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FXUS63 KLSX 020452 AFDLSX

Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1052 PM CST Fri Dec 1 2023

KEY MESSAGES

- Patchy fog is possible across much of the area late tonight through Saturday morning.

- There will be two opportunities for light rain across the area this weekend with the greatest chances being north of I-70.
Additional rounds of light rain is possible (less than a 30% chance)
across portions of the area early next week.

- Temperatures are forecast to be near normal for early December through mid-week with high confidence (90% +) in above normal temperatures towards the end of next week.



SHORT TERM
(Through Late Saturday Night)
Issued at 238 PM CST Fri Dec 1 2023

20Z surface observations show that a surface low pressure center is just to the northeast of the CWA in west-central Illinois. As the surface low continues northeastward, an associated secondary cold front behind the surface low will stretch across northern Missouri allowing winds to become more westerly. In the mid-levels, a vorticity maximum combined with increased low level moisture and lift from the cold front will provide support for a chance (20-30%) of light rain and drizzle across far northeastern Missouri late tonight. Where light rain does occur, amounts are forecast to be less than 0.1 inch. Confidence in these low rainfall amounts are further increased as less than 10% of ensemble members indicate rainfall amounts greater than 0.1 inch.

Amid primarily westerly flow, low temperatures overnight will be in the 30s, with areas across central and north central Missouri being in the lower 30s. Given that the coldest temperatures will be where light rain is forecasted, a stray snowflake or two mixed in cannot be completely ruled out. However, no impacts are expected should a snowflake reach the ground.

Patchy fog is forecast overnight tonight into early Saturday morning across much of the area with fog potentially becoming dense across northeast Missouri. Model soundings are significantly saturated near the surface. This saturated air combined with light winds will provide conditions favorable for fog and low visibilites.

Saturday will be another cloudy day as low-level moisture will continue to linger across the area. The next chance (40-70%) for light rain will be late Saturday night into Sunday morning as a shortwave trough digs across the Mid-Mississippi River Valley. The shortwave will also have an associated surface low that will move across the CWA southwest to northeast. The areas that will see the greatest chance (60-70%) for light rain will be locations north of I- 70. These locations have the greatest chance due to its location in relation to the surface low pressure and amount of available moisture. Approximately 50% of ensemble members have rainfall amounts across northeast Missouri and west-central Illinois exceeding 0.1 inch of rain, which increases Confidence in low rainfall amounts once again.

MMG/Byrd

LONG TERM
(Sunday through Next Friday)
Issued at 238 PM CST Fri Dec 1 2023

Ensemble guidance is in consensus that the next light rainfall potential (30% chance) is expected late Sunday night through Monday morning as another shortwave treks across the Mid-Mississippi River Valley. This shortwave trough will round the trough that will be positioned over the central CONUS. An associated surface low pressure system will traverse across the CWA at the same time. The available moisture will be fairly limited due to the previous shortwaves from the weekend using up much of that available moisture. There remains some uncertainty in the rainfall locations. If the surface low pressure system traverses across southern the parts of the CWA, a wider swath of the CWA will see light rainfall. Confidence in low rainfall amounts is further increased given that approximately 10% of ensemble members indicate rainfall amounts greater than 0.1 inch.

As the mid-level trough shifts east through the first half of the week, there is agreement among deterministic guidance that another shortwave will round the mid-level trough across the Mid-Mississippi River Valley. Precipitation chances with the shortwave remain well to the northeast of the CWA due to a lack of low-level available moisture in our area.

A mid-level ridge will help push the trough to the east and allow for a warm up for the later half of the work week with high temperatures forecast to be above climatological normal.
Confidence is high (over 90%) in these above normal temperatures given that the the lower quartile of NBM members have high temperatures at least 5 to 10 degrees above normal for early December.

MMG/Byrd

AVIATION
(For the 06z TAFs through 06z Saturday Night)
Issued at 1050 PM CST Fri Dec 1 2023

The biggest change since the last update is the removal of widespread visibility reductions as earlier guidance was far too pessimistic with regard to lower visibilities. Trends continue to point to gradual improvement in this aspect of the TAF with west and northwest flow remaining up around 5-10 knots. KUIN may bounce around from 4SM-P6SM in the first few hours of the TAF period, but gradually eases out of it through the morning.

MVFR/IFR ceilings continue to maintain their grip throughout the region with cloud bases generally ranging 400-1500 feet over the terminals. IFR is expected to play out through a majority of the night into Saturday morning before slowly lifting to MVFR ahead of the next system that arrives Saturday evening. There is a small window when metro terminals could briefly lift to VFR, but the gap is quite short if it does materialize. Otherwise, expect mostly MVFR ceilings with chances for rain approaching out of the west/southwest around 00z and after.

Maples

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

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