040
FXUS63 KIND 121457
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
957 AM EST Wed Feb 12 2025

.KEY MESSAGES...

- Winter Weather Advisory for combination of freezing rain and snow
  Wednesday into Wednesday night

- Total snow up to 3 inches near Lafayette, ice accumulations up to
  0.25 inches north of I-70

- Widespread Precipitation expected Friday night through Saturday
  night. Mostly rain is expected with snow mixed in at times at
  the onset and end. Heavy rain is possible at times.

- Frigid temperatures will settle in Thursday night and Sunday night
  into next week.

&&

.FORECAST UPDATE...
Issued at 957 AM EST Wed Feb 12 2025

No major changes to the current forecast for today; however
closely watching the evolution of this impactful winter storm as
very slight changes to finer details can lead to large differences
when it comes to impacts, precip types, and accumulations.

As of 10 AM EST, most locations in Central Indiana are below
freezing with a sharp south to north dew point gradient across the
state. Dew points and wet bulb temperatures are approaching freezing
from Vincennes to Seymour... north of this line, anything
precipitation that does fall will likely be freezing rain/drizzle
this morning. Latest IND ACARs sounding does show an entirely below
freezing column with a near isothermal layer along the -1 to -3C
isotherm from near the surface to 3km agl and saturation up to 2km
agl. While greatest forcing for ascent remains down in the Tennessee
Valley, weak isentropic ahead of the system into Indiana is
resulting in areas of drizzle and freezing drizzle this morning as
the temperature profile mentioned above mainly supports liquid
hydrometeors. Not too concerned with major ice issues as any drizzle
should remain light, but patchy light ice accumulations are possible
on elevated surfaces.

Will be watching the freezing line/wet bulb 0C line throughout the
day as the forecast does show it pushing north toward the I-70
corridor... cutting right through the Indy metro. North of that
line, freezing rain is likely with ice accumulations, while south of
that line, plain rain is expected. Major impact differences expected
on each side of that line, so pin pointing where the rain/freezing
rain line sets up will be very important today as heavier
precipitation pushes in.

&&

.SHORT TERM (Today and tonight)...
Issued at 216 AM EST Wed Feb 12 2025

Summary:

An impactful winter system is expected to cross central Indiana
starting in the early afternoon hours and continuing through the
early overnight, with greatest impacts to the northern half of
central Indiana. This low pressure system will bring a combination
of snow, freezing rain, and rain with snow generally isolated to
areas near Lafayette while an area of icing occurs in the corridor
north of I-70 and along/west of I-69. Areas to the south will see
primarily rain with short periods of freezing rain. Total snow of 1-
3 inches along with freezing rain of up to 0.25" expected.

Synoptic Set Up:

This system is being set up by a tightening of the ongoing
baroclinic zone and in return, strengthening the pressure gradients
over the CONUS. This then leads to a general merger of the
subtropical and polar jets, creating habitable zones for pressure
depletion, especially within the mid-Mississippi Valley. The
alignment of the jet streaks isn`t quite ideal for a deep pressure
trough, but as the subtropical jet streak becomes prominent, a
corridor of negative height tendency develops leading to a gradual
deepening of the upper level trough. This upper level trough will
move through the Ohio Valley throughout the day and tonight

The upper level dynamics will help aid in storm development but the
primary driver of the winter storm is actually more focused further
down in the troposphere, in the 700-850mb layers. Once again, high
pressure over the SE coast will act as a blocking mechanism,
tightening the pressure gradient as the upper level trough deepens.
By later today, this LLJ becomes strong and orients towards the SW,
pushing very moist air northward into the Great Lakes and Ohio
Valley. These diabatic effects along with some upper level
diffluence will likely lead to a low level low deepening with a
surface low developing over the mid Ohio Valley

Model soundings continue to show a strong warm nose in the 4-8kft
level which will be discussed in more detail below along with near
to below freezing surface temperatures. A mix of freezing rain and
snow is likely in the Lafayette area while freezing rain looks to be
the predominant precipitation type elsewhere in areas approximately
north of I-70 and west of I-69. Surface temperatures will remain
steady-state through the day with weak CAA near the surface
counteracting the latent heat fluxes in areas of ice accumulation.

Potential Impacts/Amounts:

Overall, snowfall looks to be on the lower end for central Indiana.
Portions of far NW central Indiana will likely remain below freezing
throughout the troposphere today, but the 850mb low and forcing will
be going through a pivoting point during the day as the new surface
low forms over S/SE Indiana. This will likely create a lull in QPF
where the most efficient thermodynamic zone for snow creation is
located. This in combination with low SLRs due to a near freezing
isothermal layer should push greatest snowfall totals below 3 inches
for any portion of central Indiana. With that said, there could be a
narrow timeframe for heavy snowfall rates this evening as the low
level low and LLJ strengthens. Most likely time frame for brief
heavy snowfall (+1"/hr) would be between 6-8PM over far NW central
Indiana.

The next, and likely more impactful, winter phenomenon with this
system will be a corridor of freezing rain. High resolution models
still vary on the magnitude and width of this corridor, but there is
high confidence in some portions of central Indiana getting ice
accumulation this afternoon, evening and early overnight.

One key caveat that is leading to the differing outcomes is how the
models are handling a subtle but important new surface low of which
will be a key source for strengthening and orienting a new LLJ ahead
of the main LLJ. In these solutions, the greater LLJ is pushing a
deeper surge of warm air and creating a stronger warm nose leading
to a wider corridor of greater QPF and freezing rain this evening.
If this verifies, freezing rain could push into portions of
Tippecanoe and Carrol county and some areas could approach 1/4 inch
of ice accumulation. If this occurs, it would primarily be for N/NE
central Indiana along the I-69 corridor and locations to the west.

For now, a Winter Weather Advisory remains for this area, but if
confidence increases later in the day in this solution coming to
fruition, an upgrade to a warning may be needed due to greater
potential impacts. In regards to the Indianapolis area, the zone of
impactful ice accumulation should mainly stay north of the Indy
Metro, but will likely impact northern suburbs, and could impact far
northern portions of Indianapolis proper.

*********************************************************************

Storm Summary:

Current expectation is for generally 1-3" of snowfall over NW
portions of central Indiana, with lesser amounts the closer you get
towards a Rockville, to Lebanon to Tipton line. Greatest ice
accumulations are expected to be generally just north of I-70 (west
of Indy) west/north of I-69 (east of Indy). Total ice accumulations
could be 0.2" or greater in some areas. Further south/east all rain
is currently expected with total rain amounts around 0.15 to 0.35
inches

*********************************************************************

&&

.LONG TERM (Thursday through Tuesday)...
Issued at 216 AM EST Wed Feb 12 2025

A mixed bag of weather is in store in the long term starting with
cold temperatures but fair weather Thursday into Friday under high
pressure and then mostly rainy but warmer temperatures to start the
weekend as a low pressure system with Gulf and Pacific moisture
lifts northeast across northern Kentucky Saturday night. Finally,
colder temperatures will return late weekend and the start of next
week as Arctic high pressure builds in from Saskatchewan.

Thursday through Friday...

Thursday will start off with a cold front across the Appalachians
and surface high pressure over the central Plains. The center of the
high will migrate across the Ohio Valley and Lower Great Lakes
Thursday night. Hi-Res soundings showing dry air aloft and dry
boundary layer advection support clearing from west to east on
Thursday and Thursday night before mid and high clouds move back in
on Friday as moisture returns on the back of departing high.

So, temperatures will start off very cold ahead of and under the
high with well below normal highs in the 20s over the Upper Wabash
Valley and 30s east of I-69 Thursday and single digits over the
upper Wabash Valley to the teens east of I-69 Thursday night. The
return flow/warm advection on the back of the high should allow
temperatures to climb to closer to normal Friday despite the cloud
increase with afternoon highs over most locales in the 30s.

Friday night through Sunday...

The ECMWF and ECENS and GFS and GEFS have been very consistent that
a surface low pressure system will lift northeast across northern
Kentucky Saturday and Saturday night. A broad deep upper trough will
follow and pivot through the Ohio Valley and Great Lakes on Sunday.
Strong moist and warm advection/300K isentropic lift on the heels of
a 50+ knot SW low level jet will pump anomalously deep moisture into
mainly Kentucky and Tennessee- However. PWATs to an inch look likely
to reach the I-70 corridor by Friday night. PWATs that high are in
the max moving average of SPC sounding climatology for nearby KILN.
In addition to the strong forcing and deep moisture, a 130 kt anti-
cyclonic jet will be nearby potentially including jet streak
dynamics into the equation. These factors support the potential for
heavy precipitation Friday night and Saturday, at least across areas
near and south of I-70. Frontogenetical banding will also likely be
in play somewhere across central Indiana. Highest QPF totals look
to be over southern sections, closer to the low track and deeper
moisture. Storm total rainfall could exceed an inch by Sunday.
Confidence in 90+ percent PoPs Friday night into Saturday morning
look good.

BUFKIT sounding profiles and critical thicknesses support mostly
rain for this event but snow could be mixed in over northern sections
to start and the rain will likely also mix with or change to snow
before ending in the cold advection in the wake of the surface
system and with the trough passing by.

Sunday night through Wednesday...

Arctic high pressure will build in from Saskatchewan to end the
weekend and start the new week. This will likely lead to frigid
overnight lows in the single digits over most locales Sunday night
and single digit lows over at least northern locales through next
week. Afternoon temperatures will likely not get out of the teens
and 20s. In addition, approaching northerly and southerly waves
could bring in snow chances.

&&

.AVIATION (12Z TAF Issuance)...
Issued at 605 AM EST Wed Feb 12 2025

Impacts:

-MVFR ceilings becoming IFR this afternoon

-Precipitation after 15Z Wednesday; -SN/-FZRA at KLAF, -RA at KBMG

-RA/possible -FZRA at KIND/KHUF

-Mainly east-northeast winds at 5-12KT; Gusts beginning tonight

Discussion:

Winter system will move through central Indiana today, including
diminishing flight rules at all TAF sites. Ceilings will slowing
decrease throughout the day from south to north. MVFR ceilings have
now begun at all sites except KLAF. By 15Z, most TAF sites should be
approaching or at IFR. Brief periods of LIFR are possible,
especially within heavier bands of precipitation.

Precipitation type will vary throughout the region. At KLAF,
precipitation type may begin as FZRA, but should quickly transition
to mostly SN. At times this evening, sleet and FZRA could mix at
KLAF. At KIND and KHUF, the predominant precip type should be RA,
but some FZRA may mix in at times, especially post 22Z. KBMG is
expected to remain above freezing.

Ceilings should begin to improve overnight, but pockets of snow
showers and flurries are likely, along with increasing winds/gusts.
Winds will start easterly and transition to westerly overnight.
Gusts could occur during the day, but it should be occasional.


&&

.IND WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 1 PM this afternoon to 4 AM EST
Thursday for INZ021-028>031-035>049-051-052.

&&

$$

UPDATE...CM
SHORT TERM...Updike
LONG TERM...MK
AVIATION...Updike