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FXUS63 KLSX 110434 AFDLSX

Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1034 PM CST Sun Dec 10 2023

UPDATE
Issued at 520 PM CST Sun Dec 10 2023

Made some minor adjustments to overnight clouds and temperatures.
There is an area of scattered to broken high level cirrus clouds that will be tracking through parts of the region overnight, although these won't have a strong effect on stopping the cool down tonight as compared to the low clouds that kept things more mild most of last night. Tonight will be the coldest of this air mass. Although surface high pressure is centered in Texas, its axis extends northeastward into our area with some locations already reporting calm conditions. Much of our region will see good radiational cooling conditions with temperatures varying based on exposure (valleys/fields cold while urban areas warmer)
as well as the minor impact of the high cloud cover. For a look at how cold it could get, we look at afternoon dewpoints which are largely in the low to mid 20s. This is about what the floor for temperatures tonight will be, with urban areas warmer and with the areas of high clouds helping to prevent us from cratering much below this floor. Used a blend of MOS guidance as a starting point for overnight lows as MOS does much better in radiational cooling conditions. Impactful fog or frost still looks unlikely due to low moisture content and the patches of cloud cover preventing even stronger surface cooling.

Kimble

KEY MESSAGES

- An extended period of dry weather with seasonable temperatures can be expected for much of the next week.

- While some very light rain can't be totally ruled out over the weekend, this potential is very limited (less than 15%), and any rain that does fall is very unlikely improve local drought conditions.



SHORT TERM
(Through Late Monday Night)
Issued at 227 PM CST Sun Dec 10 2023

While some stubborn cloud cover continues to linger across parts of south-central Illinois this afternoon, clear skies have emerged across the remainder of the area, resulting in a generally pleasant day as far as December is concerned. This is all being made possible by the approach of an area of high pressure from the west, which is also gradually causing winds to weaken. This latter trend will continue the remainder of the evening, and winds are likely to be nearly calm from roughly sunset through the overnight hours. While this typically would introduce the potential for fog and/or frost overnight, a dry upper level impulse is expected to bring a broken layer of mid and high level cloud cover this evening and overnight, and this will likely keep temperatures just warm enough to prevent either from developing.

By tomorrow, while northwest upper flow will persist, the axis of the surface high pressure will slide to the east, allowing for the development of modest south to southwesterly low level winds. This will result in a modest increase in temperatures, while also keeping the area dry. Expect clearing skies through the morning, with temperatures climbing from the mid to upper 20s in the morning to the upper 40s to low 50s in the afternoon. The warmest temperatures are expected to occur along and south of the Missouri River, where we may see a slight boost from downslope winds coming off of the Ozarks.

BRC

LONG TERM
(Tuesday through Next Sunday)
Issued at 227 PM CST Sun Dec 10 2023

Generally dry and seasonable conditions are expected to persist for the remainder of the week, thanks to the development of a longwave ridge across the central and western U.S. While there remains a very limited chance for some light rain over the weekend, this potential appears less likely than in previous forecasts.

By Tuesday, a weak shortwave is expected to detach from the polar jet stream and dig into the mountain west, while the aforementioned jet lifts northward. This feature is expected to slowly continue southward through the heart of a building longwave ridge, and eventually meander eastward...likely across the deep south late in the week, when it may merge with another similar shortwave somewhere across the lower Mississippi valley over the weekend. The details regarding exactly how this will unfold remain very nebulous, as there remains quite a bit of variability in the timing and track of each of these systems.

However, confidence is increasing that the potential for this pattern to produce meaningful precipitation locally is rather low, and ensemble precipitation probabilities continue to steadily decrease in kind. In fact, NBM precipitation probabilities have now fallen below 15%, which is our minimum threshold to explicitly mention precipitation in the official forecast. In other words, the chance for light rain over the weekend is not technically zero, it is now very low (less than 15%), and it is nearly certain that any rain that does manage to fall will not be significant enough to improve ongoing drought conditions.

Otherwise, the only item worth noting for the remainder of the next week will be a very gradual overall warming trend through the end of the week. This trend may not occur in a straight line as a weak backdoor cold front will act as a bit of a speed bump Tuesday and Wednesday and keep areas north of the Missouri river a few degrees cooler. Ensemble temperature forecast spreads also increase late in the week and over the weekend, reflecting the uncertainty in the evolution of the previously mentioned shortwaves. However, confidence is relatively high that temperatures will range from near to slightly above seasonal averages by the end of the week.

BRC

AVIATION
(For the 06z TAFs through 06z Monday Night)
Issued at 1032 PM CST Sun Dec 10 2023

Some high level clouds are drifting across the area overnight, but conditions are expected to remain VFR. With light winds the temperature in valley locations is falling to near the dewpoint.
Thus we cannot rule out some very isolated areas of freezing fog over the next few hours, but expectations are that this will be limited due to dry air and the effects of the high cloud cover. If it does occur, it is most likely at SUS, CPS, and JEF.

Kimble

FIRE WEATHER
Issued at 227 PM CST Sun Dec 10 2023

Modest south to southwest winds are expected to develop tomorrow, with sustained 20 ft wind speeds approaching 10 mph at times.
Meanwhile, minimum afternoon humidity values may also dip to near 30% across the Ozarks region for a few hours during the middle of the afternoon. While these conditions as forecast do not quite reach our local elevated fire weather criteria, there is a limited potential for erratic fire behavior in open areas and ridge lines in the Ozarks where wind speeds will be highest, and humidity lowest.

BRC



LSX WATCHES/WARNINGS/ADVISORIES
MO...None.
IL...None.

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