999
FXUS63 KIND 262352
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
652 PM EST Tue Nov 26 2024

.KEY MESSAGES...

- Increasing clouds and calming winds tonight, lows near freezing.

- Potential rain/snow late Wednesday and Wednesday Night; limited
accumulation and travel impacts currently expected

- Much colder Friday into the weekend.

- Potential for light snow late Saturday, highest chances across
  southern Indiana

&&

.SHORT TERM (This evening through Wednesday night)...
Issued at 145 PM EST Tue Nov 26 2024

Rest of Today and Tonight:

CAA behind the frontal passage last night has been more progressive
than originally thought, of which has brought dew points down into
the upper teens to low 20s over the Wabash Valley. Lowest dew points
have been confined near the high pressure center over S IL and SW
IN.  High pressure will continue to build over the region today
within this cold air advection. This will keep dry conditions
through at minimum the next 15 hours along with weakening winds
overnight. At the same time, a midlevel moisture plume will push
into the Ohio Valley within strong zonal westerly flow, gradually
leading to 120-200kft cloud decks.

The increase in cloud cover will help moderate overnight
temperatures a little despite near calm and dry surface conditions.
This should result in overnight lows around freezing.

Wednesday and Wednesday Night:

Focus in the short term continues to be on the arrival of a modest
surface low late Wednesday through Wednesday night. Now with
mesoscale models fully realizing this system, there is some
consensus on overall timing, with the primary variance focused on
predecessor moisture, and horizontal/vertical placement of FGEN north
of the low.

18Z satellite imagery continues to depict the advancement of a mid
level cloud deck out ahead of lee cyclogenesis. This placement of
the mid level cloud deck will generally be co-located with a
moderate baroclinic zone between 900 and 500mb. The antecedent high
pressure from Tuesday will initially provide broad subsidence
induced dry air over the region, of which will slowly saturate as
the baroclinic zone starts to precipitate and moisten the surface to
700mb column of air. Initially this could lead to light, isolated
showers Wednesday afternoon, but most of the area is expected to
remain dry until Wednesday evening when the approaching system
should act upon this baroclinic zone leading to an east-west
oriented area of frontogenesis (FGEN) and eventual deep saturation
of the lower troposphere. This FGEN is expected to remain quasi-
stationary through approximately 08-10Z Thursday morning, before
quickly moving off to the east.

As stated there is variance in expected dew points from guidance,
leading to some uncertainty. However, given current observations out
ahead of the system and a previous observed bias towards higher dew
points prior to system arrival, the current thinking is dew points
will generally fall below guidance with wetbulb temperatures
remaining at to just above freezing.

HREF guidance also has the concentration of FGEN varied in different
locations in the vertical. In general, the lower the FGEN the
further south the mesoscale banding will occur leading to less
snowfall over central Indiana. On the other hand, FGEN concentrated
around 700-600mb and a resulting mesoscale band near the I-70
corridor could result in amounts closer to the 90th percentile for
snowfall accumulation; This is due to diabatic effects of higher
precipitation rates in conjunction with near freezing wetbulb
temperatures. Fresh cold air will not be ingested into this system,
so snow will greatly depend on diabatic processes along with
evaporational cooling

Accumulations and impacts may be rate-driven, especially where one
or more mesoscale bands set up. Any areas with higher intensity
rates may also have a positive feedback loop of greater wetbulb
cooling along with increased ability to overcome ground temperatures
for isolated accumulations on roads; otherwise, accumulations should
primarily occur on elevated and grassy surfaces.

Below is the current thinking for snowfall potential given above
discussion along with trends gradually shifting towards the southern
solution. However there still remains a low chance for the
reasonably worse case scenario (northern solution).

10th-75th Percentile (Southern Solution): T-0.5" I-70 and North

75th-90th Percentile (Northern Solution): 0.5-2.0" I-70 and North

&&

.LONG TERM (Thursday through Tuesday)...
Issued at 145 PM EST Tue Nov 26 2024

Thursday through Saturday.

Any lingering precipitation is expected to end by Thursday morning
with the main focus for the long term period being on the much
colder temperatures. Clouds are likely to stick around for much of
the morning into the early afternoon before drier air gradually
begins to advect into the area.  Low level winds will remain
generally westerly through Saturday which will keep any lake effect
clouds/snow to the north of the forecast area. Temperatures Thursday
will remain seasonable with highs in the low to mid 40s with the
main cold blast arriving Friday into Saturday. Confidence is high in
overnight lows in the teens for Friday night and again the following
two nights with daytime highs staying below freezing for much of not
of central Indiana  through Tuesday.

Pressure gradients on Friday and Saturday will be strongest across
the northern counties with wind speeds of 10-15 mph which will
combine with the cold air to create periods during the morning hours
where wind chills will drop to near 0.  While this may not reach
typical Cold Advisory criteria, it will be notable how cold this
airmass will feel considering the recent warmth. These low
temperatures will also create a concern for people standing outside
ahead of Black Friday shopping.

Saturday Night Through Tuesday.

A weak shortwave within the broader northwesterly flow may bring a
quick hitting light snow to central Indiana late Saturday into early
Sunday but confidence is low as models continue to struggle in
placement of this weak wave and the general model consensus keeps
the better chances closer to the Ohio River with the better forcing
into southern Illinois. Temperatures will remain much below normal
for the rest of the weekend into early next week with daytime highs
in the low 30s and overnight lows in the teens. Later into the week
temperatures are expected to gradually return to near normal with
little chance for rain/snow.

&&

.AVIATION (00Z TAF Issuance)...
Issued at 652 PM EST Tue Nov 26 2024

Impacts:

- VFR Conditions are expected through Wednesday Afternoon
- Rain/snow and deteriorating conditions expected Wednesday evening

Discussion:

Latest satellite imagery shows high clouds streaming in from the
west ahead of the next system to impact Central Indiana. As high
pressure pushes east tonight, a weak area of low pressure will track
from the TX Panhandle to Kentucky by tomorrow night. While dry
antecedent conditions will keep cigs and vis VFR through around 20z
tomorrow, expect deteriorating conditions by the end of the TAF
period.

Rain will slowly overspread Central Indiana tomorrow evening then
likely changeover to snow. The heaviest axis of snow and worst
conditions will be confined to a narrow band, only a few counties
wide and extend W-E across the state. Confidence is lower exactly
where the worst conditions will persist; but for now it does look
like all TAF sites within Central Indiana will experience a period
of MVFR to IFR (potentially lower) conditions tomorrow evening. For
now it does look like IND, HUF, and LAF will likely be the areas to
see the lowest vis and cigs from rain/snow after 23z tomorrow
evening. Will watch the forecast closely and update the aviation
forecast accordingly.

Winds do not look impactful through the period. Expect winds out of
the SW this evening to become S then SE by the morning. By the
afternoon and evening hours, winds should become E then NE at or
under 10 kts.

&&

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

$$

SHORT TERM...Updike
LONG TERM...White
AVIATION...CM