IWX issues Area Forecast Discussion (AFD) at Aug 26, 12:12 PM EDT287
FXUS63 KIWX 261612
AFDIWX
Area Forecast Discussion
National Weather Service Northern Indiana
1212 PM EDT Wed Aug 26 2026
.KEY MESSAGES...
- There is a Slight Risk for severe weather today, mainly from
3pm EDT to 10pm EDT. Damaging wind gusts to 60 mph, isolated
large hail to 1 inch diameter are the primary risks, but an
isolated tornado cannot be ruled out.
- Seasonable temperatures continue Thursday with a very low chance
of an afternoon shower (less than 15 percent).
- Becoming warmer and more humid for the weekend.
- There is a Moderate Swim risk for southwest Lower Michigan Lake
Michigan beaches today, and again for Thursday for all of
southeast Lake Michigan beaches.
&&
.UPDATE...
Issued at 1207 PM EDT Wed Aug 26 2026
Weak trough extending from western Lower MI into NW Indiana
shows up nicely on vis sat as yet another band of agitated cu.
Showers and even some occasional lightning strikes being noted
from South Haven north with some indications of showers trying
to develop further south. CAMs continue to point towards this
activity likely remaining across N/NW sections for the next
couple of hours. Whether upscale development occurs with this
feature further inland this afternoon or with the approaching
front remains to be seen. Made a few additional tweaks to
timing and also lowered pops to less than 55% and opted for a
coverage wording.
&&
.DISCUSSION...
Issued at 315 AM EDT Wed Aug 26 2026
Convective/severe weather potential today will continue to be the
primary forecast concern for the short term period.
Clouds are expected to increase from west to east this morning as a
zone of low level warm/moist advection across
Wisconsin/northern Illinois builds eastward aligning with a
zone of stronger 800 mb positive theta-e advection. Convection
early this morning is limited to portions of southwest Wisconsin
in closer proximity to cold frontal boundary and where some
DPVA is ongoing in advance of an upper level low centered across
the arrowhead of Minnesota. Through the morning, expecting some
increase in low level forcing with a pre-frontal trough type
feature shifting across the western Great Lakes. Some isolated-
scattered convection is possible later this morning/early
afternoon along this pre-frontal feature, particularly across
NW Indiana into southwest Lower Michigan. Instability through
early afternoon should be quite limited and not expecting any
threat of severe weather through early afternoon if convection
does develop.
The primary mid/upper level trough and stronger mid/upper level
forcing should be across central/northern Lower MI today, but
southern Lower Michigan/northern Indiana will be located on southern
extent of mid/upper level height falls accompanying this trough.
Some increase in deep layer shear is expected this morning as modest
increases in 500 mb flow to 30-40 knots is expected resulting in
increasing deep layer shear to around 30 knots. Some moderate MLCAPE
on the order of 2000-2500 J/kg is possible this afternoon, but
likely oriented in narrow axis just ahead of the cold front as mid
level lapse rates across the area remain on the marginal side. Some
question persists regarding the extent of destabilization with
a possibility that some pre-frontal cloudiness/convection could
prevent instability magnitudes from reaching this potential.
There is also some question as initiation time and coverage of
convection as low level convergence fields may be a bit more
limited across the local area as stronger mid/upper level
forcing remains just north of the local area. Combination of
moderate instability potential near frontal zone and sufficient
deep layer shear could support isolated-scattered severe
thunderstorm risk this afternoon into early evening. Damaging
wind gusts to 60 mph appear to be the primary threat given steep
0-3km lapse rates, but some isolated large hail is possible
especially closer to convective initiation time. However,
limited mid level lapse rates may limit the hail potential.
Given instability/shear combination some supercell
structure/isolated tornado potential is possible, although
guidance has trended to a bit more veered nature to near sfc
flow over past several runs which could limit low level helicity
from earlier indications. The primary severe risk still looks
to be largely in the 19Z-02Z timeframe, and particularly through
00Z.
Loss of connection to stronger sfc based instability should diminish
intensity of frontal convection after 00Z or 01Z this evening. Some
patchy fog development is possible late tonight into early Thursday
morning behind the sfc front with weak low level height gradient and
residual near sfc moisture likely remaining in place. Another weak
mid level trough will dive southeast in wake of today's upper trough
for Thursday keeping the local area in broad cyclonic flow. Some
surface based instability should be present in the afternoon and
some possibility of isolated showers could materialize. Have
maintained silent 10-14 PoPs this cycle given rather nebulous mid
level forcing, but it is possible some low PoPs will be needed
Thursday PM in later cycles.
Friday should bring dry conditions as mid level ridging across
central CONUS builds eastward. Positioning of this ridge upstream of
the area should keep a modifying low level thermal trough in place
with another day of seasonable temperatures on Friday. The weekend
still looks to feature warmer conditions as favored storm track
remains across northern tier of CONUS resulting in dampening of the
upstream ridge and eventual return of low level warm air/moisture
advection. This pattern evolution should support eventual
advection of some warm mid level temperatures across the
southern Great Lakes and Ohio Valley. This leads to low
confidence in instability profiles for the weekend into early
next week. Some potential will exist for sfc frontal boundary
to approach the region in this pattern, but confidence in depth
of frontal lift is in question given background synoptic
pattern. Thus, low chance/slight chance PoPs will be maintained
for much of the days 4-7 period along with transition to above
normal temps.
&&
.AVIATION /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 631 AM EDT Wed Aug 26 2026
Forcing in the form of warm/moist advection that aided in
isolated thunderstorms earlier overnight across southwest
Wisconsin is now approaching the western Great Lakes region as
of 10Z early this morning. A few showers have developed over
past hour across east central Illinois in southern extension of
this isentropic forcing that may survive into NW Indiana toward
daybreak. The isolated thunderstorms across southern Wisconsin
have largely diminished in intensity, as this forcing has
largely outpaced better instability. May continue to carry
PROB30 shower mention at KSBN late morning/early afternoon as
pre-frontal forcing overspreads the area, but better chances of
showers/storms still looks to be in the 19Z-00Z timeframe just
ahead of an advancing cold front. Some higher res guidance does
initiate this convection just east of KSBN vicinity, but given
larger scale lift with approaching upper trough and a moderate
instability axis of 2000-2500 J/kg of MLCAPE, did continue idea
of previous TAF issuances in a TEMPO TSRA group this
afternoon/early evening. Otherwise today, VFR conditions are
anticipated outside of briefly lower cigs/vsbys in heavier
showers and storms. Southwest winds may also gust into the 15 to
20 knot range for a time ahead of the cold front. Some
consideration was given to including some patchy fog late this
period as low level gradient slackens in wake of weak cool front
late tonight, but will let later cycles fine tune this forecast
for the later periods at this time.
&&
.IWX WATCHES/WARNINGS/ADVISORIES...
IN...None.
OH...None.
MI...None.
MARINE...None.
&&
$$
UPDATE...Fisher
DISCUSSION...Marsili
AVIATION...Marsili
Source:
IWX issues Area Forecast Discussion (AFD) at Aug 26, 12:12 PM EDT---------------
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