Merriam, Indiana Weather Forecast Discussion

669
FXUS63 KIWX 231352
AFDIWX
Area Forecast Discussion
National Weather Service Northern Indiana
952 AM EDT Tue Sep 23 2025
.KEY MESSAGES...
- An active weather period will continue through Friday with
  numerous showers and scattered thunderstorms. Severe weather
  is not expected at this time.
- High temperatures will range from 70 to around 80 degrees,
  with lows in the 50s and 60s.
&&
.DISCUSSION...
Issued at 452 AM EDT Tue Sep 23 2025
Dense fog advisory continues this morning for northwest Ohio
counties; Hillsdale County, MI; and Indiana counties generally
along/east of I 69 in the north; then along/south of US 24
elsewhere. Visibilities have been generally been between 1-3SM with
drops to around 1/2SM or less becoming more frequent with time.
Didn`t add any counties with this update as it seems most of the
area outside of the advisory is lingering 1-3SM or more (as of 9z).
Additionally, based on the latest guidance I think the
expiration time of 14z is still reasonable, and will let the day
shift decide whether to keep it going beyond that.
Otherwise, precipitation chances persist through Friday. A cold
front stemming from a surface low over central Lake MI (between
Green Bay, WI and Frankfort, MI as of 5z) will drop southeastward
today, reaching our northwest by around 4-8 pm ET. It will continue
southeastward overnight, extending from near Toledo, OH
southwestward through Lima, OH and Portland, IN by 8 am ET Wed.
Relatively zonal flow at 500mb today will gradually be replaced by a
cut off low embedded within a long trough that encompasses the
entire Great Lakes down into the central/southern plains. The center
of the low is expected to linger over Lower Michigan Wed-Thu before
it weakens and lifts northeastward out of the area Thursday night
into Friday morning. At the 300mb level we have a straight/slightly
cyclonically curved jet streak that develops over Lower MI, and
another ejecting from the central plains into IL/south and central
IN by 12z Wed...which will provide us with better synoptic forcing
for precipitation particularly where any coupling occurs. The best
DPVA associated with our 500mb trough develops over the western CWA
around 21-00z today, and depending on moisture/potential AM
precip/cloud cover we`ll see between 500-1200 J/kg MUCAPE in that
region by the afternoon and early evening. With the arrival of the
surface cold front in our northwest, think we`ll have a shot at some
showers and a few thunderstorms this afternoon, with the boundary
shifting southeast overnight. Have chances in the 30-50 percent
range through this afternoon, increasing to 60 to 70 percent
overnight. Thunderstorms are possible, but it will depend on how
much instability we can get this afternoon which might be more
difficult given the foggy/cloudy start (especially in the S-SE).
In regards to this morning/early afternoon chances today-I am less
confident as forcing is limited to the outflow boundary from last
night`s convection until around 18-21z with arrival of the front.
For now, have 20-30 pops extending through the early afternoon
(increasing closer to 18z), with isolated thunderstorms after 15z.
As the stronger upper level forcing really strengthens with the
potentially coupled jets and DPVA from the digging trough through
Wednesday, a surface low develops somewhere in MO/Southern IL/IN
(00z Wed) and lifts northeastward into the Lower Great Lakes/NE
CONUS Thursday into Thursday night. This weak low will likely ride
up along/just south of the aforementioned stalled cold front that
becomes stationary in our far SE near Lima, OH--which puts our area
on the north/backside of the low as it passes through Ohio.
Additionally, with the upper low just north of us, we experience
several shortwaves that swing through in the wake of the main
surface low, prolonging precip chances into Friday morning.
The result of this will be likely to categorical rain chances Wed-
Thu, tapering off gradually through Friday morning. There could be
some lake enhancement behind this with north-northwest winds behind
the system, however there isn`t much in the way of colder air behind
it, and height rises quickly overtake the CWA (from W to E) through
much of Friday-Saturday. If the ECMWF is right and the trough/system
exits slightly slower, we could see some pop up showers and isolated
storms South of US 24/east of I 69 Friday afternoon with peak
daytime heating and lingering upper level support. Confidence is low
in that occurring.
There is another trough that passes north with another low over
James Bay Saturday afternoon, but model guidance keeps us dry with
the best forcing north and our area being just enough under height
rises that it keeps precipitation out. Mainly have an uptick in
clouds at that time for this update. High pressure builds in for
Sunday into next Tuesday, so we`ll be drying out once again.
High temperatures this week will range from the 70s to around 80
degrees, with lows in the 50s and 60s.
&&
.AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 735 AM EDT Tue Sep 23 2025
Light winds with a weak sfc ridge axis in place combined with
good deal of near sfc moisture availability have led to areas of
dense fog this morning, particularly along and east of the I-69
corridor where some measurable rain occurred yesterday. Poor
mixing may make improvement a rather slow process and scenario
should evolve into improving conditions in the form of IFR/MVFR
cloud deck before becoming VFR late morning. Otherwise for
today, an upper trough drifting across the southern Great Lakes
will be accompanied by a weak frontal zone. This could be enough
coupled with weak sfc based instability for some isolated-
scattered showers and thunderstorms in the 17Z-22Z timeframe.
Given what should be limited instability magnitudes, will
continue to omit thunder with the 12Z TAFs and limit mention to
PROB30 groups. Best chance of rain should be late in the period
as a zonally oriented upper jet streak progresses through base
of another upper low across the Rockies. Low level
southwesterlies will increase downstream of this feature likely
helping with lift in the form of warm/moist advection and some
enhancement to low level front across the area. Will include
trend back to MVFR cigs at both terminals along with rain at
KFWA tonight.
&&
.IWX WATCHES/WARNINGS/ADVISORIES...
IN...None.
OH...None.
MI...None.
MARINE...None.
&&
$$
DISCUSSION...MCD
AVIATION...Marsili

NWS IWX Office Area Forecast Discussion