St. Louis, Missouri

• A mild, largely-dry Halloween is in store with intermittent sprinkles/light rain being the only sensible weather.
• A more potent system arrives late Monday and brings the likelihood of a widespread, beneficial rainfall through at least Thursday. Hazardous weather associated with this system is very unlikely (less than 10%).
• Temperatures begin a gradual cooling trend today, but remain at least near early November normals.

Widespread mid-level cloud cover blankets the Mid-Mississippi Valley this morning, forced by diffluence and very meager isentropic ascent in advance of a dampened shortwave across the central Plains. Any precipitation of note is far to the north of our region along a wing of warm air advection across eastern Iowa and northern Illinois. The combination of the weak isentropic ascent and meager forcing boost from the approaching trough appear sufficient to threaten some sprinkles or light rain areawide today. Any drops that fall will be few, far-between, and unlikely to be measurable or cause problems for outdoor activities today. There is a reasonable (60%) chance among high-resolution guidance that the threat for this light rain ends by mid-afternoon, leaving trick-or-treaters with a mild, dry Halloween night.

The main thrust of the weak mid-level wave arrives well-after midnight, increasing rain chances mainly south of I-70 through Sunday morning. The eastward progress of this shortwave looks slower compared to 24 hours ago, which resulted in some low (20-30%) rain chances lingering into Sunday afternoon. Otherwise, expect temperatures to be noticeably cooler (by up to 10 degrees) Sunday compared to today.

Weak ridging aloft builds into the region as the shortwave departs late Sunday, leading to a dry and seasonable start to the next work week. Meanwhile, a much more potent trough spanning much of the western CONUS will track downstream and approach the Mississippi Valley. The deep southerly flow that results from such an amplified pattern will allow near-record moisture to transport poleward along a cold front to force widespread showers and a few thunderstorms. While ensemble members and deterministic guidance agree on the broad strokes of how this system will evolve as it tracks east, a few key details are less certain. Regardless of these differences, the system is forecast to bring a widespread, beneficial rain to the entire region early Tuesday through Thursday. Exact values will vary by location, and hinge to some extent on the less-certain aspects of the wave`s evolution, but NBM and LREF probabilities of 1.00 inches are now in the 50-80% range (highest across central and northeast Missouri). While lower elsewhere, appreciable rainfall is still forecast.

The most notable difference evident in the ensembles, according to EOF patterns of the individual members, is the amplitude and structure of this trough. How it evolves and interacts with a reinforcing shortwave amidst the polar jet stream will affect the track and strength of any surface features. Surface low tracks vary in the guidance from South Dakota to Kansas by Tuesday morning, impacting the system`s speed and available instability. As of now, most guidance either 1) keeps a single amplified wave that suppresses instability too far south or 2) shears off a second wave from the base of the main trough that causes additional surface cyclogenesis to pinch off instability. Either way, despite ample deep-layer shear, severe thunderstorm chances largely evade the Mid-Mississippi Valley. While model soundings vary a bit in how saturated the column becomes, and for how long it stays saturated, the factor that may inhibit heavy rain more is the lack of instability that was already discussed. While there is still time for shifts in the trough structure/surface low track to favor a particular hazard, currently there is little to suggest this system brings anything other than needed rain.

A growing number of ensemble solutions suggest the wave closes as it tracks overhead, making it more sluggish and difficult to advance downstream. As a result, rain chances will linger to some extent through much of the upcoming week. Exactly when rain chances drop off, and how much of a break we get before another system, is very uncertain with the upper-level patten depicted in wildly-different ways beyond Wednesday. The persistent rain chances and cloud cover, coupled with at least some more polar-type airmasses approaching, allow us to continue our trend cooler as we close the first week of November. Still, with normal temperatures also cooling, we likely won`t see temperatures too out of the ordinary for this time of year.

VFR flight conditions with occasional sprinkles or even a few areas of light rain will prevail this afternoon into the evening. Rain is expected to become more persistent and widespread this evening, and ceilings/visibilities will fall to MVFR from south to north tonight. Areas of IFR look likely by early Sunday morning across the eastern Ozarks, and possibly as far north as the I-70 corridor, however confidence is not high enough to include IFR in STL Metro or central Missouri TAFs at this time.

MO

None.

IL

None.