868
FXUS63 KLOT 041138
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
638 AM CDT Fri Apr 4 2025

.KEY MESSAGES...

- A period of soaking rain is expected across the Mississippi
  River Valley this afternoon into tonight. Chances for 1 inch
  of rain increase with southward extent across the area.

- A few areas may experience localized flooding tonight where
  rain is heaviest.

- Gusty snow/graupel showers possible on Monday.

&&

.DISCUSSION...
Issued at 233 AM CDT Fri Apr 4 2025

Through Saturday:

A large upper-level wave remains parked across the United
States, with deep troughing across the west and ridging to the
east. Such a regime places broad low- to upper-level flow atop a
stalled baroclinic zone draped across the central US, which
continues to support episodic rounds of showers and storms the
southern Plains to the northern Appalachian Mountains tied to
shortwaves embedded in the flow aloft. Our area is displaced
just to the north of the baroclinic zone and along the southern
periphery of the surface high pressure system centered over
Wisconsin at press time, leading to mostly clear skies,
relatively light northeasterly winds, and relatively chilly
temperatures in the mid 30s to around 40.

Over the next 6 to 12 hours, convection across west-central
Texas should expand and mature into one or more clusters (likely
tied to mid-level gravity waves) while moving northeastward
toward the middle Mississippi River Valley. Expansive mid-level
WAA and latent heat release associated with the masses of
convection, as well as an approaching northern-stream trough
from central Canada, will force a rapid development and
northward expansion of an anticyclonically-curved upper-level
jet streak into the Great Lakes. As a result, a broad region of
upper-level diffluence is expected to materialize across the
Mississippi River Valley this afternoon and tonight, setting the
stage for the low-level baroclinic zone to modestly pivot
northward and act as a highway for training showers and
thunderstorms.

It`s always tricky to pinpoint just how north baroclinic zones
will retreat in these types of scenarios, and indeed, both high-
res and global guidance offers different scenarios on exactly
where convection will parade overnight. Anytime the forecast has
a dependence on ongoing convection well upstream of our area
(vis-a-vis the thunderstorms in west-central texas at press
time), the old rule of them of "when in doubt, go south" usually
prevails. So, will favor model outcomes that depict the
baroclinic zone and region of active convection (including any
threat for flooding) remaining just to our south this afternoon
and overnight. With that said, will have to keep a close eye on
the northern edge of convection, with perhaps portions of Ford,
Iroquois, Newton, Jasper, and Benton counties still being within
striking distance of heavy rain tonight. Elsewhere, the general
area will still be within an area favorable for periods of mid-
level frontogenesis, which should still support periods of
showers from late afternoon through the overnight hours.

After daybreak, a cold front (tied to the aforementioned
Canadian shortwave) will move through our area. Around the same
time, the baroclinic zone should get shunted southeastward by
composite outflow. The net result should be a contraction of
shower coverage across our area southeastward, setting the stage
for a largely dry day. Overcast skies, north to northwesterly
winds, and modest cold air advection will lead to a chilly day
with highs in the upper 40s to lower 50s. As the next upper-
level shortwave rides the baroclinic zone toward the Ohio River
Valley Saturday afternoon, the shield of showers may expand back
toward our area. For now, we favor the highest coverage of
showers remaining across central Illinois to central Indiana,
just outside of our area.

Borchardt


Saturday Night through Thursday:

Surface high pressure will build across the region Saturday
night. As it does so, forecast soundings suggest the mid
levels will dry out rapidly, with the bulk of lingering
precipitation across our southeast likely to push out of the
forecast area. As a result, suspect that the latest NBM holds
onto mentionable PoPs a bit too deep into the overnight and
early-morning Sunday. For the time being, have left these in
place, but if trends in model guidance hold fast, we may be able
to pare these back in future grid updates. Aside from some
diurnally-building cumulus across our north, and lingering mid-
high cloud cover in our south, Sunday is looking pleasant with
highs in the lower 50s inland. An afternoon lake breeze will
hold temperatures in the mid and upper 40s lakeside.

On Monday, a vigorous shortwave is slated to push rapidly
southward out of the Arrowhead Region of Minnesota and across
lower Michigan through the afternoon. While some discrepancies
regarding (1) the availability of low-level moisture and (2) the
exact track of the parent vort max remain across the guidance
suite, the presence of strong synoptic scale forcing in the form
of DCVA and attendant 50 to 100 m/12 hour heights falls and
steepening 0-3 km lapse rates suggest there will be a
precipitation potential with this system. Have maintained chance
PoPs across the northeast half of the forecast area, with
chances decreases with inland extent/away from the lake. Based
on forecast thermodynamic profiles, with a bit of surface-based
CAPE and wet bulb zero heights just off the deck, somewhat more
intense snow or graupel showers would be possible, particularly
if the more aggressive GEM or ECMWF solutions were to verify.

A mid 1020s mb high will briefly build across the region on
Tuesday with dry and tranquil conditions expected. A series of
low amplitude/sheared shortwaves will subsequently advance
across the central CONUS Tuesday night into Wednesday as an
intensifying 150 kt jet streak scoots across the central Great
Plains. This will facilitate the development of a 40-50 kt
southwesterly low- level jet and the next round of warm
advection across the region. There`s a significant amount of
variability in the handling of these waves and whether any
notable surface cyclogenesis will occur, with the GFS and its
ensembles notably more aggressive in this regard. At this range,
the NBM-offered chance PoPs from Wednesday into Thursday look
appropriate.

Carlaw

&&

.AVIATION /12Z TAFS THROUGH 18Z SATURDAY/...
Issued at 638 AM CDT Fri Apr 4 2025

The key aviation weather messages are:

- Showers expanding in coverage this afternoon and evening.

- Cigs building down to MVFR and IFR this evening/overnight.

- East/northeasterly winds today will become northwesterly
  early Saturday morning.

VFR will prevail through the daytime hours today with
freshening east to northeasterly winds. Occasional 15-20 kt
gusts will be possible today.

Showers will then expand across the region late this afternoon
and evening with cigs expected to build down to IFR through 05z.
While the steadiest and heaviest showers are expected to set up
generally south an east of the Chicago-area terminals, MVFR
vsbys appear probable at all terminals. The threat for TS
remains too low to justify a VCTS mention at this point, but the
threat for embedded thunderstorms will exist, particularly
south and east of a PNT to VPZ line.

Showers/drizzle will linger through the overnight, before
precipitation ends from northwest to southeast through early
Saturday morning. NNE winds will become NW after about 12z
Saturday with cigs lifting to MVFR/VFR.

Carlaw

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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