IWX issues Area Forecast Discussion (AFD) at Aug 17, 1:46 AM EDT
001
FXUS63 KIWX 170546
AFDIWX
Area Forecast Discussion
National Weather Service Northern Indiana
146 AM EDT Sun Aug 17 2025
.KEY MESSAGES...
- Scattered showers and thunderstorms will continue overnight
and will be greatest in coverage from southwest Lower
Michigan into northwest Indiana. Some locally heavy rainfall
and wind gusts to 40 mph are possible.
- Remaining hot and humid with at least a slight chance of daily
thunderstorms (20% chance) until at least midweek.
&&
.DISCUSSION...
Issued at 252 PM EDT Sat Aug 16 2025
All eyes are on southern and western WI this afternoon. This
morning's MCS across MN and WI carries on this hour with renewed
development across far southern WI and a secondary area of activity
north of LaCrosse. The line segment over far southern WI,
approaching Rockford, IL, continues to be outflow dominant, yet
satellite representation to the right flank (storm relative) remains
healthy. This conflict reduces forecast confidence for this
afternoon and evening. Local conditions are unstable and moisture is
adequate for thunderstorms. Thus, a trigger is the missing
ingredient. The trigger to ignite storm can be either this line of
storms exiting southern WI or two, the activity farther west in WI
where the synoptic support is improved. In either scenario, damaging
wind gusts are the primary hazard along with heavy rainfall. The
difference between the two scenarios is timing by about 3 to 6
hours.
I'm inclined to believe that the outflow-dominance of the
line approaching Rockford will permit that activity to be somewhat
short-lived, thus, the day's best chance for storms is likely
delayed until sunset or midnight EDT. Monitor the forecast for
updates and have multiple ways to receive watch or warning
information. Given the uncertainty, the POP forecast is conservative
with plenty of 20% POPs (slight chance) this afternoon through
tonight with a peak near 40% (chance).
Depending on how this evening and overnight evolves, our hot and
humid airmass may be temporarily shunted south for Sunday allowing
for a primarily dry day. Until today's round of storms comes and
goes, forecast confidence in Sunday is low. Upper-level ridging with
west/northwest flow continues into at least Tuesday. This pattern
will allow hot and humid conditions to linger with daily
thunderstorm chances. These thunderstorm chances will be highly
dependent on daily mesoscale features, similar to today's murky
setup. Modest upper-level support from a passing shortwave looks to
enhance thunderstorm coverage Monday night. Shear continues to be a
weak point per forecast guidance at only about 20-25 knots.
This ridging pattern breaks down Wednesday as troughing occurs over
the Northeast US. This will allow high temperatures to be closer to
normal; low-80s. Dry to end the week but showers and thunderstorms
are possible later in the weekend as a Canadian cold front looks to
bring even cooler air to the region.
&&
.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 142 AM EDT Sun Aug 17 2025
The combination of remnant convective outflows and increasing
confluent/convergent flow from northern Illinois to the southern
Great Lakes region will result in a continued favor zone of
shower/thunderstorm production through the remainder of the
overnight. Orientation of instability should favor areas west of
KSBN for the greatest thunder threat, but will continue to carry
TEMPO TSRA conditions at KSBN through 09Z. A sfc cool front will
drop across the southern Great Lakes this morning eventually
resulting in a tapering of showers and thunderstorms from north
to south. This front may be accompanied by at least some MVFR
cigs/vsbys for a time in the 12Z-15Z timeframe. Will continue
trend of previous forecast in keeping TAFs dry after 15Z with
instability axis getting suppressed southwest of the terminals.
Variable winds due to convective influence overnight will become
northwest during the day.
&&
.IWX WATCHES/WARNINGS/ADVISORIES...
IN...None.
OH...None.
MI...None.
MARINE...None.
&&
$$
DISCUSSION...Brown
AVIATION...Marsili
Source: IWX issues Area Forecast Discussion (AFD) at Aug 17, 1:46 AM EDT (https://mesonet.agron.iastate.edu/p.php?pid=202508170546-KIWX-FXUS63-AFDIWX)
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