Zionsville, Indiana 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

151
FXUS63 KIND 211538
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
1010 AM EST Thu Nov 21 2024

.KEY MESSAGES...

- Breezy this afternoon and tonight with frequent gusts of 25-35
  MPH, locally higher gusts possible

- First snow of the season tonight into Thursday with as much as 1-2
  inches of snow expected, mainly in grassy and elevated surfaces,
  though localized slick spots on bridges and overpasses cannot be
  ruled out

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

- Lower than normal forecast confidence the middle or end of next
  week with some wintry precipitation potential.

&&

.FORECAST UPDATE...
Issued at 1010 AM EST Thu Nov 21 2024

The first snow of the season has arrived across much of Central
Indiana with many locations reporting over an inch of snow. The date
of the average first snowfall in Indianapolis is around Nov 24th,
with the first inch typically occurring around Dec 8th. So today`s
accumulating snow is a little ahead of schedule...very impressive
considering the anomalously warm Autumn over the past six weeks.

The first round of snow this morning did overachieve as radar
reflectivity still shows banding signatures in portions of
Southeastern and Central Indiana. Cyclonic flow around a strong
526dm upper low centered over Toledo combined with the exit region
of a 135kt upper jet has resulted in ample lift over the region.
ACARs soundings from around the area this morning show deep
saturation through the dendritic growth zone, with a 1km deep
isothermal saturated layer around the -15C isotherm. This set up has
resulted in high snowfall rates able to overcome the warm grounds
with many reports of over 1 inch around Central Indiana.

Latest satellite and radar imagery confirms the backside of the
first round of snow pushing south of the I-70 corridor as of 10am
EST. A brief break in the precipitation is expected before the
second round moves in later this afternoon. Latest observations show
a 995mb surface low pushing south through Lake Michigan, just off
the coast of Door County, WI. This is quite an interesting set up as
relatively "warm" air has wrapped around the northwest side of the
low with low level temps at or above freezing within the NW flow and
the coldest air just ahead of the low over portions of Illinois and
Indiana with 850mb temps as low as -10C and surface temps below
guidance in the 29-31 degree range. Temperatures may actually warm
up as this low passes through tonight as "warm" air advection pushes
in on the backside of the low, helping transition precip from snow
to rain, especially as the dendritic growth zone rises and dries
out.

Widespread snowfall ahead of the low pushing south is currently
near Chicago and will push southward into Indiana later this
afternoon and evening. While the low will slowly be weakening, weak
frontogenetical forcing may work to produce bands of heavy snow at
times. Despite strong winds of over 35mph and warm air advection
working to keep snow ratios lower, heavy rates will likely still
produce brief accumulations of 1-2" for many areas along and north
of I-70. Will note, temperatures and dew points are running a few
degrees below guidance this morning. Will be watching this "cooler"
trend closely as this may lead a later changeover time to a
rain/snow mix later, higher snow ratios, and potentially higher
accumulations later today. A Winter Weather Advisory is in effect
for the area for this afternoon and evening. So far the forecast
looks on track with increasing confidence in widespread
accumulations of 1-2 inches and isolated higher amounts.

&&

.SHORT TERM (Today and tonight)...
Issued at 344 AM EST Thu Nov 21 2024

THIS MORNING -

Scattered snow showers or mixed rain/snow showers will sweep across
the northern half to two thirds of the area this morning associated
with a vort lobe pivoting around the larger closed upper low. Little
to no impact expected with surface temperatures near or above
freezing across the area and road temperatures universally above
freezing, as well as sub-surface temperatures some 20-30 degrees
above freezing given recent warmth. A light dusting will be possible
on grassy and elevated surfaces, but roads should be wet for the
most part. This round will move out of the area during the first
couple of hours post daybreak, with perhaps some lingering flurries
or drizzle possible in the interim.

THIS AFTERNOON INTO TONIGHT -

A more substantial round of precipitation is expected this afternoon
into this evening as a compact shortwave and weak surface low pivot
around the upper low down Lake Michigan and into the Ohio Valley.

Profiles should be supportive of all snow for most of the area
immediately at or just after precipitation onset, with at least a
brief period of modest snow rates expected despite a relative lack
of significant signal for mesoscale banding (fairly weak
frontogenetic forcing, etc.). Snow onset appears likely in the noon-
2PM time frame in the northwest, with Indy metro following in the 2-
4PM time frame, and 4-6 PM in the southeast. A period of about 4-6
hours of snowfall is expected, with briefly stronger rates possible
somewhere in that time frame. This could possibly overwhelm the
aforementioned significant road and subsurface warmth and allow for
a minor coating on bridges and overpasses, but accumulation should
largely be limited to grassy and elevated surfaces, and a good
amount of melting should occur even as snow falls.

As the late afternoon/evening wears on, modest warming in the mid
and low levels along with drier air aloft moving in and causing a
significant drop in cloud ice content will help to bring an end to
accumulating snow and transition precipitation back over to a
rain/snow mix and then possibly a drizzle. Some lake enhancement may
contribute to additional showers late tonight, though borderline
temperature profiles and the anomalously high lake warmth may allow
this to fall as either rain/drizzle, a mix, or perhaps flurries as
elements of lake effect convection get further from the lake into
cooler air.

Winds will be a factor today and tonight as well, even in the
absence of a more favorable low level lapse rate environment.
Frequent gusts 25-35 MPH are expected with localized 40 MPH gusts
not out of the question.

SNOW AMOUNTS -

QPF and ratios at or below climatology are likely to yield snow
totals in the 1-3 inch range roughly along and northeast of a
Rockville - Bloomington - Seymour line, though a significant amount
of melting will occur even during the event as previously mentioned.
High resolution model snow depth changes, as prior discussion
alluded to, likely represents a better proxy for actual
accumulations with some nod toward the low level thermodynamic
environment incorporated, and this shows values largely on the order
of one half of that - 0.5 to 1.5/2 inches, which again would be
primarily on grassy surfaces and would likely see plentiful melting
during and after the event.

MESSAGING -

Truly on the fence about a headline given all the mitigating factors
discussed extensively above, and the extensive messaging that has
already gone into the event.

All that said, will go ahead and issue a Winter Weather Advisory for
portions of the area for the following reasons - 1) This will be the
first snowfall of the season; 2) Onset of snow will be at or near
the start of the afternoon rush, particularly for Indy metro; 3) at
least some minor potential that briefly overachieving rates could
lead to localized slick spots on bridges and overpasses.

Advisory will run 4 PM to 10 PM EST.

&&

.LONG TERM (Friday through Wednesday)...
Issued at 344 AM EST Thu Nov 21 2024

Friday:

As broad closed low continues to move eastward, we will
still experience cyclonic flow within a very moist lower
troposphere. Moisture may be augmented by trajectories off Lake
Michigan and there may be a mesoscale convergence band emanating
from the lake into central Indiana which focuses some convective
precipitation. By then, modest warm/moist advection wrapping around
the low will occur, coupled with weakening lapse rates as the low`s
parent cold core moves eastward. Therefore, any precipitation is
expected to transition to rain by early-mid morning, if not before.
Even aside from the convective showers, stratus depth and low
ceilings may support a persistent drizzle, at least until ceilings
rise some during the afternoon.

Some of the model guidance may be under-representing stratus
extensiveness and be slightly too warm. We saw that in the wake of
the last system. So, we lessened the diurnal temperature curve and
have slightly lower temperatures by a degree or two (low-mid 40s)
than in the previous forecast.

Saturday:

It is possible that slight cyclonic curvature to low-level flow and
residual moisture will keep stratus around at least during the
morning. But, clouds should decrease later in the day as low level
flow shifts in response to ridging building in. A warming trend
starts and areas that due clear may reach or at least approach 50
degrees. We tightened the temperature gradient in anticipation of
lingering stratus in the east.

Sunday-Monday:

The warming trend continues as axis of low-amplitude midlevel ridge
approaches. High level clouds/cirrus will be prevalent within
tropopause-level Pacific moisture stream.

Models have trended more amplified with the upstream shortwave
trough and have a slightly higher latitude northern extent of
subtropical moisture plume. Light rain will expand within a broad
area of moist isentropic ascent maximized in the 295-300-K layers.
Multi-model ensemble mean shows <0.25" QPF, but there are some
timing/placement differences, along with a few outliers near or
above 0.5".

Timing of the system`s cold front along with diabatic effects of
precipitation will have implications for temperatures, but in an
optimal scenario we would reach the upper 50s to lowers 60s.
Otherwise, low-mid 50s may be more reasonable compromise blending
the scenarios together.

Tuesday-Thursday:

There has been above normal run-to-run inconsistency during this
period, with a few cycles of the deterministic models showing a
strong mid-latitude system optimally placed for substantial snow
across portions of Indiana late in the week. It`s important to
understand that while such a scenario is possible, it is low
probability at this time, with only a few supporting ensemble
members. Latest cycles have trended away from this, either delaying
it beyond the 7-day period or suppressing it further south. What we
can say at this point is the synoptic pattern of strong
baroclinicity and progressive flow could favor a well-developed mid-
latitude systems passing through the area sometime late next week
into the following weekend. But, its track would determine
precipitation amounts and type, and there`s just too much chaos in
the ensembles at this time to state anything more than a low-
probability impactful precip/winter event. Cluster analysis has
overlapping between model camps but displays completely out of phase
ridge/trough positions, further underscoring the uncertainty during
this time period.

For now, we have blended multiple ensemble members into a broad time
window Wednesday night into Thursday of low-mid range precipitation
probabilities. We have used generic rain/snow wording since building
high latitude Arctic/cP air mass is contributing to an increasingly
tight baroclinic zone across our region leading to a fairly
substantial temperature spread among the ensemble suite.

Day 8-14:

This period appears active with some potential for wintry
precipitation and associated hazards, but too much chaos for
clarity. The building Arctic/cP air mass at higher latitudes
suggests potential for colder than normal temperatures for at least
part of the period. Progressive pattern and one or more mid-latitude
systems would bring temperature swings and at least normal, if not
above normal, precipitation.

&&

.AVIATION (12Z TAF Issuance)...
Issued at 641 AM EST Thu Nov 21 2024

Impacts:

- MVFR conditions in snow showers this morning

- LIFR or near LIFR conditions likely in more sustained snowfall this
  afternoon into this evening, with conditions returning to MVFR
  upon cessation of precipitation

- Gusty west or northwest winds as high as 26-31KT through much of
  the period

Discussion:

MVFR conditions in snow showers are ongoing this morning, and will
taper off in the next few hours, before more sustained snowfall
moves into the area this afternoon into this evening.

Conditions are likely to deteriorate with the second round of
snowfall to near or at LIFR in either ceilings/visibilities or both,
though fluctuations are likely as is often the case. Conditions will
return to MVFR within a short time after precipitation ends late
this evening.

Temperatures during precipitation will likely hover around or within
a couple of degrees of the freezing mark, and may rise slightly this
evening or overnight in some spots. Warm ground will likely help to
significantly mitigate snow on paved/concrete surfaces, though some
slushiness may occur during the heaviest portion of the event late
this afternoon/early this evening.

Winds will be gusty throughout much of the period, generally between
about 270-320, with sustained winds as high as 19KT and gusts as
high as 31KT. Winds will relax late tonight into Friday morning.


&&

.IND WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 4 PM this afternoon to 10 PM EST
this evening for INZ021-028>031-035>049-053>057-062>065-071-072.

&&

$$

UPDATE...CM
SHORT TERM...Nield
LONG TERM...BRB
AVIATION...Nield