161
FXUS63 KIND 210345
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
1037 PM EST Mon Jan 20 2025

.KEY MESSAGES...

- Cold Weather Advisory in effect through Wednesday morning

- Frequent wind chills of 10 below with brief periods as low as 20
  below expected through Wednesday

- Scattered flurries with a few snow showers tonight

- Gradual warming trend Wednesday onward, though still below normal

- Potential for flurries or light snow showers Thursday into
  Thursday night

&&

.FORECAST UPDATE...
Issued at 1037 PM EST Mon Jan 20 2025

The coldest week of the season persists across Indiana with
temperatures struggling to get out of the single digits and isolated
snow showers present. Deep troughing continues over much of the
United States this evening with northwesterly low to mid level flow
from Central Canada down into the Ohio Valley. A low to mid level
wave over Iowa and Illinois continues southeastward into Indiana
through the overnight hours bringing isolated snow showers and
flurries to the region. Frontogenesis within the 1000-850mb layer is
present at the leading edge of this wave with a tight NW to SE
temperature gradient present in the lower levels between Indiana and
Iowa. Just enough lift is present with this incoming wave to squeeze
out any moisture remaining in the atmosphere. The moist layer where
these snow showers are located is relatively shallow below 2 km agl
with temperatures largely colder than the dendritic growth zone. It
is not too often that temperatures this close to the surface are
colder than the DGZ... in fact 850 mb temperatures as low as -30C to
-35C are associated with this incoming wave! With temperatures this
cold, snowflakes will likely be very fine and small resulting in low
visibility under a mile in heavier snow showers. Current radar
imagery shows snow showers in Central Illinois making their way
towards Central Indiana over the next few hours. Upstream
observations do indicate quick accumulations of around a half inch
in some of the heavier snow showers. Potential is there for slick
spots to develop overnight and for the Tuesday morning commute,
mainly along and north of the I-70 corridor. Most of this activity
should depart during the morning hours Tuesday, with mostly dry
conditions, clearing skies, and frigid conditions the rest of the
day.

&&

.SHORT TERM (This evening through Tuesday)...
Issued at 145 PM EST Mon Jan 20 2025

A broad highly anomalous longwave trough covers most of the country
per upper air analysis today. This has displaced an Arctic air mass
southward across our entire region with temperatures around 20-30
degrees below mid-late January climatology. An embedded shortwave
trough will nudge even colder air southward into Indiana tomorrow.
Preceding this, high-level clouds already seen in the Southern
Plains moving our way coupled with an MSLP pattern should limit
radiative processes some preventing significantly colder
temperatures.

Lower tropospheric moistening is more in question. There is
certainly not a deep subtropical connection that would be required
for a more substantiative precipitation event. Nevertheless, a deep
enough stratus layer amid ascent from aforementioned shortwave trough
should result in some flurries or light snow showers during the
predawn hours tomorrow morning. Measurable amounts are unlikely.

Resurgence of cold advection Tuesday should be strong enough to
mostly negate the diurnal curve resulting in temperatures remaining
in the single digits for most of the area. No changes needed to the
Cold Weather Advisory as wind chills will at times (particularly
during the diurnal minimum) be 10 below or slightly lower.

&&

.LONG TERM (Tuesday night through Monday)...
Issued at 145 PM EST Mon Jan 20 2025

Tuesday Night Through Wednesday.

Another night of dangerous cold is expected for Tuesday night as
temperatures again drop to near zero with wind chills of around 10
to 15 below zero. There remains significant model differences on both
the overnight lows which impact wind chills with the NAM/GFS showing
warmer temperatures while the Euro continues to show much colder
lows and wind chills as low as 25 below.

There is growing confidence that a layer of cirrus clouds will be
advecting into central Indiana during the second half of the
overnight which would help to moderate temperatures towards daybreak
which would likely lead to the GFS/NAM solution looking more valid.
Will plan on having a sharp drop-off in temperatures after sunset
with more steady state temperatures by the late overnight timeframe.
Overall still feel confident in wind chills of -10 to -15 for much
of the area with the potential that the southern counties may only
fall as low as -5.

By Wednesday morning southerly flow will begin to ramp up with
daytime highs in the low to mid 20s. A weak upper level wave may
bring a few snow showers north of Indy during the overnight hours
Wednesday into Thursday but the better chances for accumulating snow
will be north of the forecast area.

Thursday Through Monday.

There may be some residual flurries/light snow into the daytime
hours Thursday with steep lapse rates up to the top of the boundary
layer where saturation is expected. Models hint at at least some
lift in that layer which may be enough to squeeze a few hundredths
of an inch of liquid which could equate up to a half inch of snow in
spots. Any snow should come to an end after sunset as the low level
lapse rates begin to relax with the warming trend continuing into
Friday and Saturday. Daytime highs on Saturday may reach into the
40s.

Low precipitation chances return this weekend as a low pressure
system moves in from the southwest, but there remains significant
uncertainty to timing and precipitation type as temperatures will be
near the freezing point. Looking beyond into next week there is low
confidence in the forecast as the global models have significant
differences in the broader pattern but the overall threat for
widespread rain/snow looks low.

&&

.AVIATION (00Z TAF Issuance)...
Issued at 551 PM EST Mon Jan 20 2025

Impacts:

- VFR Cigs this period.
- Snow flurries possible overnight

Discussion:

Clouds upstream over MN/IA/WI will push across Indiana and the TAF
sites overnight. These clouds are associated with a quick moving
upper wave. This may result in some weak forcing that could result
in some flurries along with the expected VFR Cigs, but any aviation
impacts will be minimal to none.

As the wave departs on Tuesday, clearing skies and unlimited ceiling
will return.


&&

.IND WATCHES/WARNINGS/ADVISORIES...
Cold Weather Advisory until 10 AM EST Wednesday for INZ021-
028>031-035>049-051>057-060>065-067>072.

&&

$$

UPDATE...CM
SHORT TERM...BRB
LONG TERM...White
AVIATION...Puma