NOTE: Some of the data on this page has not been verified and should be used with that in mind. It may and occasionally will, be wrong. The tide reports are by xtide and are NOT FOR NAVIGATION. Privacy and Cookie policy

FXUS63 KLSX 052334 AFDLSX

Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 534 PM CST Tue Dec 5 2023

KEY MESSAGES

- Near average temperatures are expected tomorrow, but well above average temperatures are almost certain Thursday and Friday (90+%).

- Widespread rain is a near certainty (90+%) between sometime late Friday and early Sunday.

- There is a limited potential for thunderstorms Friday night and Saturday, and a few wet snowflakes Saturday night and Sunday.
Confidence in either of these occurring remains low.



SHORT TERM
(Through Late Wednesday Night)
Issued at 230 PM CST Tue Dec 5 2023

While we started the day with plenty of sun early this morning, this quickly changed as a cold front swept through the area before noon.
The primary driver of this cold front is a progressive shortwave moving quickly through the Great Lakes region, along with its associated surface low now moving into central Indiana. Behind this front, cyclonic flow and cooling aloft has produced widespread low cloud cover, and a tight pressure gradient is driving gusty northwesterly winds. In other words, weather conditions are gloomy and brisk...a rude departure from the sunny skies at sunrise. This is expected to continue for the remainder of the day, and low clouds may even linger into the morning hours tomorrow before finally dissipating. Meanwhile, surface high pressure will move into the area overnight, and winds will diminish as a result.

By tomorrow, an upper level ridge will build across the central U.S.
and spread eastward into the Mississippi Valley region. Meanwhile, surface high pressure will move to our east, allowing modest southerly low level flow to resume. While lingering cloud cover may keep temperatures suppressed in areas east of the Mississippi tomorrow, the building ridge and southerly flow is likely to begin a modest warming trend, bringing afternoon maximums to near seasonal averages. No precipitation is expected, and skies are expected to mostly clear by mid afternoon in most areas.

BRC

LONG TERM
(Thursday through Next Tuesday)
Issued at 230 PM CST Tue Dec 5 2023

On Thursday and Friday, the surface pressure gradient will continue to tighten in response to large-scale surface pressure falls across the Great Plains. This will drive steadily increasing 850mb southerly wind anomalies per the NAEFS and GEFS ensembles, reaching near the 90th percentile by Friday night. This surging south southwesterly low level flow will contribute a rapid warming trend Thursday, with ensemble mean 850mb temperature anomalies reaching between the 90th and 99th percentiles. Meanwhile, ensemble temperature spreads remain narrow on both Thursday and Friday, with the interquartile ranges entirely between 10 and 15 degrees above seasonal averages. And while we'll likely see steadily increasing cloud cover during the day Friday, the end of the work week is likely to be dry.

Significant changes are expected over the weekend, though, as a dynamic trough is expected to quickly move through the area between Friday night and Sunday. While this trough is still several days away and is currently well out over the northern Pacific, confidence is growing that it will move through the area as a substantial system over the weekend. Exactly what impacts it will bring remain somewhat uncertain, but there are a few key things that are generally known at this stage.

Firstly, widespread precipitation will occur, and the majority of it will fall as rain between Friday evening and Sunday morning. As the trough approaches, it is expected to gradually take on a negative tilt, which will drive cyclogenesis somewhere across the region.
While the timing and track details of this developing surface low remain somewhat uncertain, it is very likely that the warm sector will see anomalously high moisture for this time of year (PWATS likely 90th percentile or higher per the NAEFS/GEFS/ECM). When coupled with the strong southerly flow and isentropic ascent in the warm sector, followed by the dynamic forcing associated with the trough and surging cold front, and it's difficult to envision a scenario that does not include widespread precipitation regardless of potential storm track variability. Ensemble members also unanimously agree, albeit with timing/amount differences across individual members.

Meanwhile, warm sector temperatures will keep the bulk of this precipitation as rain, with the only chance for frozen precipitation occurring behind the cold front late Saturday or early Sunday.
Considering that the post-frontal airmass does not appear to be particularly cold from a climatological perspective, and only a very small minority of ensemble members produce any snow at all (10 to 20%), the likelihood of impactful snow accumulations appears to be low at this stage. However, some wet snowflakes and even minor accumulations can't be completely ruled out just yet, and we do maintain a rain/snow mix in our forecast during this time period.

Finally, there is also a limited potential for strong or severe thunderstorms during the day Saturday, pending the very tenuous development of sufficient instability. Widespread cloud cover and precipitation is likely to be ongoing Friday night and Saturday morning, which is likely to limit heating and put an upper limit on destabilization Saturday. However, strong southerly flow and cooling aloft as the trough digs into the area may provide just enough of a boost to generate enough instability for thunderstorms. At the present time, the NBM suggests that there is a roughly 1 in 10 chance to see CAPE values up to 500 J/kg, and in a regime of strong wind fields. While those values are very marginal, it leaves the door open just wide enough for consideration. In any case, higher probabilities for strong or severe storms remain well to our south where instability is likely to be higher.

By early next week, high pressure is highly likely to settle back into the region, bringing the return of dry conditions and near average temperatures.

BRC

AVIATION
(For the 00z TAFs through 00z Wednesday Evening)
Issued at 522 PM CST Tue Dec 5 2023

Ceilings below 2000FT AGL will remain in UIN through 10Z and the St. Louis area terminals through 02Z. Otherwise MVFR ceilings are expected at COU and JEF through 04-05Z and at UIN and the St.
area terminals through 13-15Z. Chances for precipitation will remain low. Northwest winds will gust around 20 knots but diminish early this evening and become light overnight.

Britt

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

toggle option: (graph/table)

Full Color Weather Map

Link to Loop


Other links: GEOS-West     Contential US     Full GOES-East

St. Louis, MO,