510
FXUS63 KIND 210540
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
1240 AM EST Thu Nov 21 2024

.KEY MESSAGES...

- First snow of the season tonight into Thursday with as much as 1-2
  inches of snow expected, mainly in grassy and elevated surfaces

- Dry with a warming trend starting this weekend.

- Rain chances return late Monday and Tuesday.

&&

.FORECAST UPDATE...
Issued at 914 PM EST Wed Nov 20 2024

Clouds have now encompassed the bulk of the forecast area with
a few sprinkles zipping by. The radar returns look more impressive
than they really are as the ACARS sounding at KIND continues to
advertise dry air through about 850 to 800mb. The winds continue to
howl with peak gusts that have gotten to 35 to 40 mph in some areas.
Temps at 02Z were dropping quickly...ranging from mid 30s north to
lower 40s south.

The forecast for the rest of the night remains in good shape as the
deep upper low to our northwest strengthens and brings the appetizer
of light snow showers into the predawn hours...setting up the main
course of heavier and more widespread snow that will impact central
Indiana Thursday afternoon and evening. The upper low will track
southeast into northwest Ohio by daybreak Thursday...setting up an
axis of broad ascent aloft across the northern half of the forecast
area.

As mentioned above...already seeing returns on radar but with the
boundary layer still dry...have not been able to get much more than
sprinkles to this point. That will change over the next few hours as
the influx of moisture in tandem with evaporative cooling helps to
saturate the lower levels. At the same time forcing aloft will
strengthen substantially as a strong and elongated vort lobe pivots
around the base of the amplifying upper trough. All of these factors
will lead to an increase in areas of precip...first as light rain
with a few snowflakes mixed in but gradually transitioning to all
snow after 06Z as the boundary layer cools. Coverage will initially
be scattered but as the upper low intensifies...there is potential
for more widespread light snow coverage focused across the northeast
half of the forecast area in the predawn hours. This first batch of
light snow will shift east of the area shortly after daybreak
Thursday with a 5-6 hour reprieve before the widespread and heavier
snow in round 2 arrives by early to mid afternoon. Westerly winds
will remain gusty through the night but should drop back a bit.

Snowfall amounts will largely be light with this first wave late
tonight and Thursday morning at a few tenths of an inch at
best...with localized bands near to slightly greater than a half
inch possible focused over northern counties. Travel impacts will be
minimal at best as ground temps remain quite warm and overall
snowfall rates with this first wave are not likely to be high enough
to cause issues. The second wave of snow Thursday afternoon and
evening remains the greater concern with the potential for light
accums and more intense snowfall rates that will likely cause some
travel issues.

Lows will fall into the upper 20s and lower 30s over most of the
forecast area. Zone and grid updates out.

&&

.SHORT TERM (This evening through Thursday night)...
Issued at 152 PM EST Wed Nov 20 2024

Through Tonight.

Skies continue to quickly clear across central Indiana as a dry slow
pushes eastward ahead of the low pressure system that is expected to
bring the first snowflakes of the year. Even with ample sunshine,
fairly robust CAA will help to keep temperatures in the mid to upper
40s for much of central Indiana. As temperatures begin to rise,
expect to see surface wind gusts gradually ramp back upwards as the
boundary layer begins to deepen. This dry air will quickly begin to
saturate again beginning around 00Z as the previously advertised
upper level trough continues to move southeastward. Expect that by
around 11PM that skies will be fully cloudy with the first
precipitation beginning towards 3AM tonight.

The initial wave of snow will be primarily be driven on the leading
edge of an upper level vort max with broad ascent expected across
central Indiana.  Precipitation is likely to initially begin as
rain, but as the near surface saturates expect to see temperatures
drop to near freezing which will end the melting of snow towards 4-5
AM. The dendritic growth zone will be fairly deep at around 4kft,
but generally will be above where the better ascent is expected
which will help to limit the snow ratios especially when considering
the very marginal temperatures.

Thursday.

The initial wave of snow that was mentioned above should come to an
end by around 9-10AM with fairly good model agreement in a break in
the precipitation through the early afternoon. This dry time will
allow for the melting of any snow that accumulated with the first
round. Models continue to come into closer agreement on the track
and strength of the surface low with the slight southwesterly trend
in the surface low continuing in the 12Z model guidance.  This shift
will lead to a lesser influence of the Lake Michigan moisture with
850mb moisture transfer vectors showing the advection being weaker
as the LLJ is now centered to the west of the lake while also adding
a more westerly component to the winds which would further limit
that moisture.  This is being counteracted by the LLJ being tighter
with the bulk of the lift being in the exit region of the LLJ. The
DGZ will again above the area of better lift through the afternoon
which will help to keep SLRs below climatology when combined with
the 31-33 degree surface temperatures.

There is good agreement in precipitation onset across the northern
counties to be around 2-3PM with arrival in Indianapolis closer to 4-
5PM.  Road temperature forecasts show ground temperatures well above
freezing which will help to limit accumulations on the roads to
periods of higher snowfall rates.  There isn`t much of a sign for
tighter frontogenetically driven banding at this time, but that will
be driven by the mesoscale environment and will be closely monitored
tomorrow.  Model snow accumulations again look a bit high with sub-
surface ground temperatures in the 50s with the positive snow depth
change fields showing more reasonable numbers of 1-2 inches.  These
accumulations are more likely on grassy surfaces with slushy
conditions on the roads. There doesn`t look to be many breaks in the
snow, but if there are that will allow for additional melting.

Overall, expect to see a 1-2 inch snowfall that will be only fully
realized on grassy surfaces for much of central Indiana. The
northeastern and southwestern portions of the forecast area have the
greatest uncertainty with the potential that the bulk of the late
afternoon snow ends up west of Muncie with the uncertainty on the
temperatures closer to Vincennes that may impact snow accumulations.
The worst case scenario would be driven if temperatures end up 2-3
degrees colder than currently expected which would increase the snow
accumulation efficiency and could lead to amounts of closer to 4
inches. In addition to the snow, the strong 850mb jet will help to
mix down occasional wind gusts to 35 mph with the strongest winds
across west central Indiana. The thermal profiles aren`t ideal for
mixing with marginal lapse rates, but the strength of the jet should
allow for those higher gusts.

Thursday Night.

Snow will begin to taper down during the evening hours but there is
a signal for additional snowfall across the northeastern counties as
the surface low passes overhead during the early overnight hours,
but confidence that this occurs is low. Snow should come to an end
by midnight with any snow on the ground likely to begin to melt as
even with surface temperatures at or below freezing, the sub-surface
will be warm enough to help the snow from the bottom up.

&&

.LONG TERM (Friday through Wednesday)...
Issued at 152 PM EST Wed Nov 20 2024

Following the passage of the shortwave, central Indiana will
be positioned in quasi-Polar airmass with steep near surface lapse
rates. These steep near surface lapse rates along with some remnant
moisture within lake enhanced moisture advection will likely lead to
isolated showers over the area throughout Friday. Above 850mb, lapse
rates become very weak limiting vertical ascent, and should keep any
precipitation light, with low QPF numbers.

Even within a cold airmass, modest dew points and a warm layer of
stratus will keep any precipitation in liquid form. Late Friday
night into Saturday, low amplitude ridging builds in and a warming
trend occurs. This should induce substantial mixing and end our
period of stratus, with mostly clear skies beginning late day on
Saturday. This shift should also provide another period of above
normal temperatures Saturday night through Sunday night.

Chances for rainfall will will increase once again early next week
as a shortwave develops east of a trough axis. This will also be
associated with some deeper tropical moisture, but ensemble guidance
varies greatly on where the greatest moisture anomalies will be
located; general consensus is south into KY/TN.

The Day 8-14 period looks to feature large meridional swings with
periods of warm and then cool conditions, averaging to near normal.
Within these periodic waves will likely incorporate some
precipitation, but there is not any confidence in significant
precipitation hazards on the horizon at this time.

&&

.AVIATION (06Z TAF Issuance)...
Issued at 1239 AM EST Thu Nov 21 2024

Impacts:

- MVFR conditions in scattered snow showers overnight

- LIFR conditions likely in more sustained snowfall this afternoon
  into this evening

- Gusty west or northwest winds as high as 30KT at times much of the
  period

Discussion:

VFR conditions are in place across the TAF sites at this time, but
snow shower coverage should increase overnight as top down
saturation continues. Somewhat widespread MVFR ceilings should
develop, with further reductions in visibility and ceiling to lower
end MVFR on at least a TEMPO basis in the more intense snow showers.

A lull is expected sometime around or after daybreak Thursday, with
a more sustained period of snowfall this afternoon into this evening
across the sites. LIFR conditions appear likely with this second,
somewhat more intense round, at least in visibility and likely in
ceiling at times. The second round will last into the evening hours
before beginning to taper off.

Winds through the period will be westerly or northwesterly,
sustained up to 18KT at times with gusts as high as 30KT. Isolated
stronger gusts are not entirely out of the question.

&&

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

&&

$$

UPDATE...Ryan
SHORT TERM...White
LONG TERM...Updike
AVIATION...Nield