IND issues Area Forecast Discussion (AFD) at Dec 11, 5:48 AM EST757
FXUS63 KIND 111059
AFDIND
Area Forecast Discussion
National Weather Service Indianapolis IN
548 AM EST Wed Dec 11 2024
.KEY MESSAGES...
- High impact heavy snow showers and squalls this afternoon and
evening with drastically reduced visibilities and icy roadways
developing
- Sub-zero wind chills tonight into Thursday morning
- Rain chances and near normal temperatures return for this weekend
&&
.SHORT TERM (Today and tonight)...
Issued at 319 AM EST Wed Dec 11 2024
Colder air has advected into much of the region since Tuesday
afternoon with upper 20s and lower 30s as of 08Z. Mid level clouds
were predominant across central Indiana with pockets of lower
stratus...focused especially across southeast counties.
An amplifying upper level trough will expand across the area later
this afternoon with a frontal boundary accompanying at the surface.
These features in tandem with steep low level lapse rates will bring
the threat of higher impact/low accumulation snow showers and
possibly squalls for the afternoon and evening. Much colder air will
follow for tonight with subzero wind chills possible.
Bottom Line Up Front
- Scattered to numerous snow showers with squalls this afternoon and
evening
- Timeframe for main impacts from 2 PM to 8 PM
- Drastically reduced visibilities due to briefly heavy snowfall
rates and wind gusts near 40mph late today
- Evening rush hour will be significantly impacted with slick
and icy roads possible
- Minor accumulations expected
- Much colder air tonight with subzero wind chills possible
Through Early Afternoon
Cold and quiet start for early this morning with a minor exception
across far southeast portions of the forecast area currently. A
narrow band of flurries and perhaps lighter snow showers was
impacting portions of Jackson and Jennings Counties...and webcams
along Interstate 65 in south central Indiana confirm that at least
some flakes are making it to the ground. The flurries and light snow
showers will shift east over the next couple hours and will likely
produce no impact.
Mid level clouds continue to stream across the region behind the
area of low stratus and flurries in southeast Indiana followed by a
dry slot approaching the region from the southwest. The dry slot
will move across the forecast area through mid morning in
association with a narrow surface ridge and may actually provide a
couple hours with mainly clear skies. Stratus however will quickly
return from the northwest throwout the morning in advance of the
system that will bring the threat for snow showers and squalls
this afternoon.
Mid Afternoon Through Tonight
The aforementioned upper trough axis will dive across the area this
afternoon and take on a negative tilt as energy aloft carves
amplifies and strengthens the feature. A frontal boundary will track
through the forecast area this afternoon serving as a catalyst for
the development of scattered to numerous snow showers and
potentially a few squalls as well.
Some semantics to address from a pure meteorological standpoint
before we discuss the impacts. While confidence is high in
convectively induced snow showers occurring...there remains lower
confidence in whether the snow showers will achieve the intensity
level to meet the true definition of a squall. Part of the challenge
has to do with the mesoscale setup which we will not have a full
view of until later today as trends develop further. For instance...
some of the CAMs on the evening model suite have backed off ever so
slightly on the depth of the dry adiabatic flow and steep lapse
rates...likely due to the drier air aloft being pulled further down
and disrupting the low level moisture within the column.
Additionally...model guidance appears to be struggling and overdoing
potential warming of the near surface layer prior to the arrival of
the snow showers.
With that being said...the presence of shallow instability up to
around 50 j/kg with the steep lapse rates and deeper moisture which
will still extend above the dendritic growth zone for a period of
time late today will coincide with the passage of a more intense
axis of mid level forcing and frontogenesis. This will support a 3-5
hour period with greater coverage of snow showers along with more
intense but localized bursts and squalls.
Ultimately taking all of the above into account...remain very
confident in high impacts to roadways and motorists as the period
with greatest intensity to convection will coincide with the evening
rush hour when traffic volume is maximized. While the heaviest snow
bursts will produce drastically reduced visibilities due to the
intensity of the snowfall...the addition of gusty winds that will
peak at 35-40mph by late day as the boundary passes and intense cold
advection commences will further complicate and exacerbate impacts
and may serve to enhance localized and brief whiteout conditions.
While surface temps initially will be above freezing primarily in
the mid 30s prior to snow showers beginning...the infusion of
moisture into the boundary layer should aid in evaporative cooling
pulling surface temps back to near to just below freezing rather
quickly late today before temps tumble in wake of the frontal
passage this evening.
Overall accumulations will be light with most locations at a half
inch or lower and any higher amounts around an inch being isolated.
Expect the greatest potential for highest snowfall will be across
the northern half of the forecast area but most everyone across
central Indiana will be impacted in some manner. Snow will end
quickly from west to east during the first part of the evening with
skies clearing overnight as high pressure expands in. Icy conditions
on roadways will continue even after snow ends as temperatures fall
rapidly and winds remain brisk.
Temps...muted highs below the model blend with early to mid
afternoon highs in the mid 30s with upper 30s possible in southern
counties. Lows tonight will drop into the teens with subzero wind
chills north and single digit wind chills south by early Thursday.
&&
.LONG TERM (Thursday through Tuesday)...
Issued at 319 AM EST Wed Dec 11 2024
After a very cold start to the long term, temperatures will warm up
to near to slightly above normal. There will also be a couple of
upper waves moving through bringing multiple chances for
precipitation.
Quiet weather conditions are expected on Thursday with surface high
pressure settling over the region. Temperatures will still remain
cold during the day, in the 20s, due to subtle cold air advection
and troughing aloft. Wind chills for Thurs morning will range from
around 10 degrees down to around 5 below. Models are showing a
boundary setting up over north-central Indiana that could produce a
band of snow Thursday evening so keeping low chance PoPs over the
northern portion of the forecast area.
The surface high will expand through much of the day Friday as upper
ridging helps to quickly bring temps back to near normal. Conditions
should remain dry for the day. Then the first upper wave of the
period will approach and while exact timing is not yet clear,
expecting associated precipitation to arrive sometime Friday night
to Saturday morning. If it arrives overnight, some areas to the
north could see snow mix in before sunrise. This line of showers
will progress eastward through the day before exiting sometime
Sunday. Increasing dynamics and slightly anomalous moisture return
should widespread rainfall amounts between 0.25-0.75 inches through
Saturday. Some areas could slightly higher amounts.
Another quick passing ridge will allow for a break in rain until at
least Monday morning. Going into next week is when models really
start to vary on their solutions for the subsequent system or
systems. For now, can expect another upper wave to bring more
precipitation but that may arrive either Monday or may not arrive
until Tuesday.
&&
.AVIATION (12Z TAF Issuance)...
Issued at 548 AM EST Wed Dec 11 2024
Impacts:
- Snow showers with sudden visibility reductions to sub-IFR levels
and lower ceilings this afternoon and early evening
- West winds gusting to around 30 knots late this afternoon and
evening
Discussion:
Stratus already starting to build back into western Indiana early
this morning and expect that will gradually encompass most of
central Indiana by late morning. Weak forcing aloft ahead of the
main system for later already leading to scattered light snow
showers over western Illinois. May see some of these snow showers
make it to KLAF later this morning.
Numerous snow showers...some locally and briefly heavy...are
expected to develop this afternoon with the primary upper level wave
and result in sudden significant visibility reductions that may be
brief, but periodic. We have a PROB30 group covering this for now
but coverage is expected to be large enough that brief visibility
drops to 2SM or lower are likely with the potential for as low as
1/2SM within the heaviest snow showers. Gusty westerly winds peaking
at near 30kts will further complicate visibilities through the
afternoon and early evening within snow showers.
Snow showers will shift east with the front during the first part of
the evening with skies gradually clearing overnight. Westerly winds
will remain brisk through the night with gusts gradually decreasing
early Thursday.
&&
.IND WATCHES/WARNINGS/ADVISORIES...None.
&&
$$
SHORT TERM...Ryan
LONG TERM...KF
AVIATION...Ryan
Source:
IND issues Area Forecast Discussion (AFD) at Dec 11, 5:48 AM EST---------------
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