249
FXUS63 KIWX 161054
AFDIWX

Area Forecast Discussion
National Weather Service Northern Indiana
654 AM EDT Wed Jul 16 2025

.KEY MESSAGES...

- Hot and humid conditions today with heat indices in the lower to
  middle 90s expected.

- Scattered showers and thunderstorms expected today, most
  numerous along and east of Interstate 69. Severe weather is
  not expected, but brief heavy rainfall and wind gusts to
  around 40 mph possible with a few storms.

- Thunderstorms chances continue tonight, especially west of US Route
  31. There is a Marginal risk of severe storms with isolated
  damaging wind gusts possible. The most favored time period for
  an isolated severe weather threat is from approximately 7pm
  to 11pm EDT.

- Brief return of less humid conditions Thursday/Friday behind
  a cold front. This front will also likely bring an increased
  swim risk for southeast Lake Michigan beaches later Thursday
  and Friday.

&&

.DISCUSSION...
Issued at 345 AM EDT Wed Jul 16 2025

For the near term, will be continuing to watch the evolution of
a small diabatically enhanced upper vort max as it slowly lifts
northeast from southeast Illinois this morning. Broad positive
theta-e advection continues in advance of this feature. The
initial surge of better low level moisture transport late
yesterday afternoon/evening lifted across NW IN/NE IL where
pocket of better MUCAPES in the 500-1000 J/kg allowed isolated
storms to form in Lake Michigan vicinity last evening. Low level
winds have started to veer more southwesterly which will focus
better low level moisture transport into far NE IN/northern OH
through early this afternoon. This should keep the greater
scattered-numerous shower coverage along/east of the I-69
corridor today. A very high PWAT airmass will accompany the
advective surge ahead of this upstream vort max today with
MLCAPES possibly into the 1500-2000 J/kg range this afternoon.
Shear will be the missing ingredient for an appreciable severe
weather threat this afternoon, but would suspect a few instances
of 40+ mph thunderstorm wind gusts would be possible with
localized downdrafts across especially eastern portions of the
forecast area. Otherwise, today, the combination of convective
clouds/precip may keep heat indices limited to the low to mid
90s despite the highly anomalous PWATs.

For this evening, attention will turn to possibility of a
convectively enhanced short wave moving in from northern Illinois.
An MCS is currently affecting eastern NE/western Iowa, and this MCS
has been forced by a decent synoptic mid level trough ejecting
out of the Central Plains. Remnant MCV from this decaying
complex this morning may drift east-northeast into the western
Great Lakes region by this evening. The weak downstream ridging
in place across the southern Great Lakes may take favored MCV
track into northeast WI/northern Lower MI tonight with a
potential of some trailing convection on southern flank of this
MCV tracking across NW IN/SW Lower MI. Background shear will
still be on the weak side but likely locally enhanced in
vicinity of the MCV. At this time feel the better chance for any
small MCS/greater wind threat may remain west and north of the
local area, but a small temporal window centered in the 00Z-03Z
timeframe could exist for isold wind threat.

HREF consensus would tend to take an accompanying cold frontal
boundary southeast on Thursday, with a little more progression than
previous indications. Upon collaboration with surrounding WFOs, did
lower PoPs a bit from blended guidance during the day Thursday given
this recent trend. The threat of stronger storms should be southeast
of the local area however. Less humid conditions can be expected
behind the cold front on Thursday with highs from the mid 70s
far NW to the low to mid 80s far southeast. Far southeast
locations will still likely flirt with heat indices around 90
with better push of drier air arriving Thursday evening.

Friday still looks to be a pleasant day with some short-lived lower
humidity and enough suppression of stalling sfc boundary to keep the
area dry.

A series of eastern Pacific waves will provide a few rounds of
better westerly deep layer flow clipping southern Great Lakes for
the weekend, with current indications suggesting best return theta-e
advective forcing still in the later Saturday/early Sunday timeframe
with renewed convective chances.  However, given potential of so
many upstream convectively modified disturbances, predictability in
the details wanes Saturday and beyond in this pattern. Medium
range guidance continues to suggest low amplitude upper ridge
becoming re-established late weekend, with some amplification
of the longwave pattern possible early next week. It does appear
that a prolonged period of heat/humidity concerns will be
likely for much of next week, and represents a high confidence
element in the extended forecast. GEFS/EPS means generally
support deterministic idea of the longwave upper ridge being
maintained west of the local area, possibly keeping southern
Great Lakes/Ohio Valley in the proximity of a strong instability
gradient that would favor periodic higher chances of showers
and storms. Unfortunately, a broad brush approach to PoPs has to
be maintained once again given this synoptic evolution, with
each day of the extended period having some chance of showers
and thunderstorms at this forecast distance.

&&

.AVIATION /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 646 AM EDT Wed Jul 16 2025

A very moist environment will remain over the area as isolated
showers continue to develop over northern Indiana this morning.
The activity will increase with daytime heating. The latest HRRR
indicates the best chances for storms will be after 00Z. Rapid
development is expected as the cells develop over northwest
Indiana and move southeast.


&&

.IWX WATCHES/WARNINGS/ADVISORIES...
IN...Air Quality Alert until 4 AM CDT Thursday for INZ103-203.
OH...None.
MI...Air Quality Alert until 5 AM EDT Thursday for MIZ078-177-277.
MARINE...None.

&&

$$

DISCUSSION...Marsili
AVIATION...Skipper