West Lafayette, Indiana 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

647
FXUS63 KIND 210845
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
345 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.

&&

.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 (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...

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.

&&

$$

SHORT TERM...Nield
LONG TERM...BRB
AVIATION...Nield