to late Monday morning and LLWS in the meantime. Can't rule out a stray sprinkle before 14z but impact looks nil. The strong wind gusts affecting UIN and COU later on Monday, for now, still appear to remain to the west but will keep an eye on trends with the new models later tonight. TES && .CLIMATE... Record high temperatures and year Monday 01/07 STL 73 (2008) COU 75 (1965) UIN 70 (1907) && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX