Romeoville, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

606
FXUS63 KDVN 151056
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
556 AM CDT Fri Aug 15 2025

.KEY MESSAGES...

- Heat and humidity return today through the weekend, with heat
  indices near or exceeding 100 degrees. Storms/outflow could interrupt
  the heat for parts of the area, mainly north of Highway 30
  Saturday and Sunday.

- Storm chances (20-30%) return tonight/Saturday and persist
  through early next week, especially along and north of Highway
  30.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 325 AM CDT Fri Aug 15 2025

After a period of seasonal conditions earlier this week, the heat
and humidity returns today as southerly flow develops in the wake of
a departing high pressure system over the eastern Great Lakes, along
with broad anticyclonic flow across the central/southern Plains
building a thermal ridge into our region. Both the GEFS and ECMWF
ensembles suggest 850 mb temperatures warm up to around 20 to 22
degrees C this afternoon into the evening. These temperatures would
be around the 90th percentile per the SPC sounding climatology for
00z DVN 850 mb temperatures, so it will definitely feel warmer today
compared to the last few days. Additionally, it will feel more
muggy, too, as dew points increase to the lower 70s area-wide. This
will help heat indices increase to the mid/upper 90s to near 100
degrees for some locations. One thing that could keep our
temperatures down today (and really for the next few days!) may be
if we can get any cloud debris from showers and storms that could
develop over central Iowa this afternoon. CAMs have somewhat of a
mixed-bag of solutions for convection this afternoon over central
Iowa as a mid-level impulse approaches, so confidence is a bit lower
for heat indices today, especially across our northern CWA. We do
expect some locales across our south to likely touch 100 degrees F
heat indices, but this appears likely to be for a brief period of
time, so we decided not to issue any heat headlines due to the
marginal nature of the heat indices.

This evening into tonight, we will need to watch for increasing
potential of nocturnal convection ahead of the aforementioned
impulse that is progged to translate across southern MN into
southwestern WI. Again, CAMs are showing a range of possible
outcomes for convective development, ranging from a few isolated
cells to a full-blown MCS per the ARW, but the overall consensus of
the guidance indicates the potential for perhaps isolated storms,
some of which could become strong to severe. With the increased
moisture and steep mid-level lapse rates between 7 to 8 C/km, the
main threat with any severe storms would be large hail and locally
strong wind gusts. With that said, SPC has expanded the Marginal
Risk (level 1 of 5) slightly to coverage locations just north of the
Highway 30 corridor.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
Issued at 325 AM CDT Fri Aug 15 2025

The synoptic pattern doesn`t change much for Saturday through Monday
in our region, with hot and humid conditions persisting and periodic
chances (20-30%) of storms, especially for locations along and north
of Highway 30. 850 mb temperatures appear to warm even further for
Saturday and Sunday, with the latest ECMWF EFI indicating values of
0.7 to 0.9 for Saturday and Sunday high temperatures, which suggests
anomalously hot conditions are likely, with highs likely to rise to
the lower to middle 90s. Confidence in dangerous heat indices over
100 degrees F Saturday and Sunday is higher compared to today, with
both HREF and LREF ensemble exceedance probabilities of 100 degree
heat indices around 70 to 90+ percent for locations along and south
of Interstate 80. The LREF has probabilities of exceedance for the
same threshold around 60 to 80 percent for Sunday and Monday. Given
these values, our confidence is pretty high that we will need at
least Heat Advisories both Saturday and Sunday for the southern
portions of the CWA. Confidence is a bit lower farther north on heat
index values due to potential for cloud debris and showers/storm
potential which could tamper these values.

Speaking of, we continue to be on the edge of the "ring of fire"
pattern, with the thermal ridging remaining in place. The baroclinic
zone/stationary front remains to our north, but some showers/storms
could develop along this front. Some of these storms could become
strong to severe, and SPC continues to outlook our northern areas in
a Marginal Risk for both Saturday and Sunday for a damaging
wind/large hail threat. While instability will be quite impressive
over the next few days, shear will be largely lacking to support
organized convective updrafts, but mid-day Saturday along the
Highway 20 corridor looks to be the only time deep-layer shear will
be its highest of this period (between 30-40 knots), so if any
storms can develop during this time, they could become severe at
that time.

Tuesday through Thursday, the heat and humidity looks to be scoured
out by a cold front that will dive southward through the area by
sometime Wednesday (uncertain on the timing of the frontal passage).
Behind this front, guidance is in good agreement on bringing in
cooler and drier conditions for mid to late next week.

&&

.AVIATION /12Z TAFS THROUGH 12Z SATURDAY/...
Issued at 556 AM CDT Fri Aug 15 2025

Mostly VFR conditions are expected for the TAF period. Some
passing mid clouds are expected this morning, with light
south-southeasterly winds (5 knots or less). Winds are expected
to slightly strengthen to around 10 knots or so from the south
this afternoon, with some FEW to SCT daytime cumulus clouds
area-wide. Models are just starting to come into better
agreement on a window of thunderstorms moving through
northeastern Iowa by this evening, but confidence remains low on
just how far south they will go. For now, used a PROB30 group
for DBQ, but kept any TSRA mention out for CID and MLI.

&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...None.
IL...None.
MO...None.

&&

$$

SHORT TERM...Schultz
LONG TERM...Schultz
AVIATION...Schultz