464
FXUS63 KIND 180828
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
328 AM EST Sat Jan 18 2025

.KEY MESSAGES...

- Rain will mix with snow across northern portions of central Indiana
  before ending this morning. Less than an inch of snow is possible.

- Bitterly cold air is expected Sunday through Tuesday Night. Nights
  with subzero lows likely and single digit highs expected on
  Monday and Tuesday.

- EXTREME COLD WATCH for Sunday night into Monday. Wind chill values
  as low as -25 degrees possible.

- Very cold temperatures continue Tuesday and Wednesday with wind
  chills between -10 and -20 possible.

&&

.SHORT TERM (Today and tonight)...
Issued at 328 AM EST Sat Jan 18 2025

Early this morning through the daylight morning hours...

Rain will continue across central Indiana early this morning thanks
to forcing associated with an upper trough, upper jet, and ahead of
a surface cold front. Will continue with high PoPs.

Colder air was already working in aloft, and the column will cool
enough for rain to mix with or change to snow, especially across
northern portions of the area, by 12Z.

As the cold front sweeps across the remainder of the area after 12Z
and upper support moves out, the precipitation will end by noon
across most areas. Again, snow may mix in before the precipitation
ends at most locations.

Believe any snow accumulation will be less than an inch given the
timing of the cold air and low snow to liquid ratios.

This afternoon...

Low clouds will likely get trapped under a developing inversion this
afternoon, so will go pessimistic with sky cover. Wouldn`t rule out
a few flurries as well in the cold advection. Temperatures will
gradually fall with the cold advection.

Tonight...

Low clouds will continue to be trapped, and additional mid cloud
will move in as lift from an approaching upper wave from the
southwest moves into the area.

This lift may be enough for some flurries across the southern
portions of the area late tonight. Meanwhile, influence from Lake
Michigan will lead to some flurries or isolated snow showers across
portions of the northern forecast area. Little or no snow
accumulation is expected.

Even with the clouds, the colder airmass will still lead to lows in
the 10 to 20 degree range most areas.

&&

.LONG TERM (Sunday through Friday)...
Issued at 328 AM EST Sat Jan 18 2025

A transition back towards anomalously cold weather is anticipated
early in the long range, with perhaps some of the coldest
temperatures of the winter thus far.

Synoptically, ridge-building over the northern Pacific will allow
the jet pattern across North America to amplify significantly. A
deep trough centered over the Hudson Bay will span much of North
America by Sunday. Strong meridional flow across the central
portions of Canada and the US will allow Arctic air to freely flow
southward. As the continental arctic air mass filters into the
region, temperatures at 850mb look to bottom out around -25 to -30C.
This would represent a 20 to 30 degree departure from the
climatological norm for this time of the year. Temperatures at the
surface could drop well into the negatives for multiple nights with
only single digit highs.

LIGHT SNOW POTENTIAL

As for precipitation chances, we`ll see some flurries and snow
showers on Sunday. Some of the higher resolution models are a bit
more aggressive with this bringing light accumulations to portions
of the area...especially our southern zones. These models appear to
be picking up on an upper wave passing by just to our south. Given
the dry arctic air mass filtering in, moisture would appear to be
questionable. However, trajectories trace back to Lake Michigan
which may provide enough moisture to promote light snow or flurries
from the passing wave. Additionally, model soundings show some
instability as colder air arrives aloft. We will introduce chance
PoPs to most of the area on Sunday with the mention of snow showers
and flurries.

EXTREME COLD POTENTIAL

How cold we actually get depends on how efficiently we can
radiatively cool at night. This, in turn, depends on how much cloud
cover and wind are present. Additionally, how much of a snow pack is
remaining after today`s warmth and rain. Ideal conditions do not
appear until Tuesday night. Before then, enough of a MSLP gradient
is present to promote a 10 to 15kt wind which will limit cooling
potential. It will still be very cold, simply due to the magnitude
of the arctic air mass overhead. Lows Monday morning may dip into
the negatives regardless of the wind...which may allow for extremely
cold apparent temperatures / wind chills (see next paragraph). While
the wind dies down by Monday night, cloud cover may be present at
times which could limit how cold overnight lows get. Tuesday night
looks to have the most ideal conditions for cold overnight lows with
clear skies, light winds still within the core of the cold air mass.

We will issue an Extreme Cold Watch with this forecast update, as
winds Sunday night into Monday along with single digit and negative
ambient air temps lead to wind chill values between -15 and -25
degrees. This watch could then be upgraded to a Cold Weather
Advisory (values of -10 to -19) or an Extreme Cold Warning (values
of -20 or lower) depending on which side of that spectrum we
eventually end up on. As of right now there is high confidence in
apparent temperatures of at least -10, with lower confidence in -20
degree values. Our watch will span from 06Z Sunday night to 18z
Monday, since this is when the lowest wind chills are anticipated.
Apparent temperatures rise above criteria thereafter for at least 12
hours. Tuesday and Wednesday should have lighter winds as mentioned
above, which despite colder low temps could lead to wind chills not
reaching the -20 criteria. The watch may need to be expanded if this
changes, however.

By mid week, ensemble guidance shows the trough de-amplifying
somewhat with troughing still present to some extent through next
weekend. Slight moderation in temps is likely from Wednesday onward,
though values will still be below normal for January.

&&

.AVIATION (06Z TAF Issuance)...
Issued at 1206 AM EST Sat Jan 18 2025

Impacts:

- IFR conditions developing overnight with rain
- Rain at least mixing with snow by 10Z-12Z...before ending 12Z-15Z
- Wind gusts around 20kt
- Winds shifting quickly from SW to NNW within 09Z-15Z

Discussion:

Rain will continue to overspread the sites early in the period.
Ceilings are likely to fall into the IFR category as the atmosphere
saturates. Ceilings will improve to MVFR by around 15Z.

A cold front will move in, allowing rain to mix with or change to
snow from northwest to southeast 09-14Z or so. This will also shift
the winds to northwest. Winds will gust to around 20kt pre and post
front. Isolated higher gusts are possible.

Strong winds aloft will create near non-convective LLWS conditions
overnight.


&&

.IND WATCHES/WARNINGS/ADVISORIES...
Extreme Cold Watch from late Sunday night through Monday
afternoon for INZ021-028>031-035>049-051>057-062>065-072.

&&

$$

SHORT TERM...50
LONG TERM...Eckhoff
AVIATION...50