734
FXUS63 KIND 220507
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
1207 AM EST Fri Nov 22 2024

.KEY MESSAGES...

- Wind gusts to 40 mph diminishing later tonight

- Warming trend this weekend with a chance of light rain Monday.

- Lower than normal forecast confidence the second half of next
  week with some wintry precipitation potential.

&&

.FORECAST UPDATE...
Issued at 954 PM EST Thu Nov 21 2024

This afternoon/evening`s Winter Weather Advisory will be replaced by
an SPS at 1000P, which will continue into the late night for any
lingering, isolated slick roads...which will be more from at times
robust winds transporting the light (and likely melting) snowpack
onto roadways. Concern here will be mainly bridges, overpasses
and secondary roads...although any well-exposed areas next to
lower roadways may promote patchy slick spots from blown snow.

Otherwise conditions will overall continue to improve overnight with
final snow showers tapering off to flurries and drizzle as we
approach midnight.  Temperatures over the past hour have already
started to rebound as organized snow slackens and departs southward,
with 35F now at Indianapolis and 37F at Lafayette. With most isolated
readings at 32F expected to nudge above freezing through the
night.

No concerns over visibility reductions from fog with dewpoint
depressions increasing slightly and sustained winds near 10 mph or
greater. Temperatures by daybreak to range from 33F over most
eastern zones to nearly 40F in Vincennes. With road surface
readings slightly higher than air temperatures.

&&

.SHORT TERM (This evening through Friday)...
Issued at 258 PM EST Thu Nov 21 2024

The first snow of the season continues across Central Indiana this
afternoon. This snow event has come in two rounds, the initial round
this morning which produced a widespread dusting to 1.5 inches
across much of Central Indiana. The second round of snow is
currently overspreading the state from NW to SE, which will bring an
additional 1-2 inches of accumulations for much of the area. Despite
the 2nd warmest Oct 1 - Nov 20th on record, this first accumulating
snowfall is coming a few weeks early! The average first one inch of
snow for Indianapolis typically comes around December 8th.

The set up for today`s snow is quite anomalous and complex, with a
525dm occluded upper low pushing southeast towards Pittsburgh with a
surface low moving south along the western short of Lake Michigan
into Northern Indiana. The occlusion can easily be seen on
temperature profiles through the column as relatively warm air wraps
around the entire system with 850mb temps at or above 0C on the NW
and W side of the surface low while 850mb temps are nearing -10C
over Central Indiana. As this low approaches the area tonight, warm
air advection will actually be coming in from the Northwest, with
possible enhancement from the very warm waters of Lake Michigan.
This will likely result in a changeover to a mix or rain later
tonight as "warm" air advection results in temperatures rising into
the 32-36 degree range overnight.

Before we get to that part of the forecast, currently, weak 925-850
frontogenetical forcing ahead of this surface low in addition to
enhanced lift from a 130 kt jet streak to the SW and an approaching
55kt LLJ are working together to create moderate to occasionally
heavy snow bands ahead of the approaching low. Satellite and imagery
shows widespread clouds and precipitation in Northern Illinois,
Indiana, north into Wisconsin. Daycloud Phase RGB does show higher
cloud tops associated with heavier bands of snow pushing into NW and
Central Indiana. Latest ACARs sounding from IND does differ somewhat
from the latest short term forecast soundings. IND ACARs sounding
from 1935z shows an isothermal, saturated layer along the -11C
isotherm in the 800-700mb layer, whereas the short range models do
not have this layer. Also, surface temperatures and dew points have
been running 1-3 degrees below guidance  all day. With this in mind,
confidence is high that widespread 1-2 inch accumulations along and
north of the I-70 corridor through this evening, with isolated 3"
amounts possible north of Indianapolis. A Winter Weather Advisory
remains in effect through 10pm for Central and North Central
Indiana. Main accumulations should be on grassy areas; however roads
and overpasses could become slick and briefly snow covered during
heavier bursts.

While confidence in the forecast is high, there are some factors
working against snowfall accumulations. A strong 55 kt low level jet
with strong winds to 40 mph making it to the surface will cause
fracturing of dendrites and lower snow ratios. Also, ups352tream
observations do show warm air advection on the backside of this
system already raising temperatures into the mid 30s across the IL
state line and into Chicago. While snow will accumulate this
evening, there is a good chance a lot of melting will take place
overnight as temperatures warm.

For tonight, the surface low continues to weaken and push southeast,
leaving the area in a general northwest flow pattern, but with warm
air advection. Forecast soundings do show the Dendritic growth zone
drying out after sunset with precipitation ending as a period of
light rain or drizzle. Kept drizzle in through much of the night for
the area, especially along and north of I-70. Not too concerned with
icing issues, unless a few wind sheltered areas that get a good
snowpack stay at or below 32 degrees all night. Overall, the ground
will likely transition into slush tonight.

Northerly flow persists into Friday as ridging nudges closer to
Indiana. Despite warm air advection aloft, there still will likely
be a good fetch of moisture streaming SSE off Lake Michigan keeping
clouds and the chance for light showers around for portions of North
Central and Central Indiana. Thicker clouds and cool temperatures in
the low to mid 40s likely for areas stuck under the fetch coming off
the lake. Potential for SW and Southern Indiana to see some peaks of
sun by the afternoon with highs reaching the upper 40s to near 50.

&&

.LONG TERM (Friday night through Thursday)...
Issued at 258 PM EST Thu Nov 21 2024

Expect improving conditions heading into the weekend as the deep
upper level low aiding in the snow today slowly shifts east into the
Canadian Maritimes. Ridging aloft will expand into the Ohio Valley
by late weekend before transitioning to a fast quasi-zonal regime
for much of next week. Systems will quickly move along within the
flow aloft bringing chances for precip at times next week with the
possibility for a larger and more impactful storm by late in the
period with potential amplification in the upper levels over the
eastern U S.

Friday Night through Sunday

Cyclonic flow on the back side of the departing upper low will keep
stratus across much of the forecast area Friday night and probably
into Saturday as well with low level moisture trapped beneath a
strong inversion. May see lingering light rain showers over eastern
parts of the forecast area Friday night but overall expect dry
conditions developing as the system moves away to the east. Model
soundings highlight the inversion lingering into late day Saturday
as the surface ridge axis aloft moves into the region and finally
enables drier air to filter down into the boundary layer.

Dry conditions will continue Saturday night and Sunday as surface
flow transitions to a southerly direction. Skies will gradually
clear Saturday night into Sunday before increasing again late day as
a frontal boundary approaches from the west. Highs Saturday will
largely hold in the 40s with the stratus limiting warming. With the
onset of warm advection for Sunday though...will see a return to
above normal readings mainly in the mid and upper 50s.

Sunday Night through Tuesday

The aforementioned cold front will sweep across the region on Monday
with rain developing. Showers will arrive as early as Sunday evening
across the Wabash Valley but the bulk of the widespread rain will
come on Monday afternoon along the front and as stronger flow at
850mb translates across the Ohio Valley. Rainfall amounts will
generally be a quarter inch or less with rain ending Monday evening
as high pressure builds in. High temperatures will be mild ahead of
the front on Monday...running from the mid 50s to around 60.

Dry weather returns for Tuesday as the high tracks across the Great
Lakes and Ohio Valley. A more robust signal for subsidence in the
wake of the front should lead to stratus diminishing quickly Monday
night. Despite the return of sunshine on Tuesday...gusty northwest
winds and cold advection will make 352for a chillier day as highs
remain in the low and mid 40s.

Tuesday Night through Thursday

A surface low is set to eject out into the southern Plains Tuesday
night and Wednesday and track east along a quasi-stationary boundary
extending into the Tennessee and lower Ohio Valleys. Confidence is
growing in an increased risk for precipitation returning as early as
Tuesday night and persisting into Thanksgiving but low confidence
exists within the details at this early stage. It appears central
Indiana will be on the colder north side of the frontal boundary and
displaced from the deepest moisture focused closer to the boundary
over the Tennessee Valley. The presence of a colder airmass north of
the boundary could potentially present precip type challenges at
times as well...especially Tuesday and Wednesday nights as
temperatures near or go below freezing. Highs both Wednesday and
Thursday are likely to range from the upper 30s to mid 40s.

Extended guidance continues to hint at a broader piece of energy
aloft coming southeast late next week which would support the idea
of a more amplified upper level pattern developing with an
intensifying surface wave somewhere over the region. At this early
stage...expect increased chances for precipitation throughout the
second half of next week with impacts to travel possible. Will
continue to fine tune details over the next several days.

&&

.AVIATION (06Z TAF Issuance)...
Issued at 1206 AM EST Fri Nov 22 2024

Impacts:

- IFR conditions will improve to MVFR
- RASN showers will end by 08-09Z.
- Lake effect showers/sprinkles possible at IND late this morning
and early afternoon.

Discussion:

Deep low pressure in place east of Indiana will keep cool north
cyclonic flow in place across Indiana today. Forecast soundings
continue to reveal trapped lower level moisture across the TAF
sites, resulting in MVFR and IFR cigs through the period. HRRR shows
some Lake Effect showers coming off Lake Michigan, reaching Central
Indiana from Indianapolis to points east through the day. Forecast
soundings show lower levels warm enough for P-types to be rain. Thus
have included a mid day VCSH mention at IND. Otherwise, expected
continued MVFR Cigs for most of the period with NW wind.

&&

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

$$

UPDATE...AGM
SHORT TERM...CM
LONG TERM...Ryan
AVIATION...Puma