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FXUS63 KLSX 091129 AFDLSX

Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 529 AM CST Sat Dec 9 2023

KEY MESSAGES

- Showers and a few thunderstorms that line the cold front will move east and exit the forecast area early this morning.

- Other than a day of slightly below average temperatures on Sunday, near to above average temperatures will prevail through next week. Dry conditions are expected.



SHORT TERM
(Through Late Sunday Afternoon)
Issued at 231 AM CST Sat Dec 9 2023

A cold front continues to push west to east through the forecast area early this morning. A thin, broken line of showers and a few elevated thunderstorms lines front with 500-700 J/kg of MUCAPE supporting the activity , which lines up from Springfield, IL southwest through northeast Franklin County in Missouri. Southerly flow along and ahead of the cold front quickly turns west to northwest behind the front. Surface temperatures near 60 degrees ahead of the boundary (southeast Missouri and southwest Illinois)
drop into the mid to upper 40s to the west and northwest.

As of 08z, the front was running slightly ahead of estimates with the boundary approaching the metro area around 03z. Hi-res guidance shows CAPE values decreasing with eastward movement of the front as we get deeper into the night/early morning period. Given the slightly quicker movement, PoPs have been cleared to the southeast of the CWA by mid-morning (15z). Aside from an isolated sprinkle or drizzle over the far southeast fringes of the CWA, dry conditions are expected thereafter. HRRR ensembles push nearly all accumulating precipitation east of the forecast area as early as 12z/7am this morning with HRRR 1 hour ensembles showing nothing west of the Mississippi River and only fragments of perception hanging over the far eastern sections (i.e. Salem, IL) hanging onto a light shower.

Bufkit overviews show saturation evacuating 07z-08z over KCOU/KJEF, 12z-13z around the STL metro and 14z-15z over the far eastern sections of the CWA. There is even a brief 6 hour period when much of the vertical depth of the column is dry over the souther half of the forecast area, hinting that some sunshine. Despite the improvement, the morning period is about as warm as we get today with colder air advecting northwest to southeast behind the front.
The colder trend continues into Sunday with high temperatures slightly below (by about 5 degrees) Sunday. Conditions remain breezy with surface winds of 10-15 mph occasionally gust to around 25 mph.

Maples

LONG TERM
(Sunday Night through Friday)
Issued at 231 AM CST Sat Dec 9 2023

Upper level troughing results in northwest flow aloft while surface and mid-level ridging translates from west to east Monday. Surface flow turns southerly as surface high pressure departs and centers over the southeast CONUS. A slight moderation in surface temperatures can be expected on return flow with the one caveat being the strong(er) surface high building Southeast out of the northern Plains. Clockwise flow around the surface high reinforces slightly cooler air southward into the mid-Mississippi Valley.
Recall yesterday's discussion the referenced high probabilities (60- 80 percent) for high temperatures reaching 50 degrees Tuesday.
Latest LREF ensemble data has backed down to 20-40 percent with north and northeasterly flow feeding into the region around the eastern side of the ridge. NBM has also back down on highs with 50 degrees nearly hugging the I-70 corridor and points south with a trend less favorable for above normal temperatures Tuesday.

Upper flow briefly turns zonal as the upper trough exits into the northeast CONUS. Meanwhile, an upper level shortwave tracks southeast down the western side of the Rocky Mountains Tuesday into Wednesday. The shortwave evolves into a broad, closed upper level low over the Intermountain West. An upper level low of this nature can often spawn fragmented segments of vorticity to the east/northeast that support low end PoPs in support settings.
However, there is fairly high confidence in dry conditions with a surface to mid-level riding pattern that is likely to support dry conditions. This comes in the form of a strong 1040mb surface high and building mid-level ridge that hold active weather to the west.

Guidance diverges in the late week period in the handling and eastward speed of the upper level low. The general theme support a warming trend Thursday into Friday with a modest ridge building northward into the Mississippi Valley ahead of system. Specifics become less clear Friday with timing differences in the advancement of the milder air and associated rain potential. NBMs slight chance and chance (15-30 percent) PoPs seem reasonable given the distance in time and disagreement among guidance. The key message toward the end of the week will be milder temperatures with an increasing potential for light rain with current trends.

Maples

AVIATION
(For the 12z TAFs through 12z Sunday Morning)
Issued at 518 AM CST Sat Dec 9 2023

A cold front has cleared to the east of the terminals early this morning with west and northwest surface flow behind the front.
Ceilings include a mix of VFR/MVFR conditions with scattered to overcast clouds across the regio. There a trend toward VFR expected through mid-morning. The only item to note otherwise is the potential for gusty winds at central Missouri terminals along with KUIN. Metro terminal will remain between two stonger wind fields

Maples

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

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