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FXUS63 KLSX 072335 AFDLSX

Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 535 PM CST Thu Dec 7 2023

KEY MESSAGES

- Another day of well-above average temperatures is expected (near 100 percent chance) on Friday.

- A weak weather system and cold front will pass through the region Friday evening into Saturday morning. Confidence is high (80 percent) that most of the area will receive brief light rainfall with a 50 to 70 percent chance of over 0.25" of rainfall in northeastern MO/west-central IL, where rain will fall relatively longer.

- There is high confidence (80 percent) in near to above average temperatures next week, aside from short-lived slightly below average temperatures Sunday, along with dry conditions.



SHORT TERM
(Through Late Friday)
Issued at 212 PM CST Thu Dec 7 2023

An upper-level ridge over the Mississippi River Valley this evening will quickly shift eastward as a lead upper-level shortwave trough traverses the Central and Northern Plains as it rounds the east flank of a longer wavelength trough crossing the Rocky Mountains. In response, southwesterly flow will deepen and allow increasing clouds to spread into the CWA overnight into Friday. Wind gusts will also become lighter and more intermittent this evening, but a tightened surface MSLP gradient maintaining sustained winds along with increasing clouds will keep low temps tonight milder than last night and in the 40s to near 50 F--15 to 25 F above average.
Although upper-level clouds and areas of stratus/stratocumulus will be present on Friday, persistent WAA will allow temps to remain well- above average but slightly cooler than today.

Pfahler

LONG TERM
(Friday Night through Next Thursday)
Issued at 212 PM CST Thu Dec 7 2023

Model guidance continue to align in their depictions of the upper- level trough and embedded shortwave troughs as they propagate over the Great Plains and Mississippi River Valley Friday evening through Saturday, quicker and overall more progressive than projections over 24 hours ago. Another shortwave trough within the east flank of the main trough is progged to facilitate gradual deepening of an open- wave surface cyclone, tracking from near west-central MO northeastward into southeastern IA or western IL Friday evening and overnight along a cold front/baroclinic zone. Just ahead and to the northwest of this cyclone's track, more focused WAA and frontogenetical forcing is expected to support an area of numerous showers/steadier rain across northeastern MO into west-central IL blossoming after 22z Friday. To the south, large-scale ascent will be relatively weaker and broader with the development of scattered pre-frontal showers within broad WAA and a broken line of showers along the trailing cold front. This cold front will sweep through the CWA Friday evening through Saturday morning, departing to the east by around 15z Saturday.

Instability Friday evening into overnight will be limited but sufficient for a thunderstorm or two. Many CAMs suggest there will a narrow wedge of meager surface based instability along and immediately ahead of the front, but HRRR probabilities of SBCAPE in excess of 250 J/kg are 30 percent or less (highest in central MO), which does not increase confidence at this point in time that any thunderstorms will become severe despite the presence of 50 to 70 kt of deep-layer wind shear. From ensemble model guidance, probabilities are high (80 percent or higher) for measurable rainfall across the entire CWA, but probabilities of 0.25" or more of rainfall are 50 to 70 percent across northeastern MO/west-central IL where the most persistent rain is expected. Elsewhere, probabilities of 0.25" or more of rainfall are generally less than 30 percent.

After the front clears the CWA Saturday morning to the east, so should nearly all showers. As the base of the main trough pivots through the lower Mississippi River Valley Saturday evening, there will be an increase in post-frontal rain and overrunning flow but only 20 to 30 percent of ensemble model membership has any rain across far southeastern MO and southwestern IL by that time.
Otherwise, post-frontal conditions on Saturday are expected to feature peaks of sun between upper-level clouds and stratocumulus along with temps near to above average, since the strongest low- level CAA will lag behind the front. The brunt of the post-frontal airmass will be felt on Sunday but with temps only slightly below average.

Sunday into early next week, mid-level height rises will occur as upper-level flow over the central CONUS transitions from cyclonic to quasi-zonal in the wake of the upper-level trough. Model guidance advertise one or two shortwave troughs passing the Mid-Mississippi River Valley within this flow, but the background rising heights and limited moisture return support dry conditions with the probability of any measurable rainfall less than 10 percent from ensemble model guidance. Discrepancies in model solutions of the upper-level flow pattern increase by mid-next week, although there are still no indications of rainfall through that time at this juncture. Instead, the main highlight of next week will be temps returning to near or above average, with confidence increased by NBM temp 25th percentiles near or above average despite the interquartile range increasing to around 10 F.

Pfahler

AVIATION
(For the 00z TAFs through 00z Friday Evening)
Issued at 534 PM CST Thu Dec 7 2023

VFR flight conditions will persist through this evening. Then as the next weather system approaches the region, MVFR strato cu deck will begin to move in from west to east. Ceilings will lower to MVFR by 10z-12z Friday for KCOU, KJEF and KUIN, while the St.
Louis metro area will not see MVFR ceilings until 16z Friday.
Otherwise, best chances for rain will not be til just beyond current forecast period.

Still expecting a strong low level jet overnight from the southwest around 40 to 50 kts. However, the surface winds should stay up enough, south to southwest with gusts up to 20 to 25kts, to lessen chances of LLWS, thus did not add mention to the TAFs at this time.

Byrd

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

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