IWX issues Area Forecast Discussion (AFD) at Aug 28, 1:54 AM EDT997
FXUS63 KIWX 280554
AFDIWX
Area Forecast Discussion
National Weather Service Northern Indiana
154 AM EDT Wed Aug 28 2024
.KEY MESSAGES...
- Heat indices reaching back into the 90s Wednesday with a few
areas achieving 100 south of US-24. We'll also have our eyes
on Friday as a few areas may threaten 100 degree heat indices
again.
- Showers and embedded storms will continue to diminish
overnight. More scattered storms with isolated severe chances
possible again this afternoon. The best chance for storms is
Friday as cold front pushes through.
- Cooler and dry this weekend into early next week. Highs will
be between the upper 70s and mid 80s this weekend.
&&
.DISCUSSION...
Issued at 312 PM EDT Tue Aug 27 2024
An mid level anomalous ridge is still parked across the Mississippi
Valley only slowly moving eastward to the southeastern States by
Friday. Meanwhile, mid level vorticity rode from the Rockies to the
Northern Great Lakes overtop the ridge on a moreso east to west
trajectory this morning. Resultant outflow boundary and theta-e
gradient has been modeled to be pushed south, but lower confidence
exists with this placement. Additionally, a mid level upper low
rides into south central Canada and may be able to set off a
few showers and thunderstorms across IL late this
afternoon/evening. There is some chance that they edge eastward
given the westerly low level jet. Limited shear exists to
support severe weather, but given the substantial CAPE that
exists given eastward advection of 70F dew points today and mid
to upper 90s maxTs today and potential remnant boundaries in the
area, cannot totally rule out a strong to potentially severe
storm with cell interactions and boundary enhancement. 12z model
trends have been moving away from this possibility and more
towards having something form between lower MI/WI and shift
southeastward, arriving between midnight and Wednesday AM.
Perhaps our southern Lower Michigan counties could have some
shear to work with in addition to around 2k J/kg of CAPE. Again,
this would provide strong to conditionally severe storms.
This would push cooler air into the area and allow less of the area
to see 100 degree heat indices. Additionally, since the area south
of US-24 wouldn't be affected by any late night/early Wed storms,
this would be the area that has a chance to see storms refire during
Wednesday afternoon. Towards the end of the latest 12z HRRR, storms
fire along the outflow boundary bisecting the forecast area and some
semblance of this shows up in the NAM and GFS as well. The ECMWF
appears to be later with its initiation of storms. This provides
quite the uncertainty on the ceiling for temperatures on Wednesday,
especially when considering the exact location of the outflow
boundary. Low 20s Celsius at around 850 mb just above where mixing
reaches Wednesday may allow for upper 80s with a few southern areas
seeing around 90 degree temperatures. Still leaning on the ECMWF
among other models for the 70 degree dew points as opposed to the
too dry GFS allows for MaxApT values in the upper 90s to around 100.
Despite some of this uncertainty, think areas south of US-24
still require another heat advisory for Wednesday.
Thursday's chance for showers and storms depends on the placement of
the boundary after Wednesday. If recovery exists Thursday, could see
storms wait until the afternoon to fire. The too dry GFS shows that
option. The ECMWF keeps the front across the area and continues to
fire storms along it during the day. All the while, the NAM pushes
the front southward and keeps the area stable and moist. Have more
confidence in a cooler day than today for Thursday, but potentially
even cooler Wednesday by a few degrees. This would keep us out of
head headlines.
Finally, by Friday, a strong vort max associated with an upper low
traverses into south central Canada and this pushes a cold
front into the area. It appears the NAM/GFS provide some better
shear along the front, whereas the ECMWF holds off the better
shear altogether. This increases confidence in chances for
storms as well as cools things off for the weekend. As can
happen ahead of a cold front, pooling of warmer air may allow
the area to make another run at heat advisory readings with a
return to 90 degree temperatures and lingering 70 degree dew
points. Boundary-parallel flow advertised on the GFS may allude
to a straight-lined wind tendency and potential heavy rain
threat as it advances eastward.
&&
.AVIATION /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 144 AM EDT Wed Aug 28 2024
The combination of strong instability and southern Great Lakes
being located on southern fringe of Great Lakes short wave
allowed for rapid upscale growth of earlier showers and storms
across Illinois. With some development of convective inhibition
for near sfc parcels and lack of substantial shear across the
southern Great Lakes, strength of showers and storms has been on
the downward trend past few hours. This trend should continue
for the remainder of the overnight. Best MLCAPEs persist along
and south of US 24 corridor, and while cannot completely
discount isolated storm at KFWA through 08Z, the potential for
thunder should continue to diminish. Some patchy fog is possible
across northwest Indiana early this morning, but moisture
vertical profiles should be largely unsupportive.
Some redevelopment of showers and thunderstorms is expected this
afternoon but upper forcing will become increasingly diffuse
and sheared in nature. Mid level lapse rates that were also in
place for yesterday evening's storms will also be much weaker in
nature which should limit extent of surface based instability
this afternoon. With this uncertainty in thunder potential,
will limit precip mention to VCSH for the afternoon hours at
this forecast distance.
Wind forecast is of lower than normal confidence. Mesohigh
feature to begin this period will gradually break down with
preference to weak westerly flow as outflow reinforced sfc-
trough generally sags south of the terminals today. This
boundary passage is quite shallow, so it is possible a period of
weak southwest winds could mix down before front makes more
substantial southward progress later this afternoon.
&&
.IWX WATCHES/WARNINGS/ADVISORIES...
IN...Heat Advisory from 11 AM this morning to 8 PM EDT this evening
for INZ018-022>027-032>034.
OH...Heat Advisory from 11 AM this morning to 8 PM EDT this evening
for OHZ015-016-024-025.
MI...None.
MARINE...None.
&&
$$
DISCUSSION...Roller
AVIATION...Marsili
Source:
IWX issues Area Forecast Discussion (AFD) at Aug 28, 1:54 AM EDT---------------
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