IWX issues Area Forecast Discussion (AFD) at Aug 18, 2:17 PM EDT
326
FXUS63 KIWX 181817
AFDIWX
Area Forecast Discussion
National Weather Service Northern Indiana
217 PM EDT Mon Aug 18 2025
.KEY MESSAGES...
- Chances of showers and storms (30-55%) return for tonight into
Tuesday. Severe weather is not expected at this time, but a
few storms could produce some locally heavy rainfall and gusty
winds to 40 mph.
- Remaining hot and humid into Tuesday with heat indices in the
upper 80s to middle 90s.
- Confidence increasing in cooler and drier conditions mid-late
week into early next week.
- Moderate swim risk today for Lake Michigan beaches along La
Porte County, IN and Southern Berrien County, MI. Currents and
breaking waves of 2 to 3 feet expected. Waves subside to
around 1 ft by the afternoon.
&&
.DISCUSSION...
Issued at 400 AM EDT Mon Aug 18 2025
A westward advection of drier low level air continues this morning
via low level easterlies accompanying the extensive anticyclone
across Ontario/Quebec. This is a shallow push of drier air as
evident by an expanding stratocu deck across south central lower
Michigan into northwest Ohio marking the eastward displaced low
level theta-e boundary at approximately 850 mb. This
thermal/moisture stratification is depicted in 00Z DTX RAOB from
last evening with a very strong low level frontal inversion. The sfc
and near sfc theta-e gradient becomes anchored across extreme
northwest Indiana this morning and this will mark a sharp west-east
instability gradient that will make PoP forecast quite a challenge
into tonight. Have noted some isolated cells developing in
regional radar mosaics across northeast/north central IL over
the past few hours in the presence of a broad westerly advective
forcing regime.
Heading through the morning hours, a low amplitude upper level short
wave across the northern Great Lakes/Upper Midwest should allow for
slight strengthening of low level west/southwest flow across the Mid
MS Valley. This slightly stronger flow at base of this trough
coupled with the tightening near sfc theta-e gradient across the
southwest Great Lakes could be enough forcing for a few showers
to develop later this morning across southwest Lower Michigan
into northwest Indiana. An isolated storm is possible but mid
level lapse rates remain quite weak.
By this afternoon, the near sfc theta-e gradient will begin to
slowly nudge back eastward with the eastward migration of the Upper
Midwest wave. Best chance of thunder this afternoon may remain back
across northern Illinois, but some uncertainty persists regarding
how eastern NE/western IA convective complex affects
progression/strength of upstream short wave and whether any
associated outflows become focal points for additional convection.
The sharp instability gradient should persist across far western
portions of the CWA into this afternoon, so will maintain low chance
PoPs far west this afternoon/early this evening. Otherwise for
today, seasonably warm conditions are expected with heat indices
expected to reach into the low to possibly mid 90s across the far
west/southwest in closer proximity to the better low level moisture.
The better chances of storms still appear on track for tonight
as upstream upper ridge amplification allows height suppression
locally and a weak mid level trough to dig across the southern
Great Lakes. The theta-e gradient will also shift west to east
across the area providing some better instability profiles for
thunder for all but possibly far east/southeast. Some modest
shear will also accompany the aforementioned Great Lakes short
wave (~25-30 knots of 500 mb flow). Severe threat still appears
to be on the minimal side, but cannot completely rule out a few
gusty SPS level storms across SW Lower MI/far NW IN with some
localized heavy rain threat.
Mid to high chance PoPs were maintained for Tuesday as cold frontal
boundary begins to drop southeast across the area. Best instability
in more of a northwest flow pattern by this time may get pinched off
across southeast portions of the area across NE IN/NW OH, which
would be somewhat divorced from better mid/upper level support
beginning to approach the eastern Great Lakes. While severe
storms are not expected Tuesday, weakening steering flow by this
time could support some isolated heavy rain threat once again.
Latest data still supports trends of previous forecasts in a trend
to drier and less humid conditions post-Wednesday. Medium range
guidance is in decent agreement that a stable longwave pattern
(western CONUS ridge, eastern CONUS trough) will develop this
weekend which would mark a sharper trend to cooler conditions later
this weekend into early next week. Some low rain chances may need to
be introduced sometime late Friday into Saturday with possible sfc
trough marking digging nature of Canadian upper low and onset of
this cooler airmass. However, with expected limited moisture
quality, a dry forecast has been maintained.
&&
.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 130 PM EDT Mon Aug 18 2025
A weak wave continues to move into northwestern IN bringing
showers/thunderstorms over KSBN for the next couple of hours
along a baggy warm frontal boundary that is north to south
orientated over the western parts of the forecast area. Did
have vsbys drop down into IFR cat to around 1 to 2SM with the
heavier rainfall from these thunderstorms. Expectation is this
activity does keep moving slowly eastward through the remainder
of the area this afternoon/evening however the activity should
wane as it does and current thinking is that it diminishes
before it gets to KFWA.
A second batch of showers and thunderstorms currently over
northern IL will move into the area later tonight and should
approach KSBN after 06z Tue and KFWA after around 10z Tue but
will most likely be in a decaying state. Therefore, a bit of
uncertainty as to the coverage and intensity with the loss of
diurnal heating but did opt to keep Prob30 mentioning in for
both sites at this time. With the heavier showers/storms MVFR
conditions are expected, but is possible for a temporary drop of
vsbys into IFR category but confidence is very low in that
outcome.
&&
.IWX WATCHES/WARNINGS/ADVISORIES...
IN...None.
OH...None.
MI...None.
MARINE...None.
&&
$$
DISCUSSION...Marsili
AVIATION...Andersen
Source: IWX issues Area Forecast Discussion (AFD) at Aug 18, 2:17 PM EDT (https://mesonet.agron.iastate.edu/p.php?pid=202508181817-KIWX-FXUS63-AFDIWX)
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