836
FXUS63 KIWX 180739
AFDIWX

Area Forecast Discussion
National Weather Service Northern Indiana
339 AM EDT Sat May 18 2024

.KEY MESSAGES...

- Warm conditions for this weekend into Tuesday. Warmest
  conditions expected for Sunday through Tuesday with high
  temperatures in the lower to middle 80s.

- A potential for more organized storms with some strong/severe
  thunderstorm potential late Tuesday and Tuesday night, but
  continued indications of later arrival to storm chances which
  may limit overall severe threat.

- Additional chances of storms Wednesday with a cold front
  passage leading to cooler/drier air for late work week.

&&

.DISCUSSION...
Issued at 338 AM EDT Sat May 18 2024

In the near term, fog and stratus potential will be the main
forecast challenge. Through 07Z, more notable vsby restrictions
in fog development are starting to take place across far
northeast Indiana, south central Lower Michigan, and far
northwest Ohio. Weak low level convergence with weak sfc low
reflection and higher crossover temps appear to be favoring
these locations for fog development. Near sfc weak easterly flow
off Lake Erie may also be contributing to some favorable near
sfc moisture lapse rates for fog development. Satellite
imagery/sfc observations do indicate a fog/stratus combination
downwind of Lake Erie across northwest Ohio. Widespread dense
fog is currently not anticipated across the area, but will
continue to watch far northeast locations for potential areas of
dense fog early this morning.

Otherwise today, the old vort max which tracked across Lower
Michigan last evening will begin to take a southeast jog today
across western Ohio. A very subtle sfc reflection of this old
short wave may allow for redevelopment of some isolated showers
and possibly a thunderstorm this afternoon south of US 24
corridor across far northeast Indiana and northwest Ohio.
Instability should be limited by mixing down a pocket of drier
low level today combined with development of a weak subsidence
inversion. Based on track of this vort max, most favored areas
of isolated shower development may end up just southeast of the
local area. The primary story today will be the warming low
level temperatures as weak mid level height rises and moderating
low level temps under influence of large scale low/mid level
subsidence occurs. This should drive highs into the lower 80s
across much of the area.

For tonight, primarily dry conditions expected with perhaps some
more patchy fog development east of I-69 where weak near sfc
easterly flow should keep favorable moisture profiles in place.
Focusing back to the west, a stronger upper vort max will be lifting
across western Ontario overnight allowing for eastward
progression of a weak sfc trough into the western Great Lakes.
Some higher res guidance indicates just enough elevated mid
level moisture pooling for a few showers and storms along this
boundary as it reaches far NW Indiana, western Lower Michigan
late tonight into early Sunday morning. Will keep forecast dry
at this time, with expectation this weak sfc trough may have
more of an influence on isolated shower development for Sunday.

Some enhancement to sfc northwest flow behind this trough off
Lake Michigan could allow for enough sfc convergence for
isolated showers and storms on Sunday, especially considering
some weak moisture pooling near the sfc trough. Previous
forecast has this trend well in hand and no major changes made
to slight chance PoPs for Sunday. Reinforcing low level thermal
ridge with this feature will tack on another few degrees to high
temperatures Sunday in comparison to Saturday (low to mid 80s).

For Monday, upper ridge axis will being to shift east of the area
with attention turning to a progressive smaller scale short wave
lifting northeast out of the Central Plains. This strength of this
vort max appears to largely depend on diabatic processes with
Central Plains convection on Sunday which creates some lower
confidence in timing/track as it lifts northeast into Monday.
This diabatically driven PV anomaly appears to induce strongest
low level moisture advection to the NW-N of the local area
leading to low confidence in shower/storm chances. Perhaps best
chance of isolated-scattered showers and storms with this
feature will be Monday evening into early Tuesday as warm
frontal boundary begins to sharpen.

Tuesday and Wednesday are still the periods featuring potentially
the more impactful weather.  A more dynamically driven upper trough
should acquire some negative tilt as it lifts across the Upper MS
Valley Tuesday night, although strength and timing of this trough
will again depend on how upstream convection evolves. Guidance has
maintained trend of more stout upper ridging being maintained into
Tuesday evening which may delay pre-frontal forced convection until
later Tuesday evening or overnight Tuesday. Given uncertainty with
track of the upstream vort, maintained general idea of blended PoPs
with west-east ramp up of PoPs Tuesday evening/overnight.
Storms in vicinity of warm front through mid afternoon Tuesday
could be isolated strong in nature, although better shear
profiles will lag until Tuesday night. By this time, instability
magnitudes become more in question with possibility of some CIN
developing for near sfc based parcels. This later timing trend
continues to lower confidence in appreciable severe threat, but
still a period that will need to be watched Tuesday night.

A secondary lower amplitude upper trough and cold front will
allow for continued shower/storm chances into Wednesday. A few
of these could be strong with some gusty winds, but occluding
nature of Great Lakes system could pinch off better instability
to south of the local area.

Cooler and drier air settles into the region for Thursday but medium
range guidance continues to suggest low amplitude progressive pattern
of eastern Pacific waves with additional chances of precip for
next weekend.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 111 AM EDT Sat May 18 2024

Steering flow aloft and near the surface is incredibly weak, so
winds will remain light and variable throughout the day at less
than 5 knots. The main concern for this TAF cycle will be fog
this morning between 09-13Z. Uncertainty remains in just how
locally dense this fog becomes, but current thinking is that
visibilities may drop below 1 mile and ceiling could drop as
low as 300 to 500 feet. IFR/LIFR ceilings and/or visibilities
are possible, but it`s likely they won`t persist for the entire
aforementioned 4 hour window. Much like yesterday morning,
visibilities will likely vary across small distances, leading to
fluctuating conditions. Fog will lift after 12Z this morning,
with high MVFR/low VFR ceilings for the rest of the day.

&&

.IWX WATCHES/WARNINGS/ADVISORIES...
IN...Air Quality Alert until midnight EDT /11 PM CDT/ tonight for
     INZ005-012>015-022-023-103-104-116-203-204-216.
OH...None.
MI...Air Quality Alert until midnight EDT tonight for MIZ078-177-
     277.
MARINE...None.

&&

$$

DISCUSSION...Marsili
AVIATION...Johnson