Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 04/17/24

Area Forecast Discussion

National Weather Service Indianapolis IN

 817 PM EDT Tue Apr 16 2024

.KEY MESSAGES...

- Gusty winds between 20 and 30 mph this afternoon.
- Showers and thundershowers expected this evening into
  the overnight. A few strong storms possible.
- Scattered severe storms possible on Wednesday, mainly northeast of
  Indianapolis
- Locally heavy rain Thursday night with a low potential for severe
  weather

&&

.MESOSCALE UPDATE...
Issued at 817 PM EDT Tue Apr 16 2024

Discrete thunderstorms are forming across Central Illinois along a
low level convergence boundary this evening. Southeasterly winds are
being reported across all of Central Indiana while winds in Illinois
are from the SSW, where deeper moisture is being advected northward.
Daytime mixing in addition to the best axis of moisture transport
being west of the state has kept the lowest 3km fairly dry across
Central Indiana as noted on IND ACARs soundings. Surface dew points
much of the afternoon and evening are struggling to get out of the
low to mid 50s in this pattern. Storms forming in Illinois currently
have access to better moisture as observations there report dew
points in the low 60s with smaller dew point depressions as well.
With storm motion NE around 45 mph, cells in Illinois are likely
outpacing moisture advection eastward as they move into
substantially drier air in Indiana. While these storms are pulsing
to near severe levels just west of the IL/IN border, currently do
not expect storms to keep their strength as sunset approaches and
instability continues to wane. Best threat for any severe storm are
for counties right along the IL/IN border where an isolated wind
gust to 60 mph and quarter size hail are possible in the strongest
storms. Will continue to monitor storms are they push eastward into
Central Indiana over the next several hours.

&&

.SHORT TERM (This evening through Wednesday)...
Issued at 246 PM EDT Tue Apr 16 2024

The warm front that was present across the forecast area earlier
today has shifted northeast with a trailing axis of predominantly
mid and upper level moisture now progressing across the region. A
narrow band of nothing more than sprinkles or very light rain
showers essentially paralleling I-65 was drifting northeast at 18Z.
Temperatures were warm again despite the increased cloud coverage...
with upper 70s and lower 80s common and breezy conditions.

The primary focus through late Wednesday is on two opportunities for
convection...this evening into the overnight and again late morning
Wednesday into the afternoon as the region is impacted by the
large occluding low moving into the upper Midwest by Wednesday
morning. Both of these opportunities carry a risk for strong to
severe storms but the conditionality has grown with multiple
features present but out of phase to promote a more substantial
risk. Will discuss both in order below.

This Afternoon through Tonight

This band of sprinkles will progress to the east over the next few
hours with the sun returning as satellite imagery shows a well noted
dry slot present from central Illinois into the lower Ohio Valley.
The rest of the afternoon into the first part of the evening should
remain largely quiet as convection organizes along a prefrontal
trough over the mid and upper Mississippi Valley. Locally...some
work needs to be done with the atmosphere overhead until more
favorable conditions for convective maintenance will exist. The
ACARS sounding from KIND shows the big issue currently: lack of any
substantial moisture within the column through about 600mb with a
capping inversion in play too. This is a primary reason why nothing
more substantial than sprinkles can get going at the moment despite
a weakly unstable airmass.

Low level southerly flow would usually be sufficient for deeper
moisture return but a quick glance at surface analysis shows
dewpoints generally in the 50s all the way south into northern
portions of Alabama and Georgia. The deeper moisture plume is
instead displaced further west in closer proximity to the
prefrontal trough across the Ozarks east into far western portions
of Kentucky and Tennessee. Do not anticipate this axis of deeper
moisture to lift up into the Wabash Valley until near or shortly
after 00Z in tandem with a low level jet with further eastward
expansion across the forecast area into the early overnight. The
arrival of the deeper moisture will finally enable more widespread
convective coverage to shift east into the region as a result.

Despite the improving moisture profile through the boundary
layer...remain skeptical of anything more than an isolated severe
storm over the area during the second half of the evening likely
focused over western counties. Instability levels will creep up
during the evening but will remain much less favorable than points
further west in the mid Mississippi Valley. The presence of DCAPE
values rising up to 700-800 j/kg and hints of slightly better lapse
rates in play could be just enough to enable a few of the more
intense cells to throw out strong winds. But overall expect storms
to be in a largely weakened state by later in the evening...
continuing into the overnight and weakening further as the
prefrontal boundary shifts east across the forecast area with the
low level jet. The convection should be east of the entire forecast
area prior to daybreak with some clearing of the cloud deck
occurring.

Wednesday

The threat for severe convection over central Indiana always looked
more favorable for Wednesday ahead of the primary cold front. But
even that threat appears to be lowering from earlier projections as
the multiple features needed are likely to be subtly out of phase
with the best threat for severe storms focused more to the northeast
of the forecast area by the afternoon.

A negatively tilted but weakening upper low will be present over the
region with deeper moisture lingering within the boundary layer
ahead of the front. Deeper BL shear and 0-1km storm relative
helicities however will be already moving east by midday with a
focus over Ohio for the afternoon. Convective initiation ahead of
the boundary will likely occur mid to late morning across northern
portions of the forecast area with upscale growth through mid
afternoon as it moves east. Model soundings highlight the presence
of enough low level directional shear lingering to present a non-
zero tornado threat across northeast portions of the forecast area
for a few hours during the early afternoon. Large hail may be a more
prominent threat however with the upper level low aiding in cooling
heights aloft and a lower freezing level.

The main area and timeframe for concern will be to the northeast of
the Indy metro roughly between 16-19Z with a more substantial risk
for severe storms further to our northeast across Ohio where the
instability...shear and moisture will better align. Weak surface
ridging will build in from the west as the afternoon progresses...
with mild and breezy conditions as skies brighten behind the remnant
front.

Temps...lows will hold in the low to mid 60s tonight as storms move
across the forecast area. Highs will be cooler on Wednesday than the
last few days...but still expect the entire area to rise into the
70s.

&&

.LONG TERM (Wednesday night through Tuesday)...
Issued at 246 PM EDT Tue Apr 16 2024

Wednesday Night Through Friday.

By Wednesday evening the cold front will have pushed through central
Indiana with precipitation having come to an end. Precipitation will
come to an end by the evening hours with a brief break in
precipitation expected for Thursday. This break will be brief though
with another cold front expected to push through Thursday night.
Models have continued to trend upwards with the strength of the
parent low pressure system which is leading to an enhancement in the
low levels and resultant moisture advection.  This is leading to
higher QPF during the overnight hours with amounts closer to an inch
in spots.

There remains a fair amount of uncertainty as to where this axis of
heavier rain will be and will continue to monitor forecast trends
regarding the track.  There could be an isolated large hailstone
but with a near surface inversion leading to much of the convection
to be elevated, don`t expect damaging winds. Dry air will move in
quickly in the aftermath of the frontal passage with cooler air and
clear skies expected by Friday afternoon.

Saturday Through Tuesday.

The cooler than normal pattern will continue into the weekend with
persistent northwesterly flow at the surface and aloft leading to
daytime highs in the upper 50s and overnight lows in the mid 30s to
low 40s.  There may be a small potential for frost across north
central Indiana Sunday night across sheltered areas where
temperatures run a bit cooler than surrounding locations, but both
temperatures and winds are expected to be high enough to limit frost
formation outside of those ideal locations. Forecast confidence
begins to lessen on the next system early next week, but rain looks
probable in the Monday to Tuesday timeframe with uncertainty as to
timing and amounts.  Severe weather with this system looks unlikely
with a lack of instability as the system is associated with the
polar jet and flow from the Gulf of Mexico generally remains cut off.

&&

.AVIATION (00Z TAF Issuance)...
Issued at 800 PM EDT Tue Apr 16 2024

Impacts:

-  Best chance for showers and thunderstorms 02z-08z this evening
from west to east

- SE winds becoming SSW this evening then westerly tomorrow

- Wind gusts to 25 kts this evening increasing to 30-35 kts after 15z

- MVFR ceilings with overnight storms

Discussion:

Latest satellite and radar imagery show showers and thunderstorms
developing and moving NE through Central Illinois. Expect this
convection to cross the IL/IN border by around 01z-02z then continue
spreading north and eastward through Indiana. The majority of storms
should remain sub-severe, but a few of the strongest cells could
produce brief periods of damaging winds, hail up to quarter size,
MVFR to IFR cigs and vis, and lightning. Best timing for convection
is through around 08z, before everything moves north and east of
Central Indiana TAF sites. Cigs may drop to MVFR levels after the
storms move through, with improving cigs expected toward sunrise.

Outside of any storms, winds will becoming southerly, then SSW
through the overnight hours with sustained winds of 12-18 kts and
gusts of 25-30 kts at times. A shift to a more SW direction is
expected after 12-15z tomorrow with gusts upwards of 30-35 kts
during the daytime hours Wednesday.


&&

.IND WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

MESOSCALE...CM
SHORT TERM...Ryan
LONG TERM...White
AVIATION...CM