507 
FXUS63 KIWX 210810
AFDIWX

Area Forecast Discussion
National Weather Service Northern Indiana
410 AM EDT Tue May 21 2024

.KEY MESSAGES...

- Potential of strong to severe thunderstorms late tonight
  across most of the forecast area. Greatest risk appears to be
  west of Interstate 69 where some thunderstorm wind gusts of 
  70+ mph are possible.

- Warm and muggy conditions to be replaced by somewhat cooler
  and less humid air later this week into the Memorial Day
  weekend with some additional chances for showers and storms.

&&

.DISCUSSION...
Issued at 400 AM EDT Tue May 21 2024

Greatest convective impacts are still expected late tonight with a 
potential of strong to severe thunderstorms. Damaging wind gusts in 
excess of 70 mph appear to be the primary threat. 

In the near term, convection is ongoing this morning across the
Corn Belt with the greatest coverage at the nose of a 40 knot 
low level jet. Guidance shows some subtle hints of a weak vort 
max emanating from this convection and progressing across the 
western Great Lakes this morning. This should allow some 
enhancement to downstream southerly flow across the Mid Ms 
Valley and western Great Lakes pushing low level theta-e 
boundary back northward. Already have seen a few isolated 
showers develop from northeast Illinois into west central 
Illinois early this morning along this theta-e gradient. Low 
level warm front should also sharpen across extreme northern 
Indiana/southern Lower Michigan later this morning providing 
some weak low level convergence. Given nebulous large scale 
forcing, will maintain just some slight chance PoPs across the 
north for isolated shower/storm later this morning through early
to mid afternoon. By later this afternoon, expecting any low 
end thunder potential to diminish for a time as low level warm 
front shifts north and some capping likely taking hold into the 
evening hours. In terms of temperatures today, a very weak cold 
advection push is noted behind yesterday evening's departing 
MCV, but aforementioned return advection should push low level 
thermals back to close to yesterday's level which should support
another day of highs in the mid-upper 80s most locations.

Concern for tonight will turn to potential late night severe event. 
Confidence remains medium for this event with a couple of competing 
positive and negative factors to consider for severe weather 
potential locally. The short wave that will initiate severe 
weather potential later this afternoon into this evening across 
Corn Belt will eject northeast out of the northern Rockies this 
morning. A strong 50+ knot low level jet nosing into eastern 
IA/northern IL this afternoon will help to initiate convection 
into early this evening while strong deformation forcing closer 
to upper low center across central MN creates an expansive 
precip field. The past 48 hours have exhibited strong diabatic 
tendencies to modulate strength and track of the mid level 
forcing across central CONUS and it appears today will be no 
different with ultimate strength of Upper MS Valley upper level 
trough largely dictated by these diabatic effects.

Stronger upper level height falls look to just provide a glancing 
shot to southern Great Lakes region/Ohio Valley with this system
as stronger vort max lifts northeast away from the area tonight,
so some question still persists as to the longevity of 
organized convective upstream as it enters the local area. Sharp
initial inflection from Upper MS Valley trough and eastern 
Great Lakes ridge will lead to a sharp gradient in shear with a 
possibility that outflow dominance could take over across the 
southern Great Lakes late evening/overnight. Some near sfc CIN 
may also develop given late evening/overnight timing. One 
positive factor for maintaining a higher wind threat into local 
area is presence of elevated mixed layer that appears to enhance
downdraft potential (DCAPE's on the order of 1500 J/kg which is
a high end value to support strong downdrafts). Updated 06Z SPC
Day1 Outlook does indicate a fairly sharp gradient in severe 
potential from Enhanced Risk far northwest to Marginal Risk
across southeast which falls inline with above factors and the 
gradient in convective environment expected through time. Given 
strong DCAPEs some concern does exist if integrity of linear 
convection is maintained to have some 70+ mph wind gusts late 
evening/early overnight across the west/northwest. Hail looks to
be a lower end threat, and focused probably extreme 
west/northwest. This event looks to have QLCS characteristics 
upstream with a potential of QLCS tornadoes across WI/IL, but 
this potential becomes less confident into the southern Great 
Lakes given expected tendency to some outflow dominance through 
time, and the fact 0-3km shear will be more marginal with 
eastward extent compared to upstream across the MS Valley.

On Wednesday, cold front will slow its eastward progress across the 
area, especially from southern Illinois into central Indiana as 
another low amplitude upper level short wave shears out. A narrow 
moderate instability axis is expected to develop from southern 
Missouri into west central Ohio Wednesday. Still some uncertainty 
regarding frontal placement, but it does appear secondary wave lags 
quite a bit with stronger forcing not arriving until Wednesday 
evening. Guidance trends have favored areas just south/southeast of 
local area for Slight Risk (wind/hail threat) on Wednesday. A better 
intrusion of cooler air reaches southern Great Lakes briefly 
Wednesday night, but low amplitude flow will not allow this 
baroclinic zone to make much southward progress. Additional 
stronger eastern Pacific waves will allow for renewed warm/moist
advection late Thursday into Friday with periodic chances of 
showers and thunderstorms Friday into the weekend. Highs may 
nudge back into low to mid 80s again for Friday-Saturday in 
advance of expected stronger short wave, but then expected to 
cool back to near seasonable levels toward the end of the 
period. In this progressive pattern, have reluctantly kept PoPs
in numerous periods from model blend for much of the extended, 
but several dry periods are expected between systems and should 
be narrowed down in later forecasts.

&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 108 AM EDT Tue May 21 2024

Showers and storms have weakened, and dry conditions will now
prevail through the day Tuesday. VFR ceilings will persist
through much of the day Tuesday, and southerly winds will pick
up in the afternoon with gusts up to 20-25 kts possible. Showers
and storms will be likely Tuesday night (likely arriving in
Indiana between 02-06Z), so chances for storms will need to be
added in subsequent TAF forecasts as confidence increases. 
Storms may be strong to severe overnight Tuesday into early 
Wednesday, and the primary concern is strong, gusty winds.

&&

.IWX WATCHES/WARNINGS/ADVISORIES...
IN...None.
OH...None.
MI...None.
MARINE...None.

&&

$$

DISCUSSION...Marsili
AVIATION...Johnson