478
FXUS63 KIWX 211005
AFDIWX

Area Forecast Discussion
National Weather Service Northern Indiana
605 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 /12Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 604 AM EDT Tue May 21 2024

Only change made to the TAFs was to update the wind
speeds/directions for this afternoon. VFR ceilings will persist
through much of the day today, and southerly winds will pick up
in the afternoon with gusts up to 20-30 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...Small Craft Advisory from 8 PM this evening to 5 AM EDT
     Wednesday for LMZ043-046.

&&

$$

DISCUSSION...Marsili
AVIATION...Johnson