Belleville, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

389
FXUS63 KLSX 051911
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
203 PM CDT Sun Apr 5 2026

.KEY MESSAGES...

- Seasonably cool temperatures will persist through Tuesday
  including a 40 to 80 percent chance of a light freeze (30-32)
  in northeastern MO and west-central IL Tuesday morning.

- There is a 30 to 50 percent chance of light rain changing to
  light snow Monday night/Tuesday morning in northeastern MO.
  Light accumulation are possible, but confidence is not high that
  there will be impacts.

- Temperatures will return to above average Wednesday onward with
  increasing chances of showers and thunderstorms through the end
  of the week.


&&

.SHORT TERM...  (Through Late Monday Night)
Issued at 159 PM CDT Sun Apr 5 2026

A pair of surface highs will slide across parts of the Plains and
into the Midwest over the next couple of days. The first of these
tracks southeast out of the Plains, through southwest Missouri, and
into north Arkansas tonight. Wind gusts will subside this evening,
though sustained winds briefly lighten to around 5 mph before
increase to 5-10 mph after midnight. Despite that a reinforcing cold
front drifts north to south the the region, confidence is high that
frost is avoided with HREF combined probabilities showing <5% chance
of temperatures at or below 36 degrees and winds <5 mph anywhere in
the CWA. Lows will range from the mid/upper 30s for a large majority
of the area and closer to 40 degrees around urban areas.

The second, stronger (99th percentile) surface high is centered over
south-central Canada. It builds southeastward through the Upper
Midwest and into the southern Great Lakes Region late tonight
through Tuesday. An upper level shortwave presses into the central
Plains and induces a weak surface low at the lee side of the central
Rockies. The southern high glides south and east of yet another
reinforcement of colder air originating from the northern surface
ridge. While the surface low remains well departed to the west, the
increasing pressure gradient over the Plains strengthens
southwesterly flow with a strengthening southwesterly low level jet.
Guidance continues to show strong mid-level fgen somewhere over the
northern half of Missouri with disagreement in exact placement and
naturally results in difference in timing of northward advancement
through Tuesday morning.

What is more certain is that cloud cover will be on the increase
from northwest to southeast Monday morning onward. By Monday
evening, the 800-900 mb layer saturates, separated from upper level
saturation by mid/upper 900-650 mb dry layer, per NAM model
soundings. The difference may seem minor, but considering the narrow
banding of precipitation and it`s eventual northward progression, it
could determine the difference in precipitation type and intensity
over affected locations. The first concern is where the better upper
ascent will be placed as vorticity streams ahead of the parent upper
shortwave. Also, where the best omega is allocated in relation to
the saturated layers could determine precipitation type as guidance
shows a shallow mid-level layer of near/above freezing temperatures
within the saturated mid-level layer. This warm nose isn`t nearly
warm nor deep enough to create concerns of freezing rain, but could
be just warm enough to introduce localized mix with sleet/ice
pellets. Not only is this expected to be isolated, if it even
occurs, the dominant precipitation types will be rain/snow with the
weak magnitude and shallow nature of the warm layer. Therefore,
rain/snow was maintained in the forecast grids.

HREF ensembles show 40-50% probabilities for measurable
precipitation (0.01") running from Mexico, MO northwest through
Moberly, MO. However, a growing number of members show as much as
0.15" of total QPF in a very narrow band that, if it occurred, could
lead to some minor accumulations. The downside is that these bands
are very difficult to draw more exact conclusions and in some cases,
they over-perform. In collaboration with WPC, there was concern
expressed about the potential for 1"/hr rate under the strongest mid-
level fgen. This potential will likely favor the western half of the
extend of the band, which may stretch as far east as Knox, Shelby,
and Monroe Counties in Missouri. These rates would likely be brief
and limited to a 2-4 hour period between 06z-12z in the early
morning Tuesday. This does not rise to an advisory level event, even
in the worst case 90th percentile REFS (up to 2"), but it`s notable
with the late season timing. Not only does the band begin to lift
northward Tuesday morning, forcing weakens with the weakening of the
low level jet.

This setup will result in a wide range of highs Monday with low-50s
to mid-50s north of Hwy 36/I-72, mid-50s to mid-60s southward
through central and southwest Illinois, and upper 60s to near 70
degrees over southeast Missouri. In addition to the precipitation
potential, Lows Monday night drop near or just below freezing over
northeast Missouri and west-central Illinois with mid to upper 30s
elsewhere. Once again, winds remain between 5-10 mph, staving off
frost. The aforementioned boundary and cloud cover will determine
just how cold morning lows get across the northern CWA. This
continues to be monitored for potential freeze conditions, though at
this time, a hard freeze looks questionable with probabilities for
lows <30F at 10-25% north of Hwy 36/I-72.


Maples

&&

.LONG TERM...  (Tuesday through Next Sunday)
Issued at 159 PM CDT Sun Apr 5 2026

Light rain/snow lingers could linger over the northwestern sections
of the CWA Tuesday late morning or early afternoon. Precipitation
intensity should wane with the weakening low level jet to the west.
There is also some hint that the boundary could be positioned a
little farther north at this time, which would effectively shut off
all precipitation a little sooner.

It`s likely to be the coolest day of the week with 30s to start the
day under partly to mostly cloudy skies with more sunshine to the
south and thicker clouds to the north. The coldest locations over
the far northern sections of the forecast are flirt with the
freezing mark. Forecasts have been consistently plotting low-30s and
with ample cloud cover and surface winds above 5 knots, frost is not
a concern. Freeze conditions are low confidence with feedback from
agricultural partners informing us that 28-29 degrees is the tipping
point for recent blooms. While we continue to monitor this potential,
a hard freeze looks unlikely with ECM/GFS ensembles showing 6-hourly
run-to-run minimums consistently at 31-34F for the last 5 forecast
cycles. HREF ensembles show less <10% probability for temperatures
below 29F.

This cool spell will come to a rather rapid end as the strong
surface ridge moves east, the warm front lift north, and surface
flow turns out of the south Wednesday. Southerly flow around the
western side of the mid-Atlantic ridge will draw mid-level warm
northward with temperatures climbing to near 10C over the forecast
area through the day Wednesday. NBM IQR remains warm through the end
of the period with a slight decrease in spread the last few days.
One note worthy characteristic of the IQR is that it has begun to
warm, as a whole, through early next week. This suggest we may be on
the warm side of things as we progress through next weekend.

Although it looks warm, rain will become increasingly more likely
from late Wednesday evening/night onward. There will be a few things
to watch here: Surface low and upper trough moving across the
northern CONUS, tandem of upper lows traversing the west coast, and
the mid-Atlantic ridge to the east. Low pressure to the north will
send a cold front southward, but it doesn`t look to make it very far
before stalling at the back side of the ridge and being washed out
by enhancement in the southerly flow component to the south. The mid-
Atlantic ridge, while strong, sort of meanders occasionally through
early next week. It`s orientation and position will determine how
far east any showers and thunderstorms survive before decaying.

The pair of upper lows off the west coast come in piece-wise,
sending upper ascent over top the surface boundary that stalls at the
north/western side of the eastern ridge. Moisture drives northwest
from the western Gulf, bring dewpoints into the 50s/60s Thursday
through Saturday and low to mid-60s Sunday. Better CAPE (2000-2500
J/kg) remains over the Plains with decreasing values to the east.
Thunderstorms will be possible, but better potential remains west
and is very slow to advect eastward over the weekend, should the mid-
Atlantic ridge remain intact. Nonetheless, rounds of showers and
thunderstorms become more likely through the weekend with a slight
eastward shift in the overall pattern.

Temperatures will largely be in the 70s Wednesday through Friday.
We`ll tack on a few degrees each day next weekend. Low-80s become
more common by Sunday.


Maples

&&

.AVIATION...  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1200 AM CDT Sun Apr 5 2026

High pressure will maintain VFR conditions under clear skies
today. An occasionally gust can be expected through the daylight
hours before diminishing this evening. Light and variable surface
winds increase to 5-10 kts out of the northwest Monday morning.


Maples

&&

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

$$

WFO LSX