IND issues Area Forecast Discussion (AFD) at Dec 3, 9:43 PM EST823
FXUS63 KIND 040243
AFDIND
Area Forecast Discussion
National Weather Service Indianapolis IN
943 PM EST Tue Dec 3 2024
.KEY MESSAGES...
- Low chance for freezing drizzle over mainly the southwest half
of central Indiana late tonight and early Wednesday
- Snow showers Wednesday night with some travel impacts possible.
- Wind chills as low as -5 degrees Thursday morning
- Warming trend this weekend and rain early next week
&&
.FORECAST UPDATE...
Issued at 943 PM EST Tue Dec 3 2024
Stratus remains stubborn across the forecast area this evening but
it is steadily diminishing from the southwest and southeast as warm
advection and return flow has developed on the back side of high
pressure centered over the central Appalachians. Temps have moved
little this evening under the stratus...largely holding within a
range from the upper teens to the mid 20s.
Bit of a complex forecast overnight as warm advection strengthens
and interacts with a narrow axis of moisture currently moving across
Missouri. The stratus accompanying the moisture plume has been
slowly decreasing in coverage as it moves into the mid Mississippi
Valley but several of the CAMs have that increasing over the next 4-
6 hours as it approaches the region...influenced by an increase in
winds within the 925-850mb layer and a noted uptick in a band of
isentropic lift that is weak but present.
Subsidence above the boundary layer will aid in a strengthening but
shallow inversion and while the stratus over the region currently
will continue to lift to the northeast and likely lead to a period
with mainly clear skies after midnight...the arrival of the stratus
to the west will expand back across the forecast area after about
07Z. The subtle uptick in isentropic lift noted above in tandem with
the increasing warm advection regime presents a non-zero threat for
patchy drizzle to develop. While temps will slowly rise overnight...
readings will remain in the mid to upper 20s during the predawn
hours and through daybreak with any drizzle freezing to surfaces on
contact.
Will continue the freezing drizzle mention focused especially over
the southwest half of the forecast area...but have expanded the
mention slightly further to the north and east towards daybreak.
Still think any fzdz will remain generally south and southwest of
the Indy metro but will continue to monitor trends over the next few
hours and adjust as needed. Any amounts will be light and the
overall freezing drizzle will be patchy at best...but even trace
amounts will be enough to cause some slick spots on roadways. Focus
for fzdz will largely exist between 08 and 13Z from west to east.
Zone and grid updates out.
&&
.SHORT TERM (This evening through Wednesday)...
Issued at 228 PM EST Tue Dec 3 2024
A 1038mb surface high is still centered just SW of central Indiana,
most of which is collocated with a low stratus layer. Within this
area of high pressure, subsidence and weak winds have acted to
reinforce the stratus layer, keeping it around through the afternoon
and should continue to remain into the evening. There are already
some hints at mixing over NE KY, of which could erode SE IN stratus
earlier than currently in the forecast.
This increase in cloud cover led to a dampened diurnal curve this
afternoon, which has slightly suppressed afternoon highs along and
south of the I-70 corridor. This will also suppress nocturnal
cooling as well with temperatures staying in the 20s overnight.
Surface high pressure and low level ridging is expected to push
southeast this evening and overnight, resulting in continued warm
air advection just above the surface. Surface temperatures will be
slow to respond however, so a strong near surface inversion is
expected to develop. A low level moisture surge is pushing north
tonight below broader scale subsidence. This is expected to result
in a relatively thin low stratus layer over SW portions of the
state. As mentioned WAA will increase overnight, and could induce
some lift through this stratus layer. HREF models are widely spread
on the magnitude of moisture advection into central Indiana as well
as have a wide spread in location of saturated layer with respect to
vertical lift.
With this said there is a low chance(20-30%) of this saturated
layer coinciding with the vertical lift. If this occurs light
drizzle may begin to develop. Usually this would not be impactful,
however with surface temperatures lagging beneath the WAA, this
drizzle may freeze on contact with frozen objects/ground. Overall
this shouldn't result in more than a few hundredths on ice
accumulation, but that can still produce slick spots on roadways.
Greatest chance of occurrence will be over SW central Indiana
between 4AM and 7AM.
Once this stratus layer pushes off to the east, the rest of
Wednesday should be rather quiet as temperatures quickly rise beneath
strong WAA. Broken cloud cover is likely within this WAA, but
precipitation should be minimal until a shortwave trough approaches
from the north. Most of the precipitation associated with this low
should be after 00Z (7PM) but a light rain or even rain/snow mix is
possible as the frontal boundary nears, mainly over far NE portions
of central Indiana. Further discussion on precipitation after 00Z
Wednesday evening is in the Long Term.
&&
.LONG TERM (Wednesday night through Tuesday)...
Issued at 228 PM EST Tue Dec 3 2024
Steep low-level lapse rates and residual moisture behind the cold
front will result in snow showers Thursday night, until the mid-
upper level trough axis passes later in the night. These could be
briefly/locally intense causing travel impacts from some
accumulation and visibility reduction. Post-frontal winds of 20-25
mph with gusts 35-40 mph are possible. Strong cold advection and
MSLP gradient supporting 10-20 mph winds early Thursday morning will
lead to wind chill values as low as -5F.
Models have collectively trended more anomalous and therefore
stronger with the Great Lakes shortwave trough departing on
Thursday. Resulting sensible weather changes for us will be minor,
but it strengthens the case for lingering stratus with perhaps
stronger more cyclonic low-level flow remaining until Thursday
night. Residual moisture below subsidence inversion this time of
year often is more capable of stratus than models explicitly
indicate. Will trend sky cover up through the day Thursday.
Thursday night, midlevel clouds tied to a low-amplitude shortwave
trough may limit radiative cooling. If these are less prevalent or
delayed, this would increase the radiative component resulting in
colder temperatures overnight. Cold advection with tempered
radiative component would still result in low-mid teens for minimum
temperatures, but with less wind and thus wind chill values not as
cold as Wednesday night.
Multi-model ensemble shows mid-upper ridging and anomalous mean
heights this weekend, signaling a warming trend. We should return to
near normal temperatures Saturday and exceed normal values Sunday
into Monday as ridge becomes optimally positioned for warmth ahead
of deepening central CONUS trough. Ensemble clusters start to fall
out of phase by early next week with a more progressive GEFS-
weighted camp and lagging EPS-weight camp.
As the trough moves east, models differ in handling of a southern
stream shortwave that will move northeast ahead of it. There does
appear to be a strong IVT signal with broad moisture advection into
the region and ~2-sigma PWAT anomalies. This should support a decent
rain event sometime in the Sunday night through Tuesday morning
window. Right now, multi-model ensemble mean is generally 0.50-1.00
inch, with some outlier members over 1.00 inch. Higher amounts would
be possible if the moisture trajectories aren't more suppressed into
the Ohio Valley. Too much chaos in the ensemble data with southern
stream shortwave characteristics at the moment to narrow amounts
down.
In the Day 8-14 period it appears we'll see a colder trend initially
followed by a signal in the medium-range models of a warming trend
late in that period.
&&
.AVIATION (00Z TAF Issuance)...
Issued at 558 PM EST Tue Dec 3 2024
Impacts:
- MVFR ceilings persist this evening
- Low Chance -FZDZ at KHUF and KBMG early Wednesday morning
- Low level wind shear impacting all terminals predawn through mid
morning Wednesday
- W/SW winds gust speaking at 25-30kts Wednesday
Discussion:
MVFR stratocu deck remaining rather stubborn over central Indiana
early this evening and is likely to erode from the southwest slowly
over the next several hours as warm advection strengthens. Skies may
clear for a period of time overnight before another stratus layer
over western Missouri currently spreads in from the southwest. This
layer could produce patchy light FZDZ at both KBMG and KHUF for a
couple hours with an increasing shallow inversion present but
confidence in occurrence remains low at this time.
The expansion of a jet at 925mb late tonight supports the potential
for a 3-5 hour period with low level wind shear impacting the
terminals after 08-09Z. As surface winds increase Wednesday morning
courtesy of a tightening surface pressure gradient ahead of an
approaching cold front...low level wind shear concerns will
diminish. Wind gusts will peak at 25-30kts on Wednesday as winds
veer from southwest to westerly by late day with the frontal
passage.
There is potential for a period of snow showers immediately behind
the frontal passage Wednesday evening that could cause brief impacts
to the terminals.
&&
.IND WATCHES/WARNINGS/ADVISORIES...None.
&&
$$
UPDATE...Ryan
SHORT TERM...Updike
LONG TERM...BRB
AVIATION...Ryan
Source:
IND issues Area Forecast Discussion (AFD) at Dec 3, 9:43 PM EST---------------
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