Decatur, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

806
FXUS63 KILX 191414
AFDILX

Area Forecast Discussion...Updated
National Weather Service Lincoln IL
914 AM CDT Sat Jul 19 2025

.KEY MESSAGES...

- Periods of storms are forecast through Monday, with the highest
  chances (60-70%) today, tonight into tomorrow morning, and
  tomorrow night into Monday morning. Any of these storms may
  generate severe weather and heavy rain leading to localized
  flooding.

- Heat and humidity will build into the area beginning Tuesday.
  There is a 60-70% chance heat indices surpass 105 by Wednesday
  and Thursday, when heat risk is slated to reach level 3 of 4 (or
  the "major" category).

&&

.UPDATE...
Issued at 914 AM CDT Sat Jul 19 2025

MCS that formed across Iowa earlier has been holding together
fairly well, though severe warnings have eased off over the last
couple hours. The line from that is now along the IA/IL border and
extends westward along the MO border, but ahead of it, a
northwest/southeast axis of storms has been increasing quite a bit
across a large part of the forecast area. Surface obs are showing
a cold pool behind the line in southeast Iowa, suggesting this
will continue to progress along and pose a periodic damaging wind
threat. Also have concerns for areas of flash flooding, as
precipitable water on our morning sounding was at 2 inches and
orientation of the storms is suggesting some training potential.

Since it`s appearing this may be the main show, in terms of
storms, significant updates to the rain chances have been made
through the evening. Northwest CWA may recover soon enough to get
high temperatures to the original projections, but highs in the
east may be tricky.


Geelhart

&&

.DISCUSSION...
Issued at 303 AM CDT Sat Jul 19 2025

***** SCATTERED STRONG STORMS TODAY *****

Nighttime microphysics satellite imagery reveals percolating stratus
across the Midwest ahead of thunderstorm clusters in southern MN,
northwest IA, and eastern NE. South-southwesterly flow in advance of
the attendant shortwave trough continues to advect into the region a
rich supply of deep moisture from the Gulf, with PWATs over 2 inches
in southern IL according to RAP mesoanalysis at 3am. With exception
to the FV3, the 00z CAMs and successive hourly iterations of the
HRRR suggest isolated showers and storms will develop across our
southwest counties within the next couple hours, and spread
northeast mid to late morning. Given 0-6km bulk shear less than 25
kt and 1000-2000 J/kg MUCAPE, these morning storms will be sub-
severe, but should generate some beneficial rain.

The question is then: how will this mess of morning convection
affect an incoming storm cluster/MCS early this afternoon? We
still don`t have an answer to that, because the CAMs remain in
disarray on specifics (location, timing, storm morphology). For
what it`s worth, the 04z and 05z HRRR runs seem to be best
capturing what`s happening upstream, and it suggests a messy line
of storms will bring gusty to locally severe wind gusts to areas
mainly near and north of the I-74 corridor early afternoon, with
convective coverage decreasing across our area in its wake (after
4pm) as synoptic subsidence suppresses updraft initiation behind
the shortwave. Given HREF mean brings 0- 6km shear to 25-35 kt
(highest north) ahead of these storms where MLCAPE (SBCAPE) values
climb to 2000-3000 (2500-4000) J/kg, sufficient storm
organization for severe weather seems plausible. Damaging wind
gusts to 60+mph would be the main risk, though can`t completely
rule out a brief tornado given low LCLs...nor hail up to 1 inch
with any discrete cells given plentiful instability. The past
couple HRRR iterations also advertise a backbuilding cluster or
nearly stationary cell south of the I-70 corridor this afternoon,
suggesting some risk for heavy rainfall totals leading to
localized flooding there.

***** ADDITIONAL ROUNDS OF STORMS TONIGHT THROUGH MONDAY *****

This evening into tonight, another piece of shortwave energy
cresting the ridge is expected to fire additional storms upstream
across Iowa, and these will be favored to ride the instability
gradient southeast into our neck of the woods. These will bear close
watching for severe weather (evening) and heavy rain potential
(evening into overnight). The 06z HRRR is especially aggressive
in depicting localized significant severe wind gusts (75+mph) and
high VILs/reflectivities suggesting severe hail with this wave,
but this is just one iteration of one model; at this point, we`ll
label this a low probability/high impact scenario and watch how it
evolves in successive iterations (and eventually mesoanalysis and
observations) closely. Training storms tonight may result in some
localized totals over 4 inches - exceeding 6h FFG and potentially
resulting in flooding - per HREF LPMM, though most locations will
only see beneficial totals (today and tonight, combined) between
0.75 and 2.5 inches.

The next round, to arrive tomorrow night into Monday morning, seems
to stand the highest chance of being problematic for 2 reasons:
First, the pattern may be slightly more conducive here given (1)
stronger moisture transport feeding into storms with a LLJ and (2)
the ridge and instability axis shifting east, which would put the
favored MCS track across our eastern CWA with the westward portion
of any clusters (where backbuilding would be most favored) across
our area. Second, soils across parts of the area will be saturated
from whatever rain falls today and tonight.

***** HUMID, INCREASINGLY HOT NEXT WORK WEEK *****

Guidance continues to trend ever so slightly slower with the arrival
of the ridge, with LREF maintaining 40% or higher chances for rain
right through the day on Monday and even some low (15-20%) chances
into early Tuesday as the ridge takes its sweet old time building
in. However, Tuesday into Wednesday, conditions will turn
increasingly hot and humid as the ridge axis shifts overhead and
agricultural evapotranspiration, bolstered by weekend-Monday precip,
becomes efficient at adding moisture to the low levels. Wednesday
into Thursday, high temperatures are forecast to climb into the
low- mid 90s with dewpoints in the upper 70s to low 80s, bringing
heat indices into the 105-112 degF range. Consequently, we`re
likely to need heat headlines, though we`ll once again hold off
due to uncertainty which is highest on Tuesday.

Bumgardner

&&

.AVIATION...  (For the 12z TAFs through 12z Sunday Morning)
Issued at 554 AM CDT Sat Jul 19 2025

VFR flight conditions should be predominant throughout the
forecast period, though there is a 30-50% chance of a brief MVFR
ceiling this morning from 12-16z (7-11am) according to HREF. A
band of showers and thunderstorms in west-central IL will drift
east-northeast, impacting SPI and PIA (and if it holds together,
the remaining terminals) early to mid morning. An MCS across
north-central IA will arrive late morning into early afternoon,
losing organization and resulting in scattered to numerous
thunderstorms during the 17-21z (noon-4pm) timeframe. Behind it,
subsidence should limit storms for the late afternoon-early
evening timeframe, but then additional storms may develop in IA
and spread southeast into the airfields after 04z/11pm; confidence
in timing and placement of those overnight storms was too low to
add another PROB30 group at this time.

Bumgardner

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$