IND issues Area Forecast Discussion (AFD) at Dec 20, 3:08 PM EST
085
FXUS63 KIND 202008
AFDIND
Area Forecast Discussion
National Weather Service Indianapolis IN
308 PM EST Fri Dec 20 2024
.KEY MESSAGES...
- Isolated snow showers or flurries possible into the night
- Wind gusts 25-35 mph this evening, diminishing overnight
- Periods of rain and a warming trend next week
&&
.SHORT TERM (This evening through Saturday)...
Issued at 307 PM EST Fri Dec 20 2024
Today and Tonight
Snow is coming to an end across the region as surface low pressure
continues eastward towards the Atlantic coast. Little in the way of
cold air advection is occurring on the back side of the system. In
fact, temperatures have warmed slightly across central and northern
Indiana as trajectories trace back towards Lake Michigan, where
warmer water temps have modified the upstream air mass.
Nevertheless, temperatures will eventually trend colder as the core
of the continental polar (Cp) air mass approaches. Lows dip into the
20s tonight which may allow any slush or water on untreated surfaces
to refreeze.
Wind gusts, currently gusting up to 25-35mph, should diminish
through the night as the tighter MSLP gradient associated with the
low pulls eastward. Cloud cover should also diminish as most
guidance shows drying through the column as the Cp air mass arrives.
Satellite observations lend credence to this as clearing is noted
over northern Illinois and Wisconsin. However, stratus may persist
downstream of Lake Michigan as the modified Cp air may retain enough
moisture to sustain cloud cover longer than would otherwise be
expected. Some flurries are also possible as decaying lake-effect
snow showers may survive long enough to enter our northwestern
counties. Best chance of snow showers or flurries is through roughly
1am.
Saturday
Northwesterly flow is modeled to persist into Saturday with
gradually diminishing wind speeds. Cold advection should dominate
the diurnal temperature swing and highs may not rise above freezing
for a good portion of the CWA, especially where lingering snow cover
remains. Stratus may linger well into the daylight hours mainly
downstream of Lake Michigan. We increased cloud cover across our
northern and northeastern counties to account for this. Overall, a
chilly but quiet day is anticipated as surface high pressure builds
back into the region.
&&
.LONG TERM (Saturday night through Friday)...
Issued at 307 PM EST Fri Dec 20 2024
An active and progressive weather pattern continues in the long term
resulting in above normal temperatures and frequent chances for
rainfall to end out the year.
High pressure shifts east of the region Sunday and Monday ahead of
the next system developing in the Plains. With a cold airmass
already in place at the surface, the warming trend will likely start
off slow as dense cold air typically remains at the surface longer
than what guidance suggests. Therefore, going below guidance for
highs both Sunday and Monday with Sunday being the cooler of the two
days. Keeping highs near freezing for areas along and north of I-74
Sunday, especially for those locations with any snowpack. Highs
should exceed the freezing mark for South Central and Southwest
Indiana. Weak ridging and warm air advection slowly chip away at
the cold surface airmass with highs likely 5-10 degrees warmer in
the upper 30s to mid 40s.
Periodic precipitation chances are expected next week with multi-
model ensemble QPF totals in the 0.50-0.75 inch range by the end of
the week. The first round arrives Monday afternoon with a transient
shortwave trough interacting with modest moisture/PWAT anomalies.
Dry air ahead of the system will be a struggle to overcome with
latest guidance picking up on this dilemma. The best chance for
any rain development along the front in South Central Indiana where
better moisture advection is present.
For Tuesday, latest 12z guidance today keeps precipitation
development along the Ohio River with likely leftover moisture stuck
under a low level inversion across much of Central Indiana. Keeping
low PoPs Tuesday for the southern portion of the area; however this
pattern supports low clouds, fog, and maybe drizzle with cooler
temperatures than what guidance suggests (still above freezing
though).
Uncertainty builds by Christmas day and through the end of the week
as a relatively split flow pattern sets up with the polar jet
lifting well northward into Canada and numerous weak troughs
developing within the subtropical jet resulting in chances for rain
and warmer weather locally. Some ensemble members are showing over
1.00 inch QPF total for next week. Predictability with regards to
specific timing of more contiguous rain areas and breaks between is
limited at this time, especially the latter half of the week.
Refinement to precipitation probabilities will can be expected in
subsequent forecasts. This is a pattern where warm advection should
be enough for positive 2-m temperature anomalies (potentially +15 by
Thursday-Friday) and generally all rain, with little concern for
wintry precipitation and associated impacts.
Day 8-14: Medium-range models are out of phase with considerable
chaos in this period, but have a general signal for high-latitude
positive height anomalies and periodic forcing amid waves of
anomalous moisture. Thus, above normal temperatures (albeit probably
not substantially) and above normal precipitation are indicated.
An overall pattern flip is still expected by early to mid January as
the PNA goes from positive to neutral and the NAO going negative.
Ensembles have been in good agreement with this trend, which could
result in a more blocking pattern setting up across the Eastern US.
Will watch this as the new year approaches, but for now, expect a
relatively warm and rainy end of 2024.
&&
.AVIATION (18Z TAF Issuance)...
Issued at 1209 PM EST Fri Dec 20 2024
Impacts:
- Snow showers ending across eastern Parts of Central Indiana early
this afternoon.
- MVFR Ceilings will remain into the evening hours. Low VFR Cigs
expected overnight.
- Gusty winds diminish this evening.
Discussion:
Radar trends show snow showers exiting Central Indiana long with the
surface low pressure system now over Ohio. This system will continue
to push east and allow snow showers to exit with it by 19Z-20Z.
GOES16 shows extensive cloud cover upstream and across the region
within the cyclonic flow in place across Indiana. This will result
in continued MVFR ceiling as the pass across the TAF sites. Some
improvement to VFR Ceilings will be possible later this evening as
higher cigs upstream advect into central Indiana. Forecast soundings
are on board with these trends, keeping lower level saturated amid a
cold air advection regime.
The strong pressure gradient across the area this afternoon will
weaken this evening, allowing wind gusts to subside.
&&
.IND WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$
SHORT TERM...Eckhoff
LONG TERM...CM
AVIATION...Puma
Source: IND issues Area Forecast Discussion (AFD) at Dec 20, 3:08 PM EST (https://mesonet.agron.iastate.edu/p.php?pid=202412202008-KIND-FXUS63-AFDIND)
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