553
FXUS63 KIWX 220121
AFDIWX

Area Forecast Discussion
National Weather Service Northern Indiana
921 PM EDT Sat Sep 21 2024

.KEY MESSAGES...

- Chances for showers and storms increase Sunday into Sunday
  night, bringing at least some rainfall to the area.

- A break in the rain is expected Monday into Monday evening
  before another chance arrives late Monday night into Tuesday.

- Cooler temperatures will prevail through much of the period
  with highs in the 70s.

&&

.UPDATE...
Issued at 914 PM EDT Sat Sep 21 2024

No notable changes to previous forecast are warranted at this
time. Ongoing convection this evening is confined to areas well
to the west across southwest/south central Wisconsin at the nose
of instability ridge. Stable bubble and dry low level air will
persist locally through the early overnight hours. During the
overnight, a series of convectively enhanced short waves from
this upstream convection will begin to enter confluent
mid/upper level flow eventually allowing the strong low-mid
level theta-e gradient to push aggressively back across the area
into Sunday morning. Near term isentropic progs would suggest
best isentropic ascent shifting north of the local area Sunday
morning, with a few showers possible toward daybreak feeding off
this increase in elevated moisture and northward lifting
isentropic forcing. More widespread showers/iso storms may work
across the area from from west to east from mid morning through
mid afternoon Sunday in association with pocket of pre-frontal
moisture convergence and upper level short wave forcing,
followed by larger scale frontal response. Timing of precip
onset still a bit uncertain given multiple forcing mechanisms
and highly advective moisture profiles, but general theme of
previous forecast left intact at this time.

&&

.DISCUSSION...
Issued at 253 PM EDT Sat Sep 21 2024

Strong subsidence, well depicted on satellite, over the area
will hold strong until towards 12Z Sun as the next wave edges
into the region. Narrow area of showers/storms in western IL
should quickly meet its demise as it runs into the western edge
of the subsidence. Re-development will take place overnight
across northern MO into western IL, and then tracking towards
the region. Several models seem to suggest the bulk of this
initial shot may pass near/south of US-24 as noted by the last
shift. Despite this, models do agree on a better influx of
moisture (PWATs 1.75" or higher) and instability to set the
stage for development during the afternoon and evening hours
with the 15Z RAP looking the most reasonable at this point.
Overall severe threat quite low, but locally heavy rainfall
would be the main concern, something that is welcome if it
doesn`t runoff too fast. Will maintain likely pops through the
day. Showers/storms are likely to linger in eastern areas into
the evening and possibly overnight hours as the sfc boundary
takes its time working east.

Drier air will advect in Monday on N/NE winds as the upper level
trough digs in. The increasing flow getting things stirred up
beach wise with increasing waves and swim risk from Mi City to
Bridgman. Yet another wave moves towards the area Tuesday, but
models vary on potential impacts on the area partially due to
differences in available moisture and sfc low development that
will be starting to the west as a deeper wave resides somewhere
in eastern KS/W MO. System could bring another good shot of rain
to the area, but confidence is low on the evolution. Have kept
pops in the chc to low end likely for now. Thereafter, models
agree on the stronger wave cutting off but vary on how far west
and south. Some minor moderation in temps could occur, but most
likely we will stay below normal.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 700 PM EDT Sat Sep 21 2024

A strong instability gradient sprawled across the Mid MS valley
and lingering mid level subsidence across northern Indiana
should limit any eastward expansion of shower activity across
northern Illinois into this evening. By late this evening and
especially into the overnight, broad mid/upper level upstream
confluent flow will allow for more active short wave pattern to
influence western Great Lakes, allowing for an eastward shift to
strong low-mid level theta-e gradient. This should allow for
uptick in rain shower potential, particularly late Sunday
morning into Sunday afternoon as low level frontogenesis forcing
also becomes more prevalent. Weak mid level lapse rates and
moistening low/mid levels should temper overall instability.
While some isolated-scattered thunder is possible Sunday, will
continue to omit mention due to these limiting factors. Light
and variable to light southeast winds will veer southwest 5 to
10 knots on Sunday in pre-frontal environment, before shifting
west toward end of this forecast valid period with frontal
passage.

&&

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

&&

$$

UPDATE...Marsili
DISCUSSION...Fisher
AVIATION...Marsili