IND issues Area Forecast Discussion (AFD) at Dec 31, 9:47 AM EST375
FXUS63 KIND 311447
AFDIND
Area Forecast Discussion
National Weather Service Indianapolis IN
947 AM EST Tue Dec 31 2024
.KEY MESSAGES...
- Rain/showers today bringing 0.25-0.75 inches rainfall to most
locations, with higher amounts possible across northern tier
- Noticeably colder temperatures will usher in the New Year
- A chance for light snow late in the day Thursday into Thursday
night
- Potential for a significant winter storm in the Midwest Sunday
into Monday
- Bitter arctic cold in the week 2 period
&&
.FORECAST UPDATE...
Issued at 943 AM EST Tue Dec 31 2024
The day-time forecast has remained very similar with light to
moderate stratiform rain maintaining along deformation of the 850-
700mb low. Within the "warm" sector, isentropic lift is leading to
continuous shower development. There is an area of weak elevated
instability in the far E/SE portions of central Indiana that could
lead to brief heavy rain rates and isolated lightning.
The CCB will continue to develop during the day; along with diurnal
cooling, should allow for decreasing wet-bulb temperatures and
surface temperatures approaching freezing within saturation this
evening. This should lead to some snow to mix-in after 23Z, but no
accumulation is expected at this time.
Overnight, mid level vorticity along with a deep saturated layer
should allow for scattered flurries to develop. There is a low
chance for these to become isolated snow showers as well with light
accumulations, but this has not been included at this time due to
low confidence.
&&
.SHORT TERM (Today and tonight)...
Issued at 303 AM EST Tue Dec 31 2024
The last rainmaker of this late December's damp to at times rainy
period will slice across central Indiana today. Widespread light to
moderate rain during pre-dawn hours will lead to numerous showers
over the majority of the region through at least morning and midday
hours as the center of circulation tracks from Rockville to Muncie.
Rain intensity and coverage may diminish at times across southern
counties where some semblance of a mid-level dry conveyor attempts
to advance into an otherwise rather symmetrical system.
The solid majority of this system's rainfall will have fallen by
early afternoon...with expectations continuing for moderate rainfall
over roughly the region's northern half and lesser amounts south of
I-70. Precipitation will slowly taper off from west to east through
afternoon and evening hours, although at least a few wet snowflakes
should mix in over north-central and northeast counties later today
as the colder column on the backside of the system swings in from
the northwest...and surface temperatures nudge down into the mid-30s.
No impacts are expected given warm ground temperatures, although a
thin coating of slush cannot be ruled out on grassy or elevated
surfaces, especially after dark northeast of a Kokomo to New Castle
line.
This locally potent system will allow some gustier winds to mix down
to the surface on its heels later today, with northwest winds
sustained up to 20 MPH, and an area of gusts up to 30 MPH crossing
the region from northwest counties this afternoon...across the
Indianapolis Metro through the early evening rush hour...and down
southeastern portions of the I-74 corridor through the evening.
Tonight will find a slow transition to fair weather with upstream
high pressure rather weak and centered back over the High Plains.
Patchy drizzle/flurries may linger past midnight over mainly eastern
zones, with moderate confidence no measurable precipitation lasts
after the 32F isothermal's late-night passage. Temperatures through
the short term will range from highs in the 40s this morning...to
widespread mid to upper 30s around midnight tonight...with lows by
early Wednesday in the upper 20s for most locations.
&&
.LONG TERM (Wednesday through Monday)...
Issued at 303 AM EST Tue Dec 31 2024
Overview:
Those who love winter will be happy over the next several weeks as
the much colder and potentially snowier pattern is well underway to
setting up and persisting for at least two and likely three weeks.
Much of this period will see temperatures remain well below freezing
with sub zero temperatures and apparent temperatures as well at
times. Outside of heavy snowfall potential late this weekend and
early1 next week...most probable chances for snow within arctic
airmass to likely come from fast moving clipper systems that will
also reinforce the bitter cold.
Wednesday and Thursday...
These periods look mainly dry but seasonably cold as cold air
advection continues in wake of short wave. Moisture profiles dry
significantly by Wednesday but could see some flurries or very light
snow showers, especially in the east. Will keep forecast dry this
run though with flurries most likely outcome. High pressure will
slide across the Ozarks and into the TN valley through this period,
keeping northwest flow over central Indiana. Fast moving clipper
will approach late Thursday afternoon. Best chance for accumulating
snow should hold off until Thursday evening but timing will be
close. These clippers often arrive a bit faster than models indicate
riding the nose of upper level jet.
Thursday Night...
Aforementioned clipper in the form of a short wave and compact
vorticity lobe ahead of a diving 300mb jet max will quickly race
east Thursday evening and into the overnight. Gulf remains cut off
and moisture is very limited. Area of weak frontogenesis and lift
may be enough to squeeze out some light qpf, which many models are
indicating. Low levels will initially be dry so concern about how
much snow will go into saturating low levels before starting to
reach ground and accumulate. Blends and WPC giving just under a
tenth of an inch liquid which seems on the generous side. With cold
air in place and little in way of Warm air advection, snow ratios
closer to 14:1 to 16:1 being forecast which kicks out anywhere from
a few tenths of an inch to just under 2 inches in the east.
Friday through Monday...
Elongated area of high pressure to slide across the region for a
another quiet but even colder period into Sunday before potentially
impactful winter storm moves through the Ohio valley.
00z model runs have done little to give any confidence or insight
into what may happen with this system. Global NH water vapor shows
fast moving jet over the western Pacific which will be the impetus
for our developing storm system. 00z model fluctuations and run to
run inconsistency likely arising from inability to properly sample
this fast moving jetstream and embedded energy, especially closer to
the weekend as jet streak comes onshore and dives into the southern
plains and panhandle region. Obviously not making any changes to
blended ensemble solutions as status quo with median solutions is
best option for now. This leads to current forecast of mainly snow
for the entire area. However, GFS trended weaker and EC much faster
and further south. Really no confidence in either solution and agree
with WPC extended discussion on more preference toward previous 12z
runs. Think this will be a more dynamic and intensifying system
given the current Pacific kinematics and potential thermodynamics a
more amplified system would bring with strong baroclinic zone and
gulf/Pacific moisture plumes. An evolution more similar to 30/12z
runs is preferred, which would potentially bring the warmer nose and
freezing pcpn back into play for parts of Indiana.
We cannot say it enough, but please do not focus on any of the model
snowfall accumulation maps being tossed around on social media. Way
too many uncertainties not to mention about a 12-15 inch high bias
in these extended model snowfall maps. Plan and be prepared and pay
attention to reliable sources over the coming days.
Days 8 through 14...
Very cold this period with temperatures easily running 10 to 15
degrees below normal. Lows most likely to begin dipping into single
digits around morning of the 8th. If snow pack exists following
winter storm system, temps could be more like 15 to 25 degrees or
more below normals. This still looks to be an extended period of
well below normal temperatures and potentially a couple weeks of
below freezing temperatures. Plan now for extended cold and pay
attention to forecasts for possible cold weather alerts.
&&
.AVIATION (12Z TAF Issuance)...
Issued at 625 AM EST Tue Dec 31 2024
Impacts:
- Mainly IFR ceilings for most of TAF period...subtle improvement to
MVFR expected by late evening
- Visibility mainly MVFR in periods of rain...but will vary from VFR
to possibly IFR at times in heavier SHRA
- Chaotic wind directions this morning as surface low rides up I-70
corridor...northwest gusts this afternoon/evening to 23-30KT
Discussion:
Main area of organized/heavier rain has pushed northeast of TAF
sites...with isolated to numerous -SHRA expected through at least
midday today. IFR conditions will be locked in for most of the TAF
period, with low ceilings possibly briefly dropping to LIFR, and
MVFR visibility in the few heavier showers and/or areas of BR.
Wind direction will be chaotic this morning as the system's surface
low tracks east-northeastward up the I-70 corridor, with southerly
and/or easterly flow backing to north/west by 18Z. Robust breezes
from 300-340 degrees will gust up to 23-30KT this afternoon and into
most of the evening.
&&
.IND WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$
UPDATE...Updike
SHORT TERM...AGM
LONG TERM...Lashley
AVIATION...AGM
Source:
IND issues Area Forecast Discussion (AFD) at Dec 31, 9:47 AM EST---------------
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