DeKalb, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

019
FXUS63 KLOT 101139
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
639 AM CDT Sun Aug 10 2025

.KEY MESSAGES...

- Threat for thunderstorms with embedded heavy rain today,
  mainly west of the Fox Valley.

- Periodic bouts of scattered, mainly afternoon/evening,
  thunderstorms through the first half of the upcoming work
  week.

- Thunderstorms will be capable of torrential downpours the next
  several days, leading to the potential for a localized flash
  flood threat.

&&

.DISCUSSION...
Issued at 254 AM CDT Sun Aug 10 2025

Through Monday:

Early this morning, an MCS is gradually lifting north across
southern WI and central Lake Michigan. This upwind-propagating
system brought hours of torrential rainfall to the Milwaukee
metro area prompting the issuance of a couple of Flash Flood
Warnings. South of this area of convection, recent obs suggest
and outflow boundary has pushed south of the state line into
far northern Cook County before curling back up into far
southern WI farther inland, and accordingly a few isolated
pockets of precip have been stirring along the state line
lately. RAP isentropic analysis depicts efficient moist upglide
south of and across the boundary through the low- and into the
mid-levels. Occasional showers and thunderstorms will remain
possible across our northern-most counties into the morning
around the outflow.

This upglide extends west beyond the Mississippi River along a
strengthening low level jet transporting rich deep moisture into
the river valley and wrapping into southern WI. The 00Z RAOB
out of DVN last evening recorded 1.9" of PWAT, and latest RAP
guidance suggests as many as 2.2" could build up around the
river by dawn. Along the nose of the LLJ, a low level confluent
axis resides across southern and eastern IA. Scattered
thunderstorms have recently begun to blossom in this general
area, including into northwest IL, and coverage is filling in
rather quickly. And even farther upstream, an MCV is spinning
eastward along the KS-NE state line beneath a low level
shortwave impulse, and coverage is quickly expanding into
southeast IA.

Expectations for the next few hours are for coverage to
continue expanding across IA and into northwest IL in the
warm/moist advective wing. It`s not clear how much coverage
we`ll see prior to dawn in our western CWA, but recent radar
trends suggest the I-88 corridor and points north, and west of
the Fox Valley will be the focus in our CWA for pockets of heavy
rain and thunder through daybreak. Lighter showers or sprinkles
will be possible farther south and east beneath a midlevel
stratus deck blanketing much of the area.

Around daybreak, the upstream MCV will begin pushing across IA.
Recent CAM guidance suggests the dynamic forcing into the very
thermodynamically favorable environment will trigger a
widespread plume of heavy rain ahead of the wave overspreading
IA and parts of northwest IL through the morning. Again it`s
unclear what coverage will look like into our CWA as the main
focus for heavy rain looks to align just outside of the area
closer to the river during the morning hours. The MCV will lift
northeast across the Mississippi and into central WI during the
afternoon. As it does though, the greater low level moisture and
forcing look to get pulled to our north, which could result in
a downward trend in coverage and intensity across our area
through the afternoon and into the evening. The highest chances
during the latter part of the day will remain west of the Fox
Valley. CAPE profiles are of the tall, skinny variety on
forecast soundings for this afternoon and shear remains poor, so
organized severe convection may be difficult to achieve. But
the moisture profile would be supportive of plentiful lightning
in storms and possibly wet downburst winds.

The stationary front that`s been hanging out to our northwest
for a couple of days now will inch closer into southern WI
tonight and additional showers/storms may fire along and south
of it through Monday. However, there`s quite the spread in model
solutions for Monday`s precip chances and I`m sure they`ll
influenced by what ends up happening today. So rode with the
NBM`s solution of widespread chance PoPs in Monday`s forecast
for the time being.

Temperatures this afternoon are expected to make it into middle
and upper 80s around most of the CWA. Heat indices may be as
warm as the middle 90s this afternoon. Conditions will likely be
kept cooler in our northwest with the greater cloud cover and
precip chances expected. There`s uncertainty in just how much
cooler; the forecast calls for lower 80s but there`s support to
be stuck in the 70s out west today. Similar conditions are
anticipated on Monday with highs in the 80s and heat indices up
in the 90s.

Doom


Monday Night through Saturday:

The potential for continued off-and-on rounds of showers and
thunderstorms will persist through Tuesday night/Wednesday as an
axis of high theta-e air with PWATs near 2 inches remains
nearly quasistationary overhead. Several perturbations will
slingshot their way through fairly active southwest flow during
this time frame, as a robust shortwave drops out of southern
Canada and sweep through the northern Plains.

Timing each of these individual perturbations remains
challenging, as, to some degree, convective augmentation will
take place each day with successive rounds of thunderstorm
development. That said, there is a decent signal in latest model
guidance that a synoptic wave (positively tilted in most
guidance) may impinge on the region Monday evening and
overnight, yielding an uptick in broad warm and moist advection
and an associated increase in shower and some thunderstorm
activity. The ensemble signal remains fairly strong that the
greatest coverage during this period into Tuesday morning will
remain near and northwest of I-55, within the core of the
modeled deepest tropospheric moisture. While 925-850 mb
southwesterly flow will be seasonably strong (near 35 kts at
times), not currently seeing signs of particularly robust
convergence axes which would spell more of a concerning heavy
rain and flood scenario. Some training of individual convective
clusters will be possible with cloud-bearing flow generally
southwest to northeast, aligned parallel with the main moist
axis, but nothing currently stands out as overly concerning from
a widespread flash flood perspective into Tuesday.

Guidance remains in decent agreement that a cold front will
press south through the region during the Tuesday
night/Wednesday timeframe. This should mostly bring an end to
the daily shower and storm chances, although the front might end
up getting hung up in our far south on Wednesday yielding some
additional chances for a few afternoon showers and storms.

Moisture and instability looks like it`ll end up sloshing back
across the region towards next weekend. Current extended
guidance suggests that our region may remain on the northern
periphery of lingering subsidence with a belt of stronger
cyclonic mid and upper level flow displaced across northern
Wisconsin and the UP of Michigan. This may end up keeping the
more appreciable storm chances to our north, although there are
some signals that the mid-level ridge could break down towards
the Saturday night - Monday time frame ahead of another front
which could open us up to the next round of MCS chances.

Carlaw

&&

.AVIATION /12Z TAFS THROUGH 18Z MONDAY/...
Issued at 638 AM CDT Sun Aug 10 2025

The main aviation weather concerns revolve around several
chances for showers and thunderstorms.

At RFD: Showers and perhaps a few thunderstorms will likely
continue developing this morning. Prevailing SHRA to account
for this. By mid-late morning, additional activity upstream is
expected to push towards and across the terminal, and converted
previous PROB30 group to TEMPO for on-station TSRA. IFR vsbys
and variable/gusty winds will be possible. Thereafter, a brief
break is possible, but an upstream disturbance could spark off
additional storms late this afternoon/early evening. Lower
confidence on that potential, so elected to confine this to a
PROB mention. Overnight tonight into early Monday morning,
another potential for showers and embedded storms exists, but
currently the greatest signal is south of RFD. To avoid
cluttering the TAF with an additional PROB group, elected to
keep things "precip-free" for now, but additional precip chances
do exist.

Chicago-area terminals: Satellite shower growing cumulus in the
region although at present, are not particularly deep.
Lightning threat is non-zero if these continue to grow, but not
currently high enough for a formal TS mention. PROB30 groups
for TSRA have been added for this afternoon to account for
activity across central Iowa and northern Missouri. An outflow
boundary could serve as a focus for renewed thunderstorm
development as this feature advances eastward, but chances for
this remain a bit too low (and time ranges a bit too broad) to
justify longer TEMPO groups. There is yet another potential
window for storms continuing into early evening (roughly 00-03z
time frame), but latest thinking is that any storms that develop
with this next disturbance will gradually weaken with east
extent towards the c90. Finally, there`s a decent signal for
additional shower and embedded TS activity developing late
tonight into early Monday morning, for which an additional
PROB30 group for MVFR SHRA has been introduced.

East to southeasterly winds at the Chicago-area sites are
expected to become southwesterly with some intermittent
gustiness today. Winds are largely expected to remain SSW/SW,
but outflow/chaotic winds near storms could encourage E/SE wind
shifts at times.

Carlaw

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flood Watch until 10 AM CDT this morning for ILZ003-ILZ004-
     ILZ005-ILZ006.

IN...None.
LM...None.

&&

$$

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