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FXUS63 KLSX 101122 AFDLSX

Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 522 AM CST Sun Dec 10 2023

KEY MESSAGES

- Slightly below normal temperatures will be short-lived today as temperatures climb near to slightly above normal for the remainder of the week.

- Dry conditions are favored through the next several days with a slight chance (15-20 percent) for light rain next weekend.
Confidence remains low that any widespread, measurable precipitation will impact the forecast area.



SHORT TERM
(Through Late Monday Afternoon)
Issued at 305 AM CST Sun Dec 10 2023

West-northwest flow has ushered in cooler and drier air with surface temperatures in the mid/upper 30s along and east of the Mississippi River and low-30s from central Missouri westward. Colder temperatures coincide with drier air as dewpoints have dropped into the 20s. IR satellite continues to show stratus over much of the CWA, with a slow eastward movement in the back edge of the cloud cover. Locations that have cleared over central Missouri have cooled more efficiently, especially as gusts have relaxed overnight. Areas that do clear will be the cooler spots as we approach daybreak with low ranging from the mid-20s in the cooler locations to the low/mid- 30s underneath the stratus deck.

The axis to an amplified upper level trough moves west to east overhead through the day today with surface ridging building in from the west and southwest. Bufkit overviews show moisture depth eroding through the day with breaks in the cloud deck giving way to peeks of sunshine from mid morning to early afternoon. Despite sunshine, cold air advection and early morning cloud cover will inhibit much in the way of warming with highs more in line with December normals...upper 30s to low-40s over a large majority of the area. A few mid-40s will be found across the far southern extend of the CWA.

Surface high pressure to the southeast will give rise to southerly flow heading into Monday. Temperatures see a 5-10 degree boost with a return to the low-50s along and south of the Missouri River in Missouri and mid to upper 40s elsewhere. Mainly clear skies through Monday night, low temperatures will fall back into the mid-20s to low-30s.

Maples

LONG TERM
(Monday Night through Saturday)
Issued at 305 AM CST Sun Dec 10 2023

The next several days continue to look mainly dry with near to slightly above normal temperatures. Any variation in daily temperatures are subtle with a couple of boundaries passing through the region and precipitation chances look slight at best with increasing uncertainty heading into next weekend.

Global long range guidance is in good agreement in the mid-to-late work week period. The synoptic setup initially includes west- northwesterly flow aloft that gradually turns zonal as a broad upper level trough digs southward down the western side of the Rockies.
Zonal flow Tuesday into early Wednesday gives way to upper level ridge amplification as the western trough closes off over the southwest CONUS.

As much as the closed low might draw attention and bring question to precipitation chances with its eastward evolution, the bigger story is the strong surface to mid-level ridging pattern that has been consistently depicted over the Midwest in the last several runs.
Between the ECM/GFS, the surface high that builds southeast out of the northern Plains has been shown to peak around 1040mb with the latest 00z ECM now showing a 1044mb high moving west to east along the southern periphery of the Great Lakes. In addition to the surface high, broad mid-level height rises result in a mid-level ridge directly overhead that peaks at around 4 standard deviations above normal Wednesday. The only sensible weather to mention in the timeframe is a weak surface cold front that approaches from the north late Tuesday. However, the west-east orientated front signals weak cold air advection that all but washes out with a minor dip in temperatures for Wednesday. Additionally, 500mb streamlines and RH plots show high clouds will likely obscure some of the sunshine on occasion Tuesday into Wednesday. This pattern persists through Thursday before breaking down Friday, providing high confidence in a dry forecast through the end of the work week.

As the surface high moves east, southerly return flow brings a modest bump in temperatures Thursday into Friday. The ECM has made a pretty drastic change in its handling of the southwest low, now bring it along Gulf Coast, as opposed to merging with the northern stream shortwave. This leave the forecast area sandwiched between with little in the way of moisture return. Thus, I'm skeptical of the rain chances heading into next weekend, especially as a limited number of 12z LREF ensemble members (20 percent or less) supported 0.01" of rainfall Saturday in Sunday. I can only see these probabilities dropping if the ECM trends stick and eventually verify.

Maples

AVIATION
(For the 12z TAFs through 12z Monday Morning)
Issued at 517 AM CST Sun Dec 10 2023

Overcast skies are bordering high end MVFR/low end VFR levels.
The back edge of the stratus deck is slowly moving west to east with clearing skies expected to result in VFR conditions through the remainder of the period. Northwesterly surface breezes around 10 knots will be common with an occasional gust to 20 knots mainly in the 12z-18z timeframe. Gusts will diminish through the later part of the day with surface flow becoming light and variable tonight.

Maples

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

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