518
FXUS63 KIWX 200959
AFDIWX

Area Forecast Discussion
National Weather Service Northern Indiana
559 AM EDT Mon May 20 2024

.KEY MESSAGES...

- Chance of storms this afternoon into tonight. Widespread
  severe weather is not expected, but an isolated storm may
  have a hail or wind threat especially from northwest Indiana
  into southwest Lower Michigan.

- Strong to possibly severe thunderstorms still appear possible
  late Tuesday night and possibly again Wednesday afternoon or
  evening. Both of these periods are marked by low confidence in
  severe weather. Best chance of severe weather across
  northwest Indiana into southwest Lower Michigan late Tuesday
  evening into the overnight hours.

- High temperatures well into the 80s today and Tuesday, cooling
  back into the mid and upper 70s for Thursday and Friday.

- Lull in precip chances for Thursday, but additional chances of
  showers and thunderstorms next weekend.

&&

.DISCUSSION...
Issued at 407 AM EDT Mon May 20 2024

Challenging convective forecast through midweek, with several
potential rounds of convection, but all of relatively low confidence
in terms of extent and severe potential.

An upper level ridge axis will begin to shift off to the east
today allowing some return southerly low level flow. This
southerly flow will allow the narrow southwest-northeast
oriented low level theta-e maximum to migrate northward into
Lower Michigan today. A few showers or isolated thunderstorms
could accompany the northward retreat of this weak theta-e
gradient later this morning or into the mid afternoon hours. Of
greater concern will be monitoring a compact mid level short
wave which is being enhanced largely by ongoing central
Plains/Corn Belt convection. This convectively enhanced vort
appears to be positioned across northwest Missouri as of 0730Z
and will continue to track north northeast into southern
Wisconsin. Background shear will be on the marginal side today
with the exception of near this diabatically enhanced short wave
which should be accompanied by small area of better deep layer
shear. Near term guidance is exhibiting some fairly big
differences in near sfc/sfc moisture quality and low level
mixing depths today, partly in response to sensitivity in
nailing down the exact track of this small scale vort max.

Best chances of scattered showers and storms today appear to be
across west-northwest locations by late afternoon/early evening on
the southern periphery of this vort max. Best low level moisture
transport downstream of this vort max still appears to reside across
northern Illinois where a Slight Risk of severe storms persists
today (Marginal Risk for NW Indiana to south central Lower MI).
While instability may be a bit more tempered into Lower
Michigan/northern Indiana, fairly large low level dew point
depressions and steep low level lapse rates could yield some
isolated strong to near severe wind gusts across the northwest
late this afternoon into early this evening. Some hail is also
possible but will be dependent on the extent of low level
mixing and instability magnitudes. Threat for isolated severe
weather today should reside in the 20Z-03Z timeframe. Otherwise
today a continuation of the much above normal temperatures is
expected with low level thermal progs supporting highs in the
mid 80s to around 90. The biggest change from yesterday will be
warmer conditions across NW Indiana and SW Lower Michigan where
offshore flow will promote warmer temperatures closer to Lake
Michigan.

For tonight, low/mid level flow on southern periphery of departing
small scale short wave could briefly become frontogenetic in nature
helping to extend scattered showers and isolated storms into the
late evening and early overnight hours before diminishing. Mild
temps for tonight with lows only dropping into the mid 60s.

For Tuesday, mid level ridge will begin to shift across eastern
Great Lakes with northward retreat of developing low level warm
front as more substantial large scale upper level trough digs across
the northern Rockies. Fairly steep mid level lapse rates and some
low level convergence with warm front could promote a few isolated
stronger cells north of US Route 30 during the late
morning/afternoon hours. A weak small scale short wave may also be
progressing through the ridge during the day Tuesday. Given rather
nebulous forcing signals, will continue with 20-30% PoPs through
much of the day. Very warm conditions continue Tuesday with highs in
mid-upper 80s.

Tuesday night will feature the greatest risk of severe weather this
forecast cycle, but uncertainty remains for this period.
Cyclogenesis across southern MN should be ongoing Tuesday
evening with a robust 50+ knot low level jet nosing into eastern
Iowa/northern Illinois/southern Wisconsin during the afternoon
and early evening hours. Storms are likely to develop along cold
front and in pre-frontal forced zone across the Mid MS Valley
Tuesday afternoon/evening with focus for this convection
shifting into western Great Lakes late in the evening. Shear
profiles will increase from west to east locally as the
mid/upper level height evolution occurs Tuesday, but development
of some convective inhibition for surface/near surface parcels
given late timing of this forcing offers some uncertainty as to
the longevity of this convective line into the local area.
Stronger upper forcing may also be lifting northeast across the
Upper Midwest which will keep strongest height falls northwest
of the local area. Steep mid level lapse rates and outflow
boundary from better organized upstream convection could be
compensating factors to allow some convective organization to
persist into northwest half of the area late Tuesday night.
Scattered damaging wind gusts appears to be the main threat, but
cannot completely rule out an isolated tornado far west with a
spin-up possible along potential convective line given
strengthening 0-3km westerly shear.

Surface cold front will be slow to move across the area on Wednesday
as first stronger vort lifts northeast and secondary vort max
approaches from the central Plains. Some moderate instability is
expected to develop along a corridor from southern Missouri
into northwest Ohio by late Wednesday afternoon with 35-45 knots
of deep layer shear. Some question as to timing of initiation
as large scale lift from secondary vort max could provide for
favorable initiation after peak heating leading to some lower
confidence in severe weather coverage. The southeast half of the
area remains in Day 3 Slight Risk of severe storms for
wind/hail threat.

Low level anticyclone shifts across the area on Thursday with dry
conditions, but quick return to southerly flow Friday in advance of
additional eastern Pacific waves in low amplitude flow will
promote more chances of showers and storms into next weekend.
Temps will cool back into the 70s for Thursday and Friday with
much lower dew points by this time.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 556 AM EDT Mon May 20 2024

VFR conditions prevail through the first half of the day, with
operationally insignificant clouds at or above 15 kft. Southerly
winds will start to pick up after 15Z this morning, with gusts
up to 15 to 20 knots possible. Have added in a low chance for a
shower or storm into the KSBN TAF starting at 23Z, as a potent
upper level shortwave moves through the area this
afternoon/evening and may spark scattered showers and storms. A
few storms may have small hail and gusty winds. Opted to keep a
mention for a shower or storm out of the KFWA TAF for now, but
it may be needed in subsequent TAF updates. Main shower/storm
activity will be between 21-03Z.

&&

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

&&

$$

DISCUSSION...Marsili
AVIATION...Johnson