Marine Weather and Tide Forecast for Palmyra, IL

SHORT TERM
(Through Late Wednesday Afternoon)
Issued at 301 AM CDT Tue Apr 21 2026

While largely non-hazardous weather is expected in the short term (today and tomorrow), there are a few weather items to discuss, including the continuation of a warming trend, gusty southwest winds, and a very limited potential for some showers in our northern areas this afternoon/overnight.

Firstly, the resumption of southwesterly low level flow has finally put an end to the potential for patchy frost this morning, thanks to a combination of warm air advection and mechanical mixing.
Temperatures as of 1 am had only dropped into the upper 40s to mid 50s, and are only expected to drop perhaps another 5 degrees or so by sunrise. As such, this should easily take frost off the table, and set the stage for an even warmer afternoon than yesterday.

As the morning progresses, a deepening boundary layer will begin to tap into a robust southwesterly low level jet just above the surface, and surface winds will begin to increase quickly as a result. While we do not expect gust speeds this afternoon to reach Advisory levels, sustained speeds of 15 to 20 mph with gusts to 30 or 35 mph will not be difficult to achieve, especially north of the I-44 corridor. While a nuisance for some outdoor activities, these speeds should not be otherwise impactful, considering that green/wet fuels and moderate humidity will mitigate the fire threat.

Meanwhile, these persistent southwesterlies will also drive continued warm air advection, resulting in another boost in temperatures today. Afternoon maximums are likely to reach near 80 degrees area-wide, and perhaps slightly warmer along the Missouri River and St. Louis thanks to both downsloping and the urban heat island.

Later in the afternoon, a subtle shortwave is expected to drive a weak cold front through Iowa, and it is expected to stall somewhere along our northern periphery. Moisture pooling along this boundary combined with ample heating and steep mid level lapse rates will boost instability. However, a substantial capping inversion is expected to render this instability largely inaccessible for surface parcels, especially considering that the weakening front does not appear to be a strong initiating mechanism. While we may see a few elevated showers or even weak thunderstorms make a run at parts of west central Illinois and far northeast Missouri late this afternoon and overnight, we do not expect this activity to be significant. Most high resolution guidance also supports this, although we do note a few outliers that carry a few small cells into our area.

By tomorrow, a weakening surface pressure gradient will allow winds to weaken, although we will maintain southwesterlies. Meanwhile, a highly amplified upper ridge will move directly overhead, and as a result we will maintain similar temperatures and largely dry conditions Wednesday. In fact, this should produce the most pleasant weather of the week, as more rain is on the way late Thursday onward.

BRC

LONG TERM
(Wednesday Night through Monday)
Issued at 301 AM CDT Tue Apr 21 2026

The biggest story during the extended period continues to be the return of an active, wet pattern, which is expected to bring another round of soaking rain and at least a few thunderstorms between late Thursday and late Friday, followed by additional rounds of rain late in the weekend and early next week.

Driving this change is a rather complex upper flow pattern, which features the phasing of a weakening Pacific low (currently producing rain in the California Coast) with both a northern stream trough and a southern-stream subtropical jet streak. If that sounds like a lot of things happening at once, you would be correct in your assumption. The surface response to all of this is expected to manifest as a broad, deep, low pressure system across the northern high Plains by Wednesday afternoon, which will slowly move east by Thursday evening. This will drive strong southerly return flow and a narrow plume of moisture into the central Plains and Mississippi river Valley ahead of a slow-moving cold front.

Within this moisture plume, building instability and surface convergence from the front will drive widespread showers and thunderstorms during the day Thursday, initially to our west where strong/severe storms are increasingly likely. This area of showers and storms will move east and into our area sometime late Thursday and through the day Friday, and while there remains some timing uncertainty (particularly later in the event), most ensembles/clusters have converged on Thursday night/Friday morning as the window of highest probability for our area, with largely warm, dry, and breezy conditions earlier in the day. Whether or not this timing fluctuates slightly, there is high confidence (80-90%+) that most of our area will see rain sometime in this window. Meanwhile, ensemble mean PWAT values approach the 90th percentile within a narrow moisture plume, and when combined with the slow moving front that is nearly parallel to the mean flow, soaking rain is well within reason (50-70% chance for at least 1/2 inch per the NBM). While these probabilities don't immediately suggest a significant flooding threat, many places saw significant rains over the last week, and higher rain totals due to convective elements may be getting washed out a bit by ensemble spread (90th percentile NBM amounts range from 1.5 to 2.5 inches).
This will need to be monitored as the event gets closer.

As for strong/severe storms, the increasing confidence in a nocturnal event does limit our potential somewhat due to the reduced instability, not to mention our area may also be displaced from the strongest wind shear. Still, this will need to be watched, particularly in central/northeast Missouri Thursday evening at the onset of the event, and Friday afternoon/evening in the Ozarks if the front stalls and instability can redevelop in our area.

On the latter point, confidence in the evolution of this event decreases significantly beyond Friday morning, as ensembles/clusters start to diverge regarding the southward progression of the front and also the next approaching storm system later in the weekend and early next week. There are some indications that the front may stall or even lift back northward Friday thanks to the influence of the subtropical jet (and another developing surface low to the west), but this is far from unanimous, and we are now getting very far into the weeds at this time range. This slower solution would prolong shower/thunderstorm chances Friday, although thunderstorm chances especially would also depend on lingering cloud cover/precipitation/cold pools from the morning activity.

As the weekend progresses into early next week, ensembles suggest that the upper flow pattern may briefly transition into an omega block across the Midwest, followed quickly by another southern- stream trough undercutting it. Again, there is considerable model disagreement on many of these details at this stage, but in spite of this, ensemble guidance also depicts a relatively high probability (50-70%) of being impacted by another round of precipitation sometime in the Sunday/Monday timeframe.

BRC

AVIATION
(For the 06z TAFs through 06z Tuesday Night)
Issued at 1056 PM CDT Mon Apr 20 2026

Dry and VFR flight conditions are expected to persist through the TAF period with southerly winds continuing to veer to southwesterly Tuesday morning and beginning to occasionally gust 23 to 27 kt.
Winds will slacken some Tuesday night, but gusts may become more intermittent than ending altogether at KCOU and KUIN. A cold front will also approach KUIN Tuesday evening, triggering showers and thunderstorms across southeastern IA and central/south-central IL.
However, confidence is low that showers or thunderstorms will impact KUIN, precluding inclusion of such at this time.

Pfahler

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