868
FXUS63 KIWX 031823
AFDIWX

Area Forecast Discussion
National Weather Service Northern Indiana
223 PM EDT Fri May 3 2024

.KEY MESSAGES...

- Normal to above normal temperatures through the week, with
  highs in the 70s and low 80s, cooler near Lake Michigan. Lows
  will be in the 50s and 60s.

- There are chances for showers and thunderstorms at times
  between this afternoon and next week.

&&

.DISCUSSION...
Issued at 220 PM EDT Fri May 3 2024

The main surface low pressure associated with an upper level low
pressure shifts northeastward into south-central Canada and this
pushes a cold front and associated weaker area of vorticity into the
forecast area today. Fairly strong low lapse rates, surpassing 8
C/km are noted on the NAM12, but more like 7 C/km on the ECMWF, this
afternoon out in front of this eastward moving cold front. Model
discrepancy on placement of storm formation is likely as a result of
the NAM having a surface boundary advect eastward from Lake Erie on
east winds. The HRRR has a more subtle indication of this as well.
Cloud cover is a major factor in storm formation this afternoon
because the area is without substantial shear. So, storm
formation will have a better chance with reduced cloud cover.
The area yesterday had some shear to tap into and that likely
helped sustain updrafts for that one severe cell to produce
marginally severe winds and just sub-severe hail without
sufficient mid level lapse rates. Well, today, we are without
substantial shear and mid level lapse rates so agree with
today`s general thunder allotment from SPC. As a result, think
gusty winds to 40 or perhaps 50 mph may be possible with storms
and perhaps some heavy rain that could lead to ponding or
flooding with potentially training storms.

Models indicate the main temperature gradient associated with the
cold front separates from the moisture gradient and shrinks
westward, tonight. As such, could see some fog development,
especially in areas where cloud cover scatters and allows for some
radiative cooling. Areas west of I-69 would seem to be more
conducive to this outcome.

It is interesting to see the ECMWF keep its moisture farther to the
north and west in an area south of US-24 Saturday morning. Am a
little skeptical of its precip output with a lack of forcing around,
but will retain slight chance PoPs there. A somewhat better chance
for showers and storms occurs Saturday afternoon and evening as a
mix of sun and clouds allows for a reinvigoration of instability
across the area. The greatest chance for this appears to still be
south of US-24 and perhaps an axis west of US-31, but it`s much
drier over there. For whatever it`s worth, the ECMWF does not have
any afternoon showers/storms form Saturday afternoon, which is
different from the rest of the model suite. With the stalled out
baroclinic boundary to our west, models form a weak low pressure on
it and that arrives Saturday evening. There will likely be a
resurgence of showers and perhaps a thunderstorm along that boundary
with the return of forcing. Instability looks to stay lower than
1000 J/kg and perhaps even lower than 500 J/kg which would restrict
the thunder chance. Mid level lapse rates around 6 C/km may be able
to help sustain convective rain overnight into Sunday morning. Highs
in the 70s are expected across the area on Saturday.

The front moves eastward on Sunday and this likely restricts the
thunder chance until Tuesday. Although, times of rain are still
possible as weaker areas of vorticity travel along the front. As
such will populate some of the time with slight and chance PoPs in
the time periods where rain looks the most possible. Sunday and
Monday afternoons are the most likely chance PoP periods.

A slow lumbering upper low within the Western CONUS trough comes far
enough east to affect the area Tuesday. There is some disagreement
on medium range guidance if the stalled boundary can move northward
as a warm front allowing the area to be in the warm sector. The low
level jet is expected to move into the area Tuesday afternoon and
evening allowing 30 to 45 kts of effective shear to be an ingredient
for developed storms to tap into. With the placement of the front in
question, the chance for thunder is also in question. Instability
appears to be the limiting factor with 500 to 1000 J/kg MUCAPE to
tap into. 60F sfc dew points and 10C+ 850 dew points will be most
possible south of US-24 or US-30, but the ECMWF actually has them
over the whole forecast area. Boundary-parallel flow appears
possible and various areas of upper jet support may support times of
heavy rain from training/ backbuilding convective rain. Continued
rain over this unsettled period is going to need to be watched as
saturated soils could lead to standing water, especially in the more
flood prone and urbanized areas where soil has been converted to
less permeable concrete.

Either way, both models have a break in precipitation Wednesday
morning. The question then becomes, when does the front move back
northward again to cause more precipitation. A better chance for
this appears to be Wednesday night. This period also appears to have
shear and MUCAPE to tap into allowing for the possibility of more
showers and thunderstorms that could be severe and produce heavy
rain.

Chances for rain appear to be lower Thursday with instability being
restricted to less than 500 J/kg. Shear is around though so if any
thunderstorms are able to form in that meager environment, severe
weather could be possible. Still a better chance for drier weather
occurs Friday with little to no instability and shear. However, the
upper low pressure center nearby may allow for a light shower still.
Friday also sees a return to mid to upper 60s for highs, which is
normal to slightly below normal for this time of year.

&&

.AVIATION /12Z TAFS THROUGH 12Z SATURDAY/...
Issued at 1142 AM EDT Fri May 3 2024

Weak cold front making steady progress through the CWA and
will clear KFWA by 1pm. Extensive cloud cover this morning has
retarded sig heating of otherwise moist prefrontal airmass of
which will curtail additional development this aftn.
Brief/transitory MVFR based stratocu with fropa otherwise VFR
will prevail this period.

&&

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

&&

$$

DISCUSSION...Roller
AVIATION...T