269
FXUS63 KLOT 050246
AFDLOT

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

.KEY MESSAGES...

- A period of soaking rain is expected across the Mississippi
  River Valley this evening and overnight. 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.

&&

.UPDATE...
Issued at 946 PM CDT Fri Apr 4 2025

A couple changes to forecast for tonight. First, have removed
thunderstorm chances from the grids for most areas, hanging
onto thunder chances just across far southeast CWA. Also,
lowered QPF across most of the CWA.

At mid-evening, an extensive QLCS extended from southern IN
southwest to Arkansas. This QLCS is intercepting the low level
jet and largely blocking significant moisture transport to its
north. The northern flanks of the "stratiform" rain on the back
of this QLCS continues to move across areas roughly near and
south of the Kankakee River at times. The trend overnight should
be for the back edge of this precip shield to edge
progressively farther south and southeast. Areas near/south of
Highway 24 will still likely see rainfall totals near or just
over a half inch, which given the wet antecedent conditions,
will likely lead to noteworthy responses on areas rivers and
streams. Rainfall rates have been and should remain light to
moderate and unlikely to result in anything more than ponding of
water in low lying and poor drainage areas.

To the north of this precip shield, areas of light rain and
drizzle should continue into at least the first half of the
overnight hours as shortwave trough moves across the region.
Low and mid level lapse rates are exceedingly poor, as seen on
both the DVN and ILX evening soundings, so it is hard to
envision any thunderstorms occurring north of the Kankakee
River. The evening soundings at DVN and ILX also nicely depicted
the significant moisture gradient across the area, with
substantially low pwats at DVN. So despite respectable forcing
with the approaching shortwave, expecting rainfall to primarily
remain light north of the Kankakee River.

Updated grids and text products have been sent.

- Izzi

&&

.DISCUSSION...
Issued at 406 PM CDT Fri Apr 4 2025

Through Saturday Night:

A broad upper trough continues to slow drift across the
southwest CONUS with a leading shortwave trough lifting over the
Mississippi River Valley. Beneath these features is a stalled
baroclinic zone that stretches from northern OH to northern AR
which has been the focus for shower and thunderstorm development
this afternoon. This baroclinic zone is expected to lift
northward further into central IL and IN tonight as the leading
shortwave pivots across our area. Therefore, showers will
continue to expand in coverage this evening and prevail through
the night with the possibility for a few embedded thunderstorms
especially for areas south of I-80.

While most of these showers will be on the light to moderate
intensity and provide nothing more than a good soaking rainfall,
the potential remains for a narrow band of heavier rain to
develop somewhere in and/or near our southern CWA. Latest
forecast trends continue to show this band establishing just
outside of our forecast area in east-central IL and IN, but
several hi-res forecast members have shown the band shifting
northward more into our area. The reason is because of a stout
low-level jet forecast to develop tonight and force a plume of
deeper moisture northward and allowing it to converge with the
aforementioned baroclinic zone. If this does occur then a swath
of rain in excess of 2 inches could manifest as far north as a
Pontiac to Valparaiso line and result in flooding of fields and
low-lying areas in addition to river rises given the saturated
soil conditions. However, if the developing thunderstorms in the
Ohio River Valley can rob this moisture plume and/or limit the
northward extent of the low-level jet as most guidance suggests,
then lesser rain amounts (upwards of 1-2 inches) would be the
more likely scenario and thus result in a lower flooding threat.


Due to the large amount of uncertainty with where this heavier
band will set up and how much rain will be within it, we have
decided to maintain the hydrologic statement for the southern
half of our CWA (basically areas along and south of I-80) to
highlight this threat. If trends become more clear this evening
that the heaviest rainfall will reach our area, then a Flood
Watch may be considered with future forecast updates. In the
meantime, be prepared for a soggy night and check back for
updates, especially if you reside in the Illinois and Kankakee
River Valleys.

A secondary shortwave trough and associated cold front will
pivot through northern IL and northwest IN Saturday morning
which will push the widespread rainfall south of our area. As a
result, many in northern IL will actually see a mostly rain-free
Saturday but there will be the chance for some scattered
showers to redevelop Saturday afternoon especially for those
south of I-80. While rainfall with these additional showers
looks non-impactful, they will have the potential to compound
any ongoing flooding from the aforementioned rain tonight.
Regardless, these showers will taper Saturday evening as the
front fully moves through.

Outside of the rain, expect seasonable temperatures to prevail
with highs in the 50s Saturday afternoon. Though, a chilly night
is expected for Saturday night where lows are forecast to dip
into the upper 20s to lower 30s.

Yack


Sunday through Friday:

Overall the forecast expectations for Sunday through much of
next week remain generally unchanged. See the previous
discussion below for details.

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/Yack

&&

.AVIATION /00Z TAFS THROUGH 06Z SUNDAY/...
Issued at 644 PM CDT Fri Apr 4 2025

Ongoing SHRA with VFR ceilings/visibility will spread across
the area early this evening, leading to a quick lowering of
ceilings into IFR levels late this evening. IFR ceilings are
then expected to persist through the night with periods of MVFR
visibility from RA/BR. The main RA axis will shift east of the
terminals around sunrise Saturday, with isolated -SHRA lingering
for a few hours. Ceilings will also rise and SCT into MVFR
levels late in the morning before daytime mixing redevelops
MVFR ceilings during the afternoon.

Winds will remain NE around 10 knots through the evening before
slowly backing overnight and shifting west of north around
sunrise. NNW winds around or higher than 10 knots will gust to
20 knots at times during the afternoon. A lake-modified boundary
should then shift winds NE up to 10 knots early in the evening,
though exact timing of this wind shift is less certain as the
boundary may struggle to make much westward progress against the
stronger NNW flow.

Kluber

&&

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

&&

$$

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