FXUS63 KLSX 040436 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1036 PM CST Tue Dec 3 2024 .UPDATE... Issued at 719 PM CST Tue Dec 3 2024 Made a few adjustments to the temperature forecast overnight tonight which is admittedly rather complex and atypical. Currently a thin band of 3-4000 FT clouds is progressing eastward across the forecast area. This is the leading edge of warm/moist advection aloft, but thus far has only amounted to these few clouds. Outside of this band, clear skies abound. In eastern portions of the forecast area across central Illinois, low clouds lingered later into the day, holding temperatures down. These clouds have departed, and winds remain very light, allowing for good cooling conditions this evening. These areas may drop into the teens in the coming few hours. Further west, though, low level southerly flow remains modest, keeping the near-surface layer better mixed. This will reduce the overall cooling potential tonight. Meanwhile, as noted earlier, warm advection has begun aloft, and while this warm advection is primarily being driven above the near-surface layer, it is aiding in setting up a strong temperature inversion, reinforcing the inversion that would naturally set up during nocturnal cooling. Guidance suggests temperatures will be coolest this evening before warming overnight as warm advection continues. However, this may be a bit overstated as guidance tends to perform poorly on nights when warm advection helps reinforce a strong inversion. Model vertical resolution is often not good enough to account for such strong inversions and near-surface cooling can continue under a clear sky while warm advection is focused aloft. However, the surface pressure gradient is increasing overnight, which supports the idea of a modest southerly breeze keeping the mixing depth a bit higher, potentially high enough to tap into some of those warming temperatures. The bottom line for most is that temperatures are likely to be coldest this evening before they level out or rise later in the night. Coldest temperatures will be in the east where winds are lightest, allowing strong surface cooling. Warmer temperatures will be in the west where the pressure gradient is stronger and warm advection is most aggressive. Kimble && .KEY MESSAGES... - A quick warmup to near average temperatures (mid 40s to mid 50s) is expected tomorrow, with gusty west winds. - A strong cold front will bring a 30 to 40 degree drop in temperatures tomorrow night, with additional gusty winds. - While temperatures Thursday will likely stay below freezing in most places, a steady warmup is expected Friday through the weekend. - Rain is becoming increasingly likely (50 to 80% chance over 48 hours) at some point between Sunday and Monday. && .SHORT TERM... (Through Late Wednesday Night) Issued at 251 PM CST Tue Dec 3 2024 Quiet conditions have settled into the area today on the heels of yesterday's light snow, although stubborn low level clouds persisted longer than expected this morning. Overnight, a large and deepening surface low will sink into the upper Great Lakes ahead of a rather potent clipper trough. While the bulk of the significant impacts of this system will be felt to our northeast (at least initially), it will have an effect on local weather as well. First, the pressure gradient will tighten significantly overnight, leading to a strong low level jet and warm air advection. In addition to increasing wind speeds, this is likely to advect a bit of enhanced low level moisture into the area overnight as well, which is likely to only amount to a round of low to mid level cloud cover. We only bring this up because in a few very limited deterministic forecasts from the NAM there were hints that this could lead to a brief period of freezing drizzle across parts of southern Illinois overnight or early tomorrow morning. More recent runs have trended away from this, and confidence is very low (10% or less) that this will occur, with better chances remaining to our east. While a pre-frontal trough will veer winds to the west and limit warming slightly across our northern areas during the day, there will be enough warm air advection to result in much warmer temperatures tomorrow afternoon. The pre-frontal trough does limit confidence in the specific values somewhat, but there is high confidence in afternoon temperatures near average, with the potential for readings of around 5 to 10 degrees above average mainly across the Ozarks. This equates to highs in the mid 40s to mid 50s, with the greatest uncertainty along the I-70 corridor. During the evening and overnight tomorrow, the previously mentioned clipper will drive another significant cold front through the area, driving temperatures back down to well below normal values and ushering in gusty northwesterly winds. This is expected to be a dry frontal passage for us, with the best potential for precipitation remaining to our northeast. However, while model QPF is very limited (<15% chance of any accumulations per NBM ad LREF guidance), model soundings do hint at the potential for a few sprinkles along and behind the front, with a brief window for flurries as temperatures drop. This is most likely across parts of Illinois, but confidence is too low for mention in the official forecast at this time. Meanwhile, temperatures are likely to plummet to well into the 20s and perhaps as low as the mid teens, pending the persistence of low level cloud cover. Ensemble mean 850 temperatures reach below the 10th percentile and near the 2nd percentile as the core of the cold air moves through the area, but exactly how this translates to surface temperatures will likely depend at least somewhat on cloud cover. On the other hand, surface temperatures are being forecast in the mid-teens in spite of persistent surface winds, so its possible that cold air advection alone gets us close to those values regardless of cloud cover. Meanwhile, this strong cold air advection, persistent mixing, and steep low level lapse rates will also help to transport gusty winds to the surface through the night, particularly across northeast MO and south-central Illinois. Current forecasts fall short of advisory level speeds and significant impacts, but with 75th percentile HREF wind gust forecasts near 40 mph (both ahead of and behind the front), this isn't completely out of the question. Also, post-frontal winds may lead to wind chills in the single digits to possibly near 0 degrees in the colder spots. BRC && .LONG TERM... (Thursday through Next Tuesday) Issued at 251 PM CST Tue Dec 3 2024 While wind speeds and cold air advection will gradually diminish through the day Thursday, limited afternoon warming is expected and highs are only likely to reach the mid 20s to low 30s. Unlike the previous wave of cold air though, a much faster recovery is expected over the latter half of the week as southwesterly flow quickly resumes Friday and gradually increases through the weekend. This should result in a rapid warming trend that brings temperatures back to near average (mid 40s to low 50s) by Saturday, and likely above average Sunday and possibly Monday. Confidence in forecast temperatures does diminish Monday due to timing differences in the approach of another cold front and the potential for precipitation, but strong warm air advection is likely ahead of this approaching system. Meanwhile, the upper flow pattern will undergo a somewhat complex evolution over the weekend and into early next week, and one that is increasingly likely (50 to 80% over 48 hours) to result in precipitation locally. First, a broad subtropical low will gradually move east across the southwestern U.S. over the latter half of the week, drawing increasing Gulf moisture into Texas. Over time, model guidance indicates that the subtropical jet associated with this trough will arc northward and increase in strength, and at the same time, a significant northern stream trough will emerge from the northern Rockies and into the Plains. This complicated phasing of the subtropical and polar jet streams adds some inherent uncertainty to the forecast, but on the other hand, there is relatively good agreement among LREF clusters regarding the timing and strength of these two troughs at this time range. This bolsters forecast confidence in the depicted surface response as well, which features significant warm air and moisture advection Sunday and Monday, along with the aforementioned increasing precipitation chances. 75th to 90th percentile NBM dewpoint forecasts also climb well into the 50s in the warm sector across southeast Missouri and southern Illinois during this period, which is above the 90th percentile for this time of year and leaves a little bit of room to consider the possibility of thunderstorms. Given the low instability projections (90th percentile SBCAPE <200 J/kg) we will leave this potential out of the official forecast for now, but this will need to be watched considering the potential for very strong dynamics and anomalously rich moisture. BRC && .AVIATION... (For the 06z TAFs through 06z Wednesday Night) Issued at 1032 PM CST Tue Dec 3 2024 VFR conditions continue tonight with a steady southerly wind at the surface. A strong westerly low level jet has developed and will lead to low level wind shear at all sites until mixing depth increases Wednesday morning, shifting surface winds to the west and getting gusty. Winds shift to the northwest behind a cold front late Wednesday afternoon at Quincy and Wednesday evening at all other sites. Winds may be a bit stronger behind the front as cold advection begins, with gusts over 30KT possible. It's unclear how much stratus will follow the front, as there is a lot of low level cold, dry air rushing in. However, some MVFR stratus may develop behind the front. Confidence is greatest in this at Quincy Wednesday evening, although how extensive or long lasting it will be is uncertain. Kimble && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX