718 FXUS63 KLSX 071943 AFDLSX Area Forecast Discussion National Weather Service Saint Louis MO 243 PM CDT Fri Oct 7 2022 .SHORT TERM... (Through Late Saturday Night) Issued at 236 PM CDT Fri Oct 7 2022 The primary impact will be the frost/freeze conditions late tonight into early Saturday. By Sunday, the cooler-than-normal airmass will moderate back to normal. Latest surface analysis shows a cold front has pushed well to the southeast, stretching from the Tennessee Valley westward through Arkansas. Northerly surface flow has set up behind the boundary around the eastern periphery of high pressure, which will continue to build southeast through late tonight. Short term guidance depicts the center of the high elongating from the mid-Mississippi Valley westward into central Kansas. The source region for the airmass that is being funneled into the region incorporates dewpoints in the mid to upper 20s, which is consistent across multiple pieces of short range guidance. The southward extent is the question with the I-70 corridor being an average delineation where air moderates slightly to the south and east. While the DESI grand ensemble 50th percentile shows low temperatures in the low to mid-30s in the freeze warning and mid to upper 30s south and east, dry air and 850 mb temps are within a couple degrees of upstream air in the Dakotas this morning. Very dry conditions already precede this cold push, which begs to lean toward the lower end of DESI percentile data. This is especially true given the microclimate behavior in parts of the CWA with the potential for localized minimums. I would not be surprising to see notoriously cooler outlying locations fall into the upper 20s, especially through sections of northeast Missouri. Temperatures moderate slightly to the southeast, but only to the low/mid-30s. Therefore, the freeze warning and frost advisory look solid. The core of the cooler 850 mb airmass moves east Saturday. Along with dry air and sunshine, mid-level moderation will allow us to tack on 25-30 degrees for highs in the low-60s. This still runs about 10 degrees or so below normal. The surface high moves slightly east mitigating the potential for sub-freezing lows Saturday night. Nonetheless, clear skies and light wind will drop temperatures into the mid-30s to near 40 degrees with frost being a concern in some areas. By Sunday, the southern shift in the surface high bring southwesterly return flow with a slight warming of mid-level temperatures over Saturday. Temperatures will return near normal with highs in the low-70s being representative of much of the area. Maples .LONG TERM... (Sunday through Next Friday) Issued at 236 PM CDT Fri Oct 7 2022 Near and above normal temperatures are expected the first half of the week before a midweek cold front brings the potential for rainfall. Below normal temperature return behind the front later in the week. The base to the upper level trough will begin to transition to the east from Sunday into Monday. As this occurs, surface ridging will extend from the mid-Atlantic across the gulf states with west and southwesterly flow setting up from the central plains into the mid- Mississippi Valley. Initially, weaker warm air advection results in near normal temperatures with highs in the low to mid-70s Sunday. By Monday, southwesterly mid-level flow begins to strengthen underneath the axis of a modest upper level ridge. 10-15C air at 850 mb is poised to move northeast through the day Monday pushing temperatures near to above normal with highs in the mid to upper 70s. Two areas are worth watching late Monday into Tuesday with the first chance at some much needed rainfall. A northern stream shortwave tracks southeast out of the northern plains, while a southwestern cutoff upper low is kicked northeast ahead of an amplifying, positively tilted trough. Both pieces act in tandem to provide at least some upper level lift ahead of the western trough in the vicinity of a strengthen LLJ late Monday night into Tuesday. However, moisture return remains unimpressive with the southeastern half of the CWA holding onto dry air in the lower 10k feet. This keeps the potential for measurable rainfall on the order of a few hundredths of inch from Columbia to Quincy. Though several fragments of upper level vorticity are embedded withing the southwesterly flow ahead of the western trough, moisture remains limited at least until midweek. Unfortunately, that is also where guidance diverges and ensembles provide little confidence for beneficial rainfall. Little change in probabilistic guidance would lean in favor of less than one-quarter of an inch with measurable (0.01) rainfall likely (60 percent), but quickly decreasing below 30 percent as thresholds are raised above one-quarter of an inch. Both the deterministic ECM/GFS are indicating a broad, deep upper low taking shape over southern Canada by late Wednesday into Thursday. The main difference seems to be the progressive nature of the GFS as the upper trough remains positively tilted over the central U.S. On the other hand, the ECM deepens the system slightly quicker with a slower arrival of the surface front, giving rise to better moisture return and higher yield in rainfall late Wednesday. So, while guidance has been trending lower on precipitation, there is still some hope for beneficial rainfall. Northwesterly flow dominates the region behind the frontal boundary as the strong upper low traverses the U.S./Canadian border. Cooler than normal temperatures are favored late next week. Maples && .AVIATION... (For the 18z TAFs through 18z Saturday Afternoon) Issued at 1243 PM CDT Fri Oct 7 2022 VFR conditions are expected through a commanding majority of the TAF period with few, minor caveats. A secondary cold front is reinforcing cold air southward with scattered/broken MVFR level cloud bases along the boundary. Observations indicate that broken MVFR of 2500-3500 feet has been hit/miss and momentary. It is not enough to justify a prevailing group and TEMPO may even be a far stretch with these instances being isolated and only lasting a few minutes. Predominant wind direction will remain out of the north, becoming light and variable overnight, potentially calm around sunrise. Much colder temperatures will allow some steam fog in immediate vicinity of north/south oriented rivers. While steam fog could develop on west/east rivers, air may be too dry to promote spread too far from streams, limiting confidence of any direct impact. However, it may be something to watch through early Saturday. Expect winds to gradually pick up to around 5 knots Saturday morning, shifting out of the west as high pressure sinks south. Maples && .LSX WATCHES/WARNINGS/ADVISORIES... MO...Frost Advisory from 1 AM to 9 AM CDT Saturday for Boone MO- Callaway MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO- Iron MO-Jefferson MO-Lincoln MO-Madison MO-Moniteau MO- Montgomery MO-Osage MO-Reynolds MO-Saint Charles MO-Saint Francois MO-Saint Louis City MO-Saint Louis MO-Sainte Genevieve MO-Warren MO-Washington MO. Freeze Warning from 1 AM to 9 AM CDT Saturday for Audrain MO- Knox MO-Lewis MO-Marion MO-Monroe MO-Pike MO-Ralls MO- Shelby MO. IL...Frost Advisory from 1 AM to 9 AM CDT Saturday for Bond IL- Calhoun IL-Clinton IL-Fayette IL-Greene IL-Jersey IL- Macoupin IL-Madison IL-Marion IL-Monroe IL-Montgomery IL- Randolph IL-Saint Clair-Washington IL. Freeze Warning from 1 AM to 9 AM CDT Saturday for Adams IL-Brown IL-Pike IL. && $$ WFO LSX