770
FXUS63 KIWX 171036
AFDIWX

Area Forecast Discussion
National Weather Service Northern Indiana
636 AM EDT Fri May 17 2024

.KEY MESSAGES...

- Some localized dense fog possible this morning, particularly
  across northwest Indiana. Patchy dense fog possible again
  tonight.

- Isolated to scattered showers and thunderstorms possible this
  afternoon with some locally heavy downpours possible.

- Warm conditions for this weekend into Tuesday. Highs Sunday
  through Tuesday into the lower to middle 80s.

- A potential of more organized storms with some strong/severe
  potential late Tuesday, but still low confidence on timing
  and extent of severe threat.

&&

.DISCUSSION...
Issued at 352 AM EDT Fri May 17 2024

A couple of distinct areas of stronger forcing have materialized
this morning. One mid level trough continues to lift northeast
across east central Indiana and is associated with some weak
isentropic upglide and weak convergence in 925-850 mb layer. These
showers, with perhaps an isolated thunderstorm, should continue to
overspread areas mainly along and south of the US 24 corridor across
far NE Indiana/NW Ohio with this morning. Inspection of water vapor
imagery this morning also depicts a sharper vort max progressing
eastward across northwest Illinois which could play some factor in
promoting additional isolated/scattered showers and storms this
afternoon.

In the meantime, other forecast challenge this morning deals with
fog potential. Vertical moisture gradients do not appear to be
extremely supportive of widespread dense fog scenario, with
indications of more of a fog/stratus combination setup this
morning. Some pockets of locally dense fog are possible across
northwest Indiana in closer proximity to approaching weak sfc
trough axis. Fog should dissipate after 12-13Z.

This weak sfc trough and approaching short wave could provide a
focus for isolated-scattered showers and thunderstorms again this
afternoon, although exactly where this weak convergent axis
sets up by this afternoon is of low confidence. Some weak
onshore flow off Lake Michigan could sharpen this trough up
enough for isolated late morning/afternoon shower/thunderstorm
development. A secondary area of focus could be across southeast
third of the area as large scale 925-850 mb flow is confluent
in nature with large scale modest low level moisture
convergence. Steering flow will be quite weak today, so any
showers that do form will exhibit some slow storm motions and
could produce some highly localized heavier rain amounts where
they do occur.

Isolated showers may hold on into this evening, but should exhibit a
downward diurnal trend heading into the mid-late evening hours.
Breaks in clouds in wake of departing NW IL wave this evening and
high crossover temps should support some patchy fog development
across most of the area tonight, with at least a potential of some
pockets of dense fog early Saturday morning.

For Saturday, precip chances somewhat difficult to resolve as weak
upper level ridge should have some weak positive height tendencies
across the Great Lakes. However, at the same time, a weak upper
level short wave will be rippling through this ridge from the Mid MS
Valley. Some residual weak convergent axis may also persist across
southeast half of the area which could provide a focus for diurnally
enhanced isolated shower/storm. Once again, weak steering flow will
result in weak movements of any cells Saturday. Saturday will
feature the beginning of a warming trend that will continue into
early next week with highs in the low-mid 80s.

For Sunday, a primarily dry day is expected, but recent trends may
suggest a non-zero chance for an isolated afternoon shower. Warmer
conditions Sunday should drive a weak lake breeze that may provide
just enough low level convergence and pooled moisture along any lake
breeze boundary for an isolated shower. Confidence is too low to
include mentionable PoPs at this time, but perhaps something to look
at going forward.

Pattern should become more active heading into the Monday-Wednesday
timeframe. A lead short wave is expected to eject out of the Central
Plains Monday morning, reaching the western Great Lakes Monday
evening. Guidance is struggling in agreement on the strength of this
disturbance as it lifts northeast with some suspicion that GFS may
be overdone on the strength due to some convective feedback issues.
Some warm advection wing elevated convection appears to be a
possibility Monday night, but shear profiles at this point will not
be impressive with not much organization to any convection.

It still appears as though the Tuesday into possibly Wednesday
period will need to be watched for the greatest strong-severe
potential, but confidence remains low.  Some subtle dProg/dT trends
noted in various guidance of slightly more disjointed mid/upper
level forcing evolution and a bit more prolonged maintenance of
stronger upper ridging across the Great Lakes into Tuesday.
This could keep greatest convective coverage on Tuesday across
far north, or into lower central Lower Michigan with warm
frontal progression, with more widespread precip chances
possibly holding off until late Tuesday night in this slightly
slower trend. Given these trends are recent, would like to see a
little more run to run consistency before major changes to
blended PoPs. However, it is possible PoPs may need to tempered
for portions of the area Tuesday afternoon into Tuesday evening.
Strong/severe potential will hinge largely on timing of this
forcing with perhaps greater potential just upstream of the
local area. Shower and storm chances persist Wednesday as
lagging upper forcing is expected to track across the Ohio
Valley.

A trend to slightly cooler and drier conditions is expected for
end of this period, but overall pattern heading into next
weekend may remain active as medium range models continue to
hint at progressive eastern Pacific waves affecting central
CONUS.

&&

.AVIATION /12Z TAFS THROUGH 12Z SATURDAY/...
Issued at 629 AM EDT Fri May 17 2024

Widespread dense fog has formed across much of far northeast
Illinois into far northwest Indiana, in proximity of weak
surface trough and where skies have cleared overnight. KSBN is
expected to remain on the eastern fringe of more substantial fog
this morning, with some vsbys less than 1 mile and IFR cigs
possible through around 13Z. This should evolve into a brief
MVFR stratus deck at terminals this morning before mixing out to
VFR. Otherwise today the challenge will be resolving most
favored locations for isolated showers and storms. KSBN may be
in a favorable position for an isolated shower this afternoon as
onshore from from Lake Michigan could locally sharpen the weak
trough axis for a diurnal maximum in isolated showers/storms.
Due to limited coverage and uncertainty as to most favored areas
of forcing, will continue to keep dry forecast at terminals
outside of a few light showers possible in KFWA VCSH early this
morning. For tonight, patchy fog concerns likely to be more
widespread across the area with at least IFR conditions looking
like a high probability after 06Z.

&&

.IWX WATCHES/WARNINGS/ADVISORIES...
IN...Dense Fog Advisory until 10 AM EDT /9 AM CDT/ this morning for
     INZ012>015-020-103-104-203-204.
OH...None.
MI...None.
MARINE...None.

&&

$$

DISCUSSION...Marsili
AVIATION...Marsili